INSULIN INDUCES A CHANGE IN EXTRACELLULAR-MATRIX GLYCOPROTEINS SYNTHESIZED BY RAT MESANGIAL CELLS IN CULTURE

Insulin induces a change in extracellular matrix glycoproteins synthes ized by rat mesangial cells in culture. Extracellular matrix (ECM) acc umulation within the glomerular mesangium is a hallmark of progressive forms of renal disease. We recently succeeded in propagating mesangia l cells (MC) from the time of explant without supplemental insulin whi ch exhibit a matrix profile analogous to normal mesangium in vivo. We used these cells to characterize insulin-induced changes in biosynthes is and accumulation of three important matrix glycoproteins, laminin, fibronectin, and thrombospondin. Two clones of MC derived from glomeru li from a single rat were compared. MC grown in the absence of supplem ental insulin (SI(-)MC) assemble a matrix rich in fibronectin with muc h smaller accumulations of laminin and thrombospondin. In comparison, MC (SI(+)MC) grown chronically in the presence of 1 mu M insulin have a greatly expanded ECM that immunostains less intensely with antibodie s to fibronectin, but, it contains significant accumulations of lamini n and thrombospondin. Following metabolic labeling of secreted protein s with S-35-methionine, total protein synthesis was measured, and spec ific ECM components were identified and quantitated by immunoprecipita tion, SDS-PAGE and autoradiography. The rate of total protein synthesi s was increased by 50% in SI(+)MC as compared to SI(-)MC, yet, individ ual proteins were increased or decreased. The rate of synthesis of fib ronectin was decreased and the rate of synthesis of laminin and thromb ospondin was increased by insulin. These changes were directionally co rrelated with the net accumulation of these proteins as shown by immun ostaining. In addition to an increase in laminin synthesis, insulin tr eatment induced a change in the isoform of laminin expressed. To furth er delineate the role of insulin in the assembly of these different ma trices, MC chronically maintained without supplemental insulin were ex posed to 1 mu M insulin. Morphologic and ECM compositional changes occ urred within 10 days. They were identical to those described for MC pr opagated 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 insu lin plays a direct role in modulating the composition of mesangial mat rix that accumulates in diabetic glomerulosclerosis.