Effects of insulin on human pancreatic cancer progression modeled in vitro
© Chan et al.; licensee BioMed Central Ltd. 2014
Received: 7 March 2014
Accepted: 27 October 2014
Published: 6 November 2014
Pancreatic adenocarcinoma is one of the most lethal cancers, yet it remains understudied and poorly understood. Hyperinsulinemia has been reported to be a risk factor of pancreatic cancer, and the rapid rise of hyperinsulinemia associated with obesity and type 2 diabetes foreshadows a rise in cancer incidence. However, the actions of insulin at the various stages of pancreatic cancer progression remain poorly defined.
Here, we examined the effects of a range of insulin doses on signalling, proliferation and survival in three human cell models meant to represent three stages in pancreatic cancer progression: primary pancreatic duct cells, the HPDE immortalized pancreatic ductal cell line, and the PANC1 metastatic pancreatic cancer cell line. Cells were treated with a range of insulin doses, and their proliferation/viability were tracked via live cell imaging and XTT assays. Signal transduction was assessed through the AKT and ERK signalling pathways via immunoblotting. Inhibitors of AKT and ERK signalling were used to determine the relative contribution of these pathways to the survival of each cell model.
While all three cell types responded to insulin, as indicated by phosphorylation of AKT and ERK, we found that there were stark differences in insulin-dependent proliferation, cell viability and cell survival among the cell types. High concentrations of insulin increased PANC1 and HPDE cell number, but did not alter primary duct cell proliferation in vitro. Cell survival was enhanced by insulin in both primary duct cells and HPDE cells. Moreover, we found that primary cells were more dependent on AKT signalling, while HPDE cells and PANC1 cells were more dependent on RAF/ERK signalling.
Our data suggest that excessive insulin signalling may contribute to proliferation and survival in human immortalized pancreatic ductal cells and metastatic pancreatic cancer cells, but not in normal adult human pancreatic ductal cells. These data suggest that signalling pathways involved in cell survival may be rewired during pancreatic cancer progression.
KeywordsHyperinsulinemia Pancreatic cancer PANC1 HPDE Diabetes PDAC Pancreatic ductal adenocarcinoma AKT ERK
The incidence of pancreatic cancer is increasing, in parallel with the obesity and type 2 diabetes epidemics. Despite intense research efforts, the average 5-year survival rate for pancreatic cancer remains below 5%, which underscores the need to identify key risk factors and to develop preventative measures [1–3]. Multiple epidemiological studies have drawn a positive link between high levels of insulin and an increased risk of pancreatic cancer [1, 4, 5]. Obesity and early stage type 2 diabetes are both associated with elevated insulin levels, known as basal hyperinsulinemia . Given that insulin is a powerful mitogen and that its levels likely vary physiologically within the pancreas , it is possible that sustained increases in local insulin levels within the pancreas provide increased growth advantages and pro-survival effects in cells within the pancreas . It is therefore imperative to investigate the effects of insulin on different stages of pancreatic cancer progression.
The molecular mechanisms by which hyperinsulinemia may affect pancreatic cancer progression remain incompletely understood, but several studies have demonstrated the importance of the RAS-MEK-ERK pathway and the PI3K-AKT pathway. Over 90% of human pancreatic adenocarcinoma cases involve the KRASG12D gain-of-function mutation, and this mutation is sufficient to lead to pre-cancerous lesions and rare tumours in mouse models . The KRasG12D mutation leads to constitutive activation of RAF-MEK-ERK and PI3K-AKT cascades to drive uncontrolled growth, proliferation and survival of cancer cells . KRas-driven transformations can be inhibited by expression of dominant-negative Raf-1, MEK or ERK, which all lie downstream of Ras [11, 12]. It has been established that Raf-1 can promote the initiation, transformation and maintenance of neoplastic lesions in some cancer models [13, 14]. Constitutively active AKT can also transform normal mouse pancreatic duct cells into malignant pancreatic cancer cells in vivo, but the inability of PI3K-AKT inhibition to affect several Ras-driven cancers suggests that KRas acts on multiple pathways in oncogenesis [10, 16, 17].
In the present study, we examined the effects and mechanisms of insulin in three in vitro cell models designed to mimic the progression of pancreatic cancer in vivo. These cell models were: pancreatic ductal cell cultures, an immortalized human ductal epithelium cell line (HPDE), and an advanced metatstatic human pancreatic ductal cancer cell line (PANC1). We found that high levels of insulin accelerated the proliferation of immortalized and metatstatic pancreatic ductal cells but not primary ductal cells. Furthermore, the molecular signalling mechanisms activated by insulin were distinct in each model, suggesting that these processes may be rewired during the progression of pancreatic cancer. These studies reveal potential mechanisms of insulin-mediated growth and survival effects and provide a better understanding in the etiology of hyperinsulinemia-associated pancreatic cancer.
