Cancer is a multistep process that requires cells to respond appropriately to the tumor microenvironment, both in early proliferative stages and in later invasive disease. Arl8b is a lysosome localized Arf-like GTPase that controls the spatial distribution of lysosomes via recruitment of kinesin motors. Common features of the tumor microenvironment such as acidic extracellular pH and various growthfactors stimulate lysosome trafficking to the cell periphery (anterograde), which is critical for tumor invasion by facilitating the release of lysosomal proteases to promote matrix remodeling. Herein we report for the first time that Arl8b regulates anterograde lysosome trafficking in response to hepatocyte growth factor, epidermal growth factor, and acidic extracellular pH. Depletion of Arl8b results in juxtanuclear lysosome aggregation, and this effect corresponds with both diminished invasive growth and proteolytic extracellular matrix degradation in a three-dimensional model of prostate cancer. Strikingly, we found that depletion of Arl8b abolishes the ability of prostate cancer cells to establish subcutaneous xenografts in mice. We present evidence that Arl8b facilitates lipid hydrolysis to maintain efficient metabolism for a proliferative capacity in low nutrient environments, suggesting a likely explanation for the complete inability of Arl8b-depleted tumor cells to grow in vivo. In conclusion, we have identified two mechanisms by which Arl8b regulates cancer progression: 1) through lysosome positioning and protease release leading to an invasive phenotype and 2) through control of lipid metabolism to support cellular proliferation. These novel roles highlight that Arl8b is a potential target for the development of novel anti-cancer therapeutics.

The presence of reactive stroma, predominantly composed of myofibroblasts, is directly associated with and drives prostate cancer progression. We have previously shown that (−)-Epigallocatechin-3-gallate (EGCG), in the form of Polyphenon E, significantly decreases serum levels of HGF and VEGF in prostate cancer patients. Given that HGF and VEGF are secreted from surrounding tumor myofibroblasts, these observations suggested that EGCG may inhibit prostate cancer-associated myofibroblast differentiation. Herein, we demonstrate that micromolar combinations of EGCG and a second polyphenol, luteolin, synergistically inhibit TGF-β-induced myofibroblast phenotypes in prostate fibroblast cell lines, as observed primarily by potentiation of fibronectin expression. Functionally, EGCG and luteolin inhibited TGF-β-induced extracellular matrix contraction, an enhancer of tumor cell invasion. EGCG and luteolin inhibited downstream TGF-β-induced signaling, including activation of ERK and AKT, respectively, but mechanistically, only ERK appeared to be necessary for TGF-β-induced fibronectin expression. Furthermore, neither EGCG nor luteolin affected Smad signaling or nuclear translocation. Rho signaling was found to be necessary for TGF-β-induced fibronectin expression and EGCG and luteolin each reduced RhoA activation. Finally, EGCG and luteolin were shown to reverse TGF-β-induced fibronectin expression, implicating that these natural compounds may be useful not only in preventing but also in treating already activated myofibroblasts and the diseases they cause, including cancer. The ability of EGCG and luteolin to synergistically target myofibroblasts suggests that combined clinical use of these compounds could prevent or reverse cancer progression through targeting the tumor microenvironment, in addition to the tumor itself.

c-Met is a receptor tyrosine kinase whose activity can promote both mitogenic and motogenic phenotypes involved in tissue development and cancer progression. Herein, we report the first evidence that c-Met is palmitoylated and that palmitoylation facilitates its trafficking and stability. Inhibition of palmitoylation reduced the expression of c-Met in multiple cancer cell lines post-transcriptionally. Using surface biotinylation, confocal microscopy, and metabolic labeling we determined that inhibition of palmitoylation reduces the stability of newly synthesized c-Met and causes accumulation at the Golgi. Acyl-biotin exchange and click chemistry-based palmitate labeling indicated the c-Met β-chain is palmitoylated, and site-directed mutagenesis revealed two likely cysteine palmitoylation sites. Moreover, by monitoring palmitoylation kinetics during the biosynthesis and trafficking of c-Met, we revealed that stable palmitoylation occurs in the endoplasmic reticulum prior to cleavage of the 170 kDa c-Met precursor to the mature 140 kDa form. Our data suggest palmitoylation is required for egress from the Golgi for transport to the plasma membrane. These findings introduce palmitoylation as a critical modification of c-Met, providing a novel therapeutic target for c-Met-driven cancers.

