Pancreatic ductal tumors invade local parenchyma and metastasize to distant organs. Src-mediated tyrosine kinase signaling pathways promote pancreatic ductal adenocarcinoma (PDAC) metastasis, though the molecular mechanisms supporting this invasive process are poorly understood and represent important and novel therapeutic targets. The large GTPase Dynamin 2 (Dyn2), a Src-kinase substrate, regulates membrane–cytoskeletal dynamics although it is yet to be defined if it contributes to tumor cell migration and invasion. Therefore, the goal of this study was to test if Dyn2 is upregulated in human pancreatic tumors and to define its role in cell migration and metastatic invasion using in vitro assays and nude mouse models. Histological analysis showed that 81% of 85 patients had elevated Dyn2 in PDAC. To test if Dyn2 overexpression alters metastatic properties of human pancreatic tumor cells, stable clones of BxPC-3 cells overexpressing either wild-type Dyn2 or a phosphorylation-deficient mutant Dyn2Y(231/597)F known to attenuate Dyn2 function, were generated and analyzed. Importantly, tumor cells overexpressing Dyn2 protruded lamellipodia at twice the rate, migrated faster (180%) and farther (2.5-fold greater distance) on glass and through transwell chambers (2–3-fold more cells through the filter) compared with cells expressing Dyn2Y(231/597)F or vector alone. Further, depletion of Dyn2 and dynamin inhibitors Myristyl trimethyl ammonium bromides and Dynasore significantly reduced cell migration, wound healing and invasion in transwell assays compared with controls. To test the metastatic potential conferred by increased Dyn2 expression, the BxPC-3 cell lines were implanted orthotopically into the pancreas of nude mice. Cells expressing Dyn2-green fluorescent protein exhibited a threefold increase in large distal tumors compared with cells expressing Dyn2Y(231/597)F or vector alone. Finally, histological analysis revealed that Dyn2 is upregulated in 60% of human metastatic pancreatic tumors. These findings are the first to implicate dynamin in any neoplastic condition and to directly demonstrate a role for this mechanoenzyme in invasive cell migration.
Pancreatic ductal adenocarcinoma (PDAC) is a highly invasive disease with an exceptionally poor prognosis (Nieto et al., 2008) making it the fourth leading cause of cancer deaths in America with an overall 5-year survival rate of only 5% (Jemal et al., 2010). These tumors are known to disseminate aggressively from the pancreas to invade peripheral tissues and organs, including the surrounding mesentery, as well as the liver and lymph nodes (Mihaljevic et al., 2010). Currently little is known about the migratory mechanisms that support this metastatic process although it is well documented that the signaling cascades activating tumor cell migration are markedly overexpressed in these tumors. For example, the epidermal growth factor receptor family members ErbB-1 (epidermal growth factor receptor (EGFR)/Her-1) and ErbB-2 (Her-2/neu) are amplified in over a third of PDACs, whereas important downstream effectors of these receptors, such as Src and K-Ras, are increased or mutated in the majority of these tumors (Lutz et al., 1998; Thybusch-Bernhardt et al., 2001; Frame, 2002; Hakam et al., 2003; Bloomston et al., 2006; Dancer et al., 2007; Harada et al., 2008; Kim et al., 2009; Morton et al., 2010).
The adherence and migratory behavior of tumor cells activated by these aberrant signaling cascades is supported by a complex cytoskeletal network composed of filamentous actin and scores of associated linker proteins and motors. The dynamics of this network support the extension of actin-membrane protrusions, or lamellipodia, at the leading edge of the tumor cells that facilitate migration (Pollard and Borisy, 2003; Yamaguchi and Condeelis, 2007; Etienne-Manneville, 2008; Machesky, 2008). How tumor cells coordinate lamellipodial extension with cell adhesion during invasion is currently a central focus of cancer cell biology. Recently, the dynamin family of large GTPases, known to deform and sever cellular membranes (McNiven, 1998; Hinshaw, 2000; Orth and McNiven, 2003; Praefcke and McMahon, 2004; Doherty and McMahon, 2009; Hansen and Nichols, 2009), has been shown to interact with the actin cytoskeleton and thus has been implicated in cell migration (McNiven et al., 2000; Qualmann et al., 2000; Kessels et al., 2001; Schafer, 2004; Kruchten and McNiven, 2006). Along with an established role in endocytosis and membrane traffic (Urrutia et al., 1997; McNiven, 1998; Hinshaw, 2000) the ubiquitously expressed Dyn2 isoform has been shown to function in actin polymerization and organization (Lee and De Camilli, 2002; Orth et al., 2002; Schafer, 2004; Mooren et al., 2009), focal adhesion regulation (Ezratty et al., 2005, 2009; Yoo et al., 2005), and lamellipodia extension (Krueger et al., 2003; Schlunck et al., 2004), possibly through interactions with actin-binding proteins. Thus, through participation in many different cellular processes it is likely that Dyn2 may act to accentuate the neoplastic condition by supporting invasive migration. Surprisingly, a role for Dyn2 in cell migration has yet to be demonstrated, and no information exists on the expression, function or regulation of this versatile mechanoenzyme in the metastatic process.
