Hedgehog signaling in prostate cancer: Clinical relevance and its potential as a target for radiosensitization

While the Hedgehog (Hh) signaling pathway is essential for numerous processes during embryonic development, its role in adult tissues is more limited. However, there is increasing evidence of uncontrolled reactivation of the Hh signaling pathway in various types of cancer, including prostate cancer (PCa). Emerging data in the literature suggest that the Hh pathway is involved in the development of this disease as well as in the progression to more aggressive and even therapy-resistant disease states. In this project, we addressed two major problems in the management of PCa, i.e. 1) intensification of PCa treatment, especially for high-risk PCa patients for whom treatment options are still limited and 2) identification of a suitable biomarker to enable better patient stratification for a particular treatment. Therefore, we evaluated the radiosensitizing potential of different Hh inhibitors either alone or in combination with the AMPK activator metformin in in vitro and in vivo PCa models and elucidated the prognostic value of Hh signaling in different PCa patient cohorts. In the first part of this project, we demonstrated that Hh inhibition more downstream the signaling cascade, i.e. at the level of the GLI transcription factors by GANT61 was much more cytotoxic than more upstream at the level of SMO with GDC-0449. Additionally, we showed that GANT61 was able to increase the intrinsic radiosensitivity of 22Rv1 PCa cells. This GANT61-induced radiosensitizing effect appeared to be related to its effect on cell cycle distribution and apoptosis rather than DNA damage repair. Moreover, we confirmed the relationship between GLI1 expression and radiation response. Our results also demonstrated that the enhanced radiosensitivity by GANT61 in vitro was dependent on functional p53 signaling. In contrast, the observed radiosensitizing effect in vivo proved to be independent of tumoral p53 status. The radiosensitizing effect of GANT61 in vivo was due to both direct and indirect effects on the tumor since GANT61 also inhibited Hh signaling in the tumor-associated stromal cells that indirectly impacted tumor growth in our PCa xenograft models. In the next part of this study, we investigated whether the combination of Hh inhibition and AMPK activation, by means of metformin could enhance the cytotoxic and/or radiosensitizing effects of both agents, as recent reports have shown a bi-directional interaction between AMPK and Hh signaling in other tumor models. Firstly, we were able to verify that metformin interacts with the Hh signaling pathway by inhibiting the effector protein GLI1 in PCa cells both in vitro and in vivo. Secondly, combination of metformin and GANT61 significantly inhibited PCa cell growth in vitro, but this effect was absent in vivo. Worse even, a pro-proliferative effect was seen when combining both drugs in a 22Rv1 xenograft model. Furthermore, we showed that the drug combination enhanced the intrinsic radiosensitivity of 22Rv1 PCa cells, whereas in vivo we were unable to demonstrate an enhanced radiation response compared to GANT61 alone. In the last part of this project, we elucidated the prognostic value of Hh signaling in different PCa patient cohorts. In the first study, we compared protein expression of important Hh components in the cancerous and benign prostate tissue of the same patient. This PCa cohort consisted of 75 PCa patienst of which 73 patients were high-risk PCa patients. In the second study, we compared Hh protein expression in 170 PCa patients with the expression in 64 patients with benign prostatic hyperplasia. Both studies showed that Hh signaling appears more active in the cancerous tissue compared to benign prostate tissue. In addition, we showed that activation of Hh signaling is associated with more aggressive tumors, indicated by the fact that the Hh effectors, GLI1 and GLI2 were correlated with pathological Gleason score in the respective studies. Our data also demonstrated that PTCH1 expression seems to be a good prognostic marker for BCR in high-risk PCa patients. This was not the case in the second PCa cohort consisting of more intermediate-risk PCa patiens, in which GLI3 appeared to be correlated with PCa outcome. Altogether, these data highlight the potential prognostic value of Hh signaling in PCa. However, more research is needed to assess the true clinical value of Hh signaling in PCa.

Jaar van publicatie:2017

Toegankelijkheid:Closed