The collagen receptor integrins : differential regulation of their expression and distinct signaling functions

Abstract

Metazoans need cell adhesion to hold themselves together. The largest family of cell surface receptors that mediate cell-matrix interactions are the integrins. Integrins link the extracellular matrix to the intracellular cytoskeleton. Integrins are also signal transducers. Four collagen-binding integrins are known, namely αlβl, α2βl, α10βl, and α11βl. Integrin α3βl also binds collagen, but it is an assisting receptor. In this thesis, the differential regulation of the expression and distinct signaling functions of the collagen receptor integrins were studied. The expression of four collagen-receptor integrins and the regulation of their expression was investigated in human osteosarcoma cell lines: Saos-2, MG-63, and KHOS-240. Bone morphogenetic protein-2 (BMP-2), which is a peptide growth factor closely related to transforming growth factors-β (TGFs-β), decreased the expression of α2 and α3 integrins in Saos-2 cells, and in the human keratinocyte cell line, HaCaT, respectively. This way BMP-2 regulated the adhesion of Saos-2 cells to laminin-5. Integrin α2 expression has been shown to be associated with the transformed phenotype of a cell. This was studied further by testing the effects of tumor promoters, okadaic acid (OA) and 12-O-tetradecanoyl-13-phorbol acetate (IPA) on the expression of the α2 integrin gene in MG-63 cells. OA and IPA induced the integrin α2 gene expression. Expression of the novel integrin subunits, α10 and α11, was studied in osteosarcoma cells in which al and α2 subunits are expressed. Integrin subunits α10 and all were expressed in Saos-2, MG-63, and KHOS-240 cells, and their expression was regulated by TGF-β and tumor promoters, TPA and OA. Integrins αlβl and α2βl have been shown to activate different cell signaling pathways. Here the signal transduction mediated by integrin a2βl was studied further. A novel α2-related signaling mechanism was described. Integrin α2βl mediated the activation of protein serine/threonine phosphatase, PP2A, and as a result of that, PP2A inactivated Akt/PKB by dephosphorylation. Integrin α2 cytoplasmic tail was able to block a proliferative signaling pathway (Akt/PKB), and it could have an effect on cell cycle arrest in human primary fibroblasts. These results show that all four collagen-receptor integrins can be expressed simultaneously in a single cell and can be independently regulated. The results also confirm that the collagen-receptor integrins have distinct signaling functions, and their ligation may lead to opposite cellular responses.