Integrin-linked kinase regulates phosphorylation of serine 473 of protein kinase B by an indirect mechanism

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Abstract

The serine threonine kinase protein kinase B regulates cellular activities as diverse as glycogen metabolism and apoptosis. Full activation of protein kinase B requires 3-phosphoinositides and dual phosphorylation on threonine-308 and serine-473. CaM-K kinase and 3-phosphoinositide dependent-kinase-1 phosphorylate threonine-308. Integrin-linked kinase reportedly phophorylates serine-473. Consistent with this, in a model COS cell system we show that expression of wild-type integrin-linked kinase promotes the wortmannin sensitive phosphorylation of serine-473 of protein kinase B and its downstream substrates, and inhibits C2-ceramide induced apoptosis. In contrast, integrin-linked kinase mutated in a lysine residue critical for function in protein kinases is inactive in these experiments, and furthermore, acts dominantly to block serine-473 phosphorylation induced by ErbB4. However, alignment of analogous sequences from different species demonstrates that integrin-linked kinase is not a typical protein kinase and identifies a conserved serine residue which potentially regulates kinase activity in a phosphorylation dependent manner. Mutation of this serine to aspartate or glutamate, but not alanine, in combination with the inactivating lysine mutation restores integrin-linked kinase dependent phosphorylation of serine-473 of protein kinase B. These data strongly suggest that integrin-linked kinase does not possess serine-473 kinase activity but functions as an adaptor to recruit a serine-473 kinase or phosphatase.

Introduction

Protein kinase B (PKB), the cellular homologue of the viral oncogene v-akt, is a serine-threonine protein kinase (Bellacosa et al., 1991; Coffer and Woodgett, 1991; Jones et al., 1991). PKB regulates many diverse pathways and responses in a cell, depending on cell type (reviewed in Marte and Downward, 1997; Coffer et al., 1998). For example, insulin stimulation of L6 muscle cells results in PKB-dependent phosphorylation and inhibition of glycogen synthase kinase 3α (GSK-3α) (Cross et al., 1995; van Weeren et al., 1998) and stimulation of protein synthesis (Gingras et al., 1998; Kitamura et al., 1998; Ueki et al., 1998). In contrast, insulin-like growth factor-1 treatment of COS7 cells and nerve growth factor-dependent neurons confers PKB-mediated protection from apoptosis following UV treatment and serum withdrawal, respectively (Kulik et al., 1997; Dudek et al., 1997). PKB activation also protects myc transfected Rat-1 fibroblasts (KauffmanZeh et al., 1997) and MDCK epithelial cells (Khwaja et al., 1997) from the apoptosis (termed anoikis – Frisch and Francis, 1994) that normally accompanies cell detachment from extracellular matrix. PKB dependent phosphorylation of serine-136 (S136) of BAD, a protein involved in apoptotic signalling (Datta et al., 1997; Blume-Jensen et al., 1998; Chao and Korsmeyer, 1998), leads to its inactivation and may contribute to the regulation of apoptosis by PKB. Direct phosphorylation and inhibition of both caspase 9 and the forkhead transcription factor FKHLR1 by PKB are also likely to be important in the regulation of apoptosis (Cardone et al., 1998; Brunet et al., 1999). PKB is also activated by integrins (Frisch and Ruoslahti, 1997; King et al., 1997; Dimmeler et al., 1998; Guilherme and Czech, 1998) and the dual regulation of PKB by growth factors and integrins suggests that PKB activation represents a point where signals from growth factors and extracellular matrix converge in a cell.

