Tropoelastin Gene Expression in Individual Vascular Smooth Muscle Cells

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Abstract
After vascular injury, quiescent adult smooth muscle cells (SMCs) revert to a more immature synthetic-state phenotype concomitant with the onset of cell replication. The relationship between SMC proliferation and the reexpression of genes characteristic of immature SMCs (eg, tropoelastin [TE]), on an individual cell basis, has not been determined. Using a combined bromodeoxyuridine (BrdU) immunocytochemistry–TE in situ hybridization technique, we determined the relationship between DNA synthesis and TE gene expression in the rat vascular wall during development and after experimental injury. During the early development of the aortic media (embryonic days 13 to 18), low but detectable levels of TE expression occurred equally in both replicating and nonreplicating SMCs. TE message levels dramatically increased in the late fetal and early postnatal periods (fetal day 19 to 1 month postpartum), after a precipitous drop in SMC replication, and then decreased to undetectable levels by postpartum day 60. After balloon catheter injury in the adult, a developmental sequence of SMC replication followed by TE gene expression was reiterated in both the media and in the developing neointima. On an individual cell basis, adult SMCs replicating after injury expressed little or no TE message; detectable TE gene expression occurred only in nonreplicating SMCs. The most important implications of these data are that (1) adult SMCs replicating after injury appear to revert to a preelastogenic embryonic phenotype; (2) maximal TE expression occurs in SMCs only after the cessation of cell replication; and (3) in both the media and the neointima, adult SMCs responding to injury undergo temporally sequential changes in phenotype reflective of SMC development.