Inducible protective proteins: a potentially novel approach to chemotherapy

Alkylation of Inorganic Oxo Compounds and Insights on Preventing DNA Damage

Metabolism of food- and tobacco-borne procarcinogens results in the exposure of DNA to toxic alkylating agents. These assaults can bring about DNA alkylation damage, mutations, and cancer. Dietary inorganic compounds such as selenium and vanadium are known to prevent cancer, possibly by reacting directly with alkylating agents, thereby preventing DNA damage. To understand potential interactions between oxo species and alkylating toxins, we reacted a series of alkylating agents with varied classes of oxo compounds (i.e., vanadates, selenate, phosphate, sulfate, acetate, nitrate, and nitrite). A new organic-soluble selenate, [(C6H5)4P]3(O3SeOCH2OSeO3)(HSeO4), was synthesized and characterized for these studies. Vanadates were found to convert ethylating agents into ethanol, whereas other anions formed esters upon alkylation. General trends show that oxo anions of the greatest charge density were the most reactive. These studies suggest that the design of new compounds for cancer prevention should incorporate reactive oxo groups with high anionic charge density.

Inhibition of DNA alkylation damage with inorganic salts

Human exposure to alkylating agents metabolized from tobacco- and food-borne carcinogens occurs regularly. Dietary inorganic compounds such as selenium and vanadium have been shown previously to provide chemoprotective benefits in rat and human trials. Here, we present biochemical data on the ability of inorganic compounds to protect DNA from alkylation damage. An enzyme cleavage assay is used to observe alkylated DNA. Simple salts (e.g., NaCl or NiCl(2)) did not prevent DNA alkylation, whereas anionic oxo species (e.g., Na(2)SeO(4) or Na(3)VO(4)) did inhibit alkylation. We propose that these oxo species behave as nucleophilic targets for the electrophilic alkylating agents, thereby preventing DNA damage.

Radioprotective effect of zinc aspartate on mouse spermatogenesis: a flow cytometric evaluation

The radioprotective effect of zinc aspartate on spermatogonial cells of whole-body irradiated mice was studied using flow cytometry. Adult male Swiss albino mice were treated with 30 mg/kg body weight of zinc aspartate 30 min before exposure to 0, 0.5, 1, 2 and 3 Gy of gamma-radiation. The animals were killed 7 to 70 days postirradiation and the relative percentages of different germ cells were analyzed by flow cytometry. A significant increase (p<0.002, 0. 0001, 0.005 and 0.008 for 0.5, 1, 2 and 3 Gy, respectively) in the relative percentage of spermatogonial (2C) population was observed in mice treated with zinc aspartate before exposure to different doses of gamma-radiation, compared to the irradiated controls on day 35 posttreatment. Also mean of each radiation dose of all the intervals studied showed a significant (p<0.03) increase in the relative percentage of spermatogonia. Despite the increase in the relative percentage of spermatogonia, the relative percentage of tetraploid cells (4C) remained higher in the zinc aspartate treated mice, compared to the irradiated controls. However, there was no change in the haploid populations viz. round (1C) and elongated (HC) spermatids of the zinc aspartate pretreated animals compared to irradiated controls. These data suggests that zinc aspartate pretreatment protects spermatogonia and tetraploid cells from radiation-induced cell killing.

Preliminary report on the use of zinc in vivo to protect against nitrogen mustard toxicity in female Balb/cJ mice

Previous studies with cultured normal human fibroblasts indicated that treatment of cells with zinc before exposure to an alkylating agent enhanced cell survival by seven- to ninefold. To establish whether a similar zinc-induced protective response could be elicited in vivo, we conducted a preliminary experiment in which Balb/cJ female mice were treated with zinc (2 mg kg-1 body weight) or saline by intraperitoneal (i.p.) injection at 48, 36, 24 and 12 h before i.p. administration of the alkylating agent nitrogen mustard (4 mg kg-1 body weight). Of the animals that received saline before nitrogen mustard, 57% died as compared with only 20% in the group treated with zinc before administration of the alkylating agent. As was observed in the studies of cells in culture, the results described in this report may suggest the existence in Balb/cJ female mice of a zinc-mediated protective response against nitrogen mustard toxicity.