Structurally, PKB possesses an N-terminal pleckstrin homology (PH) domain that binds lipid products of phosphatidylinositol 3-kinase (PI3-kinase), a central kinase domain and a c-terminal extension containing phosphorylation sites (Marte and Downward, 1997; Coffer et al., 1998). The regulation of PKB kinase activity is complex and involves several distinct steps (Bellacosa et al., 1998; Coffer et al., 1998). The PH domain of PKB binds to 3-phosphoinositide products of PI3-kinase and this interaction is implicated in both membrane targeting and kinase activation (Anjelkovic et al., 1997; Klippel et al., 1997; Franke et al., 1997). Dual phosphorylation on both threonine-308 (T308), within the activation loop of the kinase domain, and serine-473 (S473) in the c-terminal tail is also required for full activation of kinase activity (Alessi et al., 1996). A protein kinase that phosphorylates T308 has been characterized and termed 3-phosphoinositide dependent-kinase 1 (PDK1) (Alessi et al., 1997a, b; Cohen et al., 1997; Stephens et al., 1998). Like PKB, PDK1 also contains a PH domain that binds the lipid products of PI3-kinase (Alessi et al., 1997b; Stephens et al., 1998). Phosphorylation of T308 in vivo is dependent on PI3-kinase activity, but it is unclear if this requirement is necessary for the unfolding of PKB to allow access of PDK1 to T308, direct activation of PDK1 through its PH domain, or both of these events (Bellacosa et al., 1998; Stephens et al., 1998). Ca2+/calmodulin-dependent protein kinase kinase (CaM-KK) can also phosphorylate T308 or PKB in response to calcium (Yano et al., 1998). The PI3-kinase dependent kinase activity that phosphorylates S473 or PKB in vivo, which has been termed 3-phosphoinositide dependent kinase 2 (PDK2) (Alessi et al., 1997a), is less well characterized. MAPKAP kinase 2 is capable of phosphorylating S473 in vitro, but is unlikely to be an important in vivo regulator of PKB (Alessi et al., 1996). In vitro, PDK1 bound to protein kinase-C related kinase-2 (PRK2) or a c-terminal peptide derived from PRK2 can phosphorylate S473 of PKB (Balendran et al., 1999). However it is unclear if this PDK1/PRK2 complex phosphorylates S473 in vivo. A third candidate for a S473 kinase is integrin-linked kinase (ILK) (Delcommenne et al., 1998), originally identified as a β1-integrin binding protein in a yeast 2-hybrid screen (Hannigan et al., 1996). However, the sequence of the kinase domain of ILK differs from the consensus for a protein kinase in several residues considered critical for kinase activity.

In this paper, we assess ILK S473 kinase activity in cells. Expression of exogenous ILK promotes phosphorylation of S473 of PKB resulting in phosphorylation of downstream effectors of PKB and inhibition of C2-ceramide induced apoptosis. Using sequence alignment of ILK analogues from different species combined with site-directed mutagenesis of ILK, we probe how ILK functions as a S473 kinase. We conclude that ILK regulates S473 phosphorylation by acting as an adaptor, either to recruit a distinct S473 kinase activity or to inhibit a S473 specific phosphatase activity.

Results

ILK regulates S473 phosphorylation of PKB

A COS cell transient-expression system based on expression of 2 isoforms of ErbB4 (CYT-1 and CYT-2) was established to study signalling pathways dependent on PI3-kinase activation. CYT-1 and CYT-2 differ by the presence or absence of 16 amino acids containing a consensus binding site for PI3-kinase (Sawyer et al., 1998; Elenius et al., 1999). As expected, the longer isoform CYT-1 (containing the PI3-kinase binding site), in contrast to CYT-2, promoted the recruitment and activation of PI3-kinase (data not shown) and this correlated with increased phosphorylation of both S473 and T308 of PKB (Figure 1a, compare lanes 4 and 7, Ser473 and Thr308 panels). In all other respects, CYT-1 and CYT-2 behaved identically. Both forms associated equally well with SHC, NCK and GRB2 and stimulated MAP-kinase activity (data not shown and Figure 1c lanes 4 and 7).

Figure 1
figure1

Effect of integrin-linked kinase on phosphorylation of protein kinase B, glycogen synthase kinase-3α, BAD and MAP-kinase in serum starved transiently transfected COS cells. COS cells were transiently transfected with ErbB4 CYT-1, lanes 4, 5 and 6, ErbB4 CYT-2, lanes 7, 8 and 9 or mock transfected (COS), lanes 1, 2 and 3. Certain COS cells were additionally transfected with integrin-linked kinase (WT-ILK) (lanes 2, 5 and 8) or a mutated integrin-linked kinase (DN-ILK) (lanes 3, 6 and 9). RIPA buffer extracts were fractionated by SDS – PAGE, blotted to PVDF membrane and probed with antisera specific for; (a) ILK, ErbB4, PKB, phospho ser473 PKB, phospho thr308 PKB, (b) GSK3 (α and β forms), phospho Ser21 GSK-3α, BAD or phospho Ser136 BAD, (c) MAP kinase or phospho MAP kinase as indicated. Where indicated, cells were treated with 100 nM wortmannin for 30 min prior to cell lysis. Equal amounts of protein were loaded per track

