Inhibition of F-box protein p45SKP2 expression and stabilization of cyclin-dependent kinase inhibitor p27KIP1 in vitamin D analog-treated cancer cells

Treatment of cancer cells with 1,25-dihydroxyvitamin D3 [1,25(OH)(2)D-3] or its analogs induces growth arrest and expression of the cyclin-dependent kinase inhibitor p27(KIP1). Although 1,25(OH)(2)D-3 transiently enhances p27(kip1) gene transcription in some cells, its effects on p27(KIP1) protein levels are generally more gradual and sustained. This suggests that 1,25(OH)(2)D-3 treatment may be stabilizing p27(KIP1) protein, which is sensitive to modification by the SCFSKP2 protein ubiquitin ligase and proteosomal degradation. Here, we show that treatment of AT-84 head and neck squamous carcinoma cells with the 1,25(OH)(2)D-3 analog EB1089 increases p27(KIP1) protein levels without significantly affecting expression of its mRNA. EB1089 treatment repressed expression of mRNAs encoding the F-box protein p45(SKP2), a marker of poor head and neck cancer prognosis, and the cyclin kinase subunit CKS1, which is essential for targeting p45(SKP2) to p27(KIP1). This coincided with a reduction of total p45(SKP2) protein, and p45(SKP2) associated with p27(KIP1). Consistent with these findings, turnover of p27(KIP1) protein was strongly inhibited in the presence of EB1089. A similar reduction in p45(SKP2) expression and stabilization of p27(KIP1) protein was observed in 1,25(OH)(2)D-3-sensitive UF-1 promyelocytic leukemia cells, which also respond by transiently increasing p27(kip1) gene transcription. Our results reveal that 1,25(OH)(2)D-3 analogs increase levels of p27(KIP1) in different cell types by inhibiting expression of SCFSKP2 subunits and reducing turnover of p27(KIP1) protein.