Regulation of smooth muscle α-actin expression and hypertrophy in cultured mesangial cells

Abstract

Background. Mesangial cells during embryonic development and glomerular disease express smooth muscle α-actin (α-SMA). We were therefore surprised when cultured mesangial cells deprived of serum markedly increased expression of α-SMA. Serum-deprived mesangial cells appeared larger than serum-fed mesangial cells. We hypothesized that α-SMA expression may be more reflective of mesangial cell hypertrophy than hyperplasia. Methods. Human mesangial cells were cultured in medium alone or with fetal bovine serum, thrombin, platelet-derived growth factor-BB (PDGF-BB) and/or transforming growth factor-β1 (TGF-β1). α-SMA expression was examined by immunofluorescence, Western blot, and Northern blot analysis. Cell size was analyzed by forward light scatter flow cytometry. Results. α-SMA mRNA was at least tenfold more abundant after three to five days in human mesangial cells plated without serum, but β-actin mRNA was unchanged. Serum-deprived cells contained 5.3-fold more α-SMA after three days and 56-fold more after five days by western blot. Serum deprivation also increased α-SMA in rat and mouse mesangial cells. The effects of serum deprivation on α-SMA expression were reversible. Mesangial cell mitogens, thrombin or PDGF-BB, decreased α- SMA, but TGF-β1 increased α-SMA expression and slowed mesangial cell proliferation in serum-plus medium. Flow cytometry showed that serum deprivation or TGF-β1 treatment caused mesangial cell hypertrophy. PDGF- BB, thrombin, or thrombin receptor-activating peptide blocked hypertrophy in response to serum deprivation. Conclusions. We conclude that increased α- SMA expression in mesangial cells reflects cellular hypertrophy rather than hyperplasia.