Using the COS cell system, we investigated the ability of ILK or a mutant of ILK (DN-ILK) to promote phosphorylation of S473 of PKB. DN-ILK was generated by changing a critical lysine residue (lysine 220 in ILK – see Figure 3a) conserved in most protein kinases, and essential for kinase activity, to methionine (Carrera et al., 1993; Hanks and Hunter, 1995). Figure 1a, lane 5, shows that co-expression of ILK and ErbB4 CYT-1 in serum starved COS cells augments phosphorylation of S473 or PKB. Indeed, enhanced phosphorylation of PKB on S473 is noticeable in cells expressing ILK protein alone in the absence of ErbB4 CYT-1 (for example compare lanes 1 and 2, and 7 and 8 in Figure 1a, S473 panel). Expression of DN-ILK leads to a drastic diminution of phosphorylation of S473 of PKB e.g. Figure 1a, compare lanes 4 and 6. Western blotting with an ILK-specific polyclonal antiserum confirms expression of ILK and DN-ILK in transfected cells at levels approximately fivefold higher than that of endogenous protein (Figure 1a, top). DN-ILK acts specifically to reduce S473 phosphorylation of PKB and not by a general non-specific inhibition of kinase function, since it had no effect on the phosphorylation of MAP kinase (MAPK) (Figure 1c). Control blots (Figure 1a, PKB panel) show that changes in PKB expression levels do not account for the observed changes in S473 phosphorylation. Treatment with the PI3-kinase inhibitor wortmannin (100 nM) (Ui et al., 1995) totally abolishes the ILK-dependent increase in S473 phosphorylation consistent with ILK acting via a PI3-kinase dependent mechanism (Figure 1a, bottom).

Figure 3
figure2

(a) An alignment of the human, Caenorhabditis elegans and Drosophila melanogaster ILK protein sequences, and human B-Raf (residues 445 – 719) using Clusta1W (Thompson et al., 1994). #### indicates the position of the potential FSF phosphorylation site sequence in the activation loop of ILK. The three conserved ankyrin repeats are indicated. A* marks the position of the lysine residue changed to methionine to create DN-ILK. The DFG motif, only present in B-Raf, is underlined. (b) Calculation of percentage identifies between the three ILK homologous sequences using 1Align (Huang and Miller, 1991). (c) An alignment of the predicted catalytic loop sequences of the three ILK sequences and B-Raf with the protein kinase consensus (Hanks and Hunter, 1995). A* marks the position of the catalytic aspartate residue critical for kinase function

Activation of PKB requires dual phosphorylation on T308 and S473. Since T308 is constitutively phosphorylated in serum-starved COS cells at a basal level that is unaffected by expression of ILK or DN-ILK (Figure 1a), changes in S473 phosphorylation induced by expression of ILK and ILK mutants should result in parallel changes in PKB activity towards downstream substrates.

Therefore, phosphorylation on two PKB substrate sites, serine 21 of glycogen synthase kinase 3α (GSK-3α) and serine 136 of BAD, was assessed using antibodies specific for phosphorylated forms of these proteins. As in previous studies (Blume-Jensen et al., 1998), co-transfection of a BAD expression plasmid proved necessary to investigate phosphorylation of S136 of BAD as endogenous BAD levels in COS cells are very low. Figure 1b shows that expression of ILK enhances phosphorylation of S21 of GSK-3α (for example, Figure 1b, phospho-S21 GSK-3α panel, compare lanes 1 and 2, 4 and 5, 7 and 8). A similar effect can also be seen on the phosphorylation of S136 of BAD (Figure 1b, phospho-S136 BAD panel, compare lanes 1 and 2, 4 and 5, and 7 and 8). For S136 of BAD there is a high basal level of phosphorylation that was consistently increased twofold by expression of ILK (measured by densitometry – data not shown). S136 of BAD is phosphorylated by other protein kinases and these are likely to contribute to the phosphorylation on this site seen in this experiment (Scheid and Duronio, 1998). Expression of DN-ILK leads to significant inhibition of phosphorylation of S21 of GSK-3α and S136 of BAD (Figure 1b, compare lanes 4 and 6, phospho-21GSK-3α and phospho-S136 BAD panels, respectively). Wortmannin treatment (100 nM) abolished both the S21 phosphorylation of GSK-3α and the increase in the S136 phosphorylation of BAD (Figure 1b).

