Effects of essential divalent metals on carcinogenicity and metabolism of nickel and cadmium

Interactions between the physiologically essential metals calcium, magnesium, and zinc and the carcinogenic metals nickel and cadmium were investigated to help elucidate the mechanisms of action of the carcinogenic metals. Bioassay studies revealed several significant findings, including: (1) the ability of magnesium and calcium to inhibit nickel-induced elevation of pulmonary adenoma incidence in strain A mice; (2) the ability of magnesium, but not of calcium, to prevent cadmium-induced subcutaneous sarcoma formation; and (3) the ability of magnesium, but not of calcium, to inhibit nickel-induced muscle tumor formation. Biochemical studies indicated a direct relationship between the antitumorigenic potential of magnesium and the capacity of this metal to: (1) inhibit nickel and cadmium uptake by the target tissues in vivo; (2) inhibit nickel-induced disturbances in DNA synthesis in vivo; (3) inhibit nuclear and cytosolic uptake of nickel by the target tissue cells in vivo; and (4) inhibit nickel and cadmium binding to DNA in vitro. Calcium, which in most cases did not prevent carcinogenesis, had no consistent influence on the uptake of carcinogenic metals or their biochemical effects in the target tissues. Magnesium and zinc, but not calcium, were also found to attenuate the acute toxic effects of nickel, indicating a possible correlation between prevention of acute effects and reduction in tumorigenicity. Zinc, which antagonizes cadmium tumorigenicity in the rat testis, was found to reduce markedly cadmium uptake into isolated testicular interstitial cells. Also, zinc was found to inhibit strongly cadmium binding to DNA in vitro.     

N-terminal deletion in the src gene of Rous sarcoma virus results in synthesis of a 45,000-Mr protein with mitogenic activity.

Expression of the v-src gene of Rous sarcoma virus in avian embryo neuroretina cells results in transformation and sustained proliferation of these normally resting cells. Transformed neuroretina cells are also tumorigenic upon inoculation into immunodeficient hosts. We have previously described conditional mutants of Rous sarcoma virus encoding p60v-src proteins which induce proliferation of neuroretina cells in the absence of transformation and tumorigenicity. These results suggest that p60v-src is composed of functionally distinct domains which may interact with multiple cellular targets. In this study, we describe a spontaneous variant of Rous sarcoma virus, subgroup E, which carries a deletion of 278 base pairs in the 5' portion of the v-src gene but which has retained the ability to induce proliferation of quail neuroretina cells. The deleted v-src gene encodes a 45,000-molecular-weight phosphoprotein which contains both phosphoserine and phosphotyrosine, is myristylated, and possesses tyrosine kinase activity indistinguishable from that of wild-type p60v-src. Molecular cloning and sequence analysis of the mutant v-src gene have shown that this deletion extends from amino acid 33 to 126 of the wild-type p60v-src. Therefore, this portion of the v-src protein is dispensable for the mitogenic activity of Rous sarcoma virus in neuroretina cells.     

Rous sarcoma virus mutant dlPA105 induces different transformed phenotypes in quail embryonic fibroblasts and neuroretina cells.

dlPA105 is a spontaneous variant of Rous sarcoma virus, subgroup E, which carries a deletion in the N-terminal portion of the v-src gene coding sequence. This virus was isolated on the basis of its ability to induce proliferation of quiescent quail neuroretina cells. The altered v-src gene encodes a phosphoprotein of 45,000 daltons which possesses tyrosine kinase activity. DNA sequencing of the mutant v-src gene has shown that deletion extends from amino acid 33 to 126 of wild-type p60v-src. We investigated the tumorigenic and transforming properties of this mutant virus. dlPA105 induced fibrosarcomas in quails with an incidence identical to that induced by wild-type virus. Quail neuroretina cells infected with the mutant virus were morphologically transformed and formed colonies in soft agar. In contrast, dlPA105 induced only limited morphological alterations in quail fibroblasts and was defective in promoting anchorage-independent growth of these cells. Synthesis and tyrosine kinase activity of the mutant p45v-src were similar in both cell types. These data indicate that the portion of the v-src protein deleted in p45v-src is dispensable for the mitogenic and tumorigenic properties of wild-type p60v-src, whereas it is required for in vitro transformation of fibroblasts. The ability of dlPA105 to induce different transformation phenotypes in quail fibroblasts and quail neuroretina cells is a property unique to this Rous sarcoma virus mutant and provides evidence for the existence of cell-type-specific response to v-src proteins.     

Poly(ADP-ribose)polymerase: a novel finger protein.

By Energy Dispersive X-ray fluorescence we have determined that calf thymus poly(ADP-ribose) polymerase binds two zinc ions per enzyme molecule. Using 65Zn (II) for detection of zinc binding proteins and polypeptides on western blots, we found that the zinc binding sites are localized in a 29 kd N-terminal fragment which is included in the DNA binding domain. Metal depletion and restoration experiments proved that zinc is essential for the binding of this fragment to DNA as tested by Southwestern assay. These results correlate with the existence of two putative zinc finger motifs present in the N-terminal part of the human enzyme. Poly(ADP-ribose)polymerase fingers could be involved in the recognition of DNA strand breaks and therefore in enzyme activation.     

DNA replication and poly(ADP-ribosyl)ation of chromatin

The role of poly(ADP-ribosyl)ation in chromatin replication and the activity of poly(ADP-ribose) synthetase in the newly synthesized and old chromatin was studied. It was found that 3-aminobenzamide, which is an inhibitor of poly(ADP-ribose) synthetase, had no effect on the initiation of DNA synthesis and only a moderate effect on DNA chain elongation. However, poly(ADP-ribose) synthetase activity in the newly replicated chromatin was two to three times higher than that of the unreplicated chromatin.     

An affinity matrix for the purification of poly(ADP-ribose) glycohydrolase.

The preparation of quantities of poly(ADP-ribose) glycohydrolase sufficient for detailed structural and enzymatic characterizations has been difficult due to the very low tissue content of the enzyme and its lability in late stages of purification. To date, the only purification of this enzyme to apparent homogeneity has involved a procedure requiring 6 column chromatographic steps. Described here is the preparation of an affinity matrix which consists of ADP-ribose polymers bound to dihydroxyboronyl sepharose. An application is described for the purification of poly(ADP-ribose) glycohydrolase from calf thymus in which a single rapid affinity step was used to replace 3 column chromatographic steps yielding enzyme of greater than 90% purity with a 3 fold increase in yield. This matrix should also prove useful for other studies of ADP-ribose polymer metabolism and related clinical conditions.     

Stimulation of poly(ADP-ribose) synthesis by free radicals in C3H10T1/2 cells: relationship with NAD metabolism and DNA breakage

We have studied the effect of H2O2 and O2- produced by xanthine and xanthine oxidase on NAD catabolism, poly(ADP-ribose) synthesis, and production of DNA single-strand breaks in C3H10T1/2 cells. The results show a correlation between the induction of DNA single-strand breaks, the decrease of NAD pool, and the accumulation of polymer. New techniques, based on affinity chromatography and reversed-phase high pressure liquid chromatography, have allowed an accurate determination of polymer contents and showed a 20-fold stimulation of polymer biosynthesis induced by active oxygen species. Inhibition experiments performed with 3-aminobenzamide have shown that the decrease in NAD levels after exposure of cells to active oxygen species was caused by stimulation of poly(ADP-ribosyl)ation and of another cellular process.