CONTRACTING SKELETAL MUSCLE CELLS EXPRESS AND RELEASE VEGF AND INDUCE VEGF MRNA IN ENDOTHELIAL CELLS

Abstract

An important adaptation to endurance training is new capillary formation in skeletal muscle. Several growth factors may be involved in this angiogenic process, including vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF). We sought to determine if contracting primary skeletal muscle cells express VEGF and/or bFGF mRNA and if muscle cells release VEGF. We also examined whether skeletal muscle cells produce compounds that are capable of enhancing synthesis of VEGF and bFGF in endothelial cells. Northern blot analysis of contracting primary skeletal muscle cells revealed existence of both VEGF and bFGF mRNA. Medium from contracting skeletal muscle cells was analysed for VEGF protein concentration and results indicated an increased secretion of VEGF protein into the medium from 0 hours to 37 hours of contraction. Human umbilical vein endothelial cells (HUVECs) were grown for 24 or 48 hours with (conditioned) or without medium transferred from contracting skeletal muscle cells. Northern blot analysis of endothelial cells grown for 48 hours with conditioned medium revealed a three-fold increase in VEGF mRNA expression compared to endothelial cells grown with non-conditioned medium (P < 0.05). No increase in VEGF mRNA could be detected after 24 hours of incubation (P > 0.05). Levels of bFGF mRNA in endothelial cells remained unaltered with time and with addition of conditioned medium (P > 0,05). The present data show that primary skeletal muscle cells express VEGF and bFGF mRNA and release VEGF protein. Furthermore it is demonstrated that contracting skeletal muscle cells produce not yet identified substance(s) that increase the expression of VEGF mRNA in endothelial cells. VEGF mRNA induction in the endothelial cells was time-dependent with maximal levels observed at 48 hours. These findings suggest that skeletal muscle cells participate in the regulation of angiogenesis both by releasing VEGF and by stimulating VEGF expression in endothelial cells.