Atherosclerosis is a major cause of death in industrialized nations. Part of the pathophysiology of atherosclerosis involves the dedifferentiation and proliferation of vascular smooth muscle (VSM) in response to PDGF and other growth factors. Nothing is known about the involvement of homeobox genes in the control of VSM, but these transcription factors have been identified as key regulators of cellular differentiation and proliferation in a wide variety of tissues and organs. Thus, we reasoned that alterations in homeobox gene expression may have a role in vascular disease and normal blood vessel development, and we looked for their presence in VSM. We have isolated a novel, divergent homeobox cDNA, dubbed V-Hox, that maps to a new position on mouse chromosome 12. In adults V-Hox is predominantly expressed in VSM, is present at low levels in cardiomyocytes and lung tissue, and it is not detected in visceral smooth muscle, skeletal muscle, brain nor many other tissues. A unique feature of V-Hox is that its expression is rapidly and dramatically downregulated by the addition of PDGF and serum to quiescent VSM cells.
The aims of this research are to analyze the involvement of V-Hox in the control of differentiation and proliferation in cultured VSM cells and in transgenic mice. The proposed experiments will determine the DNA-binding and gene regulatory properties of V-Hox and identify potential downstream targets for this transcription factor. Work will also focus on elucidating the molecular basis of the rapid PDGF-induced downregulation of V-Hox expression because this will provide clues about the early events of mitogen action on VSM cells. Other experiments will overexpress and underexpress V-Hox to assess the role of this factor in the control of VSM cell growth and differentiation. Genomic clones for V-Hox will be isolated. The characterization of clones will elucidate V-Hox gene structure, and the study of promoter and enhancer sequences may provide clues about the hierarchy of regulatory events that control the development of VSM from precursor cells. The genomic clone will be used to disrupt the V-Hox gene in embryonic stem cells by homologous recombination. Chimeric mice will be derived by injecting blastocysts with these modified embryonic stem cells, and the subsequent breeding of these founder mice will produce V-Hox homozygous mice. Analyses of these mice will provide clues about V-Hox function in the developing cardiovascular system, and VSM cultures derived from the null mice would provide an ideal model system to study the transcriptionaI, developmental, and growth regulatory properties of the V-Hox protein.
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