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 canc
er 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 mu M HP
R, a concentration attainable in vivo, resulted in growth inhibition and ma
rked dephosphorylation of pRb involving Ser(612), Thr(821), Ser(795) and Se
r(780), target residues for cyclin-dependent kinase 2 (Cdk2) the former two
, and Cdk4 the latter two. Interestingly, this dephosphorylation of pRb occ
urred in S-G2-M phase cells, as revealed by experiments on cells fractionat
ed by FACS according to the cell cycle phase, hence suggesting that the ret
inoid interferes with the regulation of pRb phosphorylation, The in vitro p
hosphorylation of a GST-pRb recombinant substrate by Cdk2 immunocomplexes f
rom MCF7, T47D and SKBR3 was markedly suppressed after HPR treatment, where
as 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 unaffecte
d 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 th
rough Cyclin D1 downregulation and inhibition of pRb-targeting Cdks.