S-phase delay in human hepatocellular carcinoma cells induced by overexpression of integrin beta1

To clarify the mechanisms of integrin overexpression in negatively regulating the cell cycle control of hepatocellular carcinoma cells SMMC-7721.

The cell cycle pattern was determined by flow cytometry. The mRNA and protein expression levels were assayed by RT-PCR and Western blot, respectively. Stable transfection was performed by Lipofectamine 2000 reagent, and cells were screened by G418.

Overexpression of alpha5beta1 or beta1 integrin induced S-phase delay in SMMC-7721 cells, and this delay was possibly due to the accumulation of cyclin-dependent kinase inhibitors (CKIs) p21(cip1) and p27(kip1). The decrease of protein kinase B (PKB) phosphorylation was present in this signaling pathway, but focal adhesion kinase (FAK) was not involved. When phosphorylation of PKB was solely blocked by wortmannin, p27(kip1) protein level was increased. Moreover, S-phase delay was recurred when attachment of the parental SMMC-7721 cells was inhibited by the preparation of poly-HEME, and this cell cycle pattern was similar to that of beta1-7721 or alpha5beta1-7721 cells.

S-phase delay induced by overexpression of integrin beta1 subunit is attributed to the decrease of PKB phosphorylation and subsequent increases of p21(cip1) and p27(kip1) proteins, and may be involved in the unoccupied alpha5beta1 because of lack of its ligands.