Enhancement of the effects of gemcitabine against pancreatic cancer by oridonin via the mitochondrial caspase-dependent signaling pathway

Gemcitabine is a first-line chemotherapeutic agent used in the treatment of pancreatic cancer; however resistance of the disease to the drug often develops over time. Agents that can either enhance the effects of gemcitabine, or help to overcome the chemoresistance to the drug are needed for the successful treatment of pancreatic cancer. Oridonin is one such agent which is safe and multi-targeted and has previously been shown to induce apoptosis in other tumor cells, through mitochondrial signaling pathways. The aims of the present study were to evaluate whether oridonin may enhance the effects of gemcitabine on pancreatic cancer in vitro and to investigate the possible mechanisms of this enhancement. In vitro studies have previously shown that oridonin can inhibit the proliferation of the PANC-1 pancreatic cancer cell line, and potentiate gemcitabine-induced apoptosis, which was shown to be associated with cell cycle arrest in the GI phase. Western. blot and quantitative polymerase chain reaction analyses demonstrated that the expression levels of the anti-apoptotic gene Bcl-2 and the Bcl-2/Bax ratio in the oridonin and the oridonin plus gemcitabine groups were significantly down-regulated as compared with the gemcitabine treatment and control groups. The expression levels of pro-apoptotic genes Bax, cytochrome c (cyt c), and caspase-3 and -9 in the oridonin and the combination groups were significantly upregulated as compared with the other two groups. The results suggested that oridonin improved the anti-tumor effects of gemcitabine through the enhancement of gemcitabine-induced apoptosis. This mechanism may be through the downregulation of Bcl-2 expression and the upregulation of Bax expression, resulting in the reduction of the Bcl-2/Bax ratio. These effects may promote the release of cyt c from the mitochondria into the cytoplasm thus triggering the mitochondrial apoptosis signaling pathway. Furthermore, caspase-3 and -9 were shown to be activated as a result of the induction of apoptosis.