Targeting mitochondrial glutaminase activity inhibits oncogenic transformation.

Rho GTPases impact a number of activities important for oncogenesis. We describe a small molecule inhibitor that blocks oncogenic transformation induced by various Rho GTPases in fibroblasts, and the growth of human breast cancer and B lymphoma cells, without affecting normal cells. We identify the target of this inhibitor to be the metabolic enzyme glutaminase, which catalyzes the hydrolysis of glutamine to glutamate. We show that transformed fibroblasts and breast cancer cells exhibit elevated glutaminase activity that is dependent on Rho GTPases and NF-κB activity, and is blocked by the small molecule inhibitor. These findings highlight a previously unappreciated connection between Rho GTPase activation and cellular metabolism and demonstrate that targeting glutaminase activity can inhibit oncogenic transformation.

Inhibition of lactate dehydrogenase A induces oxidative stress and inhibits tumor progression.

As the result of genetic alterations and tumor hypoxia, many cancer cells avidly take up glucose and generate lactate through lactate dehydrogenase A (LDHA), which is encoded by a target gene of c-Myc and hypoxia-inducible factor (HIF-1). Previous studies with reduction of LDHA expression indicate that LDHA is involved in tumor initiation, but its role in tumor maintenance and progression has not been established. Furthermore, how reduction of LDHA expression by interference or antisense RNA inhibits tumorigenesis is not well understood. Here, we report that reduction of LDHA by siRNA or its inhibition by a small-molecule inhibitor (FX11 [3-dihydroxy-6-methyl-7-(phenylmethyl)-4-propylnaphthalene-1-carboxylic acid]) reduced ATP levels and induced significant oxidative stress and cell death that could be partially reversed by the antioxidant N-acetylcysteine. Furthermore, we document that FX11 inhibited the progression of sizable human lymphoma and pancreatic cancer xenografts. When used in combination with the NAD(+) synthesis inhibitor FK866, FX11 induced lymphoma regression. Hence, inhibition of LDHA with FX11 is an achievable and tolerable treatment for LDHA-dependent tumors. Our studies document a therapeutical approach to the Warburg effect and demonstrate that oxidative stress and metabolic phenotyping of cancers are critical aspects of cancer biology to consider for the therapeutical targeting of cancer energy metabolism.

HIF-dependent antitumorigenic effect of antioxidants in vivo.

The antitumorigenic activity of antioxidants has been presumed to arise from their ability to squelch DNA damage and genomic instability mediated by reactive oxygen species (ROS). Here, we report that antioxidants inhibited three tumorigenic models in vivo. Inhibition of a MYC-dependent human B lymphoma model was unassociated with genomic instability but was linked to diminished hypoxia-inducible factor (HIF)-1 levels in a prolyl hydroxylase 2 and von Hippel-Lindau protein-dependent manner. Ectopic expression of an oxygen-independent, stabilized HIF-1 mutant rescued lymphoma xenografts from inhibition by two antioxidants: N-acetylcysteine and vitamin C. These findings challenge the paradigm that antioxidants diminish tumorigenesis primarily through decreasing DNA damage and mutations and provide significant support for a key antitumorigenic effect of diminishing HIF levels.

Conditional deletion of c-myc does not impair liver regeneration.

The oncogene c-myc encodes a transcription factor that has long been considered essential to liver regeneration, the process by which fully differentiated hepatocytes proliferate in an attempt to maintain a normal functional mass in response to hepatic injury. Experimental liver regeneration can be induced upon 70% partial hepatectomy and is accompanied by an increase in c-myc expression accompanying the synchronous entry of remaining hepatocytes into the cell cycle. Because liver regeneration is an essential process for achieving liver homeostasis, therapies directed at reducing MYC expression in hepatocellular carcinoma are fraught with the theoretical possibility of injuring adjacent noncancerous liver cells, thereby restricting the liver's normal regenerative response to injury. To determine if intact c-myc is required for liver regeneration, we reduced hepatic c-myc in c-myc(fl/fl) mice using an adenoviral vector that expresses Cre recombinase. Despite a 90% decrease in hepatic expression of c-myc, restoration of liver mass 7 days later was not compromised. Reconstituted liver retained the same decrease in hepatic c-myc, indicating that hepatocytes deficient in c-myc were able to proliferate in response to partial hepatectomy. Although c-myc is required for embryonic development, our findings indicate that it is not required for the maintenance of the adult liver.

