Increased Expression of NPM1 Suppresses p27(Kip1) Function in Cancer Cells

SIMPLE SUMMARY: Cancer malignancy frequently correlates with a low expression of p27(Kip1), a major cyclin-dependent kinase inhibitor, and the p27 protein level has been reportedly responsible for its antiproliferative function. However, we found the function of overexpressed p27 is suppressed in some cancer cells, suggesting that p27 function is also regulated independently of its protein level. The aim of this study was to clarify this unknown p27 regulatory mechanism and its impact on cancer proliferation. We isolated nucleophosmin isoform 1 (NPM1), which is highly expressed in variety of cancers, as a novel p27-interacting protein. Overexpressing NPM1 in normal cells suppressed and silencing NPM1 in cancer cells rescued the p27 function, respectively, in vitro. Moreover, NPM1 silencing and p27 induction in cancer cells significantly suppressed their proliferation in mouse xenografts. Our findings reveal that NPM1 is a novel p27 functional suppressor and a potential anti-cancer target, especially in cancers with normal p27 expression. ABSTRACT: p27(Kip1), a major cyclin-dependent kinase inhibitor, is frequently expressed at low levels in cancers, which correlates with their malignancy. However, in this study, we found a qualitative suppression of p27 overexpressed in some cancer cells. By proteomic screening for factors interacting with p27, we identified nucleophosmin isoform 1 (NPM1) as a novel p27-interacting factor and observed that NPM1 protein was expressed at high levels in some cancer cells. NPM1 overexpression in normal cells suppressed p27 function, and conversely, NPM1 knockdown in cancer cells restored the function in vitro. Furthermore, the tumors derived from cancer cells carrying the combination of p27 overexpression and NPM1 knockdown constructs showed significant suppression of growth as compared with those carrying other combinations in mouse xenograft models. These results strongly suggest that increased expression of NPM1 qualitatively suppresses p27 function in cancer cells.