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D S Sarma

Publications and source records attributed to D S Sarma.

At least 73 records · Page 4Linked to original sources
PubMed1987

Dietary and metabolic manipulations of the carcinogenic process: role of nucleotide pool imbalances in carcinogenesis.

Perturbations in DNA and/or membranes are considered to be important for the carcinogenic process. A search for nutritional and metabolic means of disturbing the homeostasis of DNA and membranes revealed that nucleotide pools offer an exciting possibility. An imbalance in nucleotide pools can exert a two-pronged attack on both DNA and membranes. When given to rats, orotic acid, a precursor of pyrimidine nucleotides, results in an imbalance in nucleotide pools (an increase in uridine nucleotides and a decrease in inosine/adenine nucleotides), alterations in both DNA and membranes, and promotion of carcinogenesis in the liver initiated by chemical carcinogens. Agents such as adenine and allopurinol, which inhibit the metabolism of orotic acid and thereby decrease the formation of uridine nucleotides, and galactosamine, which traps uridine nucleotides, inhibited the promotional effects of orotic acid in the liver. These results suggested that orotic acid needs to be metabolized to uridine nucleotides and the creation of a subsequent imbalance in nucleotide pools is important for the promotional effects of orotic acid. To determine whether the creation of a nucleotide pool imbalance is a more general mechanism of tumor promotion, two lines of approach were investigated. One was to determine the effect of orotic acid on promotion of carcinogenesis in other organs, and the second approach was to determine how to induce nucleotide pool imbalances by means other than orotic acid administration. It is interesting to note that orotic acid promotes carcinogenesis in duodenum initiated by azoxymethane. Regarding the second approach, it became apparent that several metabolic disturbances result in increased orotic acid synthesis and alterations in nucleotide pools.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Effect of glycine on the induction of orotic aciduria and urinary bladder tumorigenesis in the rat.

The mechanism by which amino acids increase the cellular levels of orotic acid (OA) was investigated. Administration of glycine (2.5 mmoles/100 g) to rats resulted in a 100-fold increase in urinary OA excretion, which was inhibited by pretreatment with cycloheximide or actinomycin D. The induction of OA synthesis from NH4Cl but not from carbamoylaspartate (CA) was inhibited by cycloheximide, indicating that the cycloheximide sensitive step was after the formation of ammonia and before the formation of CA. The glycine-stimulated OA synthesis was not inhibited by acivicin, a potent inhibitor of the cytosolic carbamoylphosphate (CP) synthetase, implicating the mitochondrial CP synthetase in supplying the CP for OA synthesis. Preliminary results indicated that cycloheximide did not inhibit glycine-induced urea synthesis to any significant extent. The results thus suggest that (i) the increased OA synthesis induced by glycine requires a transcription-translation dependent step and (ii) the regulatory step may be the transport of mitochondrial CP to cytosol and/or the synthesis of cytosolic CA. Attempts to determine whether increased exposure of urinary bladder to high concentrations of OA will influence bladder tumorigenesis revealed that chronic administration of glycine (2.5 mmoles/100 g, ip, daily, 5 days a week for 20 weeks) resulted in a 44% increased incidence of hyperplastic, preneoplastic, and neoplastic lesions. Some of these rats also exhibited stones in urinary bladders. The mechanism by which glycine induces tumorigenesis in the urinary bladder is currently being explored.

Ammonium Chloride↗
PubMed1987

Complementarity between two rat liver tumor promoters.

A delay in the exposure of initiated rats to orotic acid (OA) beyond a specific time frame results in a progressive loss of promotional effect in liver carcinogenesis. The current study was designed to ascertain whether the loss of promotional effect could be counteracted by pre-exposing the initiated animals to other rat liver promoting regimens such as a diet deficient in choline (CD). Male Fischer 344 rats (150 g) were initiated with diethylnitrosamine (200 mg/kg, ip); 1 week later they were given either a CD diet or a CD diet supplemented with choline for 5 weeks. Animals from these two groups were then fed either a 1% OA diet or the basal diet for another 20 weeks. The results indicated that the loss of OAs promotion efficacy from delaying the start of the promoting regimen can be counteracted by pre-exposing the initiated rats to a CD diet. Thus in rats exposed to OA from the first week of initiation, 7% of the liver developed as nodular areas, whereas only 0.8% of liver was nodular when OA feeding was delayed by 5 weeks. This loss was abolished when initiated rats were fed a CD diet for 5 weeks prior to feeding OA for 20 weeks. These results suggest that in a rat liver tumor promotion model, two tumor promoters, OA and CD, show some degree of complementarity when given sequentially.