Here we provide a novel, in depth, examination of the role of Dyn2 in pancreatic cancer. We find that Dyn2 expression is markedly upregulated in the majority of human patients with this disease and that elevated levels of this enzyme contribute substantially to lamellipodia extension that in turn enhances cell migration either along a planar substrate or through a 3-D matrix. Conversely, inhibition of Dyn2 function by expression of phosphorylation-impaired mutant protein, treatment with Dyn2 inhibitory drugs or depletion with siRNA all significantly reduce tumor cell dynamics. These experimental observations in isolated human tumor cell lines supported subsequent in vivo studies that show an enhanced dissemination and metastases of Dyn2-expressing tumor cells in an orthotopic nude mouse model. Finally, the phosphorylated state of Dyn2 is also an important factor in pancreatic cancer, as expression of Dyn2 phospho-mutants in cells leads to a marked attenuation of tumor cell migration and invasion.
The expression of Dyn2 is markedly increased in patients with pancreatic cancer
To determine the role of dynamin 2 (Dyn2) in PDAC, tissue sections from both normal and cancerous regions of pancreatic cancer patients were probed for Dyn2 expression using immunohistochemistry. An initial immunohistochemical screening showed a marked increase in Dyn2 levels between normal and cancerous regions of pancreatic tissue from 9 of 11 patients diagnosed with PDAC. Modest Dyn2 expression was detected throughout the cytoplasm of non-malignant ductal epithelial cells and acinar cells of normal tissue from the diseased patients (Figures 1a–d). In comparison, Dyn2 levels were markedly elevated in the epithelium of cancerous lesions (Figures 1a′–d′). The increased Dyn2 levels were confined to the epithelial cells and did not increase noticeably in either the stroma or the acini.
To expand the numbers of patients in the sample set, tissue microarrays (TMAs) of human PDAC from 85 patients were examined (Figure 1e). The TMA sections contained normal ducts, pancreatic intraepithelial neoplasias, and cancerous lesions. Consistent with the initial screen, Dyn2 levels were significantly higher in ductal epithelium during pancreatic intraepithelial neoplasia and cancerous stages, as compared with normal ductal epithelium within the TMA sections (Figure 1e′). Notably, three times the number of pancreatic ducts that were scored as pancreatic intraepithelial neoplasias or cancer had elevated Dyn2 levels when compared with normal ducts within the TMA (Figure 1f; P<0.0001). These data indicate that Dyn2 expression levels are upregulated in the majority of PDACs.
Elevated levels of phospho-Dyn2 promote a migratory phenotype
Dynamin expression and activity have been shown to regulate actin dynamics (Lee and De Camilli, 2002; Orth et al., 2002; Krueger et al., 2003; Mooren et al., 2009) while participating in the formation and extension of lamellipodia (McNiven et al., 2000; Schlunck et al., 2004). Importantly, it has been shown that the dynamins are phosphorylated by the tyrosine kinase c-Src (Ahn et al., 1999; Shajahan et al., 2004) that act downstream of membrane receptors such as EGFR (Ahn et al., 2002). Both EGFR and Src kinase have been shown to be markedly upregulated in PDACs (Lutz et al., 1998; Thybusch-Bernhardt et al., 2001; Hakam et al., 2003; Bloomston et al., 2006; Dancer et al., 2007; Morton et al., 2010) and activation of these signaling cascades have been shown to promote tyrosine phosphorylation of dynamin on Y231 and Y597 (Ahn et al., 1999, 2002). Phosphorylation of these specific sites increases the GTPase activity and self-assembly of dynamin and regulates its functional roles in endocytosis and Golgi reorganization (Vieira et al., 1996; Ahn et al., 2002; Cao et al., 2010; Weller et al., 2010). From these past findings we hypothesized that elevated expression of Dyn2, combined with the activated signaling cascades observed in these tumors, could enhance invasive cell migration that would in turn promote the dissemination of PDAC cells. To test this premise we used a widely studied pancreatic cancer cell line (BxPC-3) to generate stable cell lines expressing either Dyn2-green fluorescent protein (GFP), a phospho-deficient mutant Dyn2Y(231/597)F-GFP or cells expressing GFP alone as a control. Western blot analysis of these cell lines revealed that expression levels of the stably expressed wild-type (WT) or mutant Dyn2 were comparable to endogenous protein (Figure 2a). Most remarkable was the change in phenotype of the WT Dyn2-GFP-expressing cells compared with the GFP-expressing cells. Although the control cells were generally observed to clump together in non-motile colonies, the stable Dyn2-GFP-expressing cells displayed numerous, large lamellipodia (Figures 2b and c). Moreover, the cell line expressing the mutant Dyn2Y(231/597)F-GFP exhibited some motile characteristics but not to the extent of the cells expressing the WT Dyn2-GFP (Figure 2d), suggesting that phosphorylation of Dyn2 has a role in promoting a motile phenotype of these tumor cells.
