The incidence of
melanoma is increasing, particularly in young women, and the disease remains incurable for many because of its aggressive, metastatic nature and its high rate of resistance to conventional, targeted, and immunological agents.
Cathepsins are
proteases that are critical for
melanoma progression and therapeutic resistance. Intracellular
cathepsins cleave or degrade
proteins that restrict
cancer progression, whereas extracellular
cathepsins directly cleave the extracellular matrix and activate proinvasive
proteases in the tumor microenvironment.
Cathepsin secretion is markedly increased in
cancer cells. We investigated the signaling pathways leading to increased
cathepsin secretion in
melanoma cells. We found that the nonreceptor
tyrosine kinases Abl and Arg (Abl/Arg) promoted the secretion of
cathepsin B and
cathepsin L by
activating transcription factors (namely, Ets1, Sp1, and NF-κB/p65) that have key roles in the epithelial-mesenchymal transition (EMT), invasion, and therapeutic resistance. In some
melanoma cell lines, Abl/Arg promoted the Ets1/p65-induced secretion of
cathepsin B and
cathepsin L in a
kinase-independent manner, whereas in other
melanoma lines, Abl/Arg promoted the
kinase-dependent, Sp1/Ets1/p65-mediated induction of
cathepsin L secretion and the Sp1/p65-mediated induction of
cathepsin B secretion. As an indication of clinical relevance, the abundance of mRNAs encoding Abl/Arg, Sp1, Ets1, and
cathepsins was positively correlated in primary
melanomas, and Abl/Arg-driven invasion in culture and
metastasis in vivo required
cathepsin secretion. These data suggest that drugs targeting Abl
kinases, many of which are FDA-approved, might inhibit
cathepsin secretion in some
melanomas and potentially other aggressive
cancers harboring activated Abl
kinases.