Animals↗
PubMed1987

Induction of the placental form of glutathione S-transferase by lead nitrate administration in rat liver.

The administration of a single dose of lead nitrate to male Wistar rats caused an increase of a polypeptide in the liver cytosol that cross reacted with the anti-rat antibody of the placental form of glutathione S-transferase (GST-P). GST-P appeared when doses of lead that induced liver cell proliferation were given (5 and 10 micromoles/100 g of body weight). Recently, it has been shown that rat hepatic nodules also exhibited an increased content of the placental form of GST-P. The induction of GST-P by lead together with other biochemical effects exerted in the liver by this metal, suggests that some chemicals may induce in rat liver a biochemical pattern similar, in some aspects, to that exhibited by carcinogen-induced hepatocyte nodules.

Animals↗
PubMed1986

Ethionine in the analysis of the possible separate roles of methionine and choline deficiencies in carcinogenesis.

The importance of ethionine, the ethyl analogue of methionine, as a metabolic probe to study the possible roles of methionine and choline in liver carcinogenesis has been briefly reviewed. Ethionine-induced liver carcinogenesis is similar in many aspects, including initiation, promotion, and progression, to carcinogenesis with other agents. However, the special role of methionine in preventing virtually all metabolic and pathologic effects of ethionine, including liver cancer, places ethionine in a special position. On the basis of these observations and our current knowledge about choline deficiency in the genesis of liver cancer, we proposed that choline and methionine play separate but overlapping roles in the initiation and promotion of liver carcinogenesis.

Animals↗
PubMed1986

Lead nitrate induces certain biochemical properties characteristic of hepatocyte nodules.

Hepatocyte nodules in the rat exhibit a unique biochemical pattern which is characterized by a decrease in Phase I and an increase in Phase II components of the drug-metabolizing system. The present study was designed to determine whether this biochemical pattern is unique for rat hepatocyte nodules or is a property of the liver cell, but expressed only when the liver cell is perturbed. The results obtained indicate that lead nitrate (5 or 10 mumol/100 g body wt), an inducer of liver cell proliferation, caused a decrease in Phase I components such as microsomal cytochromes P-450 and in the activity of aminopyrine N-demethylase, while it caused an increase in Phase II components such as glutathione, and in the activities of glutathione-S-transferase and DT-diaphorase in rat liver. Of particular interest was the finding in liver cytosol of lead-treated rats of an increased content of a polypeptide which cross-reacts with the anti-rat placental form of glutathione-S-transferase. Recently, it has been shown that rat hepatocyte nodules exhibited an increased content of the placental form of glutathione-S-transferase. Thus, the results suggest that some chemicals, such as lead nitrate, can induce in rat liver a biochemical pattern similar in certain respects to that exhibited by hepatic nodules. These chemicals may be used as model compounds to understand the molecular mechanism(s) underlying the induction of new and unique biochemical machinery seen in hepatic nodules.

Animals↗
PubMed1986

Liver tumour promotion by chemicals: models and mechanisms.

Tumour promotion is defined as the process whereby a carcinogen initiated organ develops focal proliferations such as nodules, polyps or papillomas, one or more of which become precursors for subsequent steps in the carcinogenic process. The available models for a sequential analysis of carcinogenesis in liver have been examined within the framework of this operational definition with respect to the models themselves as well as the promoted hepatocytes. Using initiation-promotion protocols, several promoters which differ in the biological responses they elicit have been identified. The key issue in promotion pertains to the mechanisms involved in the focal proliferation of the initiated hepatocytes. The resistant hepatocyte model and perhaps the phenobarbital model suggest that focal proliferation of the initiated hepatocyte is induced by exerting a selective mitoinhibitory effect on the surrounding cells while permitting the initiated hepatocyte to respond to the proliferative stimulus, whether it is exogenous or endogenous. The promoter orotic acid, on the other hand, is a natural precursor of pyrimidine nucleotide biosynthesis and is neither an inducer nor an inhibitor of liver cell proliferation but it creates an imbalance in cellular nucleotide pools. Since nucleotides are intermediates in DNA synthesis as well as in the glycosylation of proteins and lipids including that of membranes, it has been postulated that the promotional effect of orotic acid is mediated through this imbalance affecting both DNA and membranes. There is some experimental evidence for this. The importance of this hypothesis is that it raises the possibility of achieving promotion in several organs by disturbing the normal nucleotide pool patterns. Indeed orotic acid has also been found to promote duodenal cancer. The finding that hepatic nodules, regardless of how they have been promoted, exhibit a common biochemical pattern with resistance to several agents has raised some important issues. For example, what is the basis for the resistant phenotype and how is it related to cancer development? Are all initiated hepatocytes identical? If not, do all promoters exert their effect on the same population of initiated hepatocytes? Is the heterogeneity of the initiated hepatocyte population generated by the carcinogen due to the induction of more than one critical lesion: a primary lesion responsible for initiation and secondary lesions in which different types of initiated cells are selected by different promoters?(ABSTRACT TRUNCATED AT 250 WORDS)

