Substitution of the degenerate smooth muscle (SM) α-actin CC(A/T-rich)6GG elements with c-fos serum response elements results in increased basal expression but relaxed SM cell specificity and reduced angiotensin II inducibility

We have previously demonstrated that both CC(A/T-rich)(6)GG (CArG) elements A and B of the smooth muscle (SM) alpha-actin promoter are required for smooth muscle cell (SMC)-specific expression and angiotensin If (AII)induced stimulation. Moreover, results provided evidence that AII responsiveness of SM alpha-actin was at least partially dependent on modulation of serum response factor (SRF) binding to the SIM alpha-actin CArGs by the homeodomain containing protein, MHox. The goal of the present study was to investigate whether the degeneracy of the SM alpha-actin CArGs (both contain a Gua or Cyt substitution in their A/T-rich center) and their reduced SRF binding activity as compared with c-fos serum response element (SRE) is important for conferring cell type-specific expression and AII responsiveness, Transient transfection assays using SM alpha-actin reporter gene constructs in which the endogenous SM alpha-actin CArGs were replaced by c-fos SREs demonstrated the following: 1) relaxation of cell-specific expression, 2) a 50% reduction in AII responsiveness, and 3) reduced ability to be transactivated by MHox, In addition, we also showed that the position of the SM alpha-actin CArGs was important in that interchanging them abolished both basal and AII-induced activities. Taken together these results suggest that the reduced SRF binding activities of the SM alpha-actin CArGs and CArG; positional context contribute to SMC-specific expression of SM alpha-actin as well as maximal AII responsiveness.