Rho family small G proteins play critical roles in mechanical stress-induced hypertrophic responses in cardiac myocytes

Mechanical stress induces a variety of hypertrophic responses, such as acti vation of protein kinases, reprogramming of gene expression, and an increas e in protein synthesis. In the present study, to elucidate how mechanical s tress induces such events, we examined the role of Rho family small GTP-bin ding proteins (G proteins) in mechanical stress-induced cardiac hypertrophy . Treatment of neonatal rat cardiomyocytes with the C3 exoenzyme, which abr ogates Rho functions, suppressed stretch-induced activation of extracellula r signal-regulated protein kinases (ERKs). Overexpression of the Rho GDP di ssociation inhibitor (Rho-GDI), dominant-negative mutants of RhoA (DNRhoA), or DNRac1 significantly inhibited stretch-induced activation of transfecte d ERK2. Overexpression of constitutively active mutants of RhoA slightly ac tivated ERK2 in cardiac myocytes. Overexpression of C-terminal Src kinase, which inhibits functions of the Src family of tyrosine kinases, or overexpr ession of DNRas had no effect on stretch-induced activation of transfected ERK2. The promoter activity of skeletal a-actin and c-fos genes was increas ed by stretch, and these increases were completely inhibited by either cotr ansfection of Rho-GDI or pretreatment with C3 exoenzyme. Mechanical stretch increased phenylalanine incorporation into cardiac myocytes by approximate to 1.5-fold compared with control, and this increase was also significantl y suppressed by pretreatment with C3 exoenzyme. Overexpression of Rho-GDI o r DNRhoA did not affect angiotensin II-induced activation of ERK. ERKs were activated by culture media conditioned by stretch of cardiomyocytes withou t any treatment, but not of cardiomyocytes with pretreatment by C3 exoenzym e. These results suggest that the Rho family of small G proteins plays crit ical roles in mechanical stress-induced hypertrophic responses.