To quantify the effect of the exogenously expressed Dyn2 on lamellipodia dynamics, we performed live-time imaging of cells from each established cell line, collecting one image every 15 s for 10 min. These collected images were assembled to form kymographs in which the phase-dense ruffles at the periphery of each lamellipodium could be tracked and measured for motility as performed by others (Giannone et al., 2004; Bryce et al., 2005). The extension of a lamellipodium recorded in each kymograph is reflected by the forward (upward) slope of the phase dense lines. This slope is converted to speed of protrusion in nm/s using the formula (distance extended (nm)/time (s)). When comparing kymograph slopes of >11 cells from each cell line we found that Dyn2-GFP-overexpressing cells extended lamellipodia at a greater speed than did cells expressing GFP (Figures 2e and f). These data are consistent with the observation that high levels of Dyn2 can contribute to enhanced lamellipodia protrusion rates, suggesting that Dyn2 could have a meaningful role in tumor cell migration. Further, Dyn2Y(231/597)F-GFP was not able to promote cell migration (Figures 2e and f), suggesting that the role of Dyn2 in migration is likely dependent on its phosphorylation at known Src phosphorylation sites.
Next, the role of Dyn2 function in efficient lamellipodia protrusion was tested by reducing Dyn2 levels via siRNA depletion or by perturbing Dyn2 function with dynamin inhibitors in the control GFP-expressing BxPC-3 cell line. Notably, depletion of Dyn2 resulted in an over 60% reduction in lamellipodia protrusion rate (Figures 2g–i). Similarly, a 30 min pretreatment of cells with the small molecule dynamin inhibitor MiTMAB (Myristyl trimethyl ammonium bromide), that attenuates dynamin GTPase activity by disrupting the interaction between the PH domain and phospholipids (Hill et al., 2004; Quan et al., 2007), or Dynasore, a non-competitive inhibitor of dynamin GTPase activity that locks dynamin in a GTP-bound state likely through an association with the GTPase domain (Macia et al., 2006; Newton et al., 2006), also significantly attenuated lamellipodia protrusion to the same level as Dyn2 depletion (Figures 2j and k). Taken together, these data indicate that Dyn2 expression is necessary for efficient lamellipodia protrusion and also that Dyn2 overexpression is sufficient for enhancing lamellipodia protrusion rates in pancreatic tumor cells.
Dynamin activity is required for cell migration and invasion
To test whether Dyn2 function is required for cell migration, we assessed the migratory capacity of pancreatic tumor cells in wound-healing assays. The BxPC-3 cell lines failed to close wounds consistently and when closure was realized, the rate was too slow to eliminate the confounding effects of cell division, precluding this assay from producing useful analysis of wound healing motility for these cell lines. Therefore, we utilized a highly motile pancreatic cancer cell line (Panc04.03) for wound-healing migration assays. Cells were plated to confluence and a wound was generated with a plastic pipette before the addition of 50 ng/ml EGF in the presence or absence of the pharmacological dynamin inhibitory compounds, MiTMAB or Dynasore, or following Dyn2 depletion with siRNA. Although Panc04.03 cells treated with the dimethyl sulfoxide carrier migrated into the wounded area rapidly, cells in the presence of increasing concentrations of MiTMAB (Figures 3a and b) or Dynasore (Figures 3c and d) exhibited a dose-dependent reduction in cell migration into the wound, indicating that both dynamin's GTPase activity and its association with the membrane contribute to cell migration in pancreatic cancer cells. Similarly, although Panc04.03 cells treated with non-targeting siRNA migrated rapidly into the wound, depletion of Dyn2 with siRNA resulted in a statistically significant attenuation of this directed cell migration, further supporting the requirement for Dyn2 in pancreatic cancer cell migration.
To define the contribution of Dyn2 expression and activity on the migratory behavior of individual tumor cells, the established stable BxPC-3 cell lines described in Figure 2 were next analyzed in single-cell motility assays. Cells stably expressing either WT Dyn2-GFP, Dyn2Y(231/597)F-GFP or GFP were plated at low density, tracked over 11 h, and migration paths were generated. Consistent with the observation that Dyn2 overexpression enhances lamellipodia protrusion, the migration tracks indicate that Dyn2-GFP-expressing cells migrated further than did cells expressing the phospho-deficient Dyn2Y(231/597)F-GFP or the control GFP-expressing cells (Figures 4b–d). Quantification revealed a nearly twofold increase in total distance traveled for cells overexpressing Dyn2-GFP (Figure 4e), suggesting that Dyn2 expression level enhances pancreatic cancer cell migration and that this enhancement requires phosphorylation at tyrosines 231/597.
