Growth Inhibition of Human Hepatocellular Carcinoma Cells by Overexpression of G-Protein-Coupled Receptor Kinase 2

Hepatocellular carcinomas (HCC) are a complicated human disease in terms of etiology and molecular carcinogenic mechanisms. HCC is globally the fifth most common malignancy and the third largest cause of cancer deaths [Bosch et al., 2004). The frequency of HCC in Southeast Asia and sub-Saharan Africa is greater than that in North America and Western Europe. However, recent data show that the overall frequency of HCC in developed countries is rising [Lau and Lai, 2008]. This increase is primarily due to persistent infection with hepatitis C and chronic alcohol abuse that causes cirrhosis [Kern et al., 2002; Bruix and Sherman, 2005]. HCC is a potentially curable at early stage through surgical resection and liver transplantation. Unfortunately, the majority of patients with HCC are usually in the advanced-stage with severe background liver disease which is not suitable for such treatments [Wang et al., 2002]. Moreover, HCC is a type of tumor highly resistant to conventional medical treatment such as chemotherapy and radiation. There is a critical need to develop novel strategies for effective prevention and therapy of this disease. G-protein-coupled receptor kinase 2 (GRK2) is a ubiquitously expressed serine/threonine kinase. It is the unique member of GRK family with diverse functions [Metaye et al., 2005; Ribas et al., 2007]. The role of GRK2 was first discovered in the desensitization of G-protein-coupled receptors (GPCR) signaling by phosphorylating agonist-activated 7-transmembrane receptors. The phosphorylated receptor enhances the binding of β-arrestins to form a molecular complex which prevents further coupling of the receptor from its G-protein, leading to attenuation of the receptor-mediated signalings [Aragay et al., 1998; Ribas et al., 2007]. Despite of its traditional function as a kinase in receptor desensitization, a growing body of evidence has been documented that GRK2 interacts with a variety of other cytosolic proteins involved in signaling pathways relevant to essential cellular processes, such as proliferation/apoptosis, migration, trafficking, cell cycle, and development [Penela et al., Penela et al., 2008, 2010b; Guo et al., 2009; Jiang et al., 2009; Kahsai et al., 2010]. Some of these physiological functions of GRK2 are achieved through kinase-independent mechanisms by directly binding to other proteins [Cipolletta et al., 2009; Jiang et al., 2009; Namkung et al., 2009; Chen et al., 2010]. Altered expression levels of GRK2 have been reported in many human diseases including heart failure, hypertension, rheumatoid arthritis, cystic fibrosis, and cancer [Lombardi et al., 1999; Mak et al., 2002; Vroon et al., 2004, 2005; Metaye et al., 2005; Lymperopoulos et al., 2007]. Overexpression of GRK2 has been reported to reduce cell proliferation in smooth muscle cells and thyroid cancer cells [Peppel et al., 2000; Metaye et al., 2008]. Interestingly, some thyroid tumors actually have higher GRK2 level as compared with its surrounding normal tissues. The mechanism underlying this growth inhibition is still largely unclear, given the fact that GRK2 has a complex interactome and lies in the crossroad of many signaling pathways. In this report, we tested the inhibitory effects of GRK2 overexpression on the growth of human HCC cells and examined the molecular mechanism by which GRK2 overexpression causes this growth inhibition.