Extracellular matrix proteins involved in pseudoislets formation

Extracellular matrix proteins are known to mediate, through integrins, cell adhesion and are involved in a number of cellular processes, including insulin expression and secretion in pancreatic islets. We investigated whether expression of some extracellular matrix proteins were implied in islets-like structure formation, named pseudoislets. For this purpose, we cultured the beta-cell line, RINm5F, during 1, 3, 5 and 7 days of culture on treated or untreated culture plate to form adherent cells or pseudoislets and analyzed insulin, collagen IV, fibronectin, laminin 5 and beta 1-integrin expression. We observed that insulin expression and secretion were increased during pseudoislets formation. Moreover, we showed by immunohistochemistry an aggregation of insulin secreting cells in the centre of the pseudoislets. Peripheral beta-cells of pseudoislets did not express insulin after 7 days of culture. RT-PCR and immunohistochemistry studies showed a transient expression of type IV collagen in pseudoislets for the first 3 days of culture. Study of fibronectin expression indicated that adherent cells expressed more fibronectin than pseudoislets. In contrast, laminin 5 was more expressed in pseudoislets than in adherent cells. Finally, expression of beta 1-integrin was increased in pseudoislets as compared to adherent cells. In conclusion, laminin 5 and collagen IV might be implicated in pseudoislets formation whereas fibronectin might be involved in cell adhesion. These data suggested that extracellular matrix proteins may enhance the function of pseudoislets.