Tumor progression to metastatic disease contributes to the vast majority of incurable cancer. Understanding the processes leading to advanced stage cancer is important for the development of future therapeutic strategies. Here, we establish a connection between tumor cell migration, a prerequisite to metastasis, and monocarboxylate transporter 1 (MCT1). MCT1 transporter activity is known to regulate aspects of tumor progression and, as such, is a clinically relevant target for treating cancer. Knockdown of MCT1 expression caused decreased hepatocyte growth factor (HGF)-induced as well as epidermal growth factor (EGF)-induced tumor cell scattering and wound healing. Western blot analysis suggested that MCT1 knockdown (KD) hinders signaling through the HGF receptor (c-Met) but not the EGF receptor. Exogenous, membrane-permeable MCT1 substrates were not able to rescue motility in MCT1 KD cells, nor was pharmacologic inhibition of MCT1 able to recapitulate decreased cell motility as seen with MCT1 KD cells, indicating transporter activity of MCT1 was dispensable for EGF- and HGF-induced motility. These results indicate MCT1 expression, independent of transporter activity, is required for growth factor-induced tumor cell motility. The findings presented herein suggest a novel function for MCT1 in tumor progression independent of its role as a monocarboxylate transporter.

The tumor microenvironment, primarily composed of myofibroblasts, directly influences the progression of solid tumors. Through secretion of growth factors, extracellular matrix deposition, and contractile mechanotransduction, myofibroblasts, or cancer-associated fibroblasts (CAFs), support angiogenesis and cancer cell invasion and metastasis. The differentiation of fibroblasts to CAFs is primarily induced by TGF-β from cancer cells. To discover agents capable of blocking CAF differentiation, we developed a high content immunofluorescence-based assay to screen repurposed chemical libraries utilizing fibronectin expression as an initial CAF marker. Screening of the Prestwick chemical library and NIH Clinical Collection repurposed drug library, totaling over 1700 compounds, identified cardiac glycosides as particularly potent CAF blocking agents. Cardiac glycosides are traditionally used to regulate intracellular calcium by inhibiting the Na+/K+ ATPase to control cardiac contractility. Herein, we report that multiple cardiac glycoside compounds, including digoxin, are able to inhibit TGF-β-induced fibronectin expression at low nanomolar concentrations without undesirable cell toxicity. We found this inhibition to hold true for multiple fibroblast cell lines. Using real-time qPCR, we determined that digoxin prevented induction of multiple CAF markers. Furthermore, we report that digoxin is able to prevent TGF-β-induced fibroblast contraction of extracellular matrix, a major phenotypic consequence of CAF differentiation. Assessing the mechanism of inhibition, we found digoxin reduced SMAD promoter activity downstream of TGF-β, and we provide data that the effect is through inhibition of its known target, the Na+/K+ ATPase. These findings support a critical role for calcium signaling during CAF differentiation and highlight a novel, repurposable modality for cancer therapy.

The hepatocyte growth factor receptor (HGFR or c-Met) is a driver of multiple cancer subtypes. While there are several c-Met inhibitors in development, few have been approved for clinical use, warranting the need for continued research and development of c-Met targeting therapeutic modalities. The research presented here demonstrates a particular class of compounds known as isothiocyanatostilbenes can act as c-Met inhibitors in multiple cancer cell lines. Specifically, we found that 4,4′-Diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) and 4,4′-Diisothiocyanatodihydrostilbene-2,2′-disulfonic acid (H2DIDS) had c-Met inhibitory effective doses in the low micromolar range while 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonic acid (SITS) and 4,4′-dinitrostilbene-2, 2′-disulfonic acid (DNDS) exhibited IC50s 100 to 1000 fold higher. These compounds displayed much greater selectivity for inhibiting c-Met activation compared to similar receptor tyrosine kinases. In addition, DIDS and H2DIDS reduced hepatocyte growth factor (HGF)-induced, but not epidermal growth factor (EGF)-induced, cell scattering, wound healing, and 3-dimensional (3D) proliferation of tumor cell spheroids. In-cell and cell-free assays suggested that DIDS and H2DIDS can inhibit and reverse c-Met phosphorylation, similar to SU11274. Additional data demonstrated that DIDS is tolerable in vivo. These data provide preliminary support for future studies examining DIDS, H2DIDS, and derivatives as potential c-Met therapeutics.

