A lymphocyte blastogenesis inhibitory factor (LBIF) reversibly arrests a human melanoma cell line, A375, at G1 and G2 phases of cell cycle

A lymphocyte blastogenesis inhibitory factor, LBIF, has been found in the culture supernatant of a human macrophage-like cell line, U937. The factor has been purified by fast protein liquid chromatography. Partial amino acid sequencing analysis showed that LBIF was a novel immunoregulatory factor. Recent study has demonstrated that LBIF possesses a remarkable tumor growth inhibitory activity. In this study, the cell growth inhibitory activity of LBIF was characterized on the proliferation of a human melanoma cell line A375 in vitro. LBIF strongly inhibits the proliferation of A375 cells. The inhibitory activity was cytostatic and reversible by Day 5 although the lethal effect became apparent at Day 7. Cell cycle analysis by flow cytometry showed that LBIF arrested A375 cells at both G1 and G2/M phases. Mitotic index analysis indicated that A375 cells were arrested in G1 and G2 phases. LBIF function was not attributed to the elevation of intracytoplasmic cyclic-AMP levels. Thus, these results suggest that LBIF plays an important role in controlling cell cycle and there is a similarity between the mechanisms of G1 and G2 arrests in eukaryotic cell proliferation. LBIF-induced reversible cell-cycle arrest of A375 cells can be a useful system to analyze the signal transduction for cell proliferation and cell-cycle arrest.