Tumor-initiating cells and therapy resistance

Prostate cancer is one of the most common male malignancies and has a bad prognosis when metastases have spread. Current treatment options include radiation and endocrine therapy by either orchiectomy or administration of anti-androgens. Chemotherapy with docetaxel is an option for patients that have a castration-resistant phenotype. A number of molecular mechanisms that lead to resistance against radio-, endocrine, and chemotherapy are known. Tumor-initiating cells (also known as cancer stem cells) are believed to be the cause for tumor relapse and have high resistance against a variety of therapies. Moreover, cells with a cancer stem cell-like phenotype can be found in therapy resistant tumors. We have generated three cell culture models based on the PC3 cell line that show characteristics of cancer stem cell-like cells and higher expression of cancer stem cell-like markers: a) an inflammatory model through long-term exposure to Interleukin-4; b) a radio-resistant model by repeated exposure to X-rays, and c) a docetaxel-resistant model generated through a dose-escalating exposure to the drug. We hypothesize that in all three models similar pathways are activated that lead to the appearance of the cancer stem cell-like phenotype and thus to therapy resistance. In this project it is planned to elucidate novel pathways using a cDNA microarray approach to compare gene expression in the different models with the parental PC3 cell line. The involvement of such identified pathways in the appearance of a cancer stem cell-like phenotype will be confirmed in additional cell lines and furthermore, their role in tumor-initiating cells isolated from primary tissue will be investigated. The second aim proposes to develop an experimental therapy approach by targeting a novel, selected key pathway with a RNAi approach to prevent the acquisition of a cancer stem cell-like phenotype. Such an experimental therapy will be confirmed in mouse xenograft experiments. As last aim, we will address the clinical relevance of our findings by analyzing by immunohistochemistry a tissue micro array from a patient-derived xenograft library containing specimen from patients that failed therapy. Results from this project application are of clinical importance and could lead to the development of more effective therapies against prostate cancer. Moreover, identification of novel mechanisms promoting a cancer stem cell-like phenotype and thus therapy resistance could also be of importance in other cancer types.

Prostate cancer is among the most commonly diagnosed tumors in the Western world. When detected at an early stage curability is high by surgical removal of the prostate gland. Treatment options for advanced stages are endocrine therapy and chemotherapy. Radiotherapy is an option in all stages. Although there have been advances in recent years in development of new therapies, life expectancy in late stages of the disease is still very low due to the occurrence of therapy resistances. Cells that resist anti-cancer therapies, also known as cancer stem cells are at the origin of the recurrence of the disease. The cancer stem cells can be present before therapy or can be generated by the therapy itself. Combating the cancer stem cells is an unmet therapeutic need to eradicate cancer successfully. In this project, we set out to obtain more knowledge on the regulatory pathways that mediate stemness in prostate cancer models that are resistant to radiotherapy or where stemness had been induced by an inflammatory factor called Interleukin-4. Elucidation of such regulatory pathways could unravel novel targets that are suitable for the development of a novel cancer therapy that fights the disease at the "stem". We found that radioresistance involves two pathways whose functionality is inhibited by a reduced expression of the corresponding proteins. The division of cells is a well-controlled process, where multiple pathways check the integrity of the genomic information. To our surprise these pathways checks were absent in prostate cells that were treated with radiotherapy with a schedule similar to the one used for patients. This override of the cell division checks is a characteristic of stem cells. Furthermore, non-recognition by the immune system is also a characteristic of cancer stem cells. The second identified pathway implicated in radioresistance is a decreased abundance of proteins that are activated by Interferon type 1, a molecule that is involved in the activation of the immune response. Next, we focused on the inflammatory factor Interleukin-4. This molecule was able to increase the tumor-initiating potential of prostate cancer stem cells. Inhibition by a drug targeting an Interleukin-4 regulated factor was able to reverse the tumor-initiating effect of Interleukin-4. However, Interleukin-4 seemed not to be involved in drug resistance mechanisms. In conclusion, we identified several molecular mechanisms that regulate prostate cancer stemness, which are good drug target candidates in order to develop novel prostate cancer therapies.