Serine/threonine protein phosphatase 5 regulates glucose homeostasis in vivo and apoptosis signalling in mouse Pancreatic islets and clonal MIN6 cells

Abstract

Aims/hypothesis: During the development of type 2 diabetes mellitus, beta cells are often exposed to a high glucose/hyperlipidaemic environment, in which the levels of reactive oxygen species (ROS) are elevated. In turn, ROS can trigger an apoptotic response leading to beta cell death, by activating mitogen-activated protein kinase (MAPK) signalling cascades. Here we test the hypothesis that serine/threo-nine protein phosphatase 5 (PP5) acts to suppress proapoptotic c-Jun N-terminal kinase (JNK) signalling in beta cells. Methods: Ppp5c-/- and Ppp5c+/+ mice were subjected to intraperitoneal glucose (IPGTT) or insulin tolerance tests. Pancreatic islets from Ppp5c-/- and Ppp5c+/+ mice or MIN6 cells treated with short-interfering RNA targeting PP5 were exposed to palmitate or H 2O2 to activate MAPK signalling. Changes in protein phosphorylation, mRNA expression, apoptosis and insulin secretion were detected by western blot analysis, quantitative RT-PCR or ELISA. Results: Ppp5c -/- mice weighed less and exhibited reduced fasting glycaemia and improved glucose tolerance during IPGTT, but retained normal insulin sensitivity and islet volume. Comparison of MAPK signalling in islets from Ppp5c -/- mice and MIN6 cells revealed that the lack of PP5 was associated with enhanced H2O2-induced phosphorylation of JNK and c-Jun. Cells with reduced PP5 also showed enhanced JNK phosphorylation and apoptosis after palmitate treatment. PP5 suppression in MIN6 cells correlated with hypersecretion of insulin in response to glucose. Conclusions/ interpretation: PP5 deficiency in mice is associated with reduced weight gain, lower fasting glycaemia, and improved glucose tolerance during IPGTT. At a molecular level, PP5 helps suppress apoptosis in beta cells by a mechanism that involves regulation of JNK phosphorylation. © springer-verlag 2012.