Human mixed pancreatic exocrine and ductal cell culture
Primary pancreatic exocrine cells that would normally be discarded were obtained from the Vancouver General Hospital (Vancouver, BC) as part of the Human Islet Transplant Program, from cadaver organ donors who had previously provided informed consent. Dr. Warnock’s organ retrieval protocols are approved by the University of British Columbia Clinical Research Ethics Board. Tissues were from 7 donors, males and females between the ages of 32 and 58. Procedures involved in the culturing, dissociating and sorting of primary mixed exocrine and ductal tissue were adapted from published protocols, with minor alterations [18, 19]. Briefly, human ductal cell culture was performed as follows. First, unsorted primary cells, after being dispersed by shaking incubation for 1 hour and trituration with trypsin, were plated (10 × 106 cells) in T-150 flasks, to allow preferential adhesion and removal of fibroblasts. Then, fibroblast-depleted cell suspensions were then seeded in 6-well plates at cell density of 1.5 × 106 cells per well for further treatments. For immunoblot analysis, dissociated mixed-pancreatic exocrine-ductal cells were used. For cell proliferation and cell survival assays, sorted ductal cells were used (CD90 negative population). Prior to insulin treatments, cells were cultured in basal media (CMRL1066, 0.5 mg/L transferrin, 10 mM nicotinamide, 5 μg/L sodium selenium, 0.5% BSA, 2 mM glutamine) for 6 hours, then treated with 0.2, 2, 20, 200 nM of human recombinant insulin (Sigma Aldrich, Missouri, USA), 5 μM GW5074 (Life Technologies, California, USA), or 100 nM Akti-1/2 (EMD Biosciences, Darmstadt, Germany).
HPDE and PANC1 cell culture and treatment
HPDE cells were kindly provided by Dr. Ming Tsao. HPDE cells between passages 7 to 15 were used, and were cultured in KSF medium as previously described , but switched to DMEM for the experiments because KSF medium contains 779.1 ± 87.43 nM insulin as measured by radioimmunoassay. PANC1 cells (ATCC, Manassas, USA) were cultured in DMEM as previously described . For treatments, cells were washed with PBS and starved in 1 mg/ml glucose DMEM for for 6 hours (HPDE cells), or 24 hours (PANC1 cells). Thereafter, the cells were treated with insulin, IGF-1, DMSO, 10 μM GW5074, 10 μM U0126 (Cell Signaling, USA), 200 nM Akti-1/2 or 1 μM wortmannin (EMD Biosciences). These concentrations were chosen based on the literature and were shown to block signalling in PANC1 cells.
Cell counting and cell survival assays
The number of cells, live-stained with a concentration of Hoechst-33342 (50 ng/ml) that does not affect viability , was measured over time using ImageXpressMICRO high content imaging systems (Molecular Devices, Sunnyvale, California, USA). Images were analyzed with Acuity Xpress 2.0 (Molecular Devices). Cell death was measured by quantifying the percentage of cells incorporating propidium iodide (Sigma-Aldrich, 0.5 μg/ml) [23–25]. Cell viability, as indicated by metabolic capacity, was also quantified using the XTT kit (ATCC). Bromodeoxyuridine (BrdU) incorporation (Roche, Basel, Switzerland) was also used to determine proliferation in primary cells as previously described [19, 26].
Immunoblotting and protein analysis
Cells were lysed and subjected to immunoblotting as previously described . Polyclonal mouse and rabbit secondary antibodies, monoclonal antibodies for insulin receptor, ERK1/2, p-ERK1/2(T202/Y204), AKT, p-AKT(S473), and cleaved caspase 3 were obtained from Cell Signaling. Mouse monoclonal beta-actin antibody was obtained from Novus Biologicals (Littleton, Colorado, USA). Chemiluminescence of the blots was imaged on films that were subsequently scanned. The density of individual bands was quantified using the histogram function of using Adobe Photoshop CS5 after inversion and auto-contrast functions were applied to the whole image. Protein levels were expressed as the fold change relative to control.
All data were analyzed by paired sample t-test, or one-way or two-way ANOVA, followed by post-hoc tests (Dunnett’s or Bonferroni analysis) with Prism (GraphPad, La Jolla, California, USA). Results are presented as mean ± SEM, and are considered significant if the p-value was less than 0.05.
