EFFECTS OF TUMOR-NECROSIS-FACTOR-ALPHA ON ANTIMITOGENICITY AND CELL CYCLE-RELATED PROTEINS IN MCF-7 CELLS

Tumor necrosis factor-alpha (TNF-alpha) demonstrated antimitogenic act ivity in MCF-7 cells (estrogen receptor-positive human breast cancer c ells) in a dose- and time-dependent manner (EC-50 of 2.5 ng/ml). This antimitogenic effect of TNF-alpha was accompanied by a decreased numbe r of cells in S phase in a dose- and time dependent manner. Based on g rowth arrest experiments using aphidicolin, it is apparent that TNF-al pha acted in early G(1) phase. It did not show antimitogenic effects o nce cells reentered the S phase based on [H-3]thymidine incorporation into DNA and cell cycle analysis. Specificity of TNF-alpha was establi shed by using monoclonal anti-human TNF-alpha antibody. On the basis o f Western immunoblot analysis of Rb, p53 and cell cycle inhibitory pro tein (Cip1) (p21) proteins, TNF-alpha decreased Rb protein expression in a dose- and time-dependent manner whereas it increased the expressi on level of tumor suppressor p53 protein, TNF-alpha also increased the expression level of Cip1 (p21) protein in a dose-dependent manner. Th is induction of Cip1 (p21) protein was preceded by the induction of p5 3 protein in MCF-7 cells, Cip1 (p21) protein associated with cyclin D was also increased, Tumor suppressor Rb protein expression was increas ed during G(1) to S phase progression. Cyclin D protein expression lev els were not changed in response to TNF-alpha treatment, although seri ne/threonine kinase inhibitors such as H7 and the protein kinase C inh ibitor staurosporine decreased cyclin D expression levels in MCF-7 cel ls. Based on experiments with staurosporine, it appears that TNF-alpha does not utilize a protein kinase C pathway in MCF-7 cells. Other cel l cycle-related proteins such as Cdk2, CdcB, and Cdk4 did not show any change in response to TNF-alpha. TNF-alpha did not affect complexes b etween cyclin D and Cdk2, Cdk4, and Rb proteins in RICF-7 cells. Taken together these results suggest that Rb, p53, and Cip1 (p21) proteins mediate TNF-alpha antimitogenic activity, and TNF-alpha induces growth arrest in the G(1) phase in MCB-7 cells.