Functional analysis of an ankyrin repeat protein, gankyrin, overexpressed in cancers and its applicating gene therapy

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Hepatocellular carcinoma (HCC) is one of the most common cancers in Asia and Africa, where hepatitis virus infection and exposure to specific liver carcinogens are prevalent. Although inactivation of some tumour suppressor genes such as p53 and p16ィイD1INK4AィエD1 has been identified, no known oncogene is commonly activated in hepatocellular carcinomas. Here we have isolated genes overexpressed in hepatocellular carcinomas by cDNA subtractive hybridization, and identified an oncoprotein consisting of six ankyrin repeats (gankyrin). The expression of gaukyrin was increased in all 34 hepatocellular carcinomas studied. Gankyrin induced anchorage-independent growth and tumorigenicity in NIH/3T3 cells. Gankyrin bound to the product of the retinoblstoma gene (RB1), increasing its phosphorylation and releasing the activity of the transcription factor E2F-1.Gankyrin accelerated the degradation of RB1 in vitro and in vivo, and was identical to or interacted with a subunit of the 26S proteasome. These results demonstrate the importance of ubiquitin-proteasome pathway in the regulation of cell growth and oncogenic transformation, and indicate that gankyrin overexpression contributes to hepatocarcinogenesis by destabilizing RB1.