Expression of Pirh2, a p27Kip1 ubiquitin ligase, in hepatocellular carcinoma: correlation with p27Kip1 and cell proliferation

p53-Induced ring-H2 protein (Pirh2), a recently identified ubiquitin–protein ligase, interacts with p27Kip1 to promote ubiquitination of p27Kip1 independently of p53. High Pirh2 and low p27Kip1 immunoreactivity are associated with a poor prognosis in several cancers, including resistant phenotypes. In the present study, we investigated the role of Pirh2 and p27Kip1 in human hepatocellular carcinoma (HCC) progression. Immunohistochemical analysis was performed on formalin-fixed paraffin sections of 87 specimens. Statistical analysis showed that expression of Pirh2 was negatively related to p27Kip1 expression (r = 0.787; P < .05), and Pirh2 expression correlated significantly with histologic grade (P < .001), venous invasion (P = .004), tumor size (P = .024), and the presence of multiple tumor-bearing lymph nodes (P = .017), whereas p27Kip1 expression correlated significantly with histologic grade (P < .001), venous invasion (P = .048), and cirrhosis (P = .028). By Kaplan-Meier analysis, the survival curves of low versus high expressers of Pirh2 and p27Kip1 showed significant separation (P < .01). Molecular interaction could be demonstrated between Pirh2 and p27Kip1 in three HCC cell lines. In vitro, following release of two HCC cell lines from serum starvation, the expression of Pirh2 was upregulated, whereas p27Kip1 was downregulated. Our results suggest that Pirh2 mediates the degradation of p27Kip1 and participates in cell proliferation in human HCC. These findings provide a rational framework for further development of Pirh2 inhibitors as a novel class of anti-tumor agents.