Smad and p38 MAP kinase-mediated signaling of proteoglycan synthesis in vascular smooth muscle.

Atherosclerosis is the underlying pathological process of most cardiovascular disease. A critical component of the "response to retention" hypothesis of atherogenesis is proteoglycan/low density lipoprotein (LDL) binding. Transforming growth factor beta (TGF-beta) is present in atherosclerotic lesions, regulates vascular smooth muscle cell (VSMC) proteoglycan synthesis via an unknown signaling pathway, and increases proteoglycan/LDL binding. This pathway was investigated using the activin receptor-like kinase 5 (ALK5) inhibitor SB431542 and inhibitors of p38 MAP kinase as a possible downstream or alternative mediator. TGF-beta stimulated and SB431542 inhibited the phosphorylation of Smad2/3. In human VSMC, TGF-beta increased [(35)S]sulfate incorporation into proteoglycans associated with a 19% increase in glycosaminoglycan (GAG) chain size by size exclusion chromatography. SB431542 caused a concentration-dependent decrease in TGF-beta-mediated [(35)S]sulfate incorporation with 92% inhibition at 3 mum. Two different p38 MAP kinase inhibitors, SB203580 and SB202190, but not the inactive analogue SB202474, concentration-dependently blocked TGF-beta-mediated [(35)S]sulfate incorporation. TGF-beta increased [(3)H]glucosamine incorporation into glycosaminoglycans by 180% and [(35)S]Met/Cys incorporation into proteoglycan core proteins by 35% with both effects completely inhibited by SB431542. Blocking both Smad2/3 and p38 MAP kinase pathways prevented the effect of TGF-beta to increase proteoglycan to LDL binding. TGF-beta mediates its effects on proteoglycan synthesis in VSMCs via the ALK5/Smad2/3 phosphorylation pathway as well as via the p38 MAP kinase signaling cascade. Further studies of downstream pathways controlling proteoglycan synthesis may identify potential therapeutic targets for the prevention of atherosclerosis and cardiovascular disease.