Cre-loxP-mediated DNA flip-flop in mammalian cells leading to alternate expression of retrovirally transduced genes

Cultured mesangial cells constitutively express alpha-smooth muscle actin (alpha-SMA), a marker of cellular activation. We unexpectedly found that tyrosine kinase pp60v-src, a known activator for a wide range of signalling cascades, suppressed the alpha-SMA expression in mesangial cells. The present study was conducted to elucidate molecular events involved in this phenomenon. Transfection with a reporter plasmid revealed that the serum response element (SRE), the cis-element required for alpha-SMA expression, was constitutively active in mesangial cells. When mesangial cells were transfected with pp60v-src, activity of both SRE and the alpha-SMA promoter was down-regulated. This was associated with depressed levels of phosphorylated extracellular signal-regulated kinases (ERKs), but not c-Jun N-terminal kinase. Selective inhibition of ERKs by PD098059 abrogated constitutive SRE activity, leading to suppressed alpha-SMA expression. These results uncovered a novel potential of pp60v-src for suppression of alpha-SMA via intervention in the ERK-SRE signalling pathway.