A Retinoic Acid–Induced Clonal Cell Line Derived From Multipotential P19 Embryonal Carcinoma Cells Expresses Smooth Muscle Characteristics

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Abstract
Despite intense interest in understanding the differentiation of vascular smooth muscle, very little is known about the cellular and molecular mechanisms that control differentiation of this cell type. Progress in this field has been hampered by the lack of an inducible in vitro system for study of the early steps of smooth muscle differentiation. In this study, we describe a model system in which multipotential mouse P19 embryonal carcinoma cells (P19s) can be induced to express multiple characteristics of differentiated smooth muscle. Treatment of P19s with retinoic acid was associated with profound changes in cell morphology and with the appearance at high frequency of smooth muscle α-actin–positive cells that were absent or present at extremely low frequency in parental P19s. A clonal line derived from retinoic acid–treated P19s (9E11G) stably expressed multiple characteristics of differentiated smooth muscle, including smooth muscle–specific isoforms of α-actin and myosin heavy chain, as well as functional responses to the contractile agonists phenylephrine, angiotensin II, ATP, bradykinin, histamine, platelet-derived growth factor (PDGF)-AA, and PDGF-BB. Additionally, 9E11G cells expressed transcripts for MHox, a muscle homeobox gene expressed in smooth, cardiac, and skeletal muscles, but not the skeletal muscle–specific regulatory factors, MyoD and myogenin. Results demonstrate that retinoic acid treatment of multipotential P19 cells is associated with formation of cell lines that stably express multiple properties of differentiated smooth muscle. It remains to be determined whether retinoic acid has induced commitment to a smooth muscle cell lineage as opposed to directly (or indirectly) activating genes characteristic of differentiated smooth muscle cells. However, results suggest that this cell system may be of use in attempting to identify genes involved in controlling smooth muscle differentiation and/or lineage determination.