Transforming growth factor β₁ rescues serum deprivation-induced apoptosis via the mitogen-activated protein kinase (MAPK) pathway in macrophages

Cell death and cell survival are central components of normal development and pathologic states. Transforming growth factor β₁ (TGF- β₁) is a pleiotropic cytokine that regulates both cell growth and cell death. To better understand the molecular mechanisms that control cell death or survival, we investigated the role of TGF-β₁ in the apoptotic process by dominant-negative inhibition of both TGF-β₁ and mitogen-activated protein kinase (MAPK) signaling pathways. Murine macrophages (RAW 264.7) undergo apoptosis following serum deprivation, as determined by DNA laddering assay. However, apoptosis is prevented in serum-deprived macrophages by the presence of exogenous TGF-β₁. Using stably transfected RAW 264.7 cells with the kinase-deleted dominant-negative mutant of TβR-II (TβR-II(M)) cDNA, we demonstrate that this protective effect by TGF-β₁ is completely abrogated. To determine the downstream signaling pathways, we examined TGF-β₁ effects on the MAPK pathway. We show that TGF-β₁ induces the extracellular signal- regulated kinase (ERK) activity in a time-dependent manner up to 4 h after stimulation. Furthermore, TGF-β₁ does not rescue serum deprivation-induced apoptosis in RAW 264.7 cells transfected with a dominant-negative mutant MAPK (ERK2) cDNA or in wild type RAW 264.7 cells in the presence of the MAPK kinase (MEK1) inhibitor. Taken together, our data demonstrate for the first time that TGF-β₁ is an inhibitor of apoptosis in cultured macrophages and may serve as a cell survival factor via TβR-II-mediated signaling and downstream intracellular MAPK signaling pathway.