SUMOylation protects against IL-1β-induced apoptosis in INS-1 832/13 cells and human islets

Abstract

tional modification by the small ubiquitin-like modifier (SUMO) peptides, known as SUMOylation, is reversed by the sentrin/SUMOspecific proteases (SENPs). While increased SUMOylation reduces ␤-cell exocytosis, insulin secretion, and responsiveness to GLP-1, the impact of SUMOylation on islet cell survival is unknown. Mouse islets, INS-1 832/13 cells, or human islets were transduced with adenoviruses to increase either SENP1 or SUMO1 or were transfected with siRNA duplexes to knockdown SENP1. We examined insulin secretion, intracellular Ca 2ϩ responses, induction of endoplasmic reticulum stress markers and inducible nitric oxide synthase (iNOS) expression, and apoptosis by TUNEL and caspase 3 cleavage. Surprisingly, upregulation of SENP1 reduces insulin secretion and impairs intracellular Ca 2ϩ handling. This secretory dysfunction is due to SENP1-induced cell death. Indeed, the detrimental effect of SENP1 on secretory function is diminished when two mediators of ␤-cell death, iNOS and NF-B, are pharmacologically inhibited. Conversely, enhanced SUMOylation protects against IL-1␤-induced cell death. This is associated with reduced iNOS expression, cleavage of caspase 3, and nuclear translocation of NF-B. Taken together, these findings identify SUMO1 as a novel antiapoptotic protein in islets and demonstrate that reduced viability accounts for impaired islet function following SENP1 up-regulation. islets of Langerhans; SUMOylation; inducible nitric oxide synthase; sentrin/SUMO-specific protease 1; SENP1 SMALL UBIQUITIN-RELATED MODIFIER (SUMO) peptides are involved in the posttranslational modification of numerous cellular proteins (19,20,26). SUMO modification, known as SUMOylation, occurs through a cascade of well-characterized enzymatic events that rely on the E2 conjugating enzyme Ubc9 (19,20,22). SUMOylation is made reversible by the sentrin/ SUMO-specific proteases (SENPs), which remove SUMO from target proteins (49). Due to the large number of SUMOylatable targets, the cellular outcomes of SUMO modification are diverse, ranging from the control of DNA repair to the regulation of transcription factors and plasma membrane proteins (9,10,20,28,39,42,44). SUMO1 modification has a negative impact on ␤-cell secretory function (35). Upregulation of SUMO1 decreases insulin gene expression (28, 44), glucagon-like peptide-1 (GLP-1) receptor signaling (42), and both glucose-and exendin-4-stimulated insulin secretion (10, 42), suggesting that deSUMOylation may improve secretory function. Indeed, knockdown of the SUMOylating enzyme Ubc9 enhances exendin-4-stimulated insulin secretion (42), and upregulation of the deSUMOylating enzyme SENP1 increases exocytosis from rodent ␤-cells at low glucose (10, 48). Whether this elevates insulin secretion above that stimulated by glucose is unclear, as SENP1 does not increase ␤-cell exocytosis above the levels seen with high glucose (10). Thus, while increased SUMOylation results in secretory impairment (10, 42, 48), the effect of the deSUMOylating enzyme SENP1 remains unknown.SUMOyla...

A post-translational balancing act: the good and the bad of SUMOylation in pancreatic islets

“…These observations are in line with what my laboratory [14,18,19] and others [12,20] have shown using complementary models, including in human islets. We know, for example, that increasing SUMOylation blunts glucose-stimulated insulin secretion by directly inhibiting insulin exocytosis [14,21], and that islet Senp1 knockout results in glucose intolerance without affecting islet mass or insulin content [18].…”

“…Indeed, consistent with the findings of He et al, many studies suggest that SUMOylation is protective [26], although this also may be cell-type or context specific [27]. In islets, SUMOylation protects against IL-1β-induced death and dysfunction by preventing IL-1β-induced caspase 3 cleavage, NFκB translocation and induction of inducible nitric oxide synthase (iNOS) [19]. The capacity of beta cells to handle nitrosative and oxidative stress is regulated in part by nuclear factor erythroid 2-related factor 2 (NRF2) [28], a transcription factor that controls the expression of antioxidant genes (particularly those related to the glutathione pathway).…”

“…Conversely, upregulating Senp1 causes impaired insulin secretion while inducing beta cell apoptosis and sensitising to IL-1β-induced islet dysfunction and death [19]. In this study, increasing SUMOylation either by SENP1 knockdown or Sumo1 overexpression protected islets from IL-1β-induced apoptosis [19]. Thus, the overall level of SUMOylation in beta cells is important for determining the balance between cell survival and secretory function (Fig.…”

“…We know, for example, that increasing SUMOylation blunts glucose-stimulated insulin secretion by directly inhibiting insulin exocytosis [14,21], and that islet Senp1 knockout results in glucose intolerance without affecting islet mass or insulin content [18]. Conversely, upregulating Senp1 causes impaired insulin secretion while inducing beta cell apoptosis and sensitising to IL-1β-induced islet dysfunction and death [19]. In this study, increasing SUMOylation either by SENP1 knockdown or Sumo1 overexpression protected islets from IL-1β-induced apoptosis [19].…”

Isocitrate-to-SENP1 signaling amplifies insulin secretion and rescues dysfunctional β cells

“…Indeed, SENP1 alone can enhance insulin exocytosis, but this action is blocked under oxidizing conditions (32). SUMOylation may play diverse roles in pancreatic islet biology (33), having recently been implicated in metabolism (34), incretin receptor signaling (35), excitability (36), and survival (37). Importantly, the role for SENP1 in insulin secretion and in vivo glucose homeostasis remains to be resolved.…”