The majority of the promyelocytic leukemia (PML) protein is present in nuclear bodies which are altered in several pathogenic conditions including acute promyelocytic leukemia. PML nuclear bodies are found in nearly all cells yet their function remains unknown. Here, we demonstrate that PML and the eukaryotic initiation factor 4E (eIF-4E) co-localize and co-immunopurify. eIF-4E is involved in nucleocytoplasmic transport of specific mRNAs including cyclin D1. eIF-4E overexpression leads to increased cyclin D1 protein levels; whereas, overexpression of PML leads to decreased cyclin D1 levels. Neither PML nor eIF-4E cause significant changes in cyclin D1 mRNA levels. The association with eIF-4E led us to investigate if PML could alter mRNA distribution as a possible post-transcriptional mechanism for suppressing cyclin D1 production. We show that overexpression of PML results in nuclear retention of cyclin D1 mRNA and that intact PML nuclear bodies are required. Addition of eIF-4E overcomes PML induced retention and alters the morphology of PML bodies suggesting a mechanism by which eIF-4E can modulate PML function. These results raise the possibility that PML nuclear bodies may participate in the regulation of nucleocytoplasmic transport of specific mRNAs.
Acute promyelocytic leukemia (APL) is characterized by a block in the promyelocyte stage of myeloid development and comprises approximately 10% of myelogenous leukemias (reviewed by Melnick and Licht, 1999). The promyelocytic leukemia (PML) protein was originally identified in a chromosomal translocation associated with ∼98% of APL cases. The PML protein is present in a set of nuclear organelles, PML nuclear bodies. These bodies are observed in all cell lines reported with the exception of hepatocarcinoma cells and are distinct from other nuclear organelles such as coiled bodies and nucleoli (Terris et al., 1995; Flenghi et al., 1995). PML nuclear bodies are altered by several pathogenic conditions including APL, spinocerebellar ataxia, and viral infection (reviewed in Melnick and Licht, 1999). In APL, the PML protein is fused to the retinoic acid receptor alpha (RARα) as a result of a chromosomal translocation. Pro-apoptotic and growth suppressive functions are attributed to PML and are linked to the integrity of its bodies (Wang et al., 1998b; Quignon et al., 1998; Borden et al., 1997; Ferrucci et al., 1997; Mu et al., 1994). Despite widespread interest, the function of PML and PML nuclear bodies remains elusive. Their prevalence and sensitivity to several pathogenic conditions suggest that PML nuclear bodies perform a basic function in mammalian cells.
The composition of PML bodies is heterogeneous with several components present only in a subset of bodies (Carlile et al., 1998; LaMorte et al., 1998; Borden et al., 1998b). PML associated with proteins involved in translation and transcription (Melnick and Licht, 1999). mRNA is present in some but not all nuclear bodies (LaMorte et al., 1998). Apart from nuclear bodies, PML is found as a soluble component in the nucleus and a small fraction of PML is found in bodies in the cytoplasm (Flenghi et al., 1995). Alternative splicing of PML transcripts results in co-expression of several isoforms (Fagioli et al., 1992). All isoforms have major nuclear and minor cytoplasmic distributions, although two rare isoforms are predominantly cytoplasmic in transfected cells (Fagioli et al., 1992). PML can shuttle between the nucleus and cytoplasm (Stuurman et al., 1997).
PML contains three cysteine-rich zinc-binding domains which are known as the RING and B boxes (B1 and B2) and a leucine coiled-coil domain forming a RBCC motif (Saurin et al., 1996). The RBCC motif is present in PML-RARα and in all PML isoforms. The RING motif appears in over 200 plant, animal and virus proteins including several oncoproteins and is involved in protein–protein interactions (Saurin et al., 1996; Kentsis and Borden, 2000). Mutation of the zinc-binding residues in the RING or B-boxes disrupts PML nuclear bodies (Borden et al., 1995, 1996; Le et al., 1996) and simultaneously impairs the pro-apoptotic and growth inhibitory activities of PML (Borden et al., 1997; Ferrucci et al., 1997; Mu et al., 1994).
We showed that PML interacts with the ribosomal P-proteins (P0, P1, P2) in the nucleus of NIH3T3 cells (Borden et al., 1998b). The P-proteins form part of the large ribosomal subunit and are required for protein synthesis (Borden et al., 1998b and references therein). Yeast 2-hybrid techniques indicate that PML associates with other translation factors: elongation initiation factor 1 (EF-1) and leucine zipper protein L7a (Boddy et al., 1996). Several ribosomal proteins and translation factors associate with nuclear structures and have nuclear functions in addition to their cytoplasmic ones. For instance, the ribosomal P-proteins are nuclear but excluded from the nucleoli in non-exponentially growing cells and are upregulated in colon carcinoma (Yacoub et al., 1996; Borden et al., 1998b). The nuclear fraction of L7a, again nucleolar excluded in non-exponentially growing cells, appears active in cellular transformation (Ziemieki et al., 1990). These findings raised the possibility that PML could function in some aspect of translational control and that this function may be related to its growth and transformation suppression activities.
The above results led us to investigate if PML could associate with other translation factors like eukaryotic translation initiation factor 4E (eIF-4E). Previous reports indicated that eIF-4E had nuclear and cytoplasmic functions in protein production making eIF-4E a good candidate for these studies. In the nucleus, eIF-4E selectively increases nucleocytoplasmic transport of certain transcripts including cyclin D1 (Rousseau et al., 1996; Rosenwald, 1995). A large fraction of eIF-4E is found in nuclear bodies, distinct from nucleoli, in non-exponentially growing cells (Lejbkowicz et al., 1992). In another report, we show that a related RING protein, Z, co-localizes and co-immunopurifies with eIF-4E (Campbell Dwyer et al., 2000). This viral protein binds directly to PML and translocates bodies to the cytoplasm (Borden et al., 1998a). The PML-Z association was another indication that eIF-4E would likely co-localize and co-immunopurify with PML.
