BACKGROUND: Previous studies have shown that low levels of p27(Kip1), an inhibitor of G1 cyclin-dependent kinases, are associated with high aggressiveness and poor prognosis in a variety of cancers. Decreased levels of p27 are caused, at least in part, by acceleration of the rate of its ubiquitin-mediated degradation. In cultured cells and cell-free biochemical systems, it has been shown that p27 is targeted for degradation by a ubiquitin ligase complex that contains Skp2 (S-phase kinase-associated protein 2) as the specific substrate-recognizing and rate-limiting subunit. This investigation was undertaken to examine the possible relation between levels of p27 and of its specific ubiquitin ligase subunit Skp2 in human cancers. METHODS: Quick-frozen colorectal tumor samples from 20 patients were homogenized at 0 degrees C in buffer containing a mixture of protease inhibitors. Samples were separated by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels, transferred to nitrocellulose, and probed with highly specific monoclonal antibodies directed against Skp2 and p27. The expression of Skp2 also was examined by immunohistochemistry using formalin fixed, paraffin embedded tissue sections from the same cases. RESULTS: A strongly significant inverse correlation was found between levels of Skp2 and p27 (r = -0.812; P < 0.0001). Thus, decreased levels of p27 were associated with strongly increased levels of Skp2, whereas high levels of p27 coincided with low levels of Skp2. Immunohistochemical examination of Skp2 expression agreed with immunoblot analysis in 89% of cases. CONCLUSIONS: The results are compatible with the notion that increased expression of Skp2 may have a causative role in decreasing the levels of p27 in aggressive colorectal carcinomas.