Thrombin regulates vascular smooth muscle cell proteoglycan synthesis via PAR-1 and multiple downstream signalling pathways

Introduction

Atherosclerosis is the underlying pathological process of most cardiovascular disease. Thrombin is a serine protease which can activate protease activated receptors (PAR) on vascular smooth muscle cells (VSMC) to elicit cellular responses that can contribute to the pathogenesis of atherosclerosis. Human atherosclerosis commences with the binding and retention of lipoproteins by the glycosaminoglycan (GAG) chains of chondroitin/dermatan sulfate proteoglycans. The potential effects of thrombin on the synthesis and structure of CS/DS proteoglycans produced by VSMCs was investigated.

Materials and methods

VSMCs were derived from human internal mammary arteries. Proteoglycan synthesis was assessed by [³⁵S]sulfate and [³H]glucosamine incorporation. Proteoglycan size was assessed by SDS-PAGE and size exclusion chromatography.

Results and conclusion

Thrombin caused a dose-dependent increase in [³⁵S]sulfate and [³H]glucosamine incorporation with maximum effects of approximately 150% at the highest doses tested. This increase was associated with increased size of biglycan and decorin assessed by SDS-PAGE. Chemically cleaved glycosaminoglycan (GAG) chains analyzed by SDS-PAGE and size exclusion chromatography were larger for proteoglycans from thrombin treated cells. VSMCs synthesize small GAGs when provided with exogenous xyloside and thrombin treatment also increased the size of the secreted xyloside GAGs. The effect of thrombin was not mimicked by the catalytically inactive FPRCK-HCT and was blocked in a concentration- dependent manner by the PAR-1 antagonist, JNJ5177049. Inhibition of PK C with GF 109203X resulted in concentration dependent but partial inhibition of [³⁵S]sulfate incorporation accompanied by a reduction in the size of biglycan and decorin. Epidermal growth factor (EGF) stimulated [³⁵S]sulfate incorporation and increased proteoglycan size and this was completely blocked by the EGF receptor tyrosine kinase inhibitor AG1478. AG1478 partially (32%, p < 0.01) blocked the effect of thrombin. Thrombin treatment of VSMCs increased the proportion of disaccharides sulfated at the 6 position of the GalNAc residues. Thus, thrombin has actions on VSMCs which increase the length and modify the sulfation pattern of GAG chains on proteoglycans in a manner that would enhance the binding of LDL. If manifest in vivo, this effect on proteoglycan synthesis and structure represents a new biochemical mechanism through which thrombin contributes to the development of atherosclerosis.