TRANSFORMING GROWTH-FACTOR-BETA-2 REGULATES C3 SECRETION IN MONOCYTESTHROUGH A PROTEIN-KINASE C-DEPENDENT PATHWAY

Previously, we reported that TGF-beta 2 regulates the C3 gene expressi on in a dose- and time-dependent manner in monocytes. To extend these studies, we examined the role of PKC in the TGF-beta 2-mediated induct ion of C3 expression by the human monocyte cell line, U937. Treatment of U937 cells with the PKC inhibitors, H7 and calphostin C, suppressed TGF-beta 2-mediated induction of C3 protein levels, but not mRNA leve ls, in a dose-dependent manner. At the highest concentrations of H7 an d calphostin C, C3 protein levels were inhibited 50% and 93%, respecti vely, compared to control levels. Treatment of U937 cells with HA1004, a weak PKC inhibitor used as a control for H7, did not inhibit induct ion of C3 protein levels. Down-modulating PKC with a prolonged exposur e of U937 cells to PMA also suppressed TGF-beta 2-mediated C3 protein induction by as much as 82%. Incubating cell extracts isolated from TG F-beta 2-treated U937 cells with the PKC substrate, MIBP(4-14), result ed in increased substrate phosphorylation compared to cell extracts is olated from untreated cells. Addition of calphostin C suppressed the i ncreased substrate phosphorylation by TGF-beta 2.. Furthermore, biosyn thetic labeling of U937 cells treated with TGF-beta 2 and calphostin C demonstrated an accumulation of C3 protein within cell lysates compar ed to controls. Collectively, these studies suggest a role for PKC in the secretion of C3 protein during TGF-beta 2-mediated regulation of C 3 expression in U937 cells. (C) 1998 Elsevier Science Ltd. All rights reserved.