To confirm that the enhanced migration capacity of the Dyn2-GFP-overexpressing cells was due to Dyn2 activity, individual cell migration of BxPC-3 cells was examined in the presence of the dynamin inhibitors MiTMAB and Dynasore. Inhibition of dynamin function significantly reduced the migration of BxPC-3 cells overexpressing Dyn2-GFP (Figure 4f) consistent with the inhibitory effect of these compounds on Panc04.03 migration (Figure 3). Similarly, inhibition of dynamin function or depletion of Dyn2 with siRNA in control GFP-expressing cells significantly attenuated cell migration (Figures 4g and h), consistent with the requirement of Dyn2 expression for cell migration (Figure 3) and lamellipodia protrusion (Figure 2).
To link migratory activity with chemotactic invasion, blind-well chamber assays were performed using the BxPC-3 stable cell lines. Cells were plated in a low-serum environment in the top chamber of a permeable barrier filter containing 8-μm-diameter pores coated with 0.01% gelatin, whereas the bottom of the chamber contained high serum. Notably, over twofold more Dyn2-GFP-expressing tumor cells crossed the filter than did cells expressing Dyn2Y(231/597)F or GFP vector alone (Figures 5a–c and g). Consistent with the migration assay findings from Figure 4f, Dyn2 activity was required for this enhanced behavior as MiTMAB and Dynasore both significantly attenuated the ability of the Dyn2-GFP-expressing tumor cells to invade through the porous filter (Figures 5d–f and h). To confirm that the basal level of migration in GFP-expressing control cells is dependent on Dyn2, the cells were treated with dynamin-targeting drugs or depleted of Dyn2 with siRNA. Consistent with Figures 2, 3, 4, Dyn2 inhibition or depletion significantly attenuated chemotactic invasion to levels equivalent to drug-treated Dyn2-expressing cells (Figures 5i–j). Taken together, these experiments indicate that Dyn2 expression and activity are required for chemotactic invasion of BxPC-3 cells and that Dyn2 overexpression is sufficient to enhance this invasion process in a phosphorylation-dependent manner.
Overexpression of Dyn2 enhances local dissemination and metastatic invasion of pancreatic cancer in vivo
On the basis of the observations that increased levels of Dyn2 lead to the enhanced tumor cell migration and chemotactic invasion in vitro (Figures 4 and 5), we next asked whether overexpression of Dyn2 promotes cell migration in vivo using a murine orthotopic implantation model (Alves et al., 2001). The stable BxPC-3 cell lines expressing WT Dyn2-GFP (two independent cell lines), Dyn2Y(231/597)F-GFP (one cell line) or GFP alone (two cell lines) were transduced with a lentivirus encoding a luciferase gene to monitor tumor cell metastasis in living animals, and subsequently the cells were injected directly into the pancreas of nude mice in at least three independent experiments per construct. Two weeks post-injection, these mice were anesthetized, injected with the luciferase substrate luciferin and imaged by bioluminescence to detect tumor cell dissemination away from the primary tumor (Figures 6a–c). Images obtained were auto-segmented with IVision software to detect peaks of luminescence (Figures 6a′–c′). Importantly, although luminescence emitted by the luciferin was detected at the site of injection in all mice, >2.5-fold more mice injected with the Dyn2-GFP-expressing cells displayed luminescence distal from the injection area (arrows in Figures 6b and b′) compared with GFP control cells. Most notable was the fact that markedly few mice injected with BxPC-3 cells expressing the phospho-deficient Dyn2Y(231/597)F-GFP mutation exhibited peripheral luminescence in comparison with mice injected with WT-expressing Dyn2 cells (Figure 6d), despite the fact that the same number of cells were injected into each mouse, there was no difference in the growth rates of the cell lines as performed with mitochondrial activity assays and manual cell counts (unpublished data), and the resulting primary tumor was comparable across all cell lines (Figure 7e). These data suggest that Dyn2 overexpression and phosphorylation contribute to tumor cell dissemination in vivo.
