Perivascular niche for salivary gland cancer stem cells and resistance to therapy

问题：黏液表皮样癌（MEC）是美国最常见的恶性唾液腺癌

The Problem: Mucoepidermoid carcinoma (MEC) is the most common malignant salivary gland cancer in adults and in children. Relentless growth and resistance to chemotherapy are hallmarks of MEC. Indeed, there is no FDA-approved drug for this cancer. Therefore, the primary treatment still is radical surgery, which is typically associated with high morbidity, poor quality of life, and an unacceptably low 5-year survival rate of 21- 25% for patients with stage II-IV MEC. Rationale: Lack of cell lines and animal models was identified by the NIH/NIDCR as a major roadblock to the discovery of new therapies for salivary gland cancer. In the 1st funding cycle, we generated and characterized the first panel of validated tumorigenic human MEC cell lines and xenograft tumor models (Warner et al., 2013). Using these models, we demonstrated that MEC follows the cancer stem cell hypothesis and that ALDH/CD44 identifies MEC cancer stem cells (Adams et al, 2015). Emerging evidence from other tumor types demonstrates that cancer stem cells are resistant to therapy and drive tumor recurrence, which are key challenges in the clinical management of patients with MEC. In an effort to develop an approach to overcome resistance to therapy, we focused our work on developing strategies targeting MEC cancer stem cells. We observed that the Akt-mTOR-S6K1 pathway is constitutively active in MEC cancer stem cells. In pilot studies, we showed that inhibition of mTOR with Temsirolimus results in significant decrease in the fraction of MEC cancer stem cells and inhibition of Bmi-1 (marker of self-renewal) in vivo. We have also investigated MDM2, an enhancer of tumor cell survival that is involved in the pathogenesis of MEC. We made the exciting observation that small molecule inhibitors of the MDM2-p53 interaction (e.g. MI- 773) ablate MEC cancer stem cells in vitro and in xenograft tumors. Interestingly, it has been shown that S6K1 regulates MDM2 protein stability, providing a mechanistic link between the mTOR pathway and MDM2. Our hypothesis is that therapeutic inhibition of mTOR and/or MDM2 ablates cancer stem cells and sensitizes mucoepidermoid carcinomas to chemotherapy. To address this hypothesis, we propose the following specific aims: S.A.#1: To define the effect of the mTOR pathway on the survival/self-renewal of cancer stem cells, and on the recurrence of mucoepidermoid carcinomas. S.A.#2: To define the effect of therapeutic inhibition of the MDM2-p53 interaction on the growth and recurrence of mucoepidermoid carcinomas. S.A.#3: To determine the effect of therapeutic inhibition of mTOR and/or MDM2 on the resistance of mucoepidermoid carcinomas to conventional chemotherapy (Cisplatin). Significance: This competing renewal builds upon a key discovery made in the 1st funding cycle, i.e. cancer stem cells drive MEC tumorigenesis. Here, our focus is on developing a mechanism-based therapy targeting MEC cancer stem cells for ablation with clinically relevant inhibitors of mTOR and/or MDM2-p53. As such, successful outcome of this work will provide support for a therapy that can quickly progress towards a clinical trial aiming at improving the survival of mucoepidermoid carcinoma patients.