We report here that PML and eIF-4E co-localize, co-immunopurify and have antagonistic effects on expression of cyclin D1. eIF-4E promotes protein production of cyclin D1 by preferential transport of its mRNA from the nucleus to the cytoplasm (Rousseau et al., 1996). We show that PML can suppress cyclin D1 protein production by preferential retention of this mRNA in the nucleus. Suppression is partially counteracted by additional eIF-4E. Our results show that overexpression of eIF-4E alters the morphology of PML bodies suggesting a mechanism by which eIF-4E could modulate PML function. Overexpression of PML does not alter the subcellular distribution of eIF-4E. Neither PML nor eIF-4E alter protein production of GAPDH or actin, previously established eIF-4E insensitive transcripts (Rousseau et al., 1996). We discuss the implications of translational suppression for the growth suppressive roles of PML and how disruption of this function may contribute to leukemogenesis in APL.
eIF-4E co-localizes with PML nuclear bodies
eIF-4E has a similar subcellular distribution to PML in non-exponentially growing cells where the majority of nuclear eIF-4E is localized to bodies (Lejbkowicz et al., 1992). Therefore, we investigated whether eIF-4E co-localizes with PML nuclear bodies. Immunofluorescence studies in conjunction with confocal laser microscopy were carried out (Figure 1). NIH3T3 cells were stained with a monoclonal antibody to eIF-4E, mAb eIF-4E (red; panels b,e) and a polyclonal antibody to PML (Borden et al., 1995) (green; panels a,d) with the image overlay (ov) shown in yellow (panels c,f). Immuno-staining for eIF-4E indicates that in some cells eIF-4E has a nuclear punctate pattern (Figure 1; Lejbkowicz et al., 1992) which is similar to the PML nuclear body pattern (Borden et al., 1995). Clearly, a subset of eIF-4E co-localizes with PML nuclear bodies. In some cells, eIF-4E forms a ring around the nuclear membrane. PML does not appear to localize with this population of eIF-4E. Those cells expressing higher levels of eIF-4E in the cytoplasm (Figure 1, arrow in bottom panels) appear to have higher levels of cytoplasmic PML. Like other PML body components, co-localization of eIF-4E with PML is not complete demonstrating the heterogeneous nature of these bodies. These data show that a subset of endogenous PML and eIF-4E co-localize in nuclear bodies.
PML and eIF-4E co-immunopurify
The co-localization studies raised the possibility that PML and eIF-4E could physically interact. To confirm the co-localization studies and address the possibility of an interaction, we carried out co-immunoprecipitation experiments. Human fibroblast 551 cell lysates were fractionated into nuclear and cytoplasmic components. PML in these fractions was immunoprecipitated with the monoclonal antibody mAb 5E10, which is specific to human PML (Stuurman et al., 1992). The proteins in the immunoprecipitated complexes were resolved by SDS–PAGE and subsequent Western probed with mAb eIF-4E. As shown in Figure 2a, a significant amount of eIF-4E co-immunoprecipitates with PML in both the nuclear and cytoplasmic fractions. Similar results were observed in separate experiments using NIH3T3 cells (data not shown). Results from the complementary experiment where total cell lysates were precipitated with eIF-4E and blots probed for PML indicates that a significant amount of PML co-purifies with eIF-4E (Figure 2b). This blot was probed with another monoclonal antibody to human PML, mAb PGM3 (Flenghi et al., 1995). The multiple band pattern observed indicate that more than one isoform of PML is observed, as expected. The band pattern is similar to those reported previously with this antibody (Alcalay et al., 1998; Flenghi et al., 1995). As expected, mAb eIF-4E immunoprecipitates eIF-4E (lower panel) and both PML antibodies immunoprecipitate PML (data not shown). GST pull down assays with PML and eIF-4E suggest that the interaction between these proteins is direct (unpublished observations).
To determine whether the interaction was specific we tested the ability of mAb 5E10 to immunoprecipitate other proteins. First, we examined whether PML associates with RNA polymerase II. Figure 2a shows that these proteins did not associate. Similarly, we show that actin does not precipitate with mAb 5E10 (Figure 2a). Immunoprecipitation with mouse immunoglobulin (IgG) resulted in no precipitation of eIF-4E or PML in either fraction (Figure 2d). Therefore, our data indicate that PML and eIF-4E associate specifically with each other, either directly or indirectly, consistent with our confocal data. The eIF-4E protein associates with several other translation factors, thus we cannot distinguish between the possibility that PML binds directly to eIF-4E or to eIF-4E associated factors. Nevertheless, PML clearly does co-immunopurify and co-localize with eIF-4E.
The quality of our subcellular fractionation is demonstrated in Figure 2e. The bottom panel shows that the majority of GAPDH is found in the cytoplasm with a minor nuclear distribution as observed here and reported previously (Singh and Green, 1993; Carlile et al., 1998). The top panel shows that the majority of PML is found in the nucleus, as expected. There is a faint band in the cytoplasm consistent with the observation of PML in the cytoplasm by other groups (Fagioli et al., 1992). Because of the small amount of cytoplasmic PML observed, it was necessary to use four T75 flasks to obtain sufficient cytoplasmic PML for the above immunoprecipitation experiments.