ILK expression rescues C2-ceramide induced apoptosis

Having demonstrated the ability of ILK to mediate phosphorylation of PKB and its substrates, we next tested whether ILK expression modulates PKB biological function. Treatment of COS cells with C2-ceramide induces apoptosis and this can be inhibited by the activation of PKB (Zhou et al., 1998). We examined the effects of expressing ILK or DN-ILK on the percentage of cells undergoing apoptosis in this assay. C2-ceramide treatment of cells caused several morphological changes indicative of apoptosis. These included, loss of viability, cell shrinkage, decreased adherence to tissue culture treated plastics, cell shape changes, the increased appearance of round phase bright cells and membrane blebbing (data not shown). The proportion of apoptotic cells was quantitated by two different methods (1) FACS analysis of the sub-G1 peak following staining with propidium iodide (indicative of DNA fragmentation); (2) TUNEL (a measure of DNA fragmentation and strand breakage). In addition, trypan blue exclusion from the nucleus (a measure of the integrity of the nuclear membrane) was used to measure cell viability. Figures 2a (propidium iodide staining) and 2b (TUNEL) show that markedly increased apoptosis occurs in COS cells treated with C2-ceramide but not the biologically inactive analogue, C2-dihydroceramide. Cell viability is also decreased by C2-ceramide (Figure 2c). In all three assays, ILK expression significantly reduced the number of cells undergoing apoptosis/loss of viability following treatment with C2-ceramide (Figure 2a,b and c). Expression of DN-ILK has no effect. Morphologically, C2-ceramide treated cells expressing exogenous ILK resemble those treated with C2-dihydroceramide or ethanol vehicle alone, whereas those expressing DN-ILK were rounded up, shrunken and loosely attached to the dish (data not shown). The number of apoptotic events recorded by each method of assessment varies owing to differences in sensitivity of the detection method and the differing end points i.e. the stage of apoptosis measured by each technique. In each case, however, expression of ILK results in a twofold decrease in C2-ceramide induced-apoptosis compared to control treated cells and cells expressing DN-ILK (Figure 2a and b). The extent of rescue we observe with ILK may be an underestimate of the real effect as our transfection efficiency of 50 – 80% means that 80% rescue rather than 100% is the maximal response we could anticipate. Increased PKB activity also rescues COS cells from C2-ceramide induced apoptosis (Zhou et al., 1998). Since we have shown ILK regulates PKB activity, we propose ILK regulates COS cell apoptosis through a PKB-dependent pathway. Interestingly, ILK induced S473 phosphorylation is reduced by C2-ceramide treatment compared to treatment with the inactive analogue C2-dihydroceramide (Figure 2d) suggesting in COS cells C2-ceramide exerts its effects by down-regulation of PKB activity.

Figure 2
figure3

Effects of integrin-linked kinase expression on apoptosis induced by C2-ceramide in COS cells. COS cells were transiently transfected with WT-ILK, DN-ILK or vector alone. Cells were serum starved overnight and treated with 50 μM C2-ceramide (Cera) or C2-dihydroceramide (DHC) or ethanol vehicle for 8 h and the percentage of cells undergoing apoptosis determined by FACS analysis (a) or TUNEL (b). Numbers in (a) and (b) represent the average of five and three experiments respectively, each performed in triplicate. Error bars represent standard deviation. (c) COS cells were serum starved overnight and treated with 50 μM C2-ceramide (C2-ceramide) or C2-dihydroceramide (C2 DHC) or ethanol vehicle (mock). Alternatively COS cells were transfected with WT-ILK or DN-ILK, serum starved overnight and treated with 50 μM C2-ceramide (WT-ILK+CER and DN-ILK+CER, respectively). After the indicated times, cell survival was calculated by staining with trypan blue. A minimum of 600 cells were counted per point, where possible. (d) COS cells were transiently transfected with WT-ILK, lanes 2, 5 and 8, DN-ILK, lanes 3, 6 and 9, or mock transfected, lanes 1, 4 and 7. Cells were serum starved overnight and treated with 50 μM C2-ceramide (Ceramide), lanes 4, 5 and 6, or C2-dihydroceramide (DHC), lanes 7, 8 and 9, or ethanol vehicle, lanes 1, 2 and 3, for 8 h. RIPA extracts were separated by SDS – PAGE, transferred to PVDF membrane and blotted with antisera specific for PKB (top) and phospho-S473 PKB (bottom). Equal amounts of protein were loaded per track

