Enhanced growth-dependent expression of TGFβ1 and hsp70 genes in aortic smooth muscle cells from spontaneously hypertensive rats

Abstract

Enhanced proliferation of vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) as compared with Wistar–Kyoto rats (WKY) persists in long-term culture and is characterized by an accelerated entry of these cells into the synthetic S phase of the cell cycle and a higher specific growth rate, particularly evident at high cell density. In the present study, we investigated by Northern blot experiments the expression of genes putatively involved in the regulation of VSMC growth. One of them is the transforming growth factor β1 gene (TGFβ1), a bifunctional modulator of cell growth whose action is dependent on cell density. The accumulation of TGFβ1 mRNA was enhanced in growing SHR cells at every density studied as early as 24 h after inoculation with a further increase at later times. Protooncogenes c-fos and c-myc, which have been implicated in G1/S phase transition, have also been investigated in VSMC by Northern blot analysis. At low cell density, calf serum stimulated c-fos and c-myc mRNA expression was comparable in WKY and SHR cells whereas at high cell density, c-fos induction was higher in VSMC from SHR. SHR VSMC respond more to mitogenic stimulation and to environmental (e.g., heat) stress, particularly when growing near saturation density. hsp70 constitutes a gene family responsive to environmental stimuli (heat) and to mitogenic stimulation. Our data demonstrated that heat-inducible hsp70 mRNA is expressed at higher levels in SHR as compared with WKY cells, whereas the differential expression of serum responsive hsp70 between the two strains is more evident in growing VSMC with only slight differences in quiescent cells. TGFβ1 and hsp70 proteins autoregulate their own expression. A defect in feedback regulation of TGFβ1 and hsp70 expression may be involved in the hyperproliferation and altered density-dependent inhibition of SHR VSMC.Key words: hypertension, SHR, vascular smooth muscle cells, cell cycle, transforming growth factor β1, oncogenes, hsp70.