Previously, we have shown that some interactions with PML are dependent on the presence of RNA (Carlile et al., 1998). Total cell lysates were incubated in the presence or absence of RNAse before immunoprecipitation. Lysates were immunoprecipitated with mAb 5E10 and subsequent blots probed for eIF-4E. Figure 2c indicates that PML and eIF-4E do not require RNA for association in these studies. In the lower panel (Figure 2c), we show that the interaction between PML and GAPDH is RNA dependent as reported previously and serves as a positive control of RNAse activity (Carlile et al., 1998).
Many of the functions of PML are dependent on the integrity of PML nuclear bodies. Experiments were conducted to determine whether intact PML nuclear bodies were required for co-immunoprecipitation with eIF-4E. NIH3T3 cells were transfected with human PML, subsequent cell lysates precipitated with mAb PGM3 and blots probed with mAb eIF-4E. Our use of human PML and immunoprecipitation with mAb PGM3 means that only transfected PML will be precipitated in these studies. Transfection of PML constructs with point mutations in the first zinc binding site of the RING (referred to as PMLRINGmut) or in the B-boxes are known to disrupt PML nuclear body formation (Borden et al., 1995, 1996). These constructs are listed in Figure 3a. In these experiments, the transfection efficiency is approximately 50% by confocal microscopy with transfected cells over-expressing PML to various extents. The pattern most frequently observed is shown in Figure 6a. In Figure 3b, Western analysis indicates that transfected wild-type PML and endogenous eIF-4E co-purify. Further, eIF-4E substantially co-purifies with the mutants. The lower panel in Figure 3b indicates that the associations observed in these experiments are specific as actin does not co-purify with PML. Also, immunoprecipitation with mouse IgG does not precipitate eIF-4E (Figure 2a). Thus PML nuclear bodies are not required for the co-purification of PML with eIF-4E. Co-purification with the PMLRINGmut is consistent with our previous studies which show that Z, a PML related RING protein, co-immunopurifies with eIF-4E when similar mutations are made in the first zinc-binding site of its RING (Campbell-Dwyer et al., 2000). Protein interaction experiments indicate that eIF-4E co-immunopurifies and co-localizes with both PML and Z through the second zinc binding site of the RING (our unpublished observations). Previously, our transient transfection studies suggested that transfected mutant PML, which does not form bodies, does not bind endogenous PML (Borden et al., 1995, 1996). Therefore, the interactions between mutant PML and eIF-4E are probably not mediated through endogenous wildtype PML.
PML overexpression alters production of cyclin D1 protein
The PML-eIF-4E interaction led us to investigate whether PML could affect cyclin D1 production since previous studies showed that eIF-4E overexpression increased cyclin D1 levels (Rousseau et al., 1996). The effect of eIF-4E was not general since cyclin D1 was preferentially suppressed whereas GAPDH was not (Rousseau et al., 1996). Because of the association of PML with eIF-4E and the previous reported growth suppression activities of PML, we predict that PML would decrease cyclin D1 protein production. NIH3T3 cells were transfected with PML and production of cyclin D1 and GAPDH proteins were monitored (Figure 4). We show that overexpression of PML reduces levels of cyclin D1 protein relative to untransfected (‘NT’ lane) and vector transfected cells (Figure 4a, upper panel). If PML induced suppression of cyclin D1 is mediated through eIF-4E, one would expect that eIF-4E could counteract this action of PML. Consistent with this hypothesis, eIF-4E and PML co-expression led to an increase in cyclin D1 protein production relative to PML only expressing cells (Figure 4a, lower panel). In these experiments, GAPDH levels did not vary appreciably. Blots were probed with the actin antibody to indicate the quantity of protein loaded into each lane. Consistent with these findings, previous reports indicated that cyclin D1 protein levels were reduced by PML overexpression in breast cancer cells but provided no mechanism for this observation (Le et al., 1998). Our data indicate that this suppression is mediated at least in part through eIF-4E.
Studies were carried out to assess the effects of PML mutations with all mutants studied disrupting PML nuclear body formation (Figure 4a,b). Cyclin D1 protein production was suppressed in cells expressing wild-type PML but not by expression of the PML mutant proteins. Consistent with previous studies indicating that mutation of the RING results in a loss of growth and transformation suppression actions of PML (Melnick and Licht, 1999), PMLRINGmut did not suppress cyclin D1 protein production. GAPDH and actin levels do not appear appreciably affected in these experiments.
PMLRINGmut immunoprecipitates with eIF-4E, but is unable to suppress protein production. This is consistent with recent studies which show that the Z protein co-immunoprecipitates with eIF-4E in a RING independent manner but, like PML, its ability to suppress cyclin D1 protein production is RING dependent (Campbell Dwyer et al., 2000). Z is 90 residues and the RING domain is the only feature it has in common with PML (Borden et al., 1998a). Addition of exogenous eIF-4E also relieves Z induced suppression of cyclin D1. Taken together, these data suggest that binding to eIF-4E is necessary but not sufficient to mediate these interactions and raises the possibility that other co-factors are required for PML induced suppression of cyclin D1 protein production.
PML decreases protein production selectively in cell culture
To demonstrate that the effects of PML are limited to specific proteins, we overexpressed PML in NIH3T3 cells and monitored total protein levels. Cells were transfected with PML and for comparison with PMLRINGmut, or vector. 35S-Met incorporation was monitored by autoradiography. Average signal intensity from three separate autoradiographs is shown (Figure 4c). The differences in incorporation between PML overexpressing cells and controls were insignificant. We note that slightly more 35S-Met was incorporated in cells transfected with the PMLRINGmut construct than PML. It appears that PML has no dramatic effect on total protein synthesis in cell culture. This is consistent with data from the above Western analysis which indicated that only specific proteins are affected by PML overexpression.
