Insulin induces a change in extracellular matrix glycoproteins synthesized by rat mesangial cells in culture

Abstract

Extracellular matrix (ECM) accumulation within the glomerular mesangium is a hallmark of progressive forms of renal disease. We recently succeeded in propagating mesangial cells (MC) from the time of explant without supplemental insulin which exhibit a matrix profile analogous to normal mesangium in vivo. We used these cells to characterize insulin-induced changes in biosynthesis and accumulation of three important matrix glycoproteins, laminin, fibronectin, and thrombospondin. Two clones of MC derived from glomeruli from a single rat were compared. MC grown in the absence of supplemental insulin (SI-MC) assemble a matrix rich in fibronectin with much smaller accumulations of laminin and thrombospondin. In comparison, MC (SI+MC) grown chronically in the presence of 1 μM insulin have a greatly expanded ECM that immunostains less intensely with antibodies to fibronectin, but, it contains significant accumulations of laminin and thrombospondin. Following metabolic labeling of secreted proteins with 35S-methionine, total protein synthesis was measured, and specific ECM components were identified and quantitated by immunoprecipitation, SDS-PAGE and autoradiography. The rate of total protein synthesis was increased by 50% in SI+MC as compared to SI-MC, yet, individual proteins were increased or decreased. The rate of synthesis of fibronectin was decreased and the rate of synthesis of laminin and thrombospondin was increased by insulin. These changes were directionally correlated with the net accumulation of these proteins as shown by immunostaining. In addition to an increase in laminin synthesis, insulin treatment induced a change in the isoform of laminin expressed. To further delineate the role of insulin in the assembly of these different matrices, MC chronically maintained without supplemental insulin were exposed to 1 μM insulin. Morphologic and ECM compositional changes occurred within 10 days. They were identical to those described for MC propagated in supplemental insulin from the time of explant. Furthermore, insulin treatment of SI-MC induced a prompt change in the isoform of laminin synthesized. These results support our hypothesis that insulin plays a direct role in modulating the composition of mesangial matrix that accumulates in diabetic glomerulosclerosis.