To assess whether Dyn2 overexpression might increase metastatic potential of the injected tumor cells, mice implanted with the BxPC-3 cell lines were sacrificed 8 weeks post-orthotopic injection. Macroscopic inspection of mice injected with either of the GFP-expressing control cell lines revealed locally advanced, regionally invasive tumors on the spleen and the retroperitoneum consistent with the observations of others using parental BxPC-3 cells (Bouvet et al., 2000). Few large tumors were observed outside of the injection site and no tumors were observed on the liver. In marked contrast to control mice, all 18 mice injected with Dyn2-GFP-expressing cell lines had large (>1 mm) tumors within the body cavity, particularly on the intestinal wall and within the intestinal mesentery (Figures 7a–a′ and 6b′, respectively). In addition, 3 of 18 of these mice had tumors on the liver. Quantification revealed a >7-fold increase in the average number of large intestinal tumors when compared with mice injected with GFP-expressing BxPC-3 cells and a >3-fold increase compared with mice injected with cells expressing Dyn2Y(231/597)F-GFP (Figure 7d), even though the volume of the primary pancreas tumors were equivalent (Figure 7e). These findings provide a compelling in vivo correlation to the cellular migration studies shown in Figures 2, 3, 4, 5 and further implicate Dyn2 expression and its phosphorylation state in invasive migration of pancreatic tumor cells.
To assess the status of Dyn2 expression in human metastases, the relative Dyn2 expression levels were compared on histological samples representing benign tissue, PDAC and metastatic tissue from 39 patients (Figure 8). Consistent with the TMA data described in Figure 1, >80% of the PDAC samples displayed elevated Dyn2 levels when compared with benign tissue from the same patient (Figures 8a, b and e). Notably, elevated Dyn2 expression was retained in metastatic lesions of 60% of the patients, even after the tumors had reorganized into duct-like structures (Figure 8c, enlarged in Figure 8d).
In this study, we have presented some of the first observations linking the expression levels, function and regulation of the large mechanoenzyme dynamin to the neoplastic condition. Dyn2 expression is upregulated in >80% of PDAC in patients tested (Figures 1 and 8) and 60% of metastases (Figure 8). Further, we show that overexpression of Dyn2 in the PDAC cell line BxPC-3 induces a motile phenotype characterized by large and protrusive lamellipodia that is dependent on the phosphorylation state of Dyn2, as the Dyn2 phospho-deficient mutant Dyn2Y(231,597)F failed to induce the motile phenotype (Figures 2d and f). In support of these findings, dynamin inhibitors or Dyn2 depletion by siRNA attenuated PDAC cell migration (Figures 3, 4g and h) and chemotactic invasion (Figures 5d–j), while overexpression of WT, but not phospho-deficient, Dyn2 enhanced these events (Figures 4a–f, 5a–c′ and g). Finally, this accentuation of invasion by Dyn2 function was observed by the marked dissemination and metastasis of orthotopically injected pancreatic tumor cells into nude mice (Figures 6 and 7).
The initial premise of this study was based on the hypothesis that Dyn2 would participate in promoting cell migration and metastasis. We did not predict the amplification of Dyn2 levels in cancer patients. Histological analysis revealed elevated Dyn2 protein levels in a remarkably high percentage of the ductal epithelium of neoplastic ducts, though there were no obvious changes in acinar Dyn2 expression (Figure 1). Interestingly, the Dyn2 levels were increased in the early pancreatic intraepithelial neoplasia stages, as well as the later cancer stages, suggesting that Dyn2 upregulation is an early event that precedes or coincides with cell shape changes observed in the neoplastic condition. Most relevant is the fact that 60% of metastases exhibit Dyn2 overexpression, whereas 80% of the PDAC samples excised from the pancreas exhibited Dyn2 overexpression. Thus, Dyn2 overexpression appears to contribute to the initial stages of neoplasia, while also driving the metastatic process.
An essential role of dynamin in cell migration and invasion is further supported by the capacity of siRNA-mediated Dyn2 depletion or dynamin inhibitory compounds, MiTMAB and Dynasore, to attenuate the Dyn2-mediated enhancement of these migratory processes (Figures 2, 3, 4, 5). These observations are similar to those of Schlunck et al. (2004) who observed that siRNA-mediated knockdown of Dyn2 reduces lamellipodia extension in glioblastoma cells. Macia et al. (2006) showed that Dynasore is a non-competitive inhibitor of dynamin GTPase, likely locking it in a GTP-bound state. In contrast, MiTMAB inhibits the GTPase activity of dynamin, as well as interaction between the dynamin PH domain and lipids (Quan et al., 2007), suggesting that lipid-mediated interactions, as well as GTPase of Dyn2, are required for the formation of lamellipodia.
