beta 1 integrin and organized actin filaments facilitate cardiomyocyte-specific RhoA-dependent activation of the skeletal alpha-actin promoter

Activation of RhoA GTPase causes actin filament bundling into stress fibers , integrin clustering, and focal adhesion formation through its action on a ctin cytoskeleton organization, RhoA also regulates transcriptional activit y of serum response factor (SRF), Recent studies in NM 3T3 fibroblasts have shown that SRF activation by RhoA does not require an organized cytoskelet on and may be regulated by G-actin level, In cardiac myocytes, the organiza tion of actin fibers into myofibrils is one of the primary characteristics of cardiac differentiation and hypertrophy, The primary purpose of this stu dy was to examine if RhoA regulates SRF-dependent gene expression in neonat al cardiomyocytes in a manner different from that observed in fibroblasts, Our results show that RhoA-dependent skeletal alpha -actin promoter activat ion requires beta1 integrin and a functional cytoskeleton in cardiomyocytes but not in NM 3T3 fibroblasts, Activation of the alpha -actin promoter by RhoA is greatly potentiated (up to 15-fold) by coexpression of the integrin beta 1A or beta 1D isoform but is significantly reduced by 70% with a coex pressed dominant negative mutant of beta1 integrin, Furthermore, clustering of beta1 integrin with anti-beta1 integrin antibodies potentiates synergis tic RhoA and beta1 integrin activation of the alpha -actin promoter. Cytoch alasin D and latrunculin B, inhibitors of actin polymerization, significant ly reduced RhoA-induced activation of the alpha -actin promoter, Jasplakino lide, an actin polymerizing agent, mimics the synergistic effect of RhoA an d beta1 integrin on the actin promoter, These observations support the conc ept that RhoA regulates SRF-dependent cardiac gene expression through cross -talk with beta1 integrin signal pathway via an organized actin cytoskeleto n.