Hyperplasia in multiple smooth muscle tissues in transgenic mice expressing a temperature-sensitive SV40 T-antigen under the control of smooth muscle alpha-actin regulatory sequences

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Abstract
Control of smooth muscle cell (SMC) proliferation is of fundamental importance in the development and pathology of the vasculature. To derive vascular SMC with conditional inactivation of negative cell cycle regulatory proteins in the context of smooth muscle protein expression, a 3.4 kb fragment of the mouse SMC alpha-actin promoter was used to target a temperature-sensitive mutant SV40 T antigen (tsA58) to smooth muscle in transgenic mice. Mice with this genotype display a heritable phenotype of abnormal SMC proliferation in the central tail artery, vasa deferentia, seminal vesicles, prostate, and uterus, with the latter resembling uterine leiomyomatosis and prostatic hypertrophy. Neither the aorta nor other viscera manifested abnormal proliferation. Cultures from aorta, vas deferens, seminal vesicle, and kidney tissue were characterized with regard to protein expression, stability, and matrix remodelling capacity. The alpha-actin content/cell was up to 3-4-fold higher, as well as more stable than in primary SMC cultures, suggesting successful selection for propagation of cells expressing this differentiation marker. All cells displayed enhanced growth at the permissive temperature. As an initial functional assessment, the cells were compared to non-transformed mouse aortic SMC with respect to the ability to remodel collagen gel matrices, and demonstrated conservation of this physiologic function. This in vivo analysis of the SMC alpha-actin promoter supports a broader range of smooth muscle-directed expression activity than previously recognized, and establishes the feasibility of its use to direct transgene expression to vascular as well as genito-urinary smooth muscle. The targeted expression of the tsA58 T antigen has yielded transgenic animals with several manifestations of smooth muscle hyperplasia; these animals have in turn permitted the derivation of several murine SMC lines with phenotypic stability and conditionally-modulated proliferation. These cells will allow expansion of derivative transfected smooth muscle cell lines under permissive conditions, as well as oncogene inactivation at the restrictive temperature when desired for functional studies.