The
cancer chemopreventive synthetic
retinoid N-(4-hydroxyphenyl)retinamide (HPR) can inhibit the growth and induce apoptosis of
tumor cells. In this study we analysed the growth suppressive effect of HPR on human
breast cancer cell lines in vitro and the role of the
retinoblastoma protein (pRb) in this response. Treatment of MCF7, T47D and SKBR3 for 24 - 48 h with 3 microM HPR, a concentration attainable in vivo, resulted in growth inhibition and marked dephosphorylation of pRb involving Ser612, Thr821, Ser795 and Ser780, target residues for
cyclin-dependent kinase 2 (Cdk2) the former two, and Cdk4 the latter two. Interestingly, this dephosphorylation of pRb occurred in S-G2-M phase cells, as revealed by experiments on cells fractionated by FACS according to the cell cycle phase, hence suggesting that the
retinoid interferes with the regulation of pRb phosphorylation. The in vitro phosphorylation of a GST-pRb recombinant substrate by Cdk2 immunocomplexes from MCF7, T47D and SKBR3 was markedly suppressed after HPR treatment, whereas that by Cdk4 complexes was suppressed in T47D and SKBR3 but not in MCF7. The steady-state levels of Cdk2, Cdk4 and
Cyclin A proteins were unaffected by HPR, while those of
Cyclin D1 were significantly reduced in all three cell lines. Interestingly,
Cyclin D1 downregulation by HPR correlated with transcriptional repression, but not with enhanced proteolysis of
Cyclin D1 typically elicited by other
retinoids. Collectively, our data suggest that the antiproliferative activity of HPR arises from its capacity to maintain pRb in a de-phosphorylated growth-suppressive status in S-G2/M, possibly through
Cyclin D1 downregulation and inhibition of pRb-targeting Cdks. Oncogene (2000) 19, 4035 - 41.