Matching Supply with Demand

Abstract

Insulin production in pancreatic β cells is stepped up in tandem with glucose-dependent insulin secretion. Calcium, which triggers secretion, stimulates the calpain-dependent cleavage of islet cell autoantigen 512 (ICA512), a catalytically inactive member of the receptor tyrosine phosphatase family that is associated with secretory granules. The cleaved cytosolic fragment (ICA512-CCF) translocates to the nucleus, where it stimulates insulin gene transcription. Mziaut et al. stimulated rat insulinoma INS-1 cells with glucose and a depolarizing concentration of KCl and found that a calpain inhibitor attenuated the increase in pro-ICA512 abundance seen after 2 hours of stimulation followed by 4 hours of incubation in glucose-free saline. Moreover, cells expressing an ICA512-CCF-GFP fusion protein showed increased abundance of endogenous ICA512 mRNA and pro-ICA512, as well as increased abundance of mRNA encoding other components of secretory granules. ICA512-CCF-GFP expression enhanced the nuclear abundance of total and phosphorylated STATs (signal transducers and activators of transcription), and glucose only enhanced the growth-hormone dependent nuclear accumulation of STAT5b in cultured pancreatic islets if they expressed ICA512. Both coimmunoprecipitation and fluorescence resonance energy transfer analysis indicated that ICA512 bound to STAT5b; moreover, a fusion protein containing the cytoplasmic domain of ICA512 inhibited the in vitro dephosphorylation of STAT5. ICA512-CCF-GFP enhanced STAT5-dependent transcriptional activity, and STAT5b knockdown led to a decrease in pro-ICA512 abundance. The E3 SUMO ligase PIASy bound to and sumoylated ICA512; PIASy inhibited STAT5-dependent transcriptional activity and an in vitro assay indicated that sumoylation of ICA512 decreased its binding to STAT5. Thus, the data define a retrograde pathway involving STAT5 through which ICA512 regulates production of secretory granule components. H. Mziaut, M. Trajkovski, S. Kersting, A. Ehninger, A. Altkrüger, R. P. Lemaitre, D. Schmidt, H. D. Saeger, M. S. Lee, D. N. Drechsel, S. Müller, M. Solimena, Synergy of glucose and growth hormone signalling in islet cells through ICA512 and STAT5. Nat. Cell Biol. 8 , 435-445 (2006). [PubMed]