The ubiquitin-proteasome system is inhibited by p53 protein expression in human ovarian cancer cells

Lewis y antigen promotes p27 degradation by regulating ubiquitin-proteasome activity

As a tumor-associated carbohydrate antigen, elevated expression of Lewis y promotes the malignant behaviors of tumor cells. Although our preliminary study showed that the increased expression of Lewis y antigen decreased the expression of cell cycle inhibitor protein p27, the relevant mechanism remains unclear. Autophagy and the ubiquitin-proteasome system are two main ways of intracellular protein degradation, whose abnormal activities are closely associated with progression of malignant tumors. In our present study, we constructed two stable transfected cell lines with high expression of Lewis y antigen, named CAOV3-FUT1 and SKOV3-FUT1. We showed that the proportion of cells at S phase was significantly increased after FUT1 transfection, whereas p27 protein was obviously decreased. The autophagy activity, the levels of ubiquitination, and chymotrypsin-like protease activity were increased remarkably in the transfected cells. Interestingly, Lewis y antigen promoted the degradation of p27 by increasing ubiquitin-proteasome activity. In the vivo studies, Lewis y antigen improved the tumorigenic ability of ovarian cancer cells in nude mice and reduced the expression of p27. These findings suggested that Lewis y antigen activated both the autophagy and ubiquitin-proteasome activity and promoted the degradation of p27 through the ubiquitin-proteasome pathway.

Ubiquitin pathway and ovarian cancer

The ubiquitin-proteasome pathway is a common cellular process in eukaryotic tissue. Ubiquitin binds to proteins and tags them for destruction; this tagging directs proteins to the proteosome in the cell that degrades and recycles unneeded proteins. The ubiquitin-proteasome pathway plays an important role in the regulation of cellular proteins with respect to cell cycle control, transcription, apoptosis, cell adhesion, angiogenesis, and tumour growth. This review article discusses the various ways that the ubiquitin pathway is involved in ovarian cancer, such as modulating the ovarian-cancer-related gene BRCA1 and tumour suppressor p53, and interfering with the erk pathway, the cyclin-dependent cell cycle regulation process, and ERBB2 gene expression.

Inhibitory effect of tumor suppressor p53 on proinflammatory chemokine expression in ovarian cancer cells by reducing proteasomal degradation of IκB

Ovarian cancer, one of inflammation-associated cancers, is the fifth leading cause of cancer deaths among women. Inflammation in the tumor microenvironment is associated with peritoneal tumor dissemination and massive ascites, which contribute to high mortality in ovarian cancer. Tumor suppressor p53 is frequently deleted or mutated in aggressive and high-grade ovarian cancer, probably aggravating cancer progression and increasing mortality. We therefore investigated the influence of p53 on proinflammatory chemokines in ovarian cancer cells. A PCR array of the chemokine network revealed that ovarian cancer cells with low or mutated p53 expression expressed high levels of proinflammatory chemokines such as CXCL1, 2, 3 and 8. Transient transfection of p53 into p53-null ovarian cancer cells downregulated proinflammatory chemokines induced by tumor necrosis factor-α (TNF), a proinflammatory cytokine abundantly expressed in ovarian cancer. Furthermore, p53 restoration or stabilization blocked TNF-induced NF-κB promoter activity and reduced TNF-activated IκB. Restoration of p53 increased ubiquitination of IκB, resulting from concurrently reduced proteasome activity followed by stability of IκB. A ubiquitination PCR array on restoration of p53 did not reveal any significant change in expression except for Mdm2, indicating that the balance between p53 and Mdm2 is more important in regulating NF-κB signaling rather than the direct effect of p53 on ubiquitin-related genes or IκB kinases. In addition, nutlin-3, a specific inducer of p53 stabilization, inhibited proinflammatory chemokines by reducing TNF-activated IκB through p53 stabilization. Taken together, these results suggest that p53 inhibits proinflammatory chemokines in ovarian cancer cells by reducing proteasomal degradation of IκB. Thus, frequent loss or mutation of p53 may promote tumor progression by enhancing inflammation in the tumor microenvironment.