Effects of Gossypol Acetate on Proliferation and Apoptosis in Lymphoblastoid Cell Line and Primary ALL and CLL Cells.

Gossypol is a polyphenolic compound extracted from the cotton plant of Malvaceae family. It was originally demonstrated as a male antifertility agent. In the 1960’s, its anticancer efficacy was identified. In the last few years, gossypol has been approved to have antiproliferative and apoptosis-inducing effects on some kinds of cancer cell lines in vitro. Very recently, it has been showed that gossypol is a potent small-molecule inhibitor of Bcl-2 and Bcl-X_L proteins. It can bind to Bcl-2/Bcl-X_L and block the heterodimerization of Bcl-2/Bcl-X_L with proapoptotic proteins, such as Bax, Bak and Bad. Such a blocking of antiapoptotic-proapoptotic protein heterodimerization in turn may inhibit the antiapoptotic function of Bcl-2/Bcl-X_L, and induce apoptosis of cancer cells. Overexpression of Bcl-2 has been observed in B-cell lymphoma, B-ALL, B-CLL and correlates closely with the occurrence, progress, recurrence and multidrug-resistance of these malignancies. In this study, we found that gossypol acetate was able to inhibit the proliferation and survival of Raji cells in vitro at concentrations of more than 5 μmol/L. The effect was dose and time dependent. IC₅₀ were 11.9±1.2 μmol/L at 24h and 6.8±0.4 μmol/L at 72h, which were 7.1 and 9.1 folds lower than that of normal MNCs, respectively. Gossypol acetate can remarkably increased pro-apoptosis activity of dexamethasone in Raji cells. Cell cycle analysis by flow cytometry (FCM) indicated that gossypol acetate could induce G0/G1 arrest. Apoptosis was demonstrated both morphologically by Wright-Giemsa staining, Hoechst 33342 staining and by DNA ladder formation in agarose-gel electrophoresis. By determination the percentage of subdiploid amount of DNA, apoptosis was quantified. The result showed that gossypol acetate could increase the percentage of cells with fragmented DNA in time and dose dependent manners at the concentrations of more than 1μmol/L. The fraction of annexin V⁺/PI⁻ cells was also increased with dose. Colorimetric assay showed that an activation of caspase-3 was seen at 12h and peaked at 24h at the concentration of 25μmol/L. Protein expression of Bcl-2 decreased 13.6±3.8% at 12h and 69.5±6.2% at 24h. Gossypol acetate also had apoptosis-inducing effects in primary leukemia cells from patients with ALL (n=7) and CLL (n=3). The concentration inducing apoptosis in CLL cells was higher than that in ALL cells. It was concluded that gossypol acetate could inhibit the proliferation and induce the apoptosis of Raji cells and primary cultured ALL and CLL cells in a dose and time dependent manner. It seems that down-regulation of Bcl-2 protein expression may play a role in the apoptosis.