Involvement of p27kip1 in ceramide-mediated apoptosis in HL-60 cells

Ceramide acts as a mediator of apoptosis in various cell lines, but little is known regarding the molecular mechanism linked to the cell cycle. In the present study, we examined the expression of p27kip1 and its relationship to apoptosis induced by ceramide. We demonstrated that treatment of HL-60 cells with C6-ceramide resulted in G1 phase elevation followed by apoptotic cleavage associated with increase in the level of cdk inhibitor p27kip1. Ceramide inhibited the kinase activities of cdk2 and cdk4 within 24 h of treatment. Ceramide-induced inhibition of cdk2 and cdk4 kinase activities was accompanied by increase of p27kip1 in the cdks complexes. In addition, we have shown that both the cell death and expression of p27kip1 protein induced by ceramide were significantly decreased in HL-60 cells overexpressing bcl-2. Furthermore, ceramide induced a significant increase in Bax protein expression coincided with increase in p27kip1 protein level. These findings indicate that p27kip1 may play important roles in mediating ceramide-induced apoptosis and its expression can be regulated by Bax and Bcl-2.