Hepatoma Proteins Background
Hepatocellular Carcinoma HepG2/C3A cells are cancer cells with characteristics of both hepatocytes and tumor cells. They are a subtype of HepG2 cells that are contact-inhibited in culture (unlike other HepG2 lines) and exhibit many of the characteristics of normal hepatocytes including production of serum proteins. HepG2/C3A cells were selected for “high albumin production, and ability to grow in glucose deficient medium”.
These cells display liver specific biological activity. They can be easily maintained, cultured, and used as a relevant model for in vitro studies. Studies have shown that the phenotype of this cell line evolves from exhibiting features characteristic of fetal to adult liver cells as they reach confluence in culture. Increased cell-cell interactions in this hepatoma cell line in vitro are thought to result in a more differentiated pattern of gene expression, and the phenotypic changes due to this interaction resemble hepatic differentiation and regeneration. At low density (<200,000 cells/cm²), they exhibit several characteristics of fetal liver cells. They have a rapid division characterized by a doubling time of 24 h, with a reduced synthesis of albumin. At higher density (>10⁶ cells/cm²) when the culture becomes confluent, growth slows with a doubling time of over about 200 h, with a fourfold higher level of albumin production. When a high-density culture is trypsinized and re-plated to a lower density, the cells again enter a rapid division cycle.
It has been reported that the human hepatoma cell line HepG2 possesses major neutral amino acid transport systems and the specific hepatic system N1. Prior studies have shown that IL-1β and IL-6 have no effect on glutamine uptake by HepG2 cells after various incubation times and at different concentrations (ranging from 0.1 – 5000U/ml).
Hepatoma-derived growth factor (HDGF)
Hepatoma-derived growth factor (HDGF) is a 40 kDa acidic heparin-binding growth factor originally isolated from the conditioned media of the human hepatoma cell line, HuH-7. HDGF is the first member of the HDGF related protein (HRP) family being identified. Members of the HRP family include HDGF, murine HRP-1, murine HRP-2, murine HRP-3, bovine HRP-4 and LEDGF (len epithelium-derived growth factor). These proteins share a common N-terminal homologous to the amino terminus of HDGF domain (HATH domain) of about 100 amino acid residues with a high level of sequence identity but the C-terminal regions show considerable variation.
HDGF is expressed in a broad range of tissues, with the highest expression in the liver, testis and placenta. HDGF was reported to have mitogenic activity in hepatoma cells, fibroblasts, vascular smooth muscle cells and endothelial cells. Moreover, HDGF is involved in the regulation of liver development by stimulating the proliferation of fetal hepatocyte. HDGF is believed to act primarily in the nucleus and the translocation of the growth factor into the nucleus is important for its growth-promoting function. The 240 amino acid HDGF protein contains a bipartite nuclear localization sequence and a DNA-binding PWWP domain at the HATH domain. This structure suggests that HDGF may have the ability on nuclear translocation and DNA interaction. Therefore, HDGF was also reported to exert its function by acting on DNA to regulate gene transcriptions. In addition, HDGF was also found to act in autocrine or paracrine manner that the growth factor may be secreted out of the cells and bind onto the cell surface proteins to promote cell growth.
Overexpression of HDGF can be detected in a number of human cancers including hepatoma, gastric cancer, pancreatic cancer, colorectal cancer, lung cancer and melanoma. Increased expression of HDGF was found to be correlated with high proliferating states of the cancer cells and poor clinical outcome. Therefore, the expression of HDGF was reported to be a potential prognostic factor for these human cancers.
HDGF was also believed to be involved in cancer development. A gradual increase in HDGF expression was detected before the onset of liver tumor development in FLS mice. Overexpression of HDGF in 3T3 fibroblast cells conferred a transformed phenotype and enabled the formation of tumors in nude mice. Moreover, the HDGF level in the poorly differentiated human liver cancer is less as compared with that in the well-differentiated subtype. Furthermore, HDGF was reported to stimulate the growth of vascular endothelial cells and elevated expression of HDGF may therefore stimulate the angiogenic activity and promotes the aggression of tumor. In addition, knock-down of HDGF was reported to suppress the proliferation of hepatoma cells and inhibit the anchorage-independent growth of human lung cancer cells. Therefore, HDGF is suggested to play an important role in cancer development.