Mitochondrial respiration is the most efficient form of energy production in the cell; however, in the absence of oxygen, such as within a solid tumor mass, cells rely on glycolysis as the main source of energy. Under hypoxic conditions the end product of glycolysis, pyruvate, is reduced to lactate by lactate dehydrogenase (LDH), which is then exported out of the cell. This reduction is essential to regenerate NAD+ consumed by the earlier steps of glycolysis. Recent evidence suggests that hepatocyte growth factor (HGF), the only known ligand for the c-Met receptor, causes an increase in glycolysis. This led our laboratory to hypothesize that glycolysis may be required for HGF-induced phenotypic changes in cancer cells. HGF signaling through the c-Met receptor induces an epithelial-mesenchymal transition (EMT), including loss of cell-cell adhesions through down-regulation of E-cadherin expression, as well as increased motility and cell scattering characteristic of the initiating steps leading to cancer cell invasion and metastasis. Using DU145 prostate cancer cells, we have found that inhibitors of glycolysis (2-DG) and lactate production via LDH activity (oxamate), were able to inhibit HGF-induced cell scattering. Using a wound healing assay, we observed that oxamate was able to prevent HGF-induced cell migration, suggesting a role for glycolysis in cell motility. Additionally, lactic acid was able to rescue cell scattering and wound healing blocked by oxamate, but not by 2-DG. Lactic acid measurements showed that oxamate prevented an increase in glycolysis stimulated by HGF. Measurement of ATP levels also suggested that 2-DG was able to block cell motility due to a reduction in ATP, while oxamate appeared to work through a yet undetermined method. Western blot analysis indicated that oxamate caused a decrease in HGF-induced phospho-Met and downstream phospho-Akt levels. Using H1993 lung cancer cells with constitutively active c-Met, we were able to determine that oxamate deactivated c-Met, while not affecting total c-Met levels. The ability of oxamate to reduce phosphorylation levels appeared to be Met specific based on its inability to block phosphorylation of other activated receptors. Oxamate not only inhibits LDH, but also aspartate aminotransferase (AAT), an enzyme involved in the malate-aspartate shuttle. Our data suggest, however, that the ability of oxamate to inhibit HGF-induced signaling and cell scattering is not through its ability to inhibit AAT. Taken together, our data suggest the c-Met/HGF signaling axis is dependent on glycolysis and perhaps lactic acid levels. Our long-term goals are to elucidate the specific mechanism for this dependence as a means of defining novel targets for preventing c-Met-mediated cancer cell invasion and metastasis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2914. doi:10.1158/1538-7445.AM2011-2914

Tumor cells often display a glycolytic phenotype, even in the presence of oxygen; a phenomenon known as the Warburg effect. The ability of cancerous cells to perform aerobic glycolysis is unique when compared to non-cancerous cells, and therefore, presents a possible therapeutic target for elimination of cancer cells that rely on aerobic glycolysis. Activation of the receptor tyrosine kinase, c-Met, by hepatocyte growth factor (HGF) is a major contributing event in the progression of prostate cancer (PCa). Signaling through this receptor leads to increased cell motility, proliferation, invasion and metastasis. Studies have shown that targeting this signaling axis can greatly reduce the number of bone metastases that arise from primary PCa tumors. Oxamate, a lactate dehydrogenase (LDH) inhibitor, and 2-deoxyglucose (2-DG) are known glycolytic inhibitors. Our studies have found that oxamate reduced c-Met activation and HGF-induced cell motility, while 2-DG had no effect. In order to confirm that oxamate was targeting LDH-A or LDH-B, we created LDH-A, LDH-B, and LDH-A/B knockdown (KD) DU145 cells. Although all of the clones used in the experiments had greater than 70% KD, we were unable to recapitulate the effects seen with oxamate. We believe that this could be due to incomplete KD. In support of this, use of a specific LDH-A (πFly-21) inhibitor significantly reduced c-Met activation and HGF-induced cell motility. Together, these data support a role for LDH-A in facilitating HGF-induced c-Met activation. Because oxamate is a direct LDH inhibitor, we hypothesized that oxamate would have a greater reduction in lactate (LA) export from cells than 2-DG; however, both, oxamate and 2-DG reduced LA production by approximately 75% in DU145 prostate cancer cells. Therefore, the ability of oxamate to reduce c-Met activation is not apparently due to a decrease in LA export. However, it is possible that LA import via monocarboxylate transporter 1 (MCT1) activity may be important for c-Met activation and downstream signaling events. In support of this, KD of MCT1 resulted in reduced c-Met activation and blocked HGF-induced cell motility. These data indicate a potential connection between LDH activity/lactate import and the c-Met signaling axis, suggesting that inhibition of these processes may be a new mechanism for preventing c-Met driven metastatic events. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1251. doi:1538-7445.AM2012-1251