Investigating the Roles of Fucosylation and Calcium Signaling in Melanoma Invasion

Melanoma is the deadliest form of skin cancer. Prognosis for early stage melanoma patients is excellent, and surgery is often curative for these patients. However, once patients have presented with invasive disease, the average 5-year survival rate drops significantly from over 90% to between 10 and 15%. Several therapies have been developed to target a commonly mutated oncogene BRAF, or its downstream effectors. Unfortunately, while these treatments show robust initial response, most patients relapse within a year. Moreover, therapy-resistant tumors are often more invasive and metastatic. Therefore, it is important to investigate the molecular mechanisms underlying melanoma invasion and metastasis, and to prevent melanoma cell dissemination and metastatic progression. Invadopodia are proteolytic membrane protrusions used by metastatic cancer cells to degrade the extracellular matrix and to facilitate cancer cell invasion and metastasis. In my thesis research I have focused on protein fucosylation and store-operated calcium entry, two separate mechanisms involved in invadopodial regulation. Post translational modifications of proteins are essential for their structure and function. Many cell surface proteins require modifications such as glycosylation for protein-protein interactions, cell adhesion, and signal transduction. Fucosylation is a form of glycosylation that adds L-fucose on glycan structures of proteins. There is evidence indicating that fucosylation plays an important but cancer-type and branching dependent role in cancer progression. Emerging evidence indicates that the fucose salvage pathway and protein fucosylation are altered during melanoma progression and metastasis. Here, we report that the fucose salvage pathway inhibits invadopodia formation and extracellular matrix degradation by promoting α(1,2) fucosylation of cell surface proteins. The activation of the fucose salvage pathway decreases invadopodia numbers and inhibits the proteolytic activity of invadopodia in WM793 melanoma cells. Inhibiting fucokinase, one of the critical enzymes in the fucose salvage pathway, in melanoma cells abrogates L-fucose-mediated inhibition of invadopodia, suggesting dependence on the fucose salvage pathway. The inhibition of invadopodia formation by L-Fucose treatment or fucokinase overexpression could be rescued by treatment with α(1,2), but not α(1,3/4) fucosidase, implicating an α(1,2) fucose linkage-dependent inhibitory effect. The ectopic expression of FUT1, an α(1,2) fucosyltransferase, is sufficient to inhibit invadopodia formation and ECM degradation. Our findings indicate that the fucose salvage pathway can inhibit invadopodia formation, and consequently, invasiveness in melanoma via α(1,2) fucosylation. Re-activation of this pathway in melanoma could be useful for preventing melanoma invasion and metastasis. Calcium is a critical second messenger involved in a multitude of biological processes from cell proliferation to muscle contraction. In melanoma, previous studies have found that activation of the store operated calcium entry (SOCE) channel promotes tumor invasion and metastasis, in vitro and in xenograft models. The expression levels of STIM1, an essential component of the store operated calcium channels, has been found to increase with later stages of melanoma. In melanoma cell lines, the over expression of STIM1 enhances invadopodia number whereas STIM1 knockdown inhibits invadopodia formation. Similarly, gelatin degradation activity is enhanced with STIM1 overexpression and abrogated with STIM1 knockdown, implicating STIM1 as an important factor in the regulation of invadopodia formation and melanoma invasion. Though the studies published have shown a significant role of STIM1 in tumor progression, a robust transgenic animal model has not yet been established. Here, we developed a novel transgenic mouse model which, upon 4-hydroxytamoxifen (4OHT) treatment, induces the BRAFV600E mutation and PTEN, STIM1, and STIM2 deletions in melanocytes via an inducible Cre-lox system. Our investigation found that the loss of STIM1 exacerbates tumor growth and results in tumor formation significantly more quickly than STIM1 wild type mice. Whereas PCR analysis of 4OHT-treated skin showed deletion of STIM1 and PTEN, immunohistochemical staining of these genes in tumors did not convincingly demonstrate complete deletion. Therefore, it remains to be determined whether the effects we observed are due to STIM1 and STIM2 loss. These findings need to be corroborated in the future. Our studies focus on two important mechanisms required for melanoma progression and metastasis. We found that α(1,2) fucosylation is able to inhibit invadopodia formation, and melanoma cell invasion. The reestablishment of α(1,2) fucosylation in melanoma could potentially be exploited to inhibit melanoma metastasis. Additionally, early evidence points to STIM1 having a tumor suppressive role in melanoma oncogenesis and tumor growth based on the transgenic mouse model. Although the phenotype is unexpected, further investigation of this model will likely provide important insight for the complicate roles of SOCE in melanoma initiation and progression