How Dyn2 synergizes with the action of the actin cytoskeleton to augment migration remains unclear. Dynamin has been shown to bind directly to actin-associated proteins and influence actin organization (McNiven et al., 2000; Qualmann et al., 2000; Lee and De Camilli, 2002; Orth et al., 2002; Schafer, 2004; Itoh et al., 2005; Kim et al., 2005; Kruchten and McNiven, 2006; Mooren et al., 2009; Wu et al., 2010). In particular, dynamin directly binds the actin-binding protein cortactin and together this complex regulates actin remodeling (McNiven et al., 2000; Mooren et al., 2009). This specific interaction appears to have an important role in the organization of actin bundles as dynamin and cortactin cooperate to crosslink actin into bundles that are remodeled and potentially turned over following dynamin GTPase activity (Mooren et al., 2009), as well as the formation of dendritic actin networks as demonstrated graphically by experiments with actin comet formation by Listeria monocytogenes (Lee and De Camilli, 2002; Orth et al., 2002). Thus, remodeled actin bundles and enhanced actin network branching facilitated by a Dyn2-cortactin complex would be expected to augment lamellipodial extension and cell migration. In addition, Dyn2 phosphorylation and GTPase activity could influence downstream signaling to the actin cytoskeleton as Dyn2 activity is required to traffic and activate the small GTPase Rac1 (Schlunck et al., 2004), perhaps in association with the Rac guanine exchange factor Vav1 (Gomez et al., 2005). Further, it has been shown that Dyn2 activity has an important role in the remodeling of focal adhesions, multiprotein complexes that link the internal actin cytoskeleton to the extracellular matrix (Ezratty et al., 2005, 2009; Yoo et al., 2005). In this model, elevated expression of Dyn2 would enhance turnover of focal complexes near the leading edge, allowing for more rapid transport of integrins (Chao and Kunz, 2009) and other focal adhesion molecules to the front of the cell. This model is consistent with the observation that cells expressing phospho-deficient Dyn2 continued to ruffle yet fail to extend lamellipodia at the same rate as Dyn2-overexpressing cells (Figures 2e and f) potentially due to the failure to lay down new focal contacts because of less turnover of focal adhesions. Also, Dyn2 interaction with membrane-bending F-BAR proteins (Itoh et al., 2005) and actin-regulating Spin90/WISH (Kim et al., 2005, 2006) appear to synergistically coordinate actin-dependent tubulation and scission of the plasma membrane.
Although the precise mechanism of Dyn2 action in migration remains to be determined, our data suggest that phosphorylation by Src has an important regulatory role. Overexpression of Dyn2 in pancreatic cancer cells enhanced cell migration and invasion, whereas overexpression of a phospho-deficient version of Dyn2, Dyn2Y(231,597)F, was unable to enhance these processes (Figures 2, 4b–f, 5a–c and g). Further, the expression of the phospho-mutant of Dyn2 was insufficient to enhance the Dyn2-mediated dissemination and distant metastases of pancreatic cancer cells in vivo (Figures 6–7). These data, together with previous reports showing that Src phosphorylation of Dyn2 regulates receptor endocytosis, Golgi morphology and focal adhesion turnover (Shajahan et al., 2004; Cao et al., 2010; Weller et al., 2010; Wang et al., 2011) reveal an essential regulatory role for Src in Dyn2 cellular functions. Src-mediated tyrosine phosphorylation of dynamin downstream of EGFR stimulates the GTPase activity and oligomerization of dynamin (Ahn et al., 1999, 2002). Importantly, both EGFR (Thybusch-Bernhardt et al., 2001; Bloomston et al., 2006; Dancer et al., 2007) and Src (Lutz et al., 1998; Hakam et al., 2003; Morton et al., 2010) are overexpressed in pancreatic cancer, and inhibition of Src activity is sufficient to attenuate the spread of pancreatic cancer metastasis in a mice (Morton et al., 2010).
Thus, from the combined findings reported here, Dyn2 appears to have an important role in cell migration and subsequently in the invasive process exhibited by these tumors. We have not observed any alterations in cell growth by the established Dyn2-expressing cell lines either in vitro or in vivo suggesting that the dissemination process is a reflection of metastatic activity. These observations implicate the dynamins as a new family of oncogenic proteins that could provide useful therapeutic targets. Indeed, a recent study has shown that the dynamin inhibitory compound MiTMAB preferentially attenuates the growth of multiple cancer cell types, but not fibroblasts, in culture (Joshi et al., 2010). This selectivity for neoplastic cell growth is noteworthy and further supports the premise of dynamin involvement in the neoplastic process.
