p15(Ink4b) is a critical tumour suppressor in the absence of p16(Ink4a)

The CDKN2b-CDKN2a locus on chromosome 9p21 in human (chromosome 4 in mouse) is frequently lost in cancer. The locus encodes three cell cycle inhibitory proteins: p15(INK4b) encoded by CDKN2b, p16(INK4a) encoded by CDKN2a and p14(ARF) (p19(Arf) in mice) encoded by an alternative reading frame of CDKN2a (ref. 1). Whereas the tumour suppressor functions for p16(INK4a) and p14(ARF) have been firmly established, the role of p15(INK4b) remains ambiguous. However, many 9p21 deletions also remove CDKN2b, so we hypothesized a synergistic effect of the combined deficiency for p15(INK4b), p14(ARF) and p16(INK4a). Here we report that mice deficient for all three open reading frames (Cdkn2ab(-/-)) are more tumour-prone and develop a wider spectrum of tumours than Cdkn2a mutant mice, with a preponderance of skin tumours and soft tissue sarcomas (for example, mesothelioma) frequently composed of mixed cell types and often showing biphasic differentiation. Cdkn2ab(-/-) mouse embryonic fibroblasts (MEFs) are substantially more sensitive to oncogenic transformation than Cdkn2a mutant MEFs. Under conditions of stress, p15(Ink4b) protein levels are significantly elevated in MEFs deficient for p16(Ink4a). Our data indicate that p15(Ink4b) can fulfil a critical backup function for p16(Ink4a) and provide an explanation for the frequent loss of the complete CDKN2b-CDKN2a locus in human tumours

Transforming activity of Fbxo7 is mediated specifically through regulation of cyclin D/cdk6

D cyclins (D1, D2 and D3) and their catalytic subunits (cyclin-dependent kinases cdk4 and cdk6) have a facilitating, but nonessential, role in cell cycle entry. Tissue-specific functions for D-type cyclins and cdks have been reported; however, the biochemical properties of these kinases are indistinguishable. We report that an F box protein, Fbxo7, interacted with cellular and viral D cyclins and distinguished among the cdks that bind D-type cyclins, specifically binding cdk6, in vitro and in vivo. Fbxo7 specifically regulated D cyclin/cdk6 complexes: Fbxo7 knockdown decreased cdk6 association with cyclin and its overexpression increased D cyclin/cdk6 activity and E2F activity. Fbxo7 interacted with p27, but its enhancement of cyclin D/cdk6 activity was p21/p27 independent. Fbxo7 overexpression transformed murine fibroblasts, rendering them tumorigenic in athymic nude mice. Transformed phenotypes were dependent on cdk6, as knockdown of cdk6 reversed them. Fbxo7 was highly expressed in epithelial tumors, but not in normal tissues, suggesting that it may have a proto-oncogenic role in human cancers