Release from quiescence stimulates the expression of integrin α₅β₁ which regulates DNA synthesis in human fibrosarcoma HT1080 cells

We show that integrin α₅ subunit expression is stimulated when human fibrosarcoma HT1080 cells are released from quiescence. The α₅ subunit mRNA level in quiescent HT1080 cells was increased 24 hr after their release by 10% fetal bovine serum‐containing medium reaching a maximum of 2.5 fold on day 2. Similar levels of induction of cell‐surface α₅ subunit protein as well as b̃₁ subunit protein were also observed. This resulted in a significant increase of cell attachment to fibronectin. The serum stimulation also increased α₅ subunit promoter activity by twofold which was protein synthesis independent. Subsequent deletion of α₅ subunit promoter DNA showed that the cis‐element responsible for the activation is located between ‐ 92 bp and the transcription start site. The promoter activity was not induced until 12 hr after the release. Comparison of the effect of a serum‐free medium and a 10% fetal bovine serum‐supplemented medium revealed that both the DNA synthesis and α₅ subunit induction were independent of exogenous growth factors. The increased integrin α₅b̃₁ appears to function by reducing mitogenic activity since blockade of fibronectin binding to its receptor with a RGD peptide, a monoclonal anti‐fibronectin antibody, or a monoclonal anti‐α₅ subunit antibody during the release from quiescence significantly stimulated DNA synthesis. On the other hand, stable overexpression of the α₅ subunit resulted in decreased DNA synthesis. © 1995 Wiley‐Liss, Inc.