PML affects the subcellular distribution of cyclin D1 mRNA
We investigated whether PML reduced cyclin D1 levels by repressing transcription of cyclin D1 mRNA in cells transfected with PML (Figure 5a). RNA levels were determined using standard Northern methods from two independent transfections. Values obtained from measuring band intensities were normalized to the 18S and 28S rRNA band intensities observed in the corresponding ethidium bromide stained gels (Figure 5a). Intriguingly, levels of cyclin D1 mRNA produced were slightly higher in PML overexpressing cells than in vector controls. We then investigated the effects of PMLRINGmut, and of co-expression of PML and eIF-4E on cyclin D1 mRNA production. Similar to the results observed for PML, cyclin D1 mRNA levels appeared to be slightly elevated in the PMLRINGmut expressing cells and the PML eIF-4E co-expressing cells relative to vector controls. Results from the different transfections indicate that levels of cyclin D1 mRNA did not vary significantly amongst PML, PMLRINGmut and PML/eIF-4E transfection experiments. Relative to vector transfected cells, PML does not reduce levels of cyclin D1 mRNA and therefore suppression of cyclin D1 protein production is post-transcriptional. Also, increased cyclin D1 protein levels in PMLRINGmut and PML/eIF-4E relative to PML expressing cells do not result from concomitant increases in transcript levels (Figures 4 and 5). Since RNA levels are not markedly different between the transfection experiments, it is highly unlikely that expression of PML, PMLRINGmut or eIF-4E result in differential stability of cyclin D1 transcripts.
In combination, the above data suggest that PML affects protein production post-transcriptionally. Because eIF-4E modulates nuclear cytoplasmic mRNA transport of cyclin D1 (Rousseau et al., 1996), we investigated whether PML affects cyclin D1 mRNA distribution as a possible mechanism for PML induced suppression of cyclin D1 protein production (Figure 5b,c). We assessed the consequences of overexpressing PML on the distribution of specific mRNAs in nuclear and cytoplasmic fractions. Cells transfected with the appropriate DNA were fractionated as described (Rousseau et al., 1996) resulting in the preparation of nuclei free of cytoplasmic contamination. Several experiments were carried out to assess the quality of fractionation. The RNA distribution was analysed in three fractions: cytoplasm (supernatant of the first lysis), post-nuclear fraction (nuclear wash fraction) and nucleus. Cytoplasmic and postnuclear fractions are considered as cytoplasmic fractions. Initially, RNA levels were measured spectrophotometrically to determine the overall RNA distribution in the cell. The mean and standard deviations (s.d.) for endogenous total RNA distribution from nine separate transfection experiments were as follows: cytoplasm 55±14%, postnuclear fraction 26±9% and nucleus 19±10% with no dependence on the constructs used. These results are similar to those of Rousseau et al. (1996) who find 15% of total RNA in the nucleus. Our transfection efficiency was approximately 50% (see Materials and methods). For analysis of RNA distribution we used two reference markers, lysine tRNA and U6 snRNA. The distribution of lysine tRNA was as follows: 65±6% cytoplasm; 17±2% postnuclear and 18±7% nuclear (mean±s.d.). Similar to previous reports, 10% of lysine tRNA is in the nucleus (Rousseau et al., 1996). The distribution of U6 small nuclear RNA (snRNA) in our experiments was 38±11% nuclear, 37±10% cytoplasmic and 25±5% postnuclear (mean±s.d.). Rousseau et al. (1996) found that 40% of the snRNA was confined to the nucleus. Their report indicates that the large proportion of non-nuclear U6 snRNA is due to its small size (108 nucleotides) allowing it to more readily leak out of the nucleus than larger mRNAs (Rousseau et al., 1996). Importantly, the distributions of lysine tRNA and U6 snRNA do not vary with the construct transfected. Thus, the fractionation procedure was not transfection dependent.
We examined the effect of overexpressing PML on the subcellular distribution of endogenous cyclin D1 and GAPDH mRNAs in the same cells. RNA from each fraction was analysed by Northern blotting. Values were normalized to 28S and 18S rRNA band intensities to correct for gel loading errors and corrected for subcellular distribution (Figure 5c) according to the method of Rousseau et al. (1996). Each transfection was conducted independently at least twice and for PML and vector transfections, three times. There is no significant difference in the levels of nuclear GAPDH mRNA regardless of the protein(s) overexpressed (Figure 5c). These data are consistent with previous reports, showing that GAPDH mRNA distribution is not altered by eIF-4E (Rousseau et al., 1996) and GAPDH protein levels were not altered in similar experiments (Figure 4). By contrast, PML retains cyclin D1 mRNA in the nuclear fraction of the same cells. In these cells, 50% of cyclin D1 mRNA is in the nucleus. In Figure 5b, the gels were loaded to reflect the distribution of total RNA in the cell, as seen by the relative intensities of 28S and 18S rRNA. Despite the low levels of total RNA in the nuclear fraction, cyclin D1 still has a substantial nuclear component. The nuclear retention of cyclin D1 mRNA observed here is consistent with our protein studies showing substantial decreases in cyclin D1 protein production in PML overexpressing cells (Figure 4a,b).