ILK is conserved through evolution and lacks key catalytic residues

Since ILK lacks both HRDLXXN (containing the catalytic aspartate residue) and DFG (involved in the coordination of a magnesium ion) motifs that are considered to be critical for kinase function. (Hannigan et al., 1996; Hanks and Hunter, 1995), we were intrigued by the mechanism of ILK function. We sequenced ILK analogues from C. elegans and D. melanogaster, arguing that functionally important residues should be conserved during evolution. Figure 3a shows an alignment of these three ILK sequences together with the B-Raf sequence as a reference kinase. The overall degree of sequence conservation (Figure 3b) suggests the predicted C. elegans and D. melanogaster proteins are likely to have homologous functions to human ILK. Near the N-terminus all three ILK homologues possess three ankyrin repeats (underlined in Figure 3a), followed by a linker region that may bind lipids (Delcommenne et al., 1998) and then a kinase homology domain. A fourth ankyrin repeat has been noted in human ILK (Hannigan et al., 1996) but the sequence identified is divergent from the consensus for the ankyrin repeat motif and it is not conserved through evolution.

Comparison of the kinase domain of ILK to that of other protein kinases e.g. B-Raf (Figure 3a) shows that residues important in maintaining kinase structure are conserved, but that residues involved in catalytic function are more divergent (Hubbard et al., 1994; Hanks and Hunter, 1995). In particular, ILK sequences corresponding to the catalytic loop show little similarity, implying they have not been constrained by function during evolution. None of the ILK sequences have a DFG motif or a conserved substitute for the catalytic aspartate residue found in other kinases. A phe-ser-phe (FSF) motif is conserved in all three ILK analogues within a sequence corresponding to the activation loop of protein kinases (Hubbard et al., 1994; Hanks and Hunter, 1995 – marked with ### in Figure 3a. In many kinases including insulin receptor, PKB and MAP kinase, residues in the activation loop are subject to reversible phosphorylations which control kinase activity (Her et al., 1993; Hubbard et al., 1994; Coffer et al., 1998). Substitution of a negatively charged residue e.g. aspartate or glutamate for the phosphorylated residue mimics the change in charge that accompanies phosphorylation and results in constitutive activation of the kinase. In contrast, substitution of alanine prevents phosphorylation consequently inhibiting kinase activation. FSF is similar to the consensus sequence defined for phosphorylation by PDK2, phe-X-X-phe-ser-phe/tyr (Pearson et al., 1995; Alessi et al., 1996). If ILK possesses intrinsic PDK2 activity, this site may be subject to autophosphorylation and possibly regulate ILK activity. To investigate this we changed the serine residue of the FSF motif of either wild-type ILK or DN-ILK to alanine, aspartate or glutamate, and assessed the effect on the phosphorylation of S473 of PKB.

Effects of changes in the FSF motif on PKB-S473 phosphorylation

Figure 4 shows that expression of either FDF or FEF point-mutated ILK induce phosphorylation of S473 of PKB. In contrast ILK-FAF is inactive. These data support the idea that the charge on the serine residue of the FSF motif is important in regulating ILK function and are consistent with this being a site for phosphorylation. Changes in expression levels of ILK and PKB do not account for the large changes observed in S473 phosphorylation (Figure 4, ILK and PKB panels). DN-ILK-FAF was inactive in promoting S473 phosphorylation. However, surprisingly, both DN-ILK-FDF and DN-ILK-FEF promote phosphorylation of S473 of PKB at a third of the wild type level, even though from many previous studies the lysine to methionine mutation is expected to efficiently inhibit protein-kinase activity (Figure 4, S473 panel) (Carrera et al., 1993; Hanks and Hunter, 1995). None of the mutations had a significant effect on the phosphorylation state of T308 (Figure 4). We next examined the ability of the PI3-kinase inhibitor wortmannin to inhibit the phosphorylation of S473 of PKB induced by expression of the various mutant ILK proteins. Surprisingly, S473 phosphorylation induced by expression of ILK-FDF, ILK-FEF, DN-ILK-FDF and DN-ILK-FEF was partially resistant to wortmannin treatment at concentrations that completely abolished S473 phosphorylation promoted by wild type ILK (Figure 4, bottom). In all cases phosphorylation on S21 of the PKB substrate GSK3-α was observed to change in parallel with changes in phosphorylation of S473 of PKB (data not shown). Thus by substituting a negatively charged residue in place of the serine residue of the FSF motif, we have changed the ILK induced-phosphorylation of S473 of PKB from a PI3-kinase dependent, to a partially PI3-kinase independent mechanism. Furthermore, substitution of FEF or FDF in the DN-ILK background restores function to the otherwise defective DN-ILK. This ability to restore activity to DN-ILK argues strongly against ILK being PDK2.

