ABSTRACT
Soy isoflavones have been reported to be natural chemopreventive in several types of human cancer. Daidzein and genistein are two main components of soy isoflavones. In our previous study, they were shown to be anti-proliferative and induce cell cycle arrest at S phase of SHZ-88 rat breast cancer cells. We hypothesized that soy isoflavones might exert its anticancer effect by activating cAMP/PKA pathway. The present study was designed to analyze the effect of soy isoflavones on the cAMP/PKA pathway in SHZ-88 cells. Daidzein and genistein were dissolved in DMSO. Cells were treated with 50 mug/ml daidzein and 15 mug/ml genistein, respectively, and with only equal DMSO in the culture medium as control. The cellular cAMP content was tested by radioimmunoassay (RIA). The activity of adenylate cyclase (AC), phosphodiesterase (PDE) and PKA were measured by RIA and (gamma-(32)P) ATP incorporation. Reverse transcript-polymerase chain reaction (RT-PCR) was used to analyze the expression of cAMP response element binding protein (CREB) mRNA of the cells. The results showed that the concentration of cAMP in the cells treated with 50 mug/ml daidzein and 15 mug/ml genistein was significantly increased by 9.5%and 11.0%, respectively, 5 min later (P<0.05), then increased by 31.0%and 40.3%, respectively, 10 min later (P<0.01), compared with that of the control group cells. The activity of AC was not affected during the course of experiment, but that of PDE was decreased to 71.8%and 71.6%, respectively, in the control group 5 min later (P<0.05). The PKA activity was increased to 125.8%and 122.3%, respectively, in the control group 20 min after the cells were treated with daidzein and genistein (P<0.05), and kept at high level till 40 min after treatment. CREB mRNA of the cells treated with daidzein and genistein was increased by 31.6%and 51.1%, respectively, 3 h later (P<0.05), then began to decrease 6 h after treatment. The current study suggests that soy isoflavones activate the cAMP/PKA pathway in SHZ-88 rat breast cancer cells by inhibiting the activity of phosphodiesterase.