Mutation of the RING of PML leads to a loss of PML's growth and transformation suppression activities and disrupts PML nuclear body formation. Thus, we investigated the effect of the RING mutation on cyclin D1 RNA retention. Cells expressing PMLRINGmut retain only ∼17% of this message in the nucleus (Figure 5b). This represents a threefold reduction of nuclear cyclin D1 mRNA in cells expressing PMLRINGmut versus PML. In order to highlight the reduction in cyclin D1 nuclear RNA in PMLRINGmut experiments, equal amounts of RNA from each fraction were loaded into each lane thus over-representing the population of total nuclear mRNA. Here, the major band is in the cytoplasmic fraction. The reduction in nuclear cyclin D1 mRNA between PMLRINGmut versus PML experiments is readily apparent (Figure 5b,c). Reduced nuclear mRNA retention in PMLRINGmut expressing cells is consistent with elevated levels of cyclin D1 protein observed relative to PML expressing cells (Figure 4). In contrast, the distribution of GAPDH mRNA, in the same cells, is not significantly affected by overexpression of PML or PMLRINGmut. Consistent with this result, no changes were observed in the levels of GAPDH protein produced (Figure 4). Note that levels of PML or PMLRINGmut proteins produced are similar in our transfection studies so altered activity of the mutant is not due to reduced protein levels (Borden et al., 1995; Figure 6; see Materials and methods).
If the ability to modulate RNA transport is mediated through the PML eIF-4E interaction, one would expect that expression of eIF-4E should alter this action. Overexpression of eIF-4E causes an increase in nuclear cytoplasmic transport of cyclin D1 mRNA leaving only 5% in the nucleus (Rousseau et al., 1996; Figure 5c). This is fivefold less than seen in vector transfected control cells (Figure 5c). Notably, retention in PML overexpressing cells is tenfold higher than with eIF-4E. Co-transfection with PML and eIF-4E results in 22% nuclear cyclin D1 mRNA, which is similar to values observed for PMLRINGmut. This is consistent with analysis of protein levels which showed co-expression of PML and eIF-4E resulted in substantial increases in cyclin D1 protein levels compared to PML alone expression experiments. Further, similar cyclin D1 protein levels were observed for PMLRINGmut and PML/eIF-4E experiments (Figure 4). Interestingly, nuclear levels of cyclin D1 mRNA do not fall to levels observed for eIF-4E only overexpression suggesting that PML exerts an inhibitory effect. We saw no significant variation in GAPDH mRNA distribution in these experiments consistent with the lack of significant changes observed in GAPDH protein levels (Figures 4 and 5c). The distribution of GAPDH transcripts shows no dependence on transfection indicating that alterations in RNA distribution were selective.
eIF-4E overexpression alters formation of PML nuclear bodies
The above experiments suggest that intact nuclear bodies are required for retention of transcripts by PML. Thus, it is important to establish the effect of eIF-4E on PML subcellular distribution. Cells transfected with PML show the standard pattern of nuclear body staining as described and the polyclonal PML antibody used recognizes both human and mouse PML (Figure 6a, Borden et al., 1995). Two cell nuclei are present; the larger one, outlined in white, was transfected and the smaller one on the lower left was not. The bodies formed from transfected material are typically larger than endogenous bodies (Borden et al., 1996 and references therein). The smaller nucleus typifies the punctate nuclear pattern expected for endogenous PML in NIH3T3 cells (Borden et al., 1995). Cells transfected with the PMLRINGmut show the characteristic nuclear diffuse pattern for this mutant (Figure 6b, Borden et al., 1995). Transfection of eIF-4E and subsequent staining with mAb eIF-4E gave a pattern similar to that observed endogenously (Figure 1) with both nuclear and cytoplasmic distributions (Figure 6c,d). Only one of the two cells present is transfected with eIF-4E. Although staining of endogenous eIF-4E is visible, the brightness of the transfected material meant that the laser power was reduced to observe only transfected cells in these micrographs.
To assess if eIF-4E could affect the morphology of PML nuclear bodies, we co-transfected NIH3T3 cells with genes for eIF-4E and wildtype PML (Figure 6). NIH3T3 cells overexpressing both PML and eIF-4E were stained with mAb eIF-4E (red; panels f,j) and a polyclonal antibody to PML (green; panels e,i) with the image overlay (ov) shown in yellow (panels g,k). There are many fewer PML bodies in the PML/eIF-4E transfected cells, typically only 1–2 large bodies per nuclei which is significantly less than for cells expressing PML (Figure 6a; Melnick and Licht, 1999). A similar pattern was observed for certain PML surface charge mutations of the RING (Boddy et al., 1997). As expected, expression of PML in these cells appears to be limited to the nucleus and excluded from the nucleoli. eIF-4E and PML partially co-localize as shown in yellow (Figure 6, panel g). In some cases, PML and eIF-4E bodies co-localize nearly completely. However, in other cases the PML and eIF-4E bodies only partially overlap. This suggests that other components of PML nuclear bodies, which are not upregulated in our transient transfections, may be required to optimize the overlap as observed for endogenous PML and eIF-4E in Figure 1. There is a substantial eIF-4E population, in the cytoplasm and as a diffuse distribution in the nucleus, that does not co-localize with PML. The partial co-localization of wild-type PML and eIF-4E in these experiments is consistent with our results which show that transfected and endogeous PML co-localizes and co-immunoprecipitates with eIF-4E (Figures 1–3).
