Beta-cell gene expression and functional characterisation of the human insulinoma cell line CM

Animal insulinoma cell lines are widely used to study physiological and pathophysiological mechanisms involved in glucose metabolism and to establish in vitro models for studies on β-cells. In contrast, human insulinoma cell lines are rarely used because of difficulties in obtaining and culturing them for long periods. The aim of our study was to investigate, under different experimental conditions, the capacity of the human insulinoma cell line CM to retain β-cell function, particularly the expression of constitutive β-cell genes (insulin, the glucose transporters GLUT1 and GLUT2, glucokinase), intracellular and secreted insulin, β-cell granules, and cAMP content. Results showed that CM cells from an early-passage express specific β-cell genes in response to glucose stimulation, in particular the insulin and GLUT genes. Such capacity is lost at later passages when cells are cultured at standard glucose concentrations. However, if cultured at lower glucose concentration (0.8 mM) for a longer time, CM cells re-acquire the capacity to respond to glucose stimulation, as shown by the increased expression of β- cell genes (insulin, GLUT2, glucokinase). Nonetheless, insulin secretion could not be restored under such experimental conditions despite the presence of intracellular insulin, although cAMP response to a potent activator of adenylate cyclase, forskolin, was present indicating a viable system. In conclusion, these data show that the human insulinoma cell line CM, at both early-passage and late-passage, possesses a functional glucose-signalling pathway and insulin mRNA expression similar to normal β-cells, representing, therefore, a good model for studies concerning the signalling and expression of β-cells. Furthermore, we have previously shown that it is also a good model for immunological studies. In this respect it is important to note that the CM cell line is one of the very few existing human β-cell lines in long- term culture.