Further studies on selective radioprotection by organic zinc salts and synergism of zinc aspartate with WR 2721

The organic zinc salts zinc aspartate, zinc histidine, zinc orotate and zinc acetate reduced the fall of the haematocrit, thrombocytes, erythrocytes and leucocytes in irradiated mice. In general, zinc aspartate was more effective than the other organic zinc salts. Protection of the haematocrit and thrombocytes by small doses of the aminothiol radioprotector WR 2721 was markedly improved by the concomitant administration of small doses of zinc aspartate. Zinc aspartate was the only one of the four tested organic zinc salts that did not inhibit in any instance the regression induced by radiotherapy of human tumours grown as xenografts in immunosuppressed mice. This also applied to the combination of zinc aspartate with WR 2721. In an experiment performed to determine the toxicity of the combined regimen, a dose of zinc aspartate which afforded synergistic haematological protection did not enhance the toxicity of WR 2721. The synergism of zinc aspartate with WR 2721 and the differential radioprotection of the combined protocol may make it possible in clinical cancer radiotherapy to obtain selective radioprotection at a lower toxicity giving an improved therapeutic ratio compared with WR 2721 alone.

Zinc-induced reduction in melphalan cytotoxicity: heterogeneity of response of cloned human tumor cells

Previous studies with cultured human normal fibroblasts indicated that pretreatment of the cells with zinc for 12 h prior to exposure to the alkylating agent melphalan increased survival by seven- to ninefold over survival values obtained in cultures treated with drug only. Comparable pretreatment of cells derived from a variety of human tumors resulted in an increase in survival of 1.7-fold or less. To determine whether the limited responsiveness to zinc represented a general property of tumor cells (which would be characterized by a lack of highly zinc-responsive subpopulations contained within the parental tumor populations), a series of clones was prepared from the A101D human melanoma line and the A549 human alveolar cell carcinoma line. Cells from each clone were then challenged with melphalan with and without zinc pretreatment. Twenty-five percent of the tumor clones exhibited increased resistance to melphalan following pretreatment with zinc (range of 2.1- to 5.2-fold increase in survival), indicating that the parental tumor lines were highly heterogenous in regard to inducibility to a state of reduced sensitivity to melphalan. There was no evidence of a relationship between zinc-induced reductions in toxicity and induced elevations in total intracellular glutathione content, indicating that the primary effect of zinc is not directed toward elevating intracellular levels of glutathione.

Protection against ionising radiation and synergism with thiols by zinc aspartate

Pre-treatment with zinc aspartate protected mice against the lethal effects of radiation and raised the LD50 from 8 Gy to 12.2 Gy. Zinc chloride and zinc sulphate were clearly less active. The radioprotective effect of zinc aspartate was equivalent to cysteamine and slightly inferior to S,2-aminoethylisothiourea (AET). Zinc aspartate displayed a similar therapeutic index to the thiols but could be applied at an earlier time before irradiation. Synergistic effects occurred with the combined administration of zinc aspartate and thiols. By giving zinc aspartate with cysteamine, the LD50 was increased to 13.25 Gy and, by combining it in the optimal protocol with AET, to 17.3 Gy. The radioprotection by zinc and its synergism with thiols is explained by the stabilisation of thiols through the formation of zinc complexes.

Induced resistance to alkylating agent toxicity obtainable by pretreatment with combinations of trace elements

Survival (monitored as colony-forming ability) of Chinese hamster Cd(r) 20F4 cells following exposure to the alkylating agent melphalan was enhanced by pretreating cells with combinations of the trace elements zinc, selenium, and copper. Cultures simultaneously pretreated with any two trace elements prior to exposure to melphalan exhibited survival values that were either equivalent to (Zn + Cu or Zn + Se) or somewhat greater than (Se + Cu) the sum of survivals obtained in cultures pretreated with singly administered elements. Simultaneous pretreatment with all three trace element inducers produced dramatic increases in survival level upon subsequent challenge with melphalan that ranged from 20-to 130-fold over the levels achieved in non-induced cultures.

Zinc effects on glutathione metabolism relationship to zinc-induced protection from alkylating agents

Several aspects of the effects of zinc on the metabolism of glutathione were examined in the Chinese hamster cell (line CHO) and in three derived sublines which differ in their resistance to the thiol reactive heavy metal cadmium. In the parental CHO cell, which does not induce the synthesis of metallothionein in response to zinc, glutathione levels remained approximately constant during the first 6 hr of zinc exposure. In the resistant cell lines, which induce the synthesis of metallothionein in response to zinc, the glutathione levels dropped transiently during zinc exposure. In all cell lines except the most cadmium resistant line, the glutathione levels after 12 hr were increased up to 3-fold relative to pretreatment levels. Similarly, the glutathione S-transferase activity measured by the conjugation of 1-chloro-2,4-dinitrobenzene to glutathione was increased after 9-12 hr of zinc treatment in all except the most highly cadmium resistant cell line. Glutathione reductase was not affected consistently by zinc treatment; however, the level of activity of this enzyme in the most highly cadmium resistant line was two to three times greater than that observed in the other cell lines. These effects are considered in relation to the zinc-induced protection of these cells from the toxic effect of the alkylating agent melphalan.