Abstract
The presence of macrophages within breast tumors correlates with metastatic potential. These tumor-associated macrophages often take on a pro-tumorigenic (M2-like) phenotype resulting in the secretion of growth factors and proteases, including the lysosomal protease cathepsin L. Since cathepsin L also is frequently secreted by breast cancer cells and contributes to tumor invasion, metastasis, and angiogenesis, we hypothesized that secretion of cathepsin L by both tumor-associated macrophages and neoplastic cells would facilitate the metastatic phenotype. Our results showed that the novel cathepsin L/K inhibitors KGP94 and KGP207 could inhibit in vitro M2 macrophage invasion and reduce the macrophage-stimulated invasion of 4T1 murine breast cancer cells. KGP94 and KGP207 treatment also reduced the expression of several M2-associated markers, suggesting that cathepsin L activity may be important for IL-4-driven M0 to M2 differentiation. In addition, cathepsin L shRNA knockdown studies revealed that cathepsin L from both the tumor cell and the macrophage population is important for tumor cell invasion. Thus our data suggest that tumor cells and macrophages may both contribute to the cathepsin L-driven metastatic phenotype of breast cancer. Taken together, these studies highlight the importance of cathepsin L in macrophage functions and suggest that cathepsin inhibition strategies may be therapeutically beneficial by impairing the progression of tumors with high infiltration of M2 macrophages.
Highlights
Breast cancer is the most frequently diagnosed cancer and is the second leading cause of cancer-associated deaths in American females [1]
Semi-quantitative PCR indicated that IL-4 treatment resulted in the upregulation of M2-associated transcripts, including MRC-1, IL-10, and Fizz1, suggesting that IL-4 is causing an M0 to M2 transition (Supplementary Figure 1)
Whole cell lysates were analyzed by immunoblot and revealed that cathepsin L protein levels were upregulated in response to IL-4 (Figure 1A and 1C; quantified in Supplementary Figure 2A, 2B)
Summary
Breast cancer is the most frequently diagnosed cancer and is the second leading cause of cancer-associated deaths in American females [1]. There are several families of proteases that are associated with cancer, including, but not limited to, matrix metalloproteases (MMPs) and cathepsins. Proteases facilitate many aspects of the metastatic cascade including 1) integrin turnover during motility and invasion, 2) degradation of the extracellular matrix and basement membrane, 3) matrix-sequestered growth factor release, and 4) angiogenesis initiation [2, 3]. Both the MMP and cathepsin families of proteases contain proteases with overlapping substrates and functions [4, 5]. Initial studies focused on MMPs, but the lack of specificity and resulting side effect profile of these MMP inhibitors contributed to their failure in clinical trials [6, 7]
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