Figure 4
figure4

Effect of mutations in the FSF motif of integrin-linked kinase on phosphorylation of protein kinase B in serum starved transiently transfected COS cells. COS cells were transiently transfected with wild-type ILK (lane 1) or the ILK mutant plasmids, DN-ILK (lane 2), ILK-FDF (lane 3), ILK-FEF (lane 4), ILK-FAF (lane 5), DN-ILK-FDF (lane 6), DN-ILK-FEF (lane 7), DN-ILK-FAF (lane 8), or left untransfected (lane 9). RIPA buffer extracts were fractionated by SDS – PAGE, blotted to PVDF membrane and probed with antisera specific for; PKB, phospho-Ser 473 PKB, phospho thr-308 PKB or ILK. Where indicated, cells were treated with 100 nM wortmannin for 30 min prior to cell lysis. Equal amounts of protein were loaded per track

Discussion

Our results clearly demonstrate that ILK induces phosphorylation of S473 of PKB in COS cells. PKB S473 phosphorylation correlates with the phosphorylation of PKB substrates, GSK3α and BAD, and the inhibition of apoptosis induced by treatment of COS cells with C2-ceramide. Interestingly, the same apparent degree of phosphorylation of S473 on PKB leads to different effects on substrate phosphorylation depending on whether ErbB4 is expressed or not. For example, in Figure 1a and b, the extent of phosphorylation of S473 of PKB is similar in lanes 2 and 8, but the phosphorylation on S21 of GSK-3α is reduced in lanes 8 compared to lane 2. Similarly, co-expression of DN-ILK and CYT-1 results in a large but partial loss of S473 phosphorylation of PKB, yet a complete loss of phosphorylation of S21 of GSK-3α (Figure 1a and b, lane 6, phospho S473 PKB and phospho S21 GSK-3α blots). Although the reason for this is unclear, other kinases such as PKC can also phosphorylate S21 of GSK-3α (Cook et al., 1996). Constitutive activation of ErbB4 could result in down-regulation of the activity of these kinases or up-regulation of an S21 phosphatase activity. Delcommenne et al., 1998 concluded it most likely that ILK directly regulates S473 phosphorylation of PKB. However, our data suggest this is improbable for three reasons. (1) We can detect no significant protein kinase activity in immuneprecipitates of ILK using a variety of different substrates, including PKB (data not shown). Others have also failed to detect kinase activity in ILK immuneprecipitates (Balendran et al., 1999). (2) ILK lacks both a DFG motif and the catalytic aspartate residue considered essential for efficient catalytic activity (Figure 3). In protein kinase A (PKA), mutation of the catalytic aspartate residue to alanine results in a significant reduction of catalysis to 0.4% of wild type activity (Gibbs and Zoller, 1991). Thus, although ILK may possess a low intrinsic kinase activity that explains the previously reported ability of ILK to phosphorylate β1 integrin tail, PKB and GSK-3α in vitro (Hannigan et al., 1996; Delcommenne et al., 1998), this kinase activity is unlikely to be sufficient to account for the rapid phosphorylation of S473 of PKB that is noted in vivo (Cohen et al., 1997; van Weeren et al., 1998). (3) Substitution of an FSF motif in ILK by either FDF or FEF restores activity to a kinase-dead ILK protein (DN-ILK) arguing strongly against direct phosphorylation of PKB by ILK (Figure 4).

