Activation of gene expression via CArG boxes during myogenic differentiation.

We have studied gene activation via CArG boxes in the context of myogenesis. The proximal CArG box of the human cardiac actin gene (HCA1) stimulates transcription from the herpes simplex virus thymidine kinase (TK) promoter in a tissue-specific fashion. Thus in transient transfection assays, when the expression of chloramphenicol acetyltransferase (CAT) from p(HCA1)4 TKCAT is compared to that derived from p(M1)4 TKCAT which contains an inactive mutated version (M1) of the HCA1 element, high levels of expression are seen in C2 mouse myoblasts and myotubes, and in the T4 myoblast cell line derived from the C3H10T1/2 cell line by 5-azacytidine treatment, whereas only low levels of expression are seen in the mouse L fibroblast cell line. The parental C3H10T1/2 cell line shows intermediate levels of expression. A similar situation is seen in stably transfected cell lines. Gene activation via CArG boxes was also analyzed in the course of myogenic conversion of C3H10T1/2 cells treated with 5-azacytidine. Our results indicate that activation of the CAT gene from the HCA1 element is slightly posterior to the appearance of the first MyoD1 and myogenin transcripts, concomitant with the appearance of cardiac alpha-actin transcripts, but clearly precedes the accumulation of myosin light-chain 1a transcripts and the appearance of troponin T-positive cells. These results further establish that CArG boxes can be seen as muscle-specific cis-acting regulatory element prior to terminal differentiation.