Inhibition of G sub(1) cyclin-dependent kinase activity during growth arrest of human breast carcinoma cells by prostaglandin A sub(2)

Prostaglandin A sub(2) (PGA sub(2)) potently inhibits cell proliferation and suppresses tumor growth in vivo, but little is known regarding the molecular mechanisms mediating these effects. Here we demonstrate that treatment of breast carcinoma MCF-7 cells with PGA sub(2) leads to G sub(1) arrest associated with a dramatic decrease in the levels of cyclin D sub(1) and cyclin-dependent kinase 4 (cdk4) and accompanied by an increase in the expression of p21. We further show that these effects occur independent of cellular p53 status. The decline in cyclin D and cdk4 protein levels is correlated with loss in cdk4 kinase activity. cdk2 activity is also significantly inhibited in PGA sub(2)-treated cells, an effect closely associated with the upregulation of p21. Immunoprecipitation experiments verified that p21 was indeed complexed with cdk2 in PGA sub(2)-treated cells. Additional experiments with synchronized MCF-7 cultures stimulated with serum revealed that treatment with PGA sub(2) prevents the progression of cells from G sub(1) to S. Accordingly, the kinase activity associated with cdk4, cyclin E, and cdk2 immunocomplexes, which normally increases following serum addition, was unchanged in PGA sub(2)-treated cells. Furthermore, the retinoblastoma protein (Rb), a substrate of cdk4 and cdk2 whose phosphorylation is necessary for cell cycle progression, remains underphosphorylated in PGA sub(2)-treated serum-stimulated cells. These findings indicate that PGA sub(2) exerts is growth-inhibitory effects through modulation of the expression and/or activity of several key G sub(1) regulatory proteins. Our results highlight the chemotherapeutic potential of PGA sub(2), particularly for suppressing growth of tumors lacking p53 function.