In this study, we investigated the effect of the novel
retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-
naphthalene carboxylic acid (
AHPN/CD437) on the growth of human lung
carcinoma cell lines.
AHPN inhibits the proliferation of all cell lines tested, irrespective of the lung
tumor type, in a concentration- and time-dependent manner. A dramatic reduction in cell number was observed in
adenocarcinoma H460 cells, and was shown to be related to an induction of apoptosis.
Bromodeoxyuridine (
BrdU) incorporation and flow-cytometric analyses indicated that treatment of H460 cells with
AHPN induces cell-cycle arrest at the G1 phase. We therefore investigated the effect of
AHPN on several regulatory
proteins of the G1 phase of the cell-cycle. The cell-cycle arrest induced by
AHPN was accompanied by an inhibition of the hyperphosphorylation of the
retinoblastoma (
Rb) protein, an indication of G1 arrest. Furthermore, two
cyclin-dependent kinases, cdk2 and cdk4, which are normally involved in the phosphorylation of Rb, were shown to have decreased activity. In some cell lines, the decrease in cdk activity may be partly related to an increase in p21(WAF1/Cip1) (p21), an inhibitor of
cyclin-dependent kinases. No changes were observed in the
cyclin-dependent kinase inhibitor p27(Kip1). The observed increase in p53 in response to
AHPN could at least to some extent be responsible for the increased levels of p21. The increase in p53 expression was found to be regulated at a post-transcriptional level. Our results suggest that the growth inhibition of certain lung
carcinoma cell lines by
AHPN is at least partly related to an increase in p21. However, in other cell lines, different mechanisms appear to be involved. The specificity with which
AHPN and other
retinoids induce growth arrest and p21 expression indicates that the action of
AHPN is not mediated by RAR or RXR receptors, but involves a novel signaling pathway.