The integrin, α6β1, is necessary for the matrix-dependent activation of FAK and MAP kinase and the migration of human hepatocarcinoma cells

Abstract

Expression of the integrin, ␣6␤1, a receptor for laminins, is associated with the progression of hepatocellular carcinoma (HCC). The approach to investigating the ␣6␤1 integrin signaling in HCC cells was to express a deletion mutant of the ␤4 integrin cytoplasmic domain (␤4-⌬cyt) in 2 HCC cell lines, HepG2 and Huh7. Expression of this mutant prevents formation of the ␣6␤1 heterodimer. As expected, adhesion of both the HepG2/␤4-⌬cyt and Huh7/␤4-⌬cyt transfectants to laminin, but not to collagen, was reduced compared with the mock transfectants. However, migration of the ␤4-⌬cyt transfectants toward both collagen and laminin was inhibited, suggesting a role for ␣6␤1 in the signaling of migration. The pattern of integrins expressed by human hepatocytes is strikingly different from most other epithelial cells. Normal adult hepatocytes express low levels of only 3 integrins: ␣1␤1, a collagen and laminin receptor; ␣5␤1, a fibronectin receptor; and ␣9␤1, a tenascin receptor. In contrast, other integrin receptors such as ␣2␤1, ␣3␤1, ␣6␤1, and ␣6␤4 are undetectable on normal hepatocytes. One of the most frequent alterations during liver carcinogenesis is de novo expression of the integrin, ␣6␤1. In fact, the induction of ␣6␤1 expression is an early event in hepatocellular carcinogenesis, and it is reasonable to postulate that ␣6␤1 contributes to carcinogenesis based on several lines of evidence. [1][2][3][4] For this reason, it is important to understand the mechanism by which the ␣6␤1 integrin influences the function of hepatocellular carcinoma (HCC) cells. One likely possibility is that ␣6␤1 controls signaling pathways important for HCC function.In this study, we examined the hypothesis that ␣6␤1-mediated activation of FAK and MAP kinase is important for the function of HCC cells. Both focal adhesion kinase (FAK) and the mitogen-activated protein (MAP) kinase family members, extracellular-regulated kinase 1 (ERK1) and ERK2, are of interest, because they are regulated by integrin-mediated attachment to matrix, as well as growth factor stimulation, and they regulate important functions of tumor cells such as growth and migration. [5][6][7][8] The regulation of these kinases by the ␣6␤1 integrin, however, is not well understood and somewhat controversial. A previous study concluded that ␣6␤1 was unable to activate MAP kinase in NIH 3T3 cells despite being able to stimulate FAK tyrosine phosphorylation. 9 More recently, a study performed in macrophages demonstrated that ␣6␤1 integrin, and particularly the cytoplasmic domain of the ␣6 subunit, was able to regulate MAP kinase activation without inducing FAK tyrosine phosphorylation. 10,11 Most previous studies concerning the function of ␣6␤1 integrin have relied on the use of antibodies binding ␣6␤1 to perturb the interaction of ␣6␤1 with its ligand. The hypothesis that expression of the ␣6␤1 integrin is important for FAK and MAP kinase activation in HCC cells was assessed using a dominant-negative approach to inhibit expression of this integrin in HCC cell lines. [12][13][1...

Lysophosphatidic Acid Receptor LPAR6 Supports the Tumorigenicity of Hepatocellular Carcinoma

“…Moreover, genes involved in ECM organization such as MMP19, ITGA6, and several collagen genes were significantly downregulated. It is noteworthy that the ITGA6 gene encoding integrin a6 plays a role in the tumorigenic process of many cancers including HCC (24,25). Among genes that control tissue development, we focused on Pim-3 because, being an oncogene, it could largely explain the LPAR6-mediated protumorigenic phenotype observed in HCC.…”

Role of Expression of Focal Adhesion Kinase in Progression of Hepatocellular Carcinoma

“…Studies regarding the role of FAK in cell migration in vitro have demonstrated that FAK-deficient fibroblasts display reduced migratory capacity in response to growth factor, extracellular matrix protein, and substrate rigidity-dependent stimulation that is rescued by FAK reexpression (8,37). In HCC cell lines, the activation of FAK and mitogen-activated protein kinase by integrin is important for migration (38).…”

New therapies for hepatocellular carcinoma

“…As occurs with the composition of liver ECM, the normal expression pattern of integrins is also altered early during hepatocarcinogenesis. The resulting abnormal combination of ECM ligands and integrin receptors has been proposed to play a role in liver tumor development and metastasis (Carloni et al, 2001;Nejjari et al, 2002;Yang et al, 2003;Zhang et al, 2003). In particular, overexpression of integrin b1 in human HCC cells has been demonstrated to enhance cell proliferation, survial, migratory capacity and resistance to chemotherapy (Carloni et al, 2001;Zhang et al, 2002Zhang et al, , 2003Zhang et al, , 2004.…”

“…The resulting abnormal combination of ECM ligands and integrin receptors has been proposed to play a role in liver tumor development and metastasis (Carloni et al, 2001;Nejjari et al, 2002;Yang et al, 2003;Zhang et al, 2003). In particular, overexpression of integrin b1 in human HCC cells has been demonstrated to enhance cell proliferation, survial, migratory capacity and resistance to chemotherapy (Carloni et al, 2001;Zhang et al, 2002Zhang et al, , 2003Zhang et al, , 2004. Similarly, induction of FAK gene expression has been observed in human HCC, and elevated FAK levels seem to correlate with a more aggressive tumor behavior (Itoh et al, 2004;Fujii et al, 2004).…”