Cell Division↗
PubMed1986

Sequential alterations in growth control and cell dynamics of rat hepatocytes in early precancerous steps in hepatocarcinogenesis.

This set of experiments is the second of a series designed to explore alterations in cell dynamics and growth control of new populations of hepatocytes that appear to play a role in the carcinogenic process induced in the liver by chemical carcinogens. This is part of an ongoing study of the biochemical and molecular basis for cancer development. A rat model for hepatocarcinogenesis, the resistant hepatocyte model, was chosen with its synchrony of several steps in the process. Carcinogenesis was initiated by the administration of a single necrogenic dose of diethylnitrosamine. Resistant hepatocytes so induced were stimulated to proliferate rapidly to form nodules by a mitogenic stimulus in the presence of a brief exposure to dietary 2-acetylaminofluorene sufficient to inhibit the proliferation of the majority of uninitiated hepatocytes, the nonresistant population. A small subset of these hepatocyte nodules, the persistent nodules, was examined at 2, 4, and 6 mo postinitiation. Duration of phases of the cell cycle, growth fraction, doubling time, cell death, and cell loss and the responses and subsequent recovery after the application of a strong mitogenic stimulus, partial hepatectomy, were measured. The first precancerous hepatocyte nodule, at 2 mo, showed a "normal" duration of phases of the cell cycle. The growth fractions were about 4,4, and 8% at 2, 4, and 6 mo, respectively, as compared to 0.4% in the surrounding hepatocytes. Accompanying the increased growth fractions were considerable levels of cell loss, measuring about 3% at 2 mo and 7% at 6 mo. At 6 mo, the hepatocyte nodule population, unlike the hepatocytes in the surrounding liver, shows a failure to return to its base-line level after stimulation of cell proliferation by partial hepatectomy. The results of this study have identified two new steps in the early precancerous phase of hepatocarcinogenesis relating to alterations in the control of cell proliferation and are consistent with the hypothesis that new and evolving cell populations may play an important role in the step-by-step carcinogenic process. These new populations appear to acquire alterations in growth control in a seriatim fashion, with retention of some "normal" properties.

Animals↗
PubMed1985

5-azacytidine potentiates initiation induced by carcinogens in rat liver.

To test the validity of the hypothesis that hypomethylation of DNA plays an important role in the initiation of carcinogenic process, 5-azacytidine (5-AzC) (10 mg/kg), an inhibitor of DNA methylation, was given to rats during the phase of repair synthesis induced by the three carcinogens, benzo[a]-pyrene (200 mg/kg), N-methyl-N-nitrosourea (60 mg/kg) and 1,2-dimethylhydrazine (1,2-DMH) (100 mg/kg). The initiated hepatocytes in the liver were assayed as the gamma-glutamyltransferase (gamma-GT) positive foci formed following a 2-week selection regimen consisting of dietary 0.02% 2-acetylaminofluorene coupled with a necrogenic dose of CCl4. The results obtained indicate that with all three carcinogens, administration of 5-AzC during repair synthesis increased the incidence of initiated hepatocytes, for example 10-20 foci/cm2 in 5-AzC and carcinogen-treated rats compared with 3-5 foci/cm2 in rats treated with carcinogen only. Administration of [3H]-5-azadeoxycytidine during the repair synthesis induced by 1,2-DMH further showed that 0.019 mol % of cytosine residues in DNA were substituted by the analogue, indicating that incorporation of 5-AzC occurs during repair synthesis. In the absence of the carcinogen, 5-AzC given after a two thirds partial hepatectomy, when its incorporation should be maximum, failed to induce any gamma-GT positive foci. The results suggest that hypomethylation of DNA per se may not be sufficient for initiation. Perhaps two events might be necessary for initiation, the first caused by the carcinogen and a second involving hypomethylation of DNA.

Animals↗
PubMed1985

Orotic acid, a promoter of liver carcinogenesis induces DNA damage in rat liver.

