Alpha 6 beta 4 signaling (Homo sapiens)

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The integrin alpha6beta4 was discovered in the late 1980s by two different groups and was called either alphaEbeta4 or Ic-Ic binding protein (Ic-IcBP) (1,2). The alpha6beta4 integrin is a component of Hemidesmosomes (HDs) (3, 4,5). Increased expression of alphaEbeta4 and changes in its distribution is found to be correlated with increased aggressiveness of tumors and poor prognosis (6, 7). Although integrin alpha6beta4, can interact with different laminin isoforms, its preferred ligand in the epidermal BM is laminin-5 (8, 9). The beta4 integrin is a large protein and has a cytoplasmic domain of more than 1000 amino acids (10, 11). This domain contains a Na-Ca exchanger (CalX) motif followed by two pairs of type III fibronectin (FNIII) domains separated by a connecting segment (CS). It is found to associate with the intermediate filament system through plectin and BP230, which are components of hemidesmosomes (12,13,14). Interaction of intgrin beta4 with components of hemidesmosomes including plectin, BP230 and BP180 is found to be important in signaling events associated with cell growth, survival, and migration under physiological and pathological conditions.

Studies have shown that beta4 can regulate keratinocyte migration both positively and negatively (15,16,17,18). It is also found to regulate cell survival in keratinocytes in cell culture systems under stress in a PI3K/Akt pathway dependent manner (19,18). However, alpha6beta4 was not found to have any effect on keratinocyte survival in vivo (20, 18, 16). Apart from its effects on keratinocytes, evidence suggests that integrin alpha6beta4 is important in cancer cell invasion (21,22,23,24) and survival (25,26,27,28,16,29). The cancer cell invasion is regulated through a IRS/PI3K dependent process while the effect on carcinoma cell survival in a PI3K/Akt and dependent manner. Activation of the transcription factors NFkappaB and NF-IL6 in a p38Mapk dependent pathway and subsequent activation of IL6 gene expression was shown to be mechanism of alpha6beta4 induced survival of thymocytes and proliferation of thymic epithelial cells (27,30). Integrin alpha6beta4 is also known activate the Ras/Raf/MEK/ERK cascade which is found to be involved in the regulation of cell cycle (31,32,33,34).

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