Signaling to the MITF transcription factor in melanoma

Melanoma, also known as black skin cancer, is a malignant tumor that emanates from pigment-producing cells of the skin, which are termed melanocytes. This aggressive type of cancer typically spreads to other regions of the body via the lymphatic system or the blood stream. It is therefore the most common fatal skin disease with a rapidly increasing number of new cases worldwide. Melanoma is caused by environmental factors such as UV radiation as well as by genetic factors. About 50% of all melanomas harbor changes (mutations) in the so-called BRAF gene. BRAF is involved in the transduction of signals within the cell, creating a response which alters the cells metabolism, shape or ability to divide. Mutations in BRAF cause this gene to make an altered protein that signals the cancer cells to grow and divide quickly. Thus, BRAF has become a major therapeutic target in the treatment of malignant melanoma and drugs blocking the activity of this protein are now available. Unfortunately, many patients develop a resistance to BRAF inhibitors, rendering the clinical treatment ineffective. For this reason, it is of great importance to find additional therapeutic targets for an efficient and reliable treatment of this deadly disease. One case in point is Microphthalmia-associated transcription factor (MITF). This transcription factor regulates genes that play an important role in the development, pigmentation and function of melanocytes. MITF is therefore often referred to as a master regulator of these cells. Although MITF is activated in about 15% of metastatic melanomas, its exact role in this disease remains unclear. Studies have shown that MITF cooperates with BRAF to promote disease progression and that this particular transcription factor is involved in the development of resistance to BRAF inhibitors. In order to gain a better understanding of MITF in melanoma, this research project entitled "signaling to the MITF transcription factor in melanoma" aims to clarify which signals regulate and control MITF. To answer these questions, a variety of state-of-the-art molecular biology techniques such as Western blot analysis, siRNA arrays, as well as invasion and pigmentation assays will be performed. In sum, the findings of this study could ultimately lead to the development of new therapeutic strategies for the treatment of metastatic melanoma.

Melanoma is a malignant tumor that arises from pigment-producing cells of the skin (melanocytes). This aggressive type of cancer typically spreads to other regions of the body and causes the majority of deaths related to skin cancer. About 50% of all melanomas harbor changes (mutations) in the so-called BRAF gene. Mutations in BRAF cause this gene to make an altered protein that signals the cancer cells to grow and divide rapidly. Thus, mutant BRAF has become a major therapeutic target in the treatment of malignant melanoma and drugs blocking the activity of this protein are now routinely used. Unfortunately, many patients develop resistance to BRAF inhibitors, rendering the clinical treatment ineffective. Therefore, it is of great importance to find additional therapeutic targets for an efficient and reliable treatment of this deadly disease. One case in point is Microphthalmia-associated transcription factor (MITF), which regulates genes that are essential for the development, pigmentation and function of melanocytes. Studies have shown that MITF cooperates with BRAF to promote disease progression and that this particular transcription factor is involved in the development of resistance to BRAF inhibitors. To gain a better understanding of MITF in melanoma, this research project entitled "signaling to the MITF transcription factor in melanoma" aimed to clarify which signals control MITF and how the BRAF pathway is involved. To this end, various molecular biology techniques were performed to characterize the subcellular localization, stability and activity (i.e. phosphorylation status) of MITF in BRAF-mutant melanoma. Our study revealed that MITF contains three nuclear localization signals, which target the protein to the nucleus independent of BRAF signaling. In addition, we found that - if located in the cytoplasm of melanoma cells - MITF plays an unexpected inhibitory role in the cellular degradation and recycling process, also known as autophagy. Furthermore, we demonstrated that association of two single MITF proteins (dimerization) is crucial for maintaining the stability of MITF in BRAF-mutant melanoma cells. Finally, we identified two amino acid residues in MITF that were phosphorylated in BRAF-mutant melanoma cells even in the presence of a BRAF inhibitor. Gene expression analysis indicated that these phosphorylation events in MITF may be involved in the regulation of genes associated with cell proliferation and cell cycle control. Indeed, melanoma cells expressing phospho-deficient MITF proteins showed an increased proliferative potential when compared to cells expressing the wild type protein. Interestingly, in the presence of wild type MITF, melanoma cells became resistant to BRAF inhibition, whereas cells expressing a phospho-deficient MITF mutant remained sensitive to the inhibitor. Together, our findings have contributed to a better understanding of MITF in melanoma and provided novel insights into possible means of targeting MITF localization, stability and activity for therapeutic intervention.