Abstract
Clearance of apoptotic cells by macrophages induces HGF secretion. We examined the regulatory mechanisms of HGF mRNA and protein expression in macrophages upon exposure to apoptotic cells. The interaction of RAW 264.7 macrophages with apoptotic Jurkat cells, but not with viable cells, resulted in expression of HGF mRNA and protein. Exposure of RAW 264.7 cells to apoptotic cells induced activation of RhoA, the PI3K/Akt pathway, and MAPKs, including p38 MAPK, ERK, and JNK. Down-regulation of the RhoA/Rho kinase pathway by pharmacological inhibitors or a RhoA-specific siRNA suppressed HGF mRNA and protein expression by macrophages in response to apoptotic cells through the phosphorylation of Akt and the MAPKs. Inhibition of PI3K decreased phosphorylation of Akt and the MAPKs. Inhibition of JNK, but not p38 MAPK and ERK, reduced Akt phosphorylation. The pharmacological inhibitor of PI3K and the MAPKs blocked HGF mRNA and protein expression. Other types of apoptotic cells, such as HeLa cells and murine thymocytes, could also induce HGF mRNA through the RhoA-dependent pathway. Likely, the RhoA-dependent signaling pathway was required for HGF mRNA induction in primary cells of peritoneal macrophages in response to apoptotic cells. An HGFR-blocking antibody did not alter apoptotic cell-induced activation of RhoA, Akt, and the MAPKs, as well as HGF production. Overall, the data provide evidence that activation of the RhoA/Rho kinase pathway up-regulates transcriptional HGF production in response to apoptotic cells.