Soy
isoflavone genistein exhibits growth inhibitory activity against human
pancreatic cancer cell lines. We previously reported the potential of
genistein to augment chemotherapeutic response of
pancreatic cancer cells in vitro. In the present study, we investigated whether
genistein pretreatment could be used as a novel strategy for
gemcitabine-induced killing in vitro and enhanced antitumor activity in vivo using an orthotopic
tumor model. We conducted our studies using paired isogenic human
pancreatic cancer cell line with differences in metastatic behavior (COLO 357 and L3.6pl). In vitro studies were done to measure growth inhibition and degree of apoptotic cell death induced by either
genistein alone,
gemcitabine alone, or
genistein followed by
gemcitabine. Our results show that pretreatment of cells with
genistein for 24 hours followed by
gemcitabine resulted in 60% to 80% growth inhibition compared with 25% to 30% when
gemcitabine was used alone. The overall growth inhibition was directly correlated with apoptotic cell death irrespective of the metastatic potential of cells. Several genes that are known to inhibit apoptosis and contribute to chemoresistance such as
nuclear factor-kappaB (
NF-kappaB) and Akt were assessed to investigate the basis for the observed chemosensitizing effects of
genistein.
Genistein potentiated the
gemcitabine-induced killing by down-regulation of
NF-kappaB and Akt. In contrast, Akt and
NF-kappaB were found to be up-regulated when
pancreatic cancer cells were exposed to
gemcitabine alone, suggesting the potential mechanism of acquired chemoresistance. In addition to in vitro results, we show here for the first time, that
genistein in combination with
gemcitabine is much more effective as an
antitumor agent compared with either agent alone in our orthotopic
tumor model. But most importantly, our data also showed that a specific target, such as
NF-kappaB, was inactivated in
genistein-treated animal
tumors and that
gemcitabine-induced activation of
NF-kappaB was completely inhibited in animal
tumors treated with
genistein and
gemcitabine. These results provide strong molecular in vivo evidence in support of our hypothesis that inactivation of
NF-kappaB signaling pathway by
genistein could also abrogate
gemcitabine-induced activation of
NF-kappaB resulting in the chemosensitization of pancreatic
tumors to
gemcitabine, which is likely to be an important and novel strategy for the treatment of
pancreatic cancer.