Quantitative assessment of hemoglobin-induced endothelial barrier dysfunction.

Hemoglobin (Hb)-based O2 carriers (HBOC) are undergoing extensive development as potential "blood substitutes." A major problem associated with these molecules is an increase in microvascular permeability and peripheral vascular resistance. In this paper, we utilized bovine lung microvascular endothelial cell monolayers and simultaneously measured Hb-induced changes in transendothelial electrical resistance, diffusive albumin permeability, and diffusive Hb permeability (PDH) for three forms of Hb: natural tetrameric human Hb-A and two polymerized recombinant HBOCs containing alpha-human and beta-bovine chains designated Hb-Polytaur (molecular mass: 500 kDa) and Hb-(Polytaur)n (molecular mass: approximately 1,000,000 Da), respectively. Hb-Polytaur and Hb-(Polytaur)n are being evaluated for clinical use as HBOCs. All three Hb molecules induce a rapid decline of transendothelial electrical resistance to 30% of baseline. Diffusive albumin permeabiltiy increases, on average, approximately ninefold (2.78 x 10(-7) vs. 2.47 x 10(-6) cm/s) in response to Hb exposure. All three Hb molecules induce an increase in their own permeability, a process that we have called Hb-induced Hb permeability. The magnitude of change of PDH is also related to Hb size. When PDH is corrected for the diffusive coefficient for each Hb species, no evidence of restricted diffusion is found. Immunofluorescent images demonstrate Hb-induced actin stress fiber formation and large intercellular gaps. These data provide the first quantitative assessment of the effect of polymerized HBOC on their own diffusion rates over time. We discuss the importance of these findings in terms of Hb extravasation rates, molecular sieving, and clinical consequences of HBOC use.

Lung endothelial heparan sulfates mediate cationic peptide-induced barrier dysfunction: a new role for the glycocalyx.

The endothelial glycocalyx is believed to play a major role in microvascular permeability. We tested the hypothesis that specific components of the glycocalyx, via cytoskeletal-mediated signaling, actively participate in barrier regulation. With the use of polymers of arginine and lysine as a model of neutrophil-derived inflammatory cationic proteins, we determined size- and dose-dependent responses of cultured bovine lung microvascular endothelial cell permeability as assessed by transendothelial electrical resistance (TER). Polymers of arginine and lysine >11 kDa produced maximal barrier dysfunction as demonstrated by a 70% decrease in TER. Monomers of l-arginine and l-lysine did not alter barrier function, suggesting a cross-linking requirement of cell surface "receptors". To test the hypothesis that glycosaminoglycans (GAGs) are candidate receptors for this response, we used highly selective enzymes to remove specific GAGs before polyarginine (PA) treatment and examined the effect on TER. Heparinase III attenuated PA-induced barrier dysfunction by 50%, whereas heparinase I had no effect. To link changes in barrier function with structural alterations, we examined actin organization and syndecan localization after PA. PA induced actin stress fiber formation and clustering of syndecan-1 and syndecan-4, which were significantly attenuated by heparinase III. PA-induced cytoskeletal rearrangement and barrier function did not involve myosin light chain kinase (MLCK) or p38 MAPK, as ML-7, a specific MLCK inhibitor, or SB-20358, a p38 MAPK inhibitor, did not alter PA-induced barrier dysfunction. In summary, lung endothelial cell heparan sulfate proteoglycans are key participants in inflammatory cationic peptide-induced signaling that links cytoskeletal reorganization with subsequent barrier dysfunction.