Intracellular signaling in rat cultured vascular smooth muscle cells: roles of nuclear factor-kappaB and p38 mitogen-activated protein kinase on tumor necrosis factor-alpha production.

Lipopolysaccharide (LPS) is responsible for initiating host responses leading to septic shock, and tumor necrosis factor-alpha (TNF alpha) is thought to be its primary mediator. In addition, TNF alpha is one of the major components of the pathogenesis of insulin resistance in various conditions. It has been shown that LPS induced TNF alpha production in rat vascular smooth muscle cells (VSMC). However, little is known about the signaling pathway by which VSMC in culture produce TNF alpha. We investigated the possible signaling components involved in this pathway. LPS elicited phosphorylation of p42/44 mitogen-activated protein kinase (MAPK) and p38 MAPK, degradation of inhibitor of kappaB (IkappaB), and an increase in nuclear binding activity of activating protein-1 and nuclear factor-kappaB (NF-kappaB). Different types of NF-kappaB inhibitors, pyrrolidine dithiocarbamate and MG132, which specifically abolished IkappaB degradation and subsequent NF-kappaB activation by LPS, suppressed TNF alpha secretion from VSMC. Although PD98059, a specific MAPK kinase inhibitor and SB203580, a specific p38 MAPK inhibitor, had no effect on NF-kappaB activity, SB203580 suppressed TNF alpha secretion; however, PD98059 did not. A cotransfection assay showed that transfection of dominant negative IkappaB or pretreatment with SB203580 suppressed the TNF alpha gene promotor-dependent transcription. TNF alpha messenger RNA expression induced by LPS was inhibited by pyrrolidine dithiocarbamate, MG132, and SB203580, but not by PD98059. These observations indicate that TNF alpha production in VSMC is stimulated by LPS, and its transcription and translation are dependent on NF-kappaB activation through proteasome-mediated IkappaB degradation. It is likely that p38 MAPK may play a critical role in regulating transcription of the TNF alpha gene in VSMC, unlike in other cell lines.