Materials and methods
The polyclonal Dyn2 and pan-dynamin MC63 antibodies havebeen described previously (Cao et al., 1998; Henley et al., 1998). Monoclonal cortactin antibody, 4f11, was purchased from Upstate Biotechnology (Lake Placid, NY, USA). Mouse diclonal anti-GFP antibody was purchased from Roche Applied Science (Indianapolis, IN, USA). Polyclonal anti-actin antibody, TRITC-labeled phalloidin, 4,6-diamidino-2-phenylindole (DAPI), and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) were purchased from Sigma (St Louis, MO, USA). Dynamin inhibitory compounds, Dynasore and MiTMAB (Calbiochem, Darmstadt, Germany; Cat # 324410 and 324411), were resuspended in dimethyl sulfoxide to 25 mM stock concentrations and stored at −20 °C until use. Non-targeting siRNA#1 (Cat # D-001810-01) and Dyn2 siRNA 5′-GACAUGAUCCUGCAGUUCA-3′ (D2-02; Cat # D-004007-02;) from Dharmacon (Lafayette, CO, USA) were used for knockdown experiments with Invitrogen Lipofectamine RNAiMax Reagent (Invitrogen, Paisley, UK; Cat # 13778-075) according to the manufacturer's protocol, except that three time the recommended siRNA and RNAiMAX was used for BxPC-3 cells. Cells were used after 3 days of siRNA treatment.
Pancreatic epithelial tumor cell lines, from American Type Culture Collection (ATCC, Manassas, VA, USA), were grown in a 5% CO2/95% air incubator at 37 °C. Panc04.03 cells (ATCC CRL-2555) and BxPC-3 cells (ATCC CRL-1687) were maintained in RPMI-1640 medium and supplemented with 10% fetal bovine serum (FBS) (Sigma), 100 U/ml penicillin and 100 g/ml streptomycin.
Stable cell lines
BxPC-3 cells were electroporated in the presence of expression plasmids pEGFP-N1 (Clontech, Mountain View, CA, USA) encoding WT Dyn2, Dyn2Y(231/597)F or empty vector as described previously (Cao et al., 1998). Subsequently the cells were grown for 10–15 days before resistant clones were isolated, clonal lines were expanded and expression was confirmed by fluorescence microscopy and western blot. Growth medium for BxPC-3 stable cells was supplemented with 250 mg/l G418 (Invitrogen).
Cells were fixed in formaldehyde and processed for immunofluorecence as described previously (Cao et al., 1998). Fluorescent micrographs were acquired with a Zeiss Axio Observer D.1 epifluorescence microscope (Carl Zeiss, Inc., Goettingen, Germany) equipped with a Hamamatsu Orca II camera (Hamamatsu Photonics, Hamamatsu City, Japan) and analyzed with IVision software (BioVision, Mountain View, CA, USA).
Western blot analysis was performed as described (Cao et al., 1998) with minor modification. Cellular protein was harvested in RIPA buffer supplemented with EDTA-free complete protease inhibitor cocktail (Roche Applied Science) and the protein concentration was determined using the Pierce BCA Protein Assay Kit (Pierce Chemical Co., Rockford, IL, USA) using bovine serum albumin as a standard. Cellular proteins separated by continuous SDS–PAGE were transferred to polyvinyldifluoride membranes, which were subsequently blocked in phosphate buffered saline (150 mM NaCl, 10 mM NaH2PO4, pH 7.2) containing 5% milk and reacted with appropriate primary antibodies, washed in phosphate buffered saline and incubated with secondary antibodies conjugated to horseradish peroxidase. Immunoreactive bands were detected with SuperSignal West Pico Chemiluminescent Substrate (Pierce) using an autoradiography film (Denville Scientific Inc., Denville, NJ, USA).
Live-time imaging and kymography
BxPC-3 stable cells plated at low density were grown overnight on imaging dishes in a growth medium containing 10% FBS. Images of live cells were taken every 15 s for 10 min with the microscope system described above using a × 40 objective. Kymography of membrane protrusion was performed using the ImageJ (NIH) Kymography macro (http://www.embl.de/eamnet/html/downloads.html). Briefly, IVision movies were converted into .tiff stacks and then into ImageJ movies. Subsequently, ImageJ was used to select a line along advancing lamellipodia in each cell. The line of interested was processed through the kymography macro to generate kymograph images that were then copied into Photoshop (Adobe Systems Incorporated, San Jose, CA, USA) for measurement. Lamellipodium protrusion speed was calculated from the distance and time that the leading lamellipodial membrane traveled during advancing phases of membrane protrusion.
Cell migration and invasion assays
Wound healing assay
Untreated Panc04.03 cells or cells treated 48 h with siRNAs were plated to confluence on gridded coverslips (Bellco Biotechnology, Vineland, NJ, USA), serum starved (0.2% FBS) for 16 h and wounded with a plastic pipette tip. After washing thoroughly, wounds were stimulated with low-serum media containing 50 ng/ml EGF or carrier, supplemented with dimethyl sulfoxide, Dynasore or MiTMAB. Phase contrast images of the wound area were acquired 1 and 5 h after stimulation with a × 10 objective using the grid pattern to locate the exact same area of the wound at different times. Resulting images were compiled in Adobe Photoshop. Migration distance was determined by measuring the difference in wound-edge location in four regions of every image. Average migration speed was calculated from the average distance traveled and the time elapsed between images.
