The Role of Extracellular Proteases in Tumor Progression and the Development of Innovative Metal Ion Chelators that Inhibit their Activity

Abstract

The crucial role of extracellular proteases in cancer progression is well-known, especially in relation to the promotion of cell invasion through extracellular matrix remodeling. This also occurs by the ability of extracellular proteases to induce the shedding of transmembrane proteins at the plasma membrane surface or within extracellular vesicles. This process results in the regulation of key signaling pathways by the modulation of kinases, e.g., the epidermal growth factor receptor (EGFR). Considering their regulatory roles in cancer, therapeutics targeting various extracellular proteases have been discovered. These include the metal-binding agents di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT) and di-2-pyridylketone-4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), which increase c-MET degradation by multiple mechanisms. Both the direct and indirect inhibition of protease expression and activity can be achieved through metal ion depletion. Considering direct mechanisms, chelators can bind zinc(II) that plays a catalytic role in enzyme activity. In terms of indirect mechanisms, Dp44mT and DpC potently suppress the expression of the kallikrein-related peptidase-a prostate-specific antigen-in prostate cancer cells. The mechanism of this activity involves promotion of the degradation of the androgen receptor. Additional suppressive mechanisms of Dp44mT and DpC on matrix metalloproteases (MMPs) relate to their ability to up-regulate the metastasis suppressors N-myc downstream regulated gene-1 (NDRG1) and NDRG2, which down-regulate MMPs that are crucial for cancer cell invasion.

Keywords: Keywords: cancer therapeutics; matrix metalloproteases; prostate specific antigen; thiosemicarbazones.

Figure 1

Interplay between EMMPRIN and MMP leads to metastasis. Soluble EMMPRIN (sEMMPRIN) and membrane-bound EMMPRIN form a bidirectional feedback between tumor cells and stromal cells. (A) Plasma membrane bound-EMMPRIN of the tumor cells interacts with cancer-associated stromal cells, such as fibroblasts. (B) This interaction between the cell-types induces up-regulation of MMP expression and secretion by the fibroblasts. (C) The MMPs secreted by fibroblasts cleave membrane-bound EMMPRIN to generate sEMMPRIN. (D) sEMMPRIN further augments MMP and EMMPRIN expression either in fibroblasts in the local tumor microenvironment or at distal sites. (E) This process then stimulates cancer cell invasion and metastasis. Modified from [56].

Figure 2

Line drawings of the structures of: (A) batimastat (BB-94) and (B) marimastat (BB-2516). These compounds are synthetic inhibitors of MMPs that act to potently inhibit these enzymes via their hydroxamate groups, which act to chelate zinc(II) that is required for MMP catalytic activity [106].

Figure 3

Line drawing of the structure of oxaprozin (4,5-diphenyl-2-oxazolepropionic acid). This agent is also a chelator and a non-steroidal, analgesic, and antipyretic propionic acid derivative that inhibits MMP9 activity by binding zinc(II) [125].

Figure 4

Line drawings of the structures of thiosemicarbazones: (A) di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT) and (B) di-2-pyridylketone-4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC). Both agents exert potent and selective anti-cancer activity in a wide variety of cancer models in vitro and in vivo [138,139,140,141,142] via a variety of molecular mechanisms [148,149,150,151,152,153,154,155].

Figure 5

The thiosemicarbazone Dp44mT down-regulates c-MET expression in cancer cells. (A) Under control conditions, the c-MET protein undergoes proteolytic cleavage by membrane-bound proteases, such as ADAM-10 and/or -17. This process generates the c-MET N-terminal fragment (NTF) that is liberated from the cell and the c-MET C-terminal fragment (CTF). The c-MET CTF is then further cleaved by γ-secretase to produce a smaller fragment—the c-MET intracellular domain (ICD)—which is then readily degraded by the proteasome. The cleavage of c-MET could also occur intracellularly, and the c-MET protein is also internalized and degraded by lysosomal activity after the incubation of cells with Dp44mT. Dp44mT enhances (B) metalloproteinase-mediated intracellular shedding of the c-MET protein and (C) the lysosomal degradation of c-MET. Taken from [152].