Smooth muscle-specific transcriptional regulation by SRF and homeodomain proteins

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While serum response factor (SRF)/CArG box interaction has been well documented for a variety of transcription systems, including immediate early and muscle genes, it cannot solely be responsible for transcription in respective tissues. Here, we identified two cis-elements in the alphal-integrin promoter region in addition to CArG box : a TAAT sequence, a consensus-binding site for homeoproteins, and a GATA family-binding box. We further cloned a homeobox cDNA, Nkx-3.2, which is mainly expressed in smooth muscle tissues and skeletal structures. Gel-shift assays showed a ternary complex formation of SRF and Nkx-3.2 or GATA-6 with their corresponding cis-elements. Further, Nkx-3.2, SRF, and GATA-6 or their respective functional domains transactivate synergistically the alphal-integrin gene in vascular SMC-derived cell line and heterologous cells. We conclude that transcription of alphal-integrin in vascular SMCs is regulated by coordinated interactions between Nkx-3.2, SRF, GATA-6 and their corresponding cis-elements. In addition, we characterized the transcriptional machinery of the beta-TM gene in SMCs. Promoter and gel mobility shift analyses revealed an obligatory role for serum response factor (SRF) and its interaction with the CArG box sequence in the SMC-specific transcription of the beta-TM gene in differentiated SMCs. We further isolated a novel homologue of the Barx homeoprotein family, Barx1b, from chicken gizzard. Barx1b was exclusively localized to SMCs of the upper digestive organs and their attached arteries and to craniofacial structures. SRF and Barx1b bound each other directly, coordinately transactivated the beta-TM gene in differentiated SMCs and heterologous cells, and formed a temary complex with a CArG probe. Taken together, these results suggest that SRF, homeodomain proteins, and/or GATA family transcription factors are coordinately involved in the tissue-specific transcription of the smooth muscle genes.