TGF-β stimulates biglycan core protein synthesis but not glycosaminoglycan chain elongation via Akt phosphorylation in vascular smooth muscle

Abstract

Transforming growth factor-β (TGF-β) can mediate proteoglycan synthesis via Smad and non-Smad signalling pathways in vascular smooth muscle (VSM). We investigated whether TGF-β-mediated proteoglycan synthesis is via PI3K/Akt. TGF-β induced a rapid phosphorylation of Akt that continued upto 4 h. Akt phosphorylation was blocked by Akt1/2 inhibitor SN30978; however, it did not block Smad2 phosphorylation at either the carboxy or linker regions indicating that TGF-β-mediated Akt phosphorylation is independent of Smad2 signalling. The role of Akt in TGF-β-mediated proteoglycan synthesis was investigated. Treatment with SN30978 showed a concentration-dependent decrease in TGF-β-mediated [35S]-sulphate and [35S]-Met/Cys incorporation into secreted proteoglycans; however, SDS-PAGE showed no change in biglycan size. In TGF-β-treated cells, biglycan mRNA levels increased by 40–100% in 24 h and was significantly blocked by SN30978. Our findings demonstrate that Akt is a downstream signalling component of TGF-β-mediated biglycan core protein synthesis but not glycosaminoglycan chain hyper-elongation in VSM.