Mitogen-induced up-regulation of non-smooth muscle isoform of α- tropomyosin in rat aortic smooth muscle cells

Correlation between the expression of the α-tropomyosin isoforms and cell growth was investigated in rat aortic smooth muscle cells. The levels of exon 1a, exons 1a + 2a (smooth muscle type) and exons 1a + 2b (non-smooth muscle type) were determined by reverse transcription-polymerase chain reaction (RT-PCR). When the cells were cultured, the level of exons 1a + 2b transiently increased while reaching a maximum at 3-5 days. When the serum- deprived confluent cells were stimulated with 3-20% serum for 1.5 h, the level of exons 1a + 2b increased by about twofold. The 1-(5- isoquinolinesulphonyl)-2-methylpiperazine (H-7) but not 2-[1-(3- dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide (GF 109203X) inhibited this up-regulation. Phorbol-12,13-dibutyrate (PDB) mimicked the effect of serum. The DNA synthesis as determined by the incorporation of 5- bromo-2'-deoxy-uridine (BrdU) was not enhanced by the 1.5 h stimulation with serum or phorbol ester. The up-regulation of non-smooth muscle isoform of α- tropomyosin occurred during G₀/G₁ transition before entering S phase. Protein phosphorylation is suggested to be involved in the up-regulation. However, the responsible kinase(s) remain to be elucidated. (C) 2000 Elsevier Science B.V.