Metastasis is one of the main causes of death in cancer patients. Metastatic melanoma is a death sentence, as chemotherapeutic agents have a 5% success rate or do not extend survival beyond 10 months. The lack of effective chemotherapeutic agents for treating metastatic melanoma indicates a dire need to identify new drug targets and develop new therapies. Our lab has previously shown that the kinase activity of Abelson family of non-receptor tyrosine kinases (c-Abl and Arg) is elevated in invasive breast cancer cell lines as compared to non-invasive cell lines. Previous studies from our lab have shown that Abl kinases are convergent point of ErbB2 and Src Kinases in melanoma cells and Abl kinases promote invasion by an undefined mechanism. Although Abl kinases promote invasion, it is not known whether they are important for metastastic potential. For the first time, we report that Abl kinases promote melanoma cell proliferation, survival, matrigel-invasion and single-cell 3D invasion. To investigate the mechanism by which Abl kinases promote invasion, we found out that active c-Abl transcriptionally upregulates MMP-1, and using rescue approaches we show that c-Abl promotes invasion via a STAT3àMMP-1 pathway. In contrast, active Arg drives invasion in a STAT3-independent manner, and upregulates the expression of MMP-3 and MT1-MMP, in addition to MMP-1. We also found that Abl kinases promote invasion via lysosomal degradation of a metastasis suppressor, NM23-H1 by activating lysosomal cathepsins B and L, which directly cleave and degrade NM23-H1. Furthermore, c-Abl and Arg are activated in primary melanomas and cAbl/Arg activity is inversely correlated with NM23-H1 expression both in primary melanoma and human melanoma cells. We also demonstrate, for the first time that active Abl kinases promote metastasis in vivo, as inhibition of c-Abl/Arg with nilotinib, dramatically inhibits lung colonization/metastasis in a mouse model using two different melanoma cell lines. In summary, we identify Abl kinases as critical, novel, drug targets in metastatic melanoma, and our data indicate that nilotinib may be useful in preventing metastasis in a select group of patients, harboring active Abl kinases.

Cancer patient mortality is caused by the ability of tumor cells to invade the extracellular matrix and metastasize. Our lab was the first to identify the role of Abl family of non-receptor tyrosine kinases (c-Abl and Arg) in the progression of solid tumor cancers. In our previous studies, we showed that high c-Abl/Arg activity promotes proliferation, invasion, and metastasis in melanoma and breast cancer cells lines. Here, we demonstrate that our previous findings are clinically relevant by showing increased c-Abl/Arg kinase activity in primary melanoma tumor tissue in comparison to low activity as compared to benign nevi. Additionally, in breast cancer tissue, we found aggressive tumor subtypes (triple-negative and high-grade breast cancer) had increased c-Abl/Arg activity as compared to less aggressive subtypes. To define the mechanism by which c-Abl and Arg promote melanoma and breast cancer metastasis, we searched for novel pathways by which c-Abl and Arg promote invasion, a key step in metastasis. Significantly, we found that c-Abl and Arg decrease the expression of non-metastatic protein, NM23-H1, a metastasis suppressor that is lost during metastatic progression. We demonstrate that NM23-H1 is localized and degraded within the lysosome via proteases, cathepsins L and B. Moreover, we show that c-Abl and Arg upregulate cathepsin mRNA levels and activate the cathepsins, which in-turn degrade NM23-H1. We demonstrate that this pathway is functionally significant as c-Abl and Arg require the downregulation of NM23-H1 to promote invasion in melanoma and breast cancer cell lines. We show that the pathway is clinically significant as c-Abl/Arg activity is inversely correlated with NM23-H1 expression in mouse lung metastases, as well as in human primary melanoma and primary breast cancer tissue. In summary, we are the first to demonstrate novel crosstalk between oncogenic and metastasis suppressor signaling pathways, and provide evidence that pharmacological inhibition of Abl family kinases in melanoma and breast cancer patients may prevent metastatic progression by stabilizing a metastasis suppressor.