Orotic acid, a precursor of pyrimidine nucleotide biosynthesis and a promoter for liver carcinogenesis, when fed at 1% level in a diet for 5 weeks resulted in liver DNA damage. The damage can be monitored as alkali-labile lesions using alkaline sucrose gradients as well as alkaline elution technique. Furthermore, the induced DNA damage persists for up to three weeks after withdrawal of the orotic acid diet. The fact that several skin-tumour promoters also induce DNA damage raises the question whether DNA damage is a component in tumour promotion.

Animals↗
PubMed1985

A common biochemical pattern in preneoplastic hepatocyte nodules generated in four different models in the rat.

Hepatocyte nodules, structures consistently seen in every model of liver carcinogenesis well before the first appearance of cancer, were examined with respect to some Phase I and Phase II components considered to be important in the metabolism of carcinogens and other xenobiotics. Phase I components are those related to the metabolism of xenobiotics and include microsomal cytochromes P-450 and mixed-function oxygenase activities. Phase II components are those related to the conjugation and detoxification reactions of xenobiotics and their metabolites and include glutathione S-transferases and glutathione. Nodules were induced by the resistant hepatocyte, choline-deficient, methionine-low diet, phenobarbital and orotic acid models of liver carcinogenesis. Also, nodules generated by the resistant hepatocyte model were examined after transplantation to the spleen of syngeneic animals. The hepatocyte nodules show a common biochemical pattern, consisting of decreased microsomal cytochromes P-450, cytochrome b5, and aminopyrine N-demethylase activity and increased glutathione and gamma-glutamyltransferase in whole homogenates and glutathione S-transferase activity in the cytosol. This similarity, appropriate to a resistance phenotype, adds additional support for the hypothesis that hepatocyte nodules may be a common step in liver carcinogenesis in several different models.

Aminopyrine N-Demethylase↗
PubMed1984

Is there more than one critical lesion relevant for experimental liver carcinogenesis?

The present study was designed to determine whether there is one or more than one critical lesion, induced by a carcinogen, relevant for initiation. The experimental approach consisted of administering a non-necrogenic dose of the carcinogen 1,2-dimethylhydrazine 2HCl (100 mg/kg, i.p.) to male Fischer 344 rats (120-140 g) and completing the initiation process by two different methods: (i) induction of liver cell proliferation by partial hepatectomy, or (ii) creation of hypomethylation in DNA by giving 5-azacytidine, an agent that inhibits DNA methylation. The initiated hepatocytes were assayed as gamma-glutamyltransferase positive foci. The rationale for the approach was based on the premise that the two methods used for completing the initiation step might give either the same or a different pattern in the incidence of initiated hepatocytes depending on whether one or more than one lesion was important for initiation, particularly if some of the lesions were allowed to repair before applying the cell proliferative stimulus or administering the 5-azacytidine. The results obtained indicated that 5-azacytidine facilitated the induction of the same number of foci whether given 12 or 96 hours after the carcinogen indicating that the critical lesion involved in this mode of initiation persisted up to at least 96 hours. In contrast, our earlier results showed that there was a reduction in the number of gamma-glutamyltransferase positive foci when partial hepatectomy was delayed beyond 24 hours after the carcinogen administration, indicating that the critical lesion involved in this mode of initiation has a half-life of not more than 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

1,2-Dimethylhydrazine↗
PubMed1984

Orotic acid, a new promoter for experimental liver carcinogenesis.

Male Fischer 344 rats initiated with 1,2-dimethylhydrazine 2HCl (100 mg/kg) given 18 hr after partial hepatectomy and exposed to a diet containing 1% orotic acid for 13 months developed a 100% incidence of hepatocellular carcinoma. The creation of nucleotide pool imbalances by dietary orotic acid, for e.g., an increase in uridine nucleotides and a decrease in adenine nucleotides, was considered as a possible mechanism for the promotional effect of orotic acid on liver carcinogenesis. The significance of this hypothesis is that altered nucleotide pools affect both genomic as well as membrane organization. Consistent with this hypothesis is our finding that feeding rats with a diet containing 1% orotic acid for 10 weeks resulted in a liver DNA damage as monitored by its slower sedimentation in alkaline sucrose gradients compared to the corresponding controls. To assess the general applicability of this hypothesis, nucleotide pool imbalances were created by using methods other than feeding orotic acid and their effect on the incidence of gamma-glutamyltransferase positive foci in carcinogen initiated rats was determined. The results obtained indicated that rats initiated with 1,2-dimethylhydrazine.2HCl (100 mg/kg) given 18 hr after partial hepatectomy and exposed to diet deficient in arginine, a regimen that causes an increased synthesis and excretion of orotic acid, or were fed diets containing 1% thymidine or 1% thymine developed greater number of gamma-glutamyltransferase positive foci compared to the corresponding controls fed the basal diets. These results were interpreted to indicate that orotic acid exerts its promotional effect probably by creating an imbalance in nucleotide pools. One of the mechanisms by which an imbalance of nucleotide pools influences the pathogenesis of the carcinogenic process may be by inducing perturbations in the DNA.