If ILK isn't responsible for the direct phosphorylation of S473 of PKB then this raises the questions of how does ILK function and what is the identity of PDK2? Two related models can be proposed for ILK function. In both models, ILK initially binds ATP in its ATP binding pocket and is then able to become phosphorylated on the FSF motif. If this phosphorylation is mediated by a low intrinsic kinase activity this then explains the ability of DN-ILK to act as a dominant negative inhibitor, since autophosphorylation on the FSF motif will be blocked in this instance. 3-phosphoinositides regulate access of ATP into the ILK ATP binding pocket in an analogous way to that proposed for the regulation of ATP access into the ATP binding site by phosphorylation in the atrial natriuretic receptor (Foster and Garbers, 1998; Potter and Hunter, 1998). Indeed the glycine rich loop of the ILK ATP binding pocket overlaps with sequences predicted by Delcommenne et al., 1998 to be responsible for the binding of 3-phosphoinosides by ILK. Subsequently, ILK phosphorylated on the FSF motif either (a) activates PDK2 activity or (b) inhibits a S473 phosphatase activity. Both scenarios result in increased phosphorylation of S473 of PKB. Although this model predicts the ILK-FDF and ILK-FEF mutants no-longer require 3-phosphoinositides for function, wortmannin does partially inhibit the ability of these mutants to induce phosphorylation on S473 of PKB (Figure 4). We suspect this is because 3-phosphoinositides are still required for proper membrane targeting and unfolding of PKB. Current experiments are aimed at distinguishing between these two possible methods of action of ILK. Model A predicts that PDK2 activity lies in a wortmannin insensitive kinase that associates with ILK phosphorylated on the FSF motif. However, this model is still consistent with PDK2 activity being 3-phosphoinositide dependent in vivo, since there is a requirement for 3-phosphoinositides for the initial phosphorylation of ILK on the FSF motif. In vitro, PDK1 gains PDK2 activity when complexed with PRK2 or c-terminal peptides from PRK2 containing an FEF sequence motif (Balendran et al., 1999). However, it is unlikely that the phosphorylated FSF motif of ILK functions in a similar manner, as, unlike the PRK2/PDK1 complex, which retains kinase activity towards T308, we see no increase in T308 phosphorylation when ILK is expressed and phosphorylation on S473 of PKB is increased. In addition, we are unable to detect PDK1 in ILK-containing immuneprecipitates (data not shown). Model B, where ILK inhibits a S473-phosphatase activity may be more likely given the large increase in S473 phosphorylation promoted by ILK expression. Whilst serum-starvation of COS cells only decreases PKB activity twofold (Meier et al., 1997), treatment with phosphatase inhibitors increases activity several-fold (Andjelkovic et al., 1996; Meier et al., 1997; Millward et al., 1999).

In this paper, we show that full activation of PKB in a model COS cell system requires both the recruitment and activation of PI3-kinase by ErbB4 and ILK. Since ILK is regulated by integrins (Hannigan et al., 1996), PKB activation is therefore dependent on integrin- and growth factor-mediated signals. Dual regulation of PKB activity in this way may be required to drive cell-cycle progression, and/or cell survival, both biological responses known to require joint input from growth factor- and extracellular matrix-mediated signals. The mechanism by which ILK regulates S473 phosphorylation on PKB is unclear, although our results strongly suggest it is unlikely to be due to direct phosphorylation of PKB by ILK. Rather, our results suggest that PDK2 activity residues in a distinct molecule and ILK functions as an adaptor protein that either activates PDK2 or inhibits a S473 phosphatase.

Materials and methods

Antibodies

ErbB4, Santa Cruz C-18; phosphoserine 473 Akt (PKB), phosphothreonine 308 Akt (PKB), Akt (PKB), BAD, phosphoserine 136 BAD (New England Biolabs); GSK-3 (α and β), phosphoserine 21 GSK-3α (Upstate Biotechnologies Inc.); phospho-Map kinase, Map kinase (Promega). ILK antiserum was raised against a TrpE-ILK fusion protein encompassing the N-terminal ankyrin repeat region of ILK (residues 1 – 171).

Clones

Expression constructs were made by subcloning cDNAs for ErbB4 CYT-1 and CYT-2 and ILK from pBluescript into the expression vector pMT2 (Sambrook et al., 1987) with modified cloning sites including NotI. DN-ILK was constructed by PCR mutagenesis. Oligonucleotides including restriction sites for BsrD1 5′-CACAGAATTCGCAATGACATCGTTGTGATGGTGCTGAAGGTTCGAG (5 prime) and NdeI 5′-TAGAGGGATCCATATGGCATCCAG (3 prime) were used to amplify a 188 bp fragment containing the desired lysine to methionine change (residue 220 – in bold). This was cloned into pBluescript as a EcoRI-BamHI fragment and the change confirmed by DNA sequencing. A BsrDI-NdeI fragment (a silent G-A polymorphism creates a NdeI site in our ILK cDNA) was re-cloned into pBluescript-ILK and the ILK cDNA with the lysine to methionine mutation subcloned into pMT2 as a NotI fragment.

Mutation of the FSF motif was performed by PCR directed mutagenesis. PCR primers were constructed covering and including the NdeI (5′-CTCTAAGCTTCACACACTGGATGCCATATGGAT) and BstXI sites. Primers covering the BstXI site included the following amino-acid substitutions FSF to FEF: (5′-TGTGTCTAGACCAGGACATTGGAATTCGAACTTGACATCAG); FSF to FDF (5′-TGTGAATTCCAGGACATTGGAAATCGAACTTGACATCAG); FSF to FAF (5′-TGTGAATTCCAGGACATTGGAACGCGAACTTGACATCAG). Fragments were amplified by PCR using Pfu DNA polymerase (Stratagene) and subcloned into pMT2-ILK or pMT2 DN-ILK as NdeI-BstXI fragments. Insert segments of all vectors were confirmed by DNA sequence analysis.

