Tumorigenesis and Neoplastic Progression Regulation of Skp 2p 27 Axis by the Cdh 1 / Anaphase-Promoting Complex Pathway in Colorectal Tumorigenesis

Abrogated entry into S phase is a common hallmark of cancer cells. Skp2, a subunit of ubiquitin ligase, is critical for regulating the G1/S transition. Uncontrolled Skp2 activity is detected frequently in human tumors, often correlated with poor prognosis. Current studies have suggested that the regulation of Skp2 turnover is mediated by another critical ubiquitin ligase, the anaphase-promoting complex (APC), in association with its substrate-specific factor Cdh1. To dissect the potential role of Cdh1/APC in tumorigenesis through the degradation of Skp2, we analyzed the Cdh1/APC-Skp2-p27 axis in colorectal tumorigenesis using a human tumor array and biochemical analyses. Our results show that the percentage of Cdh1and p27-positive samples in colon cancer tissues was significantly lower than that in adjacent nonmalignant tissue. Conversely, the percentage of Skp2-positive colon cancer samples was significantly higher than that in normal tissue. Furthermore, results from clinicopathological analysis revealed that elevated Cdh1 expression was associated with lower histological grade tumors. In addition, depletion of Cdh1 by RNA interference in nonmalignant colon cells resulted in increased cellular proliferation, whereas knockdown of Skp2 significantly suppressed cancer cell growth. Our result suggests a pathological correlation between Skp2 and Cdh1/APC in colorectal cancer. Thus, Cdh1 may function as a component in tumor suppression via proteolysis of Skp2 in colorectal tumorigenesis and may serve as a prognostic marker in colon cancer patients. (Am J Pathol 2008, 173:217–228; DOI: 10.2353/ajpath.2008.070957) S phase kinase-associated protein 2 (Skp2) acts as the substrate-specific factor for the Skp1-Cul1-F-box (SCF) complex that catalyzes ubiquitylation of multiple functional protein for degradation, including p27, p21, Rb, and p53. Dysregulated regulation of Skp2 is often detected in human tumor tissue, which is correlated with poor prognosis. The well understood role of Skp2 is to govern the transition from G1 to S phase through degradation of p27 thereby regulating cyclin E/CDK2. Both Skp2 transcripts and protein oscillate during the cell cycle. Current studies have shown that anaphase-promoting complex (APC) is the critical E3 ligase that targets Skp2 for ubiquitylation and degradation, therefore averting abnormal entry into S phase. However, the correlation between Cdh1 and tumorigenesis through regulation of Skp2 has not yet been validated although Skp2 has served as a clinical marker for several types of cancer. The APC is a multifunctional E3 ligase, regulating several critical cellular events, including mitotic progression, DNA replication, cellular differentiation, genomic integrity, and signal transduction. Recent evidence has drawn our attention to connect APC function with human diseases. Pathological and epigenetic studies have demonstrated that dysfunction in several components of the APC pathway, including APC6, Cdc16, Cdc23, and Cdh1 or Cdc20, is correlated with different types of cancer such as colon cancer, B-lymphoma, gastric, and lung cancer. Activation of APC is controlled by two WD40 family proteins, Cdh1 and Cdc20. Cdh1 in association with APC results in determining APC function in G1 and postmitotic processes, whereas interaction of Cdc20 with APC leads to controlling chromatid separation during