Initiation and transduction of stretch-induced RhoA and Rac1 activation through caveolae - Cytoskeletal regulation of ERK translocation

The Rho family small GTPases play a crucial role in mediating cellular responses to stretch. However, it remains unclear how force is transduced to Rho signaling pathways. We investigated the effect of stretch on the activation and caveolar localization of RhoA and Rac1 in neonatal rat cardiomyocytes. In unstretched cardiomyocytes, RhoA and Rac1 were detected in both caveolar and non-caveolar fractions as assessed using detergent-free floatation analysis. Stretching myocytes for 4 min activated RhoA and Rac1. By 15 min of stretch, RhoA and Rac1 had dissociated from caveolae, and there was decreased coprecipitation of RhoA and Rac1 with caveolin-3. To determine whether compartmentation of RhoA and Rac1 within caveolae was necessary for stretch signaling, we disrupted caveolae with methyl beta-cyclodextrin (MbetaCD). Treatment with 5 mM MbetaCD for 1 h dissociated both RhoA and Rac1 from caveolae. Under this condition, stretch failed to activate RhoA or Rac1. Stretch-induced actin cytoskeletal organization was concomitantly impaired. Interestingly the ability of stretch to activate extracellular signal-regulated kinase (ERK) was unaffected by MbetaCD treatment, but ERK translocation to the nucleus was impaired. Stretch-induced hypertrophy was also inhibited. Actin cytoskeletal disruption with cytochalasin-D also prevented stretch from increasing nuclear ERK, whereas actin polymerization with jasplakinolide restored nuclear translocation of activated ERK in the presence of MbetaCD. We suggest that activation of RhoA or Rac1, localized in a caveolar compartment, is essential for sensing externally applied force and transducing this signal to the actin cytoskeleton and ERK translocation.