Ca2+ signaling in BRAF mutant tumors: therapeutic resistance and drug combinations

Therapeutic targeting of mutant BRAF fundamentally changed melanoma therapy. Nevertheless, BRAF mutant melanomas often show limited response due to intrinsic resistance and initially responding patients frequently relapse due to acquired resistance. While a number of resistance mechanisms have been identified, the role of Ca2+ signaling a critical regulator of migration and thus tumor progression - as well as BRAF inhibition in other BRAF mutant malignancies is rather unexplored. Our preliminary data suggest that BRAF mutant melanomas demonstrate a unique plasma membrane Ca2+ ATPase (PMCA) expression and that vemurafenib alters this pattern and hence, produces distinct alterations of Ca2+ signals. Accordingly, we plan to determine the PMCA expression pattern in colon cancer as well as melanoma cells and tissues with known BRAF status. We will investigate Ca2+ signaling, migration and proliferation of BRAF mutant colon cancer and melanoma cells. Combination treatments affecting Ca2+ levels aiming at both the Ca2+ entry and extrusion pathways together with BRAF inhibition will be studied on cellular and tumor growth of BRAF mutant melanoma and colorectal cancer. We will analyze the pathological consequences of changes in the PMCA expression and activity in these tumor types. The comprehensive analysis of Ca2+ signaling components in resistant cells and tumor tissues may identify biomarkers that could predict response or indicate the appearance of therapeutic resistance. The proposed international collaboration will combine expertise from the field of cell biology, pathology and preclinical cancer research that may lead to new combination therapies in the future.

Calcium is not only a major building block of our bones but also an important messenger involved in regulating the behavior of all cells of our body. The concentration of calcium in our cells therefore needs to be tightly controlled. Deregulation of calcium levels leads to severe diseases including cancer. Members of the PMCA family of proteins (plasma membrane calcium ATPases) are located in the cell membrane and pump calcium out of the cell, thereby fulfilling a critical role in the regulation of calcium levels in our body. In this project an international team of researchers from the Semmelweiss University in Budapest and the Medical University of Vienna has investigated the role of PMCA proteins in skin cancer (melanoma) and colorectal cancer. A major finding was that drug-induced blockade of growth signals in cancer cells leads to enhanced production of a specific PMCA protein called PMCA4b. This led to a more efficient removal of calcium form the cells and was accompanied by reduced cell migration. Migration of cancer cells is a prerequisite for metastasis and indeed subsequent experiments demonstrated that bringing MCA4b into skin cancer cells by artificial means led to reduced formation of metastases by these cells. Thus the project on the one hand led to new insights about the regulation of an important mediator of cellular calcium levels and on the other hand identified a new mechanism for inhibiting metastasis. Ways to use these findings in clinical settings should be addressed in follow-up studies.