We wish to thank Polaris Inc. likely lead to autophagy, a possible mechanism to survive by recycling intracellular arginine. However apoptosis does occur which can be both caspase-dependent or independent. In order to increase the therapeutic efficacy of this form of treatment, one should Gata2 consider adding other agent(s) which can drive the cells toward apoptosis or inhibit the autophagic process. [3C5]. In the same year Miyazaki reported on the potent growth inhibition of human tumor cells in culture by arginine deiminase purified from culture medium infected with mycoplasma [6]. Later, Takaku described the antitumor activity of arginine deiminase [7, 8] in melanoma cell lines. Why Some Tumor Cells are Sensitive to Arginine Deprivation by Arginine Deiminase? The amino acid arginine is involved in several important Lavendustin A cellular functions. These include polyamine synthesis, creatine production and nitric oxide production [9C11]. Arginine is the only endogenous source of nitric oxide in humans [9]. In adult humans, arginine is considered a nonessential amino acid since it can be synthesized from citrulline. However, endogenous production of arginine may be insufficient under certain circumstances such as cell proliferation or wound healing. In tumor cells with high proliferation rate, exogenous arginine is required for their growth. Subsequently, it was discovered that melanoma cells are more sensitive to arginine deprivation. Arginine is synthesized from citrulline via the urea cycle (see Fig. 1). There are two key enzymes involved: (i) argininosuccinate synthetase (ASS) which converts L-citrulline and aspartic acid to argininosuccinate and (ii) argininosuccinate lyase (ASL) which then converts argininosuccinate to L-arginine and fumaric acid. L-arginine can also be degraded by the urea cycle enzyme arginase to L-ornithine. L-ornithine is converted back to L-citrulline by ornithine carbamoyltransferase (OCT) and then is recycled back to arginine. Wheatley has suggested that ASS and ASL are tightly coupled [12C14]. The sensitivity of tumor cells to arginine deprivation may depend on their ability to synthesize arginine from alternative intermediates in the urea cycle such as citrulline. Open in a separate window Fig. 1 Enzymes in urea cycle. The dotted line represents the conversion of arginine to citrulline by ADI, a mycoplasma enzyme. We and others have shown that melanoma cells do not express argininosuccinate synthetase Lavendustin A (ASS) and hence are very sensitive to arginine deprivation. Treatment with arginine deiminase inhibits growth and induces cell death in melanoma cells [15C17]. In tumor or normal cells Lavendustin A which possess ASS, treatment with ADI results in no growth inhibitory effects. In fact, transfection of ASS mRNA in melanoma cells results in resistance to ADI treatment [16, 17]. This concept has been exploited to treat tumor cells which lack ASS. Which Arginine Degrading Enzyme Should be Chosen to Degrade Arginine? There are three enzymes which can catabolize arginine: arginase, arginine decarboxylase and arginine deiminase (ADI). Arginase has a low affinity for arginine, and larger amounts of the enzyme may be required to produce a response. Furthermore, for arginase the optimal pH is 9.5 which is higher than the physiological Lavendustin A value. On the other hand, Wheatley has shown that arginase may be effective at lower pH of 7.2 to 9 [18]. It is important to note that arginase catabolizes arginine to ornithine. The liver and small bowel are known to be able to convert ornithine to citrulline due to the presence of OCT. It is not clear whether other normal tissues are able to do this since OCT gene is hypermethylated and not expressed in other normal tissues. Thus, there is the potential for normal tissue toxicity from the use of arginase. Despite these potential problems, pegylated arginase has been shown to have both and activity in hepatocellular carcinoma [19, 20]. Arginine decarboxylase converts arginine to agmatine, which cannot be converted back to arginine and is relatively toxic to normal cells. Therefore, arginine decarboxylase is less favorable. Arginine deiminase degrades arginine to citrulline and ammonia. Tumor cells which lack ASS expression will not be able to synthesize arginine from citrulline while normal cells are able to do so. Thus, normal tissue toxicity can be.
