Arginase added to culture medium reduced arginine to negligible levels within B6 h, and enzyme activity persisted relatively undiminished for at least 3 days. Human and bovine arginase proved equally effective. The response of normal cells was to enter G1 (G0) arrest, from which most of the cells could be recovered weeks later. In contrast, malignant cell lines treated with unpegylated or pegylated enzyme resulted in cell death on a massive scale within 3 -5 days, with a very low to negligible percentage of cells (o0.01%) being recoverable on restoration with arginine. Although pegylation resulted in a 40% drop in specific activity, arginase was considerably more stable and remained active for b8 days. Arginine decarboxylase caused malignant cell arrest at the same units per millilitre as arginase. Its breakdown product, agmatine, was relatively nontoxic in the presence of arginine, but exacerbated cell death above millimolar concentration in its absence. Although ornithine failed to rescue cells from deprivation, citrulline recovered cells in all cases, although less well in fast-growing tumour cell populations, whereas readdition of arginine failed to work unless a complete medium change was given (because of the persistence of the enzymes in the medium catabolising its destruction). The advantages and disadvantages of these two arginine-catabolising enzymes are discussed, and compared with arginine deiminase.
Arginine deprivation causes death of up to 80% of cancer cell lines in vitro, but in the body, citrulline would be available as a convertible source of this amino acid in vivo. Some tumour cell lines, notably the vast majority of melanomas and hepatocellular carcinomas, tend to be deficient in argininosuccinate synthetase (EC 6.5.4.3.), and therefore cannot recycle citrulline to arginine. Argininosuccinate synthetase is present at levels that convert enough citrulline to arginine to allow limited growth in about half of a modest range of malignant cell types analysed in this study. Attempts to rescue cells that are unable to utilise citrulline with the immediate downstream product, argininosuccinate, had very limited success in a few tumour cell lines. Particularly noteworthy is the demonstration that argininosuccinate was totally incapable of rescuing cells that utilise citrulline efficiently, consistent with tight channelling (coupling) of argininosuccinate synthetase and argininosuccinate lyase in the urea cycle. The findings suggest that an excellent opportunity exists for further exploitation of arginine deprivation in the selective killing of tumour cells. Arginine is a vital amino acid for many metabolic processes other than protein synthesis (e.g. creatine production, polyamine synthesis and nitric oxide (NO) generation). Its removal from culture medium by medium formulation or arginase treatment quickly leads to death in B80% of tumour cell lines (Scott et al, 2000). The addition of citrulline can often circumvent this deficiency, but few cells are capable of its biosynthesis in culture. Arginine deficiency in vivo has been induced by various means, but is less effectively achieved because body homeostasis is too robust, and citrulline generation is difficult to control. Having successfully reduced L1210 cells in culture to negligible viability in 3-5 days with arginase (EC 3.5.3.1), citrulline supplementation in the continued presence of the enzyme partially circumvented the arginine deficiency. The rate of conversion became a rate-limiting factor, most noticeably in such fast-growing leukaemia as L1210 (Philip et al, 2003). In a corresponding in vivo study, arginase treatment of DBA/2 mice injected intraperitoneally with 1210 cells increased neither their survival time nor decreased tumour burden, despite plasma arginine falling to B1-2 mM level. Plasma citrulline remained normal, and therefore the tumour cells must have converted citrulline to arginine fast enough to sustain relatively normal tumour growth rate (Wu and Morris, 1998;Philip et al, 2003). This makes it difficult to understand how previous in vivo work with arginase and arginine deiminase could have achieved significant inhibition of tumour growth under similar conditions (Bach and Swaine, 1965;Miyazaki et al, 1990), and begs further analysis of the ability of tumour cells to use citrulline in lieu of arginine (Wheatley and Campbell, 2002). Sugimura et al (1992) found that four out of five human melanoma cell lines deprived of arginine w...
Although it is self evident that cells will not grow in amino acid deficient medium, an observation less well appreciated is that malignant cells are particularly vulnerable to such deprivation, which can lead to their rapid demise. Indeed, the more flagrantly malignant the phenotype (anaplastic the tumor), the more susceptible the cells seem to be to deprivation. While some attempts to employ this strategy in cancer treatment have been made, the difference between normal and malignant cells should be more fully exploited as a means of selectively eliminating tumor cell populations. To be successful, information on differences between the normal and the deranged cell cycle engine and checkpoints, especially how these are affected by deprivation, is of crucial importance. Since it is only recently that the controls at restriction points have been elucidated, it is little surprise that earlier attempts to control tumor cell growth by limiting the availability of an essential amino acid have met with limited success. Studies have been sporadic and isolated, often with little more than anecdotal descriptions as far as clinical work was concerned. This review concentrates on what has been accomplished primarily in vitro and since about 1950 with regard to arginine catabolism, while recognising that other essential amino acids have also been the focus of attention by some investigators. Treatments have included medium and plasma manipulation, dietary control, enzymatic degradation, and the use of liver extracts. On some occasions, substitution of amino acid analogues has been explored. It is argued that current knowledge, combined with past experience, calls for a much closer examination of the full potential of amino acid (and specifically arginine) deprivation as a means of controlling tumor growth, with greater attention to protocols that might be used to treat human cancers.
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