Plasticity of vascular smooth muscle α-actin gene transcription: Characterization of multiple, single-, and double-strand specific DNA-binding proteins in myoblasts and fibroblasts

Transcriptional activity of the mouse vascular smooth muscle (VSM) α-actin promoter was governed by both cell type and developmental stage-specific mechanisms. A purine-rich motif (PrM) located as -181 to -176 in the promoter was absolutely required for activation in mouse AKR-2B embryonic fibroblasts and partially contributed to activation in undifferentiated mouse BC3H1 myoblasts. Transcriptional enhancer factor 1 recognized the PrM and cooperated with other promoter-binding proteins to regulate serum growth factor-dependent transcription in both myoblasts and fibroblasts. Two distinct protein factors (VAC-ssBF1 and VAC-ssBF2) also were identified that bound sequence-specifically to single-stranded oligonucleotide probes that spanned both the PrM and a closely positioned negative regulatory element. VAC-ssBF1 and BF2 binding activity was detected in undifferentiated myoblasts, embryonic fibroblasts, and several smooth muscle tissues in the mouse and human. A myoblast-specific protein (VAC-RF1) also was detected that bound double-stranded probes containing a CArG-like sequence that previously was shown to impart strong, cell type specific repression. The binding activity of transcription enhancer factor 1, VAC-RF1, and VAC-ssBF1 was significantly diminished when confluent BC3H1 myoblasts differentiated into myocytes and expressed VSM α-actin mRNA after exposure to serum-free medium. The results indicated that cell type-specific control of the VSM α-actin gene promoter required the participation of multiple DNA-binding proteins, including two that were enriched in smooth muscle and had preferential affinity for single-stranded DNA.