Individual cell motility assay
BxPC-3 stable cell lines were plated in imaging dishes containing gridded coverslips in growth media (10% FBS). At 4 h after plating, images were captured hourly for 11 h using a × 10 objective. The resultant images of each selected grid were aligned using the grid pattern as reference and compiled into movies. The motion paths of individual cells within each grid were generated with IVision software by tracking the center of area of each cell in the field in each frame acquired. Total distance, the sum of the distances that each cell traveled between all frames of the 11 h movies, and net distance, the distance traveled by each cell from the first to last frame, were measured using IVision. Cells that migrated out of the field of view during the experiment were eliminated from the analysis. For individual cell motility assays using dynamin inhibitors, the drugs or carrier was added after the first image was acquired.
Polycarbonate membranes (#PFA8, Neuro Probe, Inc., Gaithersburg, MD, USA) containing 8 μm pores were coated with 0.01% gelatin and assembled into Blind Well Chambers (#BW200L, Neuro Probe, Inc.) that contained a growth medium (RPMI-1640 supplemented with 10% FBS) in the bottom chamber. Subsequently, BxPC-3 stable cell lines were trypsinized (1 × trypsin-EDTA, 0.5% Trypsin, 1 mM 4Na-EDTA; GIBCO, Grand Island, NY, USA), resuspended in low-serum media (0.2% FBS) and counted with a hemocytometer. A total of 2 × 105 cells were plated into the top chamber. At 40 min after plating, the top chamber was supplemented with or without dimethyl sulfoxide or dynamin inhibitors. At 4 h after plating, the filters were fixed with formaldehyde, stained with TRITC phalloidin and DAPI, and imaged on a Zeiss 510 confocal microscope (Carl Zeiss, Inc.). The number of nuclei on the top and bottom of the filter was counted, and the result was expressed as a percentage of cells that migrated through the filter.
Histology and TMA
Histological samples containing normal and PDAC tissue from 9 de-identified patients or benign, PDAC and metastasis tissue from 39 de-identified patients were obtained from the Mayo Clinic SPORE in Pancreatic Cancer tissue core, were stained with the Dyn2 antibody by Tissue and Cell Molecular Analysis facility (Mayo Clinic, MN, USA) and relative Dyn2 expression was compared with normal or benign tissue. TMAs representing 85 patients, described previously (Muders et al., 2006), were stained with Dyn2 antibody by Tissue and Cell Molecular Analysis, staged using the International Union Against Cancer guidelines, graded according to WHO, and Dyn2 expression was scored semi-quantitatively by a pathologist (LZ) with respect to intensity (scale: 0–3). Normal ducts within the TMAs generally exhibited an intensity score of 1; therefore, ductal epithelium or PDAC within the TMAs with intensity scores of 2–3 were characterized as overexpressed.
Orthotopic implantation, xenogen imaging and autopsy
BxPC-3 stable clones were transduced with lentivirus encoding cDNAs for expression of FRα, firefly luciferase as reported previously (Hasegawa et al., 2006). Orthotopic implantation of 1 × 106 clonal BxPC-3 cells into 5–6-week-old female athymic BALB/c mice (nu/nu) obtained from Harlan was performed similar to (Alves et al., 2001). General anesthesia and analgesics were performed by intraperitoneal injection of ketamine (100 mg/kg) and xylazine (10 mg/kg) followed by intraperitoneal injection of buprenorphine (0.1 mg/kg). At 2 weeks after implantation, tumor cell spreading was visualized in anesthetized mice injected with 200 μl of 15 mg/ml D-Luciferin (Gold Biotechnology, St Louis, MO, USA) using a Xenogen IVIS 200 imaging system (Caliper LifeSciences, Hopkinton, MA, USA). The resulting luminescent images were imported into IVision to analyze tumor area and spatial distribution. The IVision auto-segmentation feature was used to automatically identify distinct tumors that were subsequently scored for location. At 8 weeks after implantation, mice were euthanized by intraperitoneal injection of Sleepaway sodium pentobarbital euthanasia solution (Fort Dodge Animal Health, Fort Dodge, IA, USA) and autopsied for macroscopic tumors. All experiments were carried out under IACUC approval.
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We are grateful for support from the National Institutes of Health NCI R01-CA104125 (MAM), T32-DK007198 (RDE), the Mayo Clinic Center for Cell Signaling in Gastroenterology (NIDDK P30DK084567), the Mayo Clinic SPORE in Pancreatic Cancer (P50 CA 102701), the Lustgarten Foundation for Pancreatic Cancer Research and the Fraternal Order of the Eagles.
The authors declare no conflict of interest.
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