To further investigate the effect that eIF-4E has on PML nuclear body assembly, we monitored the subcellular distribution of PMLRINGmut and eIF-4E. In single transfection experiments, PMLRINGmut was nuclear diffuse with no evidence of a cytoplasmic distribution (Figure 6, panel b; Borden et al., 1995). Surprisingly, there is a significant cytoplasmic staining observed for PMLRINGmut in the presence of overexpressed eIF-4E (panel i). PMLRINGmut and eIF-4E co-localize both in the nucleus and the cytoplasm. Both proteins form a ring around the nucleoli (top transfected cell, panel k). Interestingly, the co-localization between PMLRINGmut and eIF-4E is more pronounced than between wild-type PML and eIF-4E (Figure 6, panel g). Consistent with these co-localization findings, our co-immunoprecipitation studies show PMLRINGmut and eIF-4E co-precipitate (Figure 3). The disruption of assembly of normal PML bodies in the PML co-transfection experiments (Figure 6, panels e–h) and the dominance of eIF-4E over the nuclear diffuse PMLRINGmut phenotype (Figure 6, panels i–l) suggests that eIF-4E is able to disrupt normal PML body assembly. Thus, eIF-4E may alter the retention activity of PML by altering the morphology of PML bodies.
The eIF-4E subcellular distribution does not appear affected by co-expression of PML (Figure 6, panels f,j) as patterns are similar to those observed when only the eIF-4E gene is transfected (Figure 6, panels c,d) and to the endogenous pattern (Figure 1). There is significant eIF-4E present as bodies in nuclei and a significant staining around the nucleus as well as as an intense cytoplasmic distribution (Figure 6, panels c,d,f,j). For comparison, untransfected cells adjacent to those of interest are shown. The endogenous staining pattern is significantly weaker than that observed in transfected cells, as expected. In some cases the endogenous staining is not observed in the micrographs due to the relative brightness of the transfected material. Inspection of the phase contrast views (Figure 6, panels d,h,l) and fluorescent panels (Figure 6, panels c,f,j) indicates when this is the case. It appears that the subcellular distribution of eIF-4E is not altered by co-expression of PML or PMLRINGmut.
PML and PML nuclear bodies are crucial for normal functioning of the cell. Disruption of the PML protein and of PML nuclear bodies appears to contribute to leukemogenesis of APL and other pathogenic conditions (Melnick and Licht, 1999). However, there is no molecular mechanism known for the function of either this protein or the bodies themselves. Earlier findings and those presented here indicate that PML associates either directly or indirectly with components of the translational machinery including eIF-4E, P0, P1, P2, L7a and EF1 (Boddy et al., 1996; Borden et al., 1998b). A consequence of these interactions appears to be that PML inhibits protein production at the post-transcriptional level in cell culture. We show that eIF-4E is involved directly or indirectly because exogenous eIF-4E is able to partially rescue protein production. Since PML suppresses protein but not RNA production of cyclin D1, we investigated whether PML could alter cyclin D1 mRNA distribution. Using standard fractionation procedures like those previously reported (Rousseau et al., 1996), we show that PML overexpression causes cyclin D1 mRNA to accumulate in the nucleus. In the same cells, the distribution of GAPDH mRNA, lys tRNA and U6 snRNA do not vary amongst transfection experiments. Thus, differential nuclear accumulation of cyclin D1 is not a result of differential nuclear integrity in our preparations. Our results suggest that PML-induced nuclear accumulation has a similar specificity to eIF-4E.
We show that there is a strong link between the subcellular distribution of PML and its ability to alter RNA distribution and suppress protein production. Overexpression of PML results in formation of more nuclear bodies (Borden et al., 1995 and references therein) and increases nuclear retention of cyclin D1 mRNA (this study). PML is overexpressed when cells are subjected to certain kinds of stress. Interferon treatment of NIH3T3 increases the number of PML nuclear bodies (Maul et al., 1995), and increases nuclear retention of cyclin D1 mRNA (our unpublished observations). Expression of PMLRINGmut results in disruption of nuclear bodies (Borden et al., 1995; Figure 6) and an inability to retain cyclin D1 mRNA in the nucleus (this study). Co-expression of PML and eIF-4E results in fewer bodies than observed in cells expressing only PML and subsequently, less nuclear retention of cyclin D1 mRNA. These data indicate that eIF-4E has a dominant effect over assembly of PML nuclear bodies with no converse effect of PML on eIF-4E distribution. Disruption of PML nuclear bodies, as with our PMLRINGmut studies, has wide ranging effects including a loss of growth suppression activity (Le et al., 1996), loss of apoptotic activity (Borden et al., 1997), and now the inability to suppress transport of cyclin D1 mRNA (this study). Thus in pathogenic conditions which disrupt PML nuclear bodies, the loss of growth suppression and apoptotic actions may be linked to incorrect control of selected RNA transport.
Consistent with this report, recent studies indicate that the viral RING protein Z has similar actions to those we show for PML (Campbell Dwyer et al., 2000). The only identifiable domain in Z is the RING. The 90 residue Z protein co-localizes and immunoprecipitates with eIF-4E in the nucleus and cytoplasm. Like PML, the first zinc-binding site of the RING of Z is not required for co-immunopurification with eIF-4E. For both PML and Z, suppression is mediated at least in part through eIF-4E as exogenous eIF-4E alleviates suppression. In both cases, the RING is required for suppression of cyclin D1 production as mutation of this domain alleviates suppression. PML and Z have similar proline rich regions adjacent to their RINGs which may be required for association with eIF-4E or other necessary intermediary components. Taken together, these data suggest that additional protein(s), which binds the RING, is required for the suppression action for both PML and Z. Importantly, neither PML nor Z repress cyclin D1 mRNA production. These data show that both PML and Z suppress protein production of cyclin D1 in a RING dependent manner. Since Z directly binds to PML during infection (Borden et al., 1998a), it is not surprising that these proteins have parallel functions.