1,2-Dimethylhydrazine↗
PubMed1984

Occurrence of cell death (apoptosis) in preneoplastic and neoplastic liver cells. A sequential study.

A sequential study was performed to investigate the occurrence of cell death in preneoplastic and neoplastic liver cells of F-344 rats. The animals were administered a single initiator dose of 1,2-dimethylhydrazine and were then subjected to a liver carcinogenesis promotion regimen, consisting of a diet containing 1% orotic acid. Cell death, morphologically similar to that described as apoptosis, was evident in foci of preneoplastic hepatocytes at 10 weeks after orotic acid feeding. An increased frequency of apoptotic bodies was observed in nodules, but not in the surrounding liver, 20 weeks after starting the dietary regimen, and in hepatocellular carcinomas that developed after 1 year of continuous promotion. Occurrence of this type of cell death was also observed in liver foci of rats subjected to two other promoting regimens, suggesting, thus, a possible relevance of apoptosis to the carcinogenic process in the liver.

Animals↗
PubMed1984

Promotion by orotic acid of liver carcinogenesis in rats initiated by 1,2-dimethylhydrazine.

Our earlier experiments revealed that orotic acid, a precursor for pyrimidine nucleotides, selectively stimulated the growth of carcinogen-modified liver cells to grow into enzyme-altered hepatocytes (Cancer Lett., 16: 191-196, 1982). The present study was designed to determine whether prolonged feeding of orotic acid will result in hepatocellular carcinoma in initiated rats. Accordingly, groups of rats were given i.p. either 1,2-dimethylhydrazine dihydrochloride (100 mg/kg) or an equivalent volume of 0.9% sodium chloride solution 18 hr after two-thirds partial hepatectomy. After 1 week of recovery, they were continued on either the basal diet or the basal diet containing 1% orotic acid for 10 to 13 months. Some groups of rats, in addition, received a single necrogenic dose of CCl4 8 weeks following exposure to orotic acid diet. The results obtained indicated that 87.5% of initiated rats exposed to orotic acid developed hepatocellular carcinomas in 10 months and 100% in 13 months. Initiated rats exposed to orotic acid diet coupled with a single administration of CCl4 developed 100% hepatocellular carcinoma by 10 months. In contrast, the incidence of hepatocellular carcinoma in initiated rats fed basal diet alone for 13 months was 37.5%, while, in those that received CCl4 in addition, the incidence was 25% in 10 months. Interestingly, a significant number of liver cancers (29 to 36%) in the orotic acid-fed group metastasized to lungs, whereas none of the liver cancers in rats exposed to basal diet metastasized.(ABSTRACT TRUNCATED AT 250 WORDS)

1,2-Dimethylhydrazine↗
PubMed1983

Dietary orotic acid enhances the incidence of gamma-glutamyltransferase positive foci in rat liver induced by chemical carcinogens.

Feeding male Fischer F-344 rats for 5 weeks a diet containing 1% orotic acid, a precursor for pyrimidine nucleotide biosynthesis, resulted in an increased incidence of gamma-glutamyltransferase (EC 2.3.2.2) positive foci induced by chemical carcinogens including 1,2-dimethylhydrazine, diethylnitrosamine, benzo[a]pyrene, and aflatoxin B1. This unique effect of orotic acid can be accentuated by supplying a liver cell proliferative stimulus. The enzyme altered hepatocytes have a higher labelling index (4.4%) compared with that of the hepatocytes in the surrounding liver (0.26%). The effect of orotic acid on the increased incidence of foci cannot be attributed to either the induction of liver cell proliferation or the imposition of a preferential inhibitory effect on the proliferation of normal hepatocytes while permitting the carcinogen-modified hepatocytes to respond to an endogenous or exogenous liver cell proliferative stimulus and grow to form foci. Orotic acid also did not behave like some of the promoters of liver carcinogenesis such as phenobarbital and polychlorinated biphenyls in that it did not induce either the phase I or phase II components of hepatic drug metabolizing enzyme systems. Some of the possible mechanisms by which orotic acid enhances the incidence of gamma-glutamyltransferase positive foci by carcinogens are discussed.

Acyltransferases↗