Anti-tumor activity of miR-17 in melanoma

Abstract

A large body of evidence suggests that the crosstalk between cancer cells and the surrounding microenvironment plays a critical role in primary tumor establishment and its dissemination to distant sites. Such interplay is important for sustaining an adaptive microenvironment for tumor cells encountering different conditions during tumor dissemination. MicroRNAs are a small class of non-coding RNAs that have important roles in cancer progression through downregulation of the expression of their target mRNAs. Accumulating evidence shows that abnormal expression of miRNAs in tumors can lead to changes in the intrinsic properties of cancer cells, which include cell proliferation, migration, apoptosis, and cellular senescence.1 Recent research from our lab has implicated the importance of miR-17 expression in the adaptability of tumor cells to environmental fluctuations using the glioblastoma cell line. These cells, which are highly adaptive and over-express miR-17, appeared to adopt a new strategy to cope with the lack of nutrition in the environment by lowering their metabolic rate. Interestingly, reduced metabolism not only enabled them to survive better in serum-free conditions, but also helped them to resist chemotherapeutic drug toxicity, as these drugs are designed to eradicate fast growing tumor cells.2 This adaptability to environmental conditions conferred to tumor cells by miR-17, led us to investigate the immune response properties of these cells. Our previous study showed that B-cell and T-cell development was impaired in miR-17 transgenic mice, indicating the importance of miR-17 in lymphomagenesis and lymphopoiesis.3 In the current study, we observed that transgenic mice over-expressing miR-17 developed fewer tumors compared to wild type mice when they were challenged with B16 melanoma cells. Systemic analysis of CD45+ and CD8+ T-cells, in peripheral blood and spleen, had revealed that there was a significant increase in the percentage of CD8+ T-Cells in transgenic mice compared to control, directly following tumor implantation. In contrast, the percentage of CD8+ T-cells was initially lower in transgenic mice compared to control mice before the tumor was implanted. The expansion of CD8+ T-cell populations and the subsequent infiltration of T-cells into the tumors, suggest that the miR-17 transgenic mice were able to induce a stronger anti-tumor immune response compared to control. But, how is miR-17 able to instigate this rapid adaptive immune response by expansion of CD8+ T-cells in these transgenic mice? CD8+ or cytotoxic T-cells are tumor-specific effector T-cells that are capable of attacking and eliminating cancer cells. However, the ability of CD8+ T-cells to eliminate cancerous cells relies on the tumor microenvironment, but due to the presence of immunosuppressive factors, tumor environment is hostile to T-cell proliferation and activation. So, to explain the increase in T-cells seen in transgenic mice, we reasoned there is an important signaling molecule or transcription factor – promoting immune suppression in the tumor environment – that has been targeted by miR-17. Hence, the down-regulation of this molecule by miR-17, is allowing the increased immune response. A fascinating candidate is Signal Transducer and Activator of Transcription 3 (STAT3), which is commonly activated both in tumor cells and in immune cells of the microenvironment. STAT3 is known to promote tumor-mediated immunosuppression at many levels.4 For instance, STAT3 signaling can lead to the secretion of immunosuppressive cytokines IL-2 and IFN-γ; therefore, it is a negative regulator of CD8+ T-cell proliferation and activation.5 In addition, miR-17 over-expression in Jurkat cells can lead to their faster proliferation and better survival in the presence of B16 melanoma cells, owing to the blockade of STAT3. Furthermore, after being co-cultured with B16 melanoma cells, Jurkat cells over-expressing miR-17 remained in G1 phase for a longer duration, and stayed in S phase for less, thus, indicating their fast mitosis. This data suggest that microRNA-17 can be a negative regulator of STAT3 and therefore facilitate an anti-tumor response through a complex interconnected network.6 Despite recent advances in immunotherapy for cancer, this modality still suffers from a number of limitations. As discussed, failure to initiate a proper anti-tumor immune response by host cells and the development of immune tolerance by cancer cells are 2 key factors in limiting the success of cancer immunotherapy. However, targeting STAT3 by inhibitors has emerged as a potential strategy in providing immunotherapy for cancer.7 Conversely, our study revealed that over expressing miR-17 can be used as an alternative to direct STAT3 inhibition to potentially restore T-cell activation in the microenvironment. Ultimately, this may induce a tumor specific immune response, allowing the hosts own immune system to battle cancer cells more effectively, thus, impacting the outcome of the therapy positively.