Gene expression heterogeneity in human islet endocrine cells in vitro: the insulin signalling cascade

Abstract

Insulin secretion is a highly regulated mechanism involving a complex insulin-dependent network of communication between alpha, beta and delta cells. However, whereas the role of insulin in beta cells has been well documented, very little is known about its role in alpha and delta cells. Having recently demonstrated heterogeneity of insulin receptor (INSR) isoform expression in these three endocrine cell types, our current study aimed to characterise the expression pattern of the multiple isoforms involved in the insulin signal transduction cascade in human alpha, beta and delta cells in vitro. cDNA samples prepared from single human islet cells were subjected to nested PCRs. Of 706 cells analysed, 15% were alpha cells, 28% beta cells, 8% delta cells and 46% non-endocrine cells. Profiling of expression of the insulin signalling cascade elements showed a heterogeneity between islet cell types, although at least one member of each protein family was expressed in the three populations of endocrine cells. Thus, the mRNAs coding for INSR-B, phosphoinositide-dependent protein kinase-1 and the human homologue of v-akt murine thymoma viral oncogene homologue 1 (AKT1) could not be detected in alpha cells, but were expressed by beta and delta cells. In addition, while the insulin receptor substrates IRS1 and IRS2, phosphoinositide-3-kinase, catalytic, beta polypeptide (PIK3CB) and AKT2 were expressed with relatively low frequencies in alpha and delta cells (<17% for IRS1, IRS2, PIK3CB; <25% for AKT2), their frequencies of expression in beta cells were 50, 33, 33 and 100%, respectively. Our results suggest that insulin signalling cascade elements in human alpha, beta and delta cells have distinct expression patterns.