Date of Graduation
Masters of Science (MS)
The mitogen-activated protein kinase (MAPK) pathway is a cascade of serine-threonine kinases involved in cell growth, proliferation, and apoptosis; it is one of the most well-known pathways associated with melanoma progression. The MAPK pathway is constitutively activated in melanomas due to mutations in the signaling components, particularly the proto-oncogene BRAF V600E that accounts for (40-50%) of these cases. Metastatic melanoma is one of the deadliest and most aggressive forms of cancer, with a 3-year survival rate of less than 15%. Immunotherapies that utilize cytotoxic T lymphocytes (CTLs) have proven to be very effective at inducing regressions of large, bulky tumors, and in improving melanoma patient survival. Two MAPK targeted inhibitors, vemurafenib and dabrafenib, have produced positive results in clinical trials thus far, but they are not without limitations. Recent studies have shown that oncogene activation in tumor cells can affect the level of expression of major histocompatibility complex I molecules (MHC-I) on the cell surface, and potentially allow melanoma cells to escape immune system surveillance. MHC-I molecules expressed by tumor cells are the crucial targets recognized by CTLs to kill tumor cells; thus, strategies to improve MHC-I antigen presentation to T-cells is very likely to improve the efficacy of current immunotherapies. In DCs and other hematopoietic cell types the rapid internalization and recycling of MHC-I through endocytic compartments has been characterized as a cytoplasmic tail dependent process. The MHC-I cytoplasmic tail possesses two conserved phosphorylation sites, a tyrosine (Y320) and serine (S335). It is known that MAPK pathway activation can induce a phosphorylation signaling cascade in tumor cells, but the mechanism behind the regulation of MHC-I internalization and modulation has yet to be identified in tumor cells.
In our study we show that both BRAF and MEK inhibitors increase MHC-I surface expression in BRAF V600E melanoma mutant cell lines. Additional studies have shown that the level of MHC-I surface expression directly affects CTL recognition and cytokine release. We also reveal a novel potential mechanism for MAPK pathway regulation of MHC-I through the highly conserved serine encoded by exon 7 of the MHC-I cytoplasmic tail. These studies suggest that the mechanism behind MAPK regulation of MHC-I is through serine phosphorylation and inhibition of this process allows for a longer surface half-life of MHC-I molecules leading to better CTL responses. Ultimately, we have shed light on MHC-I surface expression, trafficking, internalization and antigen presentation in melanoma. Knowledge gained through this study could aid in the development of cancer treatment strategies whereby MAPK pathway inhibition is used to augment the effectiveness of CTL-based immunotherapies. Furthermore, these types of therapeutic approaches may be generalizable to other tumor types that also demonstrate constitutive MAPK pathway activation.
MAPK pathway, Immunology, Melanoma, Oncogenes, BRAF