Implications for PML growth control actions
PML and eIF-4E affect cell growth in contrasting ways. eIF-4E transforms cells through preferential transport and translation of mRNAs that are normally suppressed (Rosenwald et al., 1995; Rousseau et al., 1996). Overexpression of eIF-4E blocks apoptosis in growth factor restricted fibroblasts (Polunovsky et al., 1996); while, overexpression of PML under similar conditions decreases cell survival (Borden et al., 1997). The apoptotic action of PML is independent of any ability to regulate gene transcription and does not require new RNA synthesis (Quignon et al., 1998; Wang et al., 1998b). Our findings suggest that disruption of eIF-4E mediated RNA transport could serve as a possible mechanism. Our results suggest that the slow growth observed in cells overexpressing PML likely results from the preferential downregulation of cyclin D1 protein which functions at the G1/S transition of the cell cycle (Hunter and Pines, 1994). Consistent with our findings, overexpression of PML in breast cancer lines results in decreased cyclin D1 protein levels (Le et al., 1998). The disruption of PML nuclear bodies in APL suggests that transport of cyclin D1 mRNA may not be properly regulated. Disruption of these bodies may contribute to APL since loss of cell-cycle control and subsequent uncontrolled proliferation are important steps in leukemogenesis.
Implications for the transcriptional function of the PML
Transcription and chromatin remodeling factors are associated with PML. However, PML nuclear bodies do not associate with sites of active transcription and do not localize with nascent DNA (Melnick and Licht, 1999). General transcription factors like TFIIH and RNA Polymerase II do not co-localize with PML bodies (Grande et al., 1996). Consistent with these findings, we observe no association between PML and RNA polymerase II (Figure 2a). Overexpression of PML does not significantly alter production of endogenous cyclin D1 mRNA although cyclin D1 protein levels are reduced. Thus like eIF-4E, PML can alter protein production without altering transcription. Transcription could be altered by modulating transport of cyclin D1 mRNA and subsequent cyclin D1 protein production. Recent reports suggest that cyclin D1 itself can alter transcription of some steroid receptors (Knudsen et al., 1999; McMahon et al., 1999; Zwijsen et al., 1998). For instance, cyclin D1 is able to recruit P/CAF to the estrogen receptor (McMahon et al., 1999). Association of some transcription factors with PML may occur because these factors are transporting transcripts to PML nuclear bodies. None of these possibilities are mutually exclusive and may all play a role in PML and PML nuclear body function. PML may play dual roles in transcription and translation similarly to other proteins (Mayfield, 1996).
PML function has been understood mainly at the phenomenological level. Many groups showed that PML is growth suppressive and apoptotic. Disruption of PML nuclear bodies in APL is devastating to the cell and likely contributes to leukemogenesis. However no clear-cut molecular function has been assigned to PML or PML nuclear bodies. This is the first report to suggest that PML and PML nuclear bodies play a role in translational control through partner proteins such as eIF-4E by modulating eIF-4E mRNA transport function in the nucleus. PML nuclear bodies could be depots for a subset of transcripts which would be deposited at the nuclear bodies for preferential regulation of export, stabilization and translation adding another level of control for protein production from these mRNAs. In this way, PML nuclear bodies could regulate trafficking of certain transcripts without directly transporting mRNA. The presence of mRNA in some PML nuclear bodies is consistent with this possibility. Alternatively, PML may sequester transport components which must be released to expedite transport of RNAs housed elsewhere. Further study is required to elucidate the molecular details of these new functions for PML and to distinguish between these two possibilities for PML nuclear body function.
Materials and methods
NIH3T3 (ATCC CRL 1658) and human 551 fibroblasts (ATCC CCL-110) were grown and maintained in 10% fetal bovine serum (FBS) and DMEM (Gibco).
PML antibodies include a monoclonal antibody to the human PML, mAb 5E10 (Stuurman et al., 1992) or a polyclonal antibody to both human and mouse PML (Borden et al., 1995) which was used to detect PML in transgenic mice experiments (Wang et al., 1998a). The pattern of bands observed for mAb 5E10 in Figure 2d,e are similar to those previously reported for these antibodies (Weis et al., 1994; Koken et al., 1994; Stuurman et al., 1992). The specificity of mAb eIF-4E (Transduction Laboratories) was verified in (Campbell Dwyer et al., 2000).
Mammalian overexpression constructs containing PML, and PMLRING were described (Borden et al., 1995, 1998a). Mutations in the B-boxes were described in (Borden et al., 1996), and deletion of the leucine coiled coil and C-terminal region in (Borden et al., 1998a). eIF-4E was obtained as an expressed sequence tag (ATCC 600222) in pCMVSPORT which contained the entire coding region of eIF-4E except for the last seven residues. Despite this deletion, eIF-4E retained its activity to alter transport which centers on the integrity of Ser 53 (Rousseau et al., 1996) and actively alleviated repression by the viral RING protein, Z and PML (Campbell Dwyer et al., 2000). The sequence of the eIF-4E gene was confirmed using standard methods.
Immunofluorescence and confocal laser microscopy studies
Immunofluorescence methods are described (Borden et al., 1998a,b). The PML polyclonal antibody and the eIF-4E monoclonal antibody (Transduction Laboratories) with appropriate fluoroscein isothioconjugate (FITC) and Texas Red secondary antibodies (Jackson Immunoresearch) were used. Fluorescence was observed using a Leica confocal laser microscope with excitation at 568 nm (red) or 488 nm (green). The two channels were recorded independently to avoid cross-talk between them. The pinhole was set to 20. Under these conditions, there was no breakthrough of FITC signal into the red channel or vice versa. Experiments were repeated at least twice with at least 500 cells in each sample. Images were overlaid in Photoshop.