COS Transfections

COS1 cells were grown in Glasgow's MEM (Life Technologies) supplemented with 10% v/v newborn calf serum and 2 mM L-glutamine. Transfections were performed with Fugene 6 (Boehringer) and cells left for 72 h before harvest. For analysis of BAD, pEBG-mBAD (New England Biolabs), encoding a 49KD murine-BAD fusion protein, was transiently co-transformed with other plasmids. Cells were serum starved for 16 h, washed twice in ice-cold phosphate buffered saline (PBS) and lysed for 15 min on ice in radioimmune precipitation assay (RIPA) buffer 0.5% w/v sodium deoxycholate, 150 mM sodium chloride, 1% v/v NP40, 50 mM Tris-HCL pH 8.0, 0.1% w/v SDS, 10% v/v glycerol, 5 mM EDTA, 1 mM sodium orthovanadate, 1 mM PMSF, 10 mg ml−1 aprotinin, 10 mg ml−1 leupeptin. Insoluble material was removed by centrifugation (12 000 g for 10 min at 4°C).

Western blots

Cells were lysed in 200 μl RIPA lysis buffer and protein content estimated using Bicinchoninic acid Protein Assay Reagent (Pierce). 20 μg protein was boiled in SDS sample buffer and resolved on a 7.5% SDS-polyacrylamide gel. Proteins were transferred to Immobilon PVDF membrane (Millipore) by electro-blotting and probed with appropriate antibodies at the recommended dilutions. Protein detection was performed using ECL (Amersham Life Science) followed by autoradiography.

C2-ceramide-induced apoptosis

This was performed as described in Zhou et al. (1998).

Detection of apoptosis

TUNEL staining was performed using the Apoptosis Detection System, Fluorescein (Promega) Flow-cytometry/FACS analysis with propidium iodide. Cells (106) were harvested by mild trypsinisation and pelleted (1500 r.p.m., 5 min). Cell pellets were permeabilized with 1 ml 3.7% paraformaldehyde (5 min 22°C), pelleted, washed with PBS and resuspended in 1 ml 0.1% Triton X-100, 1% BSA in PBS, incubated for 5 min at 4°C, repelleted (3000 r.p.m., 5 min) and resuspended in 1 ml PBS containing 5 μg ml−1 propidium iodide and 100 μg ml−1 ribonuclease. Cells were incubated for 1 h at 22°C in the dark and cell fluorescence measured at 620 nm (excitation 488 nm) using a FACscan flow cytometer and CELLQuest acquisition and analysis software (Becton Dickinson).

Trypan Blue exclusion assay. Cells were harvested by mild trypsinization, pelleted (1500 r.p.m., 5 min) and resuspended in 1 ml growth medium. 50 μl cells was added to 50 μl 0.4% Trypan Blue in PBS with mixing. After 2 min, stained and unstained cells were quantitated by counting with a haemocytometer. A minimum of 600 cells were counted per point.

ILK sequences

Partial C. elegans sequences with homology to human ILK were identified in EST databases (GenBank). PCR primers were constructed and RT – PCR carried out using mRNA from whole embryos to obtain full-length cDNAs. Screening of EST databases (GenBank) identified EST clones covering the N-terminus of ILK from D. melanogaster. Two of these (LD02317 and LD08711: Berkeley Drosophila Genome Project) were obtained from Research Genetics Inc and sequenced. DNA sequence analysis was performed using Perkin-Elmer ABI Prism 377 Sequencers and BigDye Terminator Chemistry (Perkin-Elmer, Foster City, CA., USA).

Accession numbers for sequences are C. elegans (AJ249344) and D. melanogaster (AJ249345).

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Acknowledgements

We thank Simon Barry, Harren Jhoti, Rob Harris and Peter Soden for helpful discussions and technical advice. This work was supported by a MRC collaborative studentship award between GlaxoWellcome and Cambridge University (DK Lynch) and a grant from the Association for International Cancer Research (PAW Edwards).

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Correspondence to Ian D Hiles.

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Keywords

  • Akt
  • ErbB4
  • integrin-linked kinase
  • phosphatidylinositol 3-kinase
  • protein kinase B
  • site-directed mutagenesis

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