INHIBITION OF G(1) CYCLIN-DEPENDENT KINASE-ACTIVITY DURING GROWTH ARREST OF HUMAN BREAST-CARCINOMA CELLS BY PROSTAGLANDIN A(2)

Prostaglandin A(2) (PGA(2)) potently inhibits cell proliferation and S uppresses tumor growth in vivo, but little is known regarding the mole cular mechanisms mediating these effects. Here we demonstrate that tre atment of breast carcinoma MCF-7 cells with PGA(2) leads to G(1) arres t associated with a dramatic decrease in the levels of cyclin D1 and c yclin dependent kinase 4 (cdk4) and accompanied by an increase in the expression of p21. We further show that these effects occur independen t of cellular p53 status. The decline in cyclin D and cdk4 protein lev els is correlated with loss in cdk4 kinase activity. cdk2 activity is also significantly inhibited in PGA(2)-treated cells, an effect closel y associated with the upregulation of p21. Immunoprecipitation experim ents verified that p21 was indeed complexed with cdk2 in PGA(2)-treate d cells. Additional experiments with synchronized MCF-7 cultures stimu lated with serum revealed that treatment with PGA(2) prevents the prog ression of cells from G(1) to S. Accordingly, the kinase activity asso ciated with cdk4, cyclin E, and cdk2 immunocomplexes, which normally i ncreases following serum addition, was unchanged in PGA(2)-treated cel ls. Furthermore, the retinoblastoma protein (Rb), a substrate of cdk4 and cdk2 whose phosphorylation is necessary for cell cycle progression , remains underphosphorylated in PGA(2)-treated serum-stimulated cells . These findings indicate that PGA(2) exerts its growth-inhibitory eff ects through modulation of the expression and/or activity of several k ey G(1) regulatory proteins. Our results highlight the chemotherapeuti c potential of PGA(2), particularly for suppressing growth of tumors l acking p53 function.