Baseline abundance of insulin signalling proteins in human primary pancreatic ductal cells, human HPDE cells and human PANC1 cells
Insulin signaling in primary human exocrine and ductal pancreas cells
Insulin signalling in HPDE cells
Insulin signalling in PANC1 cells
Effects of three insulin analogs on PANC1 cells
Insulin and IGF1 are growth factors with putative regulatory roles in proliferation, survival and cancer progression . Given that hyperinsulinemia has been identified as an independent risk factor for pancreatic cancer [1, 2, 39, 41], it is imperative to understand how changes in insulin signalling may promote cancer progression. To date, not much is known about the action of insulin on normal human pancreatic exocrine and ductal cells. Furthermore, direct comparisons of insulin signalling effects across models of different stages of pancreatic cancer have not been reported. In the present study, we demonstrated that pancreatic cancer progression is associated with changes in insulin signalling pathways that underlie cell survival, proliferation and viability. We found that primary human ductal cells are responsive to insulin and exhibit reduced cell viability when AKT signalling is disrupted. Immortalized HPDE ductal cells were also responsive to insulin, but less so than to IGF1, perhaps due to an abundance of IGF1 receptors. In contrast to the primary cells, HPDE cells required MAPK signaling and not AKT signaling to survive. The metastatic PANC1 cell model responded to insulin, more so than to IGF1, and also had a strong dependence on MAPK signalling and not AKT signalling. Collectively, our results imply a re-wiring of ductal cell dependence on the MAPK signalling axis for cell survival. Further understanding of how cells favor one pathway over another in pancreatic cancer progression may lead to novel approaches to halt early carcinogenesis and improve the long-term survival of pancreatic cancer patients.
In the present study, we found that these cell models derived from exocrine tissue required higher doses of insulin to elicit responses when compared to our previous experience with pancreatic exocrine cells that respond to physiological insulin doses in the high picomolar range [6, 26, 34, 35, 37, 38, 42]. This finding suggests the possibility that the exocrine cells and their cancerous descendants may be somewhat refractory to low concentrations insulin and may require sustained hyperinsulinemia to accelerate cancer progression. Multiple epidemiological studies have demonstrated that the hyperinsulinemic states of obesity and recent onset type 2 diabetes are associated with different types of cancer [43, 44], and this has been replicated in some animal models. For example, elevated insulin levels have been implicated in in vivo mouse models of breast cancer [45, 46]. The metabolic changes that result from both conditions make it difficult to discern causal factors that promote carcinogenesis. Hyperinsulinemia can precede and lead to the development of obesity , which suggests that it may contribute to carcinogenesis indirectly as well. Indeed, high levels of circulating insulin have been associated with increased risk of breast cancer in post-menopausal women [47, 48]. Given the association between hyperinsulinemia and pancreatic cancer , it has been suggested that excessive secretion of insulin by pancreatic β-cells required to maintain glucose homeostasis may directly influence pancreatic carcinogenesis in at-risk individuals.
The mitogenic actions of insulin have been well described in vitro and in vivo, but little is known of insulin’s proliferative effects on the endocrine and exocrine compartments of the pancreas. We previously demonstrated that insulin, even at physiological picomolar doses , promotes the proliferation of pancreatic endocrine β-cells , but whether similar effects occur on the exocrine compartment was not known. In the present study, we did not observe any proliferative effects of insulin in primary ductal cells or transformed HPDE cells. Instead, we found that insulin and closely related IGF1 promoted cell viability and survival in multiple models of pancreatic cancer progression. Collectively, these findings suggest that the oncogenic properties of insulin may be due to its effects on survival as opposed to its mitogenic effects. The downstream mechanisms of insulin action in these three models remain unclear. However, a recent report has suggested that HPDE proliferation depends on Pdx1 , which we have shown is an anti-apoptotic transcription factor controlled by low doses of insulin [42, 51]. Additional studies are warranted to fully elucidate the mechanisms.
The authors thank Caitlin Der, Ling Mu, Qinya Zhang, Roger Kiang, and others in the Johnson laboratory for their efforts throughout this project. We thank Dr. Sylvia Ng (University of British Columbia) and Dr. Ming Tsao (University of Toronto) for the HPDE cell line. This study was supported by a grant from the Cancer Research Society to J.D.J. and a grant from the Vancouver Hospital Foundation to G.L.W and J.D.J.
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