Forced expression of hic-5, a senescence-related gene, potentiates a differentiation process of RCT-1 cells induced by retinoic acid.

The hic-5 gene encodes the novel LIM protein which has been implicated in cellular senescence and differentiation processes. Previous studies of rat calvarial cells stimulated to differentiate by addition of retinoic acid (R.A.) showed a four-fold increase in hic-5 expression which preceded an increase in the expression of the differentiation markers, alkaline phosphatase and alpha (I) pro-collagen mRNA. These data suggest involvement of hic-5 in osteoblast differentiation. This hypothesis was further examined using rat calvarial RCT-1 cells containing expression vectors of hic-5. The over-expressing clones showed a decrease in proliferation and displayed the differentiation-related phenotypes such as increased basal levels of alpha (I) collagen mRNA expression and high inducibility by R.A. of alkaline phosphatase activity. Conversely, introduction of hic-5 anti-sense vector leads to the inhibition of alpha (I) collagen mRNA following induction by R.A. These results suggest that hic-5 is one of the regulatory molecules involved in the R.A. induced differentiation process of RCT-1 cells.