High glucose induces suppression of insulin signalling and apoptosis via upregulation of endogenous IL-1β and suppressor of cytokine signalling-1 in mouse pancreatic beta cells

Abstract

Chronic hyperglycemia and inflammatory cytokines disrupt and/or attenuate signal transduction pathways that promote normal β-cell survival, leading to the destruction of endocrine pancreas in type 2 diabetes. There is convincing evidence that autocrine insulin signalling exerts protective anti-apoptotic effects on beta cells. Suppressors of cytokine signalling (SOCS) were induced by several cytokines and inhibit insulin-initiated signal transduction. The aim of this study was to investigate whether high glucose can influence endogenous interleukin-1β (IL-1β) and SOCS expression thus affecting insulin signalling and survival in insulin-producing mouse pancreatic beta cells (βTC-6). Results showed that prolonged exposure of βTC-6 cells to increased glucose concentrations resulted in significant inhibition of insulin-induced tyrosine phosphorylation of the insulin receptor (IR), and insulin receptor substrate-2 (IRS-2) as well as PI3-kinase activation. These changes were accompanied by impaired activation of the anti-apoptotic signalling protein Akt and annulment of Akt-mediated suppression of the Forkhead family of transcription factors (FoxO) activation. Glucose-induced attenuation of IRS-2/Akt-mediated signalling was associated with increased IL-1β expression. Enhanced endogenous IL-1β specifically induced mRNA and protein expression of SOCS-1 in βTC-6 cells. Inhibition of SOCS-1 expression by SOCS-1-specific small interfering RNA restored IRS-2/PI3K-mediated Akt phosphorylation suppressed by high glucose. The upregulation of endogenous cytokine signalling and FoxO activation were accompanied by enhanced caspase-3 activation and increased susceptibility of cells to apoptosis. These results indicated that glucose-induced endogenous IL-1β expression increased βTC-6 cells apoptosis by inhibiting, at least in part, IRS-2/Akt-mediated signalling through SOCS-1 upregulation.

Introduction

Increasing evidence suggests that enhanced pancreatic beta cell apoptosis is an important factor contributing to beta cell loss in type 2 diabetes [1], [2]. Chronic hyperglycemia and inflammatory cytokines disrupt and/or attenuate signal transduction pathways that promote normal beta cell turnover and survival, leading to the destruction of the endocrine pancreas in type 1 and type 2 diabetes [3], [4]. Pro-inflammatory cytokines, such as interleukin-1β (IL-1β), tumor necrosis factor (TNF-α) or γ-interferon (IFN-γ), induce beta cell death, acting alone or in combination [5]. Although IL-1β is typically produced by activated macrophages during the inflammatory process, it was reported that prolonged exposure of human islets to increased glucose concentrations induced IL-1β production by resident beta cells leading to nuclear factor-κB (NF-κB) activation and upregulation of Fas signalling, thus triggering cell apoptosis [3], [6], [7], [8], [9]. However, these results were debated by other studies indicating that exposure of human islets to increasing glucose concentrations did not lead to IL-1β or Fas expression and NF-κB activation [4], [10]. Pro-inflammatory cytokines stimulate the expression of suppressors of cytokine signalling (SOCS) via activation of the JAK/STAT signalling pathway [11]. Cytokine-induced SOCS proteins participate in a negative feedback loop in cytokine signalling and in the negative regulation of insulin signalling in insulin-sensitive tissues, thus providing a mechanism which partly explains insulin resistance in type 2 diabetes [11]. A recent study demonstrated that exposure of the RINm5F pancreatic beta cells to exogenous IL-1β, induced desensitization of insulin signalling by inducing SOCS-3 expression, an effect which could potentially alter insulin-mediated beta cell survival [12].

Activation of insulin/IGF signalling in pancreatic beta cells was suggested to protect from apoptosis [13], [14], [15], [16], [17]. The importance of autocrine/paracrine insulin signalling in pancreatic beta cells, especially on beta cell mass plasticity was demonstrated in the beta cell-specific insulin receptor knockout (βIRKO) mouse model as well as in IRS-2 null mice, both of which had impaired beta cell growth [18], [19] and a marked reduction of beta cell mass caused by increased beta cell apoptosis, similar to that found in type 2 diabetes [20]. Moreover, increased IRS-2 expression in beta cells promoted beta cell survival, protecting them from streptozotocin- and free fatty acid (FFA)- or hyperglycemia-induced apoptosis [16], [21], [22]. Signal transduction downstream IRS-2 also contributes to control of beta cell growth and survival through activation of the serine–threonine kinase PKB/Akt [2], [23], [24]. Expression of a constitutively active Akt in pancreatic beta cells prevented FFA-induced apoptosis [25]; transgenic expression of the same variant in these cells prevented streptozotocin-induced diabetes and increased beta cells mass [14]. The anti-apoptotic function of Akt is mediated by its ability to phosphorylate apoptotic regulatory molecules including the Forkhead family of transcription factors (FoxO). Phosphorylation by Akt inhibits FoxO activity by promoting nuclear to cytoplasmic translocation, away from cell-death inducing genes [26].

The importance of insulin signalling in the control of pancreatic beta cell survival is well appreciated; however, the adverse effects of high glucose on insulin signalling pathway, in association with endogenous cytokine signalling and beta cell survival have not been examined. In the present study we investigated whether high glucose concentrations could interfere with endogenous IL-1β/SOCS expression and insulin-induced signalling in the mouse βTC-6 pancreatic beta cell line, thus increasing susceptibility of beta cells to apoptosis.