Regulation of smooth muscle α-actin promoter by vasopressin and platelet-derived growth factor in rat aortic vascular smooth muscle cells

Abstract

Vasoconstrictors such as arginine vasopressin (AVP) and angiotensin II (Ang II) have been shown to increase protein and mRNA levels of smooth muscle α-actin (SM-α-actin) in vascular smooth muscle cells. In the same cells, platelet-derived growth factor (PDGF) decreased SM-α-actin protein and mRNA. The rat SM-α-actin promoter that has recently been isolated contains two E-boxes and three CC(A/T)6GG (CArG) elements. To examine regulation of the SM-α-actin promoter, a 765-bp region of the rat SM-α-actin gene was ligated into chloramphenicol acetyltransferase (CAT)-containing vectors and transfected into rat aortic vascular smooth muscle cells. Stimulation of cells with either AVP or Ang II increased CAT activity 5- to 10-fold. PDGF was able to completely block the AVP-induced increase in CAT activity. To identify regions of the promoter responsible for both the AVP stimulation and PDGF inhibition of promoter activity, a series of truncation mutants were prepared and transfected into vascular smooth muscle cells. Truncation of both E-boxes and the most distal CArG element did not qualitatively alter either AVP-induced stimulation of CAT activity or PDGF inhibition. However, removal of the middle CArG element resulted in a loss of AVP stimulation. These studies indicate that the AVP-induced elevation and PDGF-induced inhibition of SM-α-actin levels in vascular smooth muscle cells are mediated at least in part through regulation of the SM-α-actin promoter. The critical region of the promoter mediating this effect involves at a minimum one of the CArG elements.