Appropriate constructs were transfected into cells with lipofectamine/superfect as directed by the manufacturer (Gibco/Qiagen). 48–72 h later, experiments were carried out. Efficiency of transfection, ∼50%, was determined by immunofluorescence and confocal microscopy. Our previous studies (Borden et al., 1995, 1996; Boddy et al., 1997) indicated that there were no significant differences between constructs in terms of transfection efficiency and protein production. When appropriate, lysates were prepared from cells as described below. The total protein concentrations were determined in duplicate using the Protein Assay kit (Bio-Rad) and equal amounts of proteins were loaded onto SDS–PAGE for Western analysis. For metabolic labeling, after transfection cells were washed, and methionine free DMEM media (Gibco) was added. Cells were starved for 15–30 min. Cells were incubated for 3 h with 50 μCi/ml 35S-Methionine (Amersham). Cells were resuspended in lysis buffer (150 mM NaCl, 20 mM Tris-HCl pH 7.4, 100 μM PMSF and protease inhibitors as described in the subcellular fractionation section). 20 μg of total protein from each experiment was subjected to SDS–PAGE. Results were observed by autoradiography and analysed using NIH Image 1.58 software. In control experiments, vector refers to empty mammalian expression vector used (Borden et al., 1998a).
Cells were fractionated as described (Borden et al., 1998a,b). In order to obtain enough cells to readily detect cytoplasmic PML, four T75 flasks were required. After harvesting, cells were washed in cold PBS, spun, placed in buffer A (110 mM potassium acetate, 2 mM magnesium, 2 mM DTT and 10 mM HEPES, pH 7.3) and then spun and resuspended with protease inhibitors and 20 μM cytochalasin B in buffer B (10 mM potassium acetate, 2 mM magnesium acetate, 2 mM DTT, 5 mM HEPES, pH 7.3). Typically, protease inhibitors included: 2 μg/ml each of leupeptin, pepstatin A, and 0.05% aprotinin. Cells were disrupted by passage through 18, 21 and 23 gauge needles on ice. Lysates were spun at 1500 g for 15 min at 4°C to yield a pellet and a supernatant designated the nuclear and cytoplasmic fractions respectively.
Protein–protein interactions were demonstrated by coimmunoprecipitation assays (see Carlile et al., 1998). Cell lysates were mixed with the appropriate antibody: mAb eIF-4E, mAb 5E10 or mAb PGM3 (PML antibody from Santa Cruz). These antibodies were covalently bound to protein A sepharose beads. PML, eIF-4E or mouse immunoglobulin (IgG) were immunoprecipitated in separate experiments. Fractions were precleared as described (Carlile et al., 1998). Protein A-antibody beads were added to precleared supernatants and mixed for 2 h at 4°C. Beads were washed three times with IPB buffer (150 mM NaCl, 20 mM Tris-HCl pH 7.4, 1% Nonidet P40, 100 μM PMSF, and 5 μg/ml of leupeptin, pepstatin A, and 0.05% aprotinin) and three times with modified IPB buffer (0.1% deoxycholate and no nonidetP-40). Beads were subjected to SDS–PAGE and blotted (Western method) onto Immobolin P membranes, using ECL (Amersham) to visualize bound antibodies. Blots were probed with eIF-4E antibody. Complementary experiments were carried out where lysates were precipitated with mAb eIF-4E and blots probed with mAb 5E10. Monoclonal antibodies to RNA Polymerase II (Santa Cruz), and actin (Sigma) were used. Cyclin D1 (Santa Cruz) and GAPDH (Chemicon) antibodies were mouse monoclonals and revealed single bands on subsequent Westerns.
RNA extraction and analysis
NIH3T3 cells were fractionated and preparation of cytoplasm-free nuclei was done according to Rousseau et al. (1996). RNA was extracted using Trizol (Life Technologies) according to the manufacturer. RNA from the nuclear fractions was extracted directly with Trizol and treated with RNAse free DNAse I. Purified RNA was quantitated by spectrophotometry. For Northern analysis, typically 20 μg of RNA was analysed on agarose/formaldehyde gels. RNA was transferred onto positively charged membrane using the Northern Max-Plus kit (Ambion). Hybridization was performed using psoralen-biotin labeled non-radioactive cDNA probes or with 5′ biotinylated oligonucleotides to 18-mer complementary to the 3′ end of lysine tRNA or 48-mer to U6 snRNA. Bands were detected using the Brightstar Biodetect kit (Ambion). Band intensities were measured using NIH Image 1.58 software. Variation in loading was corrected for by normalizing against 28S and 18S rRNA band intensities.
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We are grateful for the kind gifts of the mAb 5E10 from L de Jong, and the polyclonal PML antibody from E Solomon and P Freemont. We thank N Gray, G Carlile, L Etkin, J Licht, M Salvato, A Melnick and especially S Pinol-Roma for critical discussions. KLB Borden acknowledges financial support from the NIH RO1 CA80728.
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Lai, HK., Borden, K. The promyelocytic leukemia (PML) protein suppresses cyclin D1 protein production by altering the nuclear cytoplasmic distribution of cyclin D1 mRNA. Oncogene 19, 1623–1634 (2000). https://doi.org/10.1038/sj.onc.1203473
- nuclear bodies
- RNA transport
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