Polyubiquitin is a new phenotypic marker of contractile vascular smooth muscle cells

Medial vascular smooth muscle cells (VSMCs) in healthy vessels are phenotypically distinct from their intimal counterparts in vascular disease. To compare the genes expressed in these phenotypes we have previously performed a differential cDNA library screen on cultured rat VSMCs. The aim of this study was to identify and characterise a 2.8 kb transcript, 2E10, which was highly expressed in freshly dispersed rat aortic VSMCs and downregulated in multiply passaged cultured VSMCs.

Sequence analysis was used to identify the 2.8 kb rat cDNA. After trypsinisation of proliferating cultured rat and human VSMCs, or enzymatic digestion of aortic tunica media, total cytoplasmic RNA was isolated from VSMCs by lysis in Nonidet P-40 and extraction in phenol; 15 micrograms of total cytoplasmic RNA was used in Northern blot analysis with a 32P-[dCTP]-labelled 2E10 cDNA probe. 35S-[dATP]-labelled 2E10 riboprobe was hybridised in situ to frozen sections of normal and diseased human coronary arteries.

DNA sequencing identified 2E10 as a rat polyubiquitin which is homologous to the human polyubiquitin, UbC. Northern blot analysis showed that this polyubiquitin was more highly expressed in differentiated, freshly dispersed rat and human aortic VSMCs compared with their dedifferentiated proliferating counterparts. This also identified a 3.2 kb transcript cross-reacting with the polyubiquitin probe which is specific to differentiated rat VSMCs only. However, expression in growth arrested and proliferating VSMCs was identical, suggesting that UbC does not have a role in VSMC growth arrest. In situ hybridisation of the polyubiquitin riboprobe to sections of diseased human coronary arteries indicated much higher expression in medial than in intimal VSMCs. Northern blot analysis of RNA from the developing rat aorta showed that polyubiquitin expression increased substantially after week 2 of neonatal life, coincident with expression of VSMC-specific contractile proteins.

The greater expression of a UbC polyubiquitin transcript in contractile, differentiated VSMCs compared with proliferating, synthetic VSMCs provides a new gene marker for the phenotypic characterisation of VSMCs in vivo. This, and the finding that the developmental induction of expression of polyubiquitin (UbC) mirrors that of VSMC contractile proteins, suggests that ubiquitin, a protein known to associate with and degrade contractile proteins in skeletal muscle, is involved in the function or maintenance of the contractile phenotype of VSMCs.