THE NOVEL STRUCTURAL MOTIF GLN(795)-GLN(802) IN THE INTEGRIN BETA(1C) CYTOPLASMIC DOMAIN REGULATES CELL-PROLIFERATION

Alternative splicing of the integrin beta(1) subunit mRNA generates a variant form, beta(1C), with a unique cytoplasmic domain that differs from beta(1) for a 48-amino acid COOH-terminal sequence. The potential role of this unique sequence in modulating cellular functions was investigated using Chinese hamster ovary (CHO)(1) cells transiently transfected with cDNAs coding for human integrin beta(1C) or beta(1) subunits or mutants containing truncated forms of the beta(1C) cytoplasmic domain. A differential effect of beta(1C) and beta(1) on cell proliferation was ob served. Expression of wild type beta(1) was associated with a 6-10-fold increase in cell proliferation in response to serum, as measured by [H-3]thymidine incorporation. In contrast, only a a fold increase in cell proliferation was observed in transfectants expressing comparable levels of beta(1C). Cells expressing the beta(1C) mutant truncated at Leu794 and lacking the last 31 amino acids of the cytoplasmic domain showed a 12-fold proliferation increase in response to serum. However, three beta(1C) deletion mutants, lacking the COOH-terminal 23, 13, and 8 amino acids, which all contained residues Gln(795)-Gln(802) of the variant cytoplasmic domain responded to serum stimulation with a 2-fold increase in [H-3]thymidine uptake. The effect of beta(1C) expression on cell proliferation was not associated with changes in exposure of integrin functional epitopes, as judged by the finding that CHO transfectants expressing beta(1C) full-length or deletion mutants, or beta(1) equally adhered to a functionally inhibitory monoclonal antibody against human beta(1) integrin. Expression of beta(1C) inversely correlated with the mitogenic potential of vascular cells. Absent on growing cultured endothe lial cells, surface expression of beta(1C) was induced in growth arrested, tumor necrosis factor-stimulated endothelial cells. These findings suggest that integrin alternative splicing may provide an accessory mechanism to modulate cell type-specific growth regulatory pathways during vascular cell injury in vivo.