In the current study, we found that HH pathway signaling components are overexpressed in chemoresistant ovarian cancer cells. Moreover, targeting the HH pathway decreased ovarian cancer cell viability and sensitized chemoresistant ovarian cancer cells to paclitaxel therapy through decreased MDR1 expression. The participation of HH signaling in ovarian cancer cell survival and chemotherapy resistance makes it an attractive target for therapy, especially since most ovarian cancer patients develop tumor recurrence and succumb to chemoresistant disease.
Currently, it has not been shown what role HH signaling might play in mediating ovarian cancer chemoresistance, a persistent obstacle in the treatment of this disease. While the clinical behavior of ovarian cancer suggests that most cancer cells are initially sensitive to chemotherapy, they subsequently either develop resistance or contain a population of cells that are inherently resistant. The latter hypothesis is consistent with what has become known as tumor initiating cells or cancer stem cells (CSCs). These CSCs are commonly believed to have enhanced tumorigenicity, differentiation capacity and resistance to chemotherapy in comparison to non-CSCs. It is because of these features that CSCs have been examined for molecular pathways and markers that could be targeted for therapeutic purposes. Recent studies have suggested that developmental pathways, including HH, play important roles in the maintenance of CSCs (10
) and that inhibiting these pathways may provide enhanced chemosensitivity when combined with traditional chemotherapies. In our study, we sought to define a role for HH signaling in ovarian cancer chemoresistance. Both in vitro
and in vivo
, we observed significant sensitization to paclitaxel following Smo antagonism (LDE225) in taxane-resistant ovarian cancer cells. This sensitization was also present in ALDH-positive cells, a subpopulation of cancer cells with enhanced tumorigenicity and chemoresistance. The mechanism underlying this sensitization appears to involve downregulation of P-glycoprotein (ABCB1/MDR1), a well-characterized mediator of multi-drug resistance. By downregulating MDR1
expression, uptake of paclitaxel by cancer cells would be increased, resulting in a greater response to the chemotherapeutic agent. This mechanism would explain why Smo antagonists did not sensitize chemoresistant cells to carboplatin, since this compound is not a substrate for the P-glycoprotein drug efflux pump. In addition, this model of HH inhibition and chemosensitization agrees with a previous study performed by Sims-Mourtada et al., in which it was demonstrated that cyclopamine sensitized prostate cancer cells to a variety of chemotherapy agents in vitro
(including the taxane docetaxel), through modulation of MDR1 expression (12
). The observation that Smo antagonism did not sensitize cells to platinum therapy highlights the specificity of this effect.
Previous studies have demonstrated aberrant expression of the HH pathway in primary specimens of ovarian cancer compared to normal ovarian epithelium (7
), including a study that found elevated Gli1 expression is associated with decreased survival (9
). These studies have also demonstrated decreased ovarian cancer cell growth/viability following treatment with the Smo antagonist cyclopamine, results that our study supports. We have previously shown that GLI1
mRNA levels were significantly higher in cancer cells isolated from persistent/chemoresistant tumors compared to those isolated from matched primary tumors (29
). Smoothened expression was also increased (3.7-fold) in persistent tumors; however, this increase was not statistically significant. Patients from whom persistent tumors were obtained had failed both taxane and platinum chemotherapies, making it difficult to determine whether this increase in hedgehog pathway genes is a taxane-specific effect. The in vitro
data presented in this study, however, would suggest that Smoothened, as well as Gli1 and Gli2, are associated with taxane resistance. In our initial experiments examining the effects of targeting HH alone, either with Smo antagonists or RNAi, ovarian cancer cell viability was significantly decreased in vitro
, indicating that the HH pathway is important for ovarian cancer survival. However, this effect did not seem to translate to our xenograft models, in which the Smo antagonist LDE225 had no significant impact on tumor growth when used alone, even in models with relatively high Gli1 expression. These findings suggest that survival pathways are activated in the murine tumor microenvironment that allows resistance to HH antagonist monotherapy. Given the recognized importance of crosstalk between the tumor stromal cells and malignant cells in the HH pathway (6
), and the failure of this model to target both murine and human compartments, more efficacy may be noted with monotherapy in humans.
Collectively, the data presented in this study demonstrate that increased expression of HH signaling components is associated with taxane resistance, which can be overcome by targeting multiple effectors of the HH signaling pathway. With the ability to identify subsets of cancer patients with HH pathway overexpression, antagonism of HH signaling in combination with taxane therapy could ultimately provide a useful therapeutic strategy for recurrent, chemoresistant ovarian cancer.