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Biomedical subjects

A K Chan

Publications and source records attributed to A K Chan.

At least 19 recordsLinked to original sources
PubMed1995

Clinical manifestations of primary hyperparathyroidism before and after parathyroidectomy. A case-control study.

BACKGROUND: There has been an National Institutes of Health consensus meeting concerning the management of patients with "asymptomatic" primary hyperparathyroidism, yet there is no clear definition of this condition. The authors, therefore, documented the clinical manifestations and frequencies of these manifestations in unselected patients with primary hyperparathyroidism and determined whether these clinical manifestations resolved after parathyroidectomy. METHOD: The authors studied 152 unselected consecutive patients with primary hyperparathyroidism and 132 control patients with nontoxic thyroid disorders who were treated by parathyroidectomy or thyroidectomy, respectively, between January 1986 and June 1991. All patients received a questionnaire during their initial office visits and the same questionnaire again after their operations. Patients were also questioned about their perception of the success of the operation. Eighty percent of the parathyroid patients and 70.5% of the thyroid patients completed the questionnaires, and the mean follow-up time was 20 months. RESULTS: Only 7 (4.6%) patients with primary hyperparathyroidism had no symptoms, and 26 (17.1%) had no associated conditions despite 74.3% of these patients having serum calcium levels less than 12 mg/dL. Symptoms including fatigue, exhaustion, weakness, polydipsia, polyuria, nocturia, joint pain, bone pain, constipation, depression, anorexia, nausea, heartburn, and associated conditions, including nephrolithiasis, and hematuria occurred more often in patients with primary hyperparathyroidism than in the thyroid control patients (p < 0.05). After parathyroidectomy, only eight (5.3%) patients failed to have any improvement in symptoms or associated conditions. Fifty-seven percent of the parathyroid patients versus 30% of the thyroid patients felt better overall after the operation, strength subjectively improved in 29% of parathyroid patients versus 13% in thyroid patients; thirty-seven percent of the parathyroid patients versus 13% of the thyroid patients claimed they were less depressed. CONCLUSION: When examined thoroughly, few patients with primary hyperparathyroidism were asymptomatic or without associated conditions, and most patients experienced subjective improvement after successful parathyroidectomy.

Adolescent↗
PubMed1995

Neonatal haemochromatosis associated with Down syndrome.

METHODOLOGY: The clinical course of a child with neonatal haemochromatosis associated with Down syndrome is described, using case notes and post-mortem examination. RESULTS: The patient died of liver failure on day 36. Post-mortem examination showed siderosis of liver, pancreas, heart, kidney and thyroid. CONCLUSIONS: Neonatal haemochromatosis is a rare paediatric disease, the aetiology of which is unknown. Evidence suggests a disorder of fetoplacental iron handling with implications for recurrence in future pregnancies.

Down Syndrome↗
PubMed1993

Phosphorylation of ribonucleotide reductase R2 protein: in vivo and in vitro evidence of a role for p34cdc2 and CDK2 protein kinases.

Ribonucleotide reductase is responsible for supplying the deoxyribonucleotides required for DNA synthesis and repair. The active enzyme consists of two dissimilar protein components called R1 and R2. Immunoprecipitation of R1 and R2 proteins from [32P]orthophosphate-labeled exponentially growing mouse L cells showed that the R2 protein but not the R1 protein of ribonucleotide reductase could be phosphorylated in vivo. Two-dimensional phosphopeptide mapping experiments of trypsin-digested R2 protein showed a major spot containing more than 90% of the total radioactivity and a minor spot with the remaining radioactivity. Phosphoamino acid analysis of R2 phosphorylated protein indicated that phosphorylation occurred exclusively on serine. Protein kinase C, cAMP-dependent protein kinase, p34cdc2, and CDK2 were capable of phosphorylating the R2 protein in vitro, whereas casein kinase II was not. To determine whether any of these enzymes could phosphorylate peptides observed to be phosphorylated in actively growing cells, tryptic phosphopeptide maps of R2 that had been phosphorylated in vitro were compared with maps of R2 that had been isolated from [32P]-labeled cells. Only the phosphopeptide maps obtained with p34cdc2 and CDK2 matched the pattern found in [32P]-labeled cells. Experiments in which tryptic digests from different samples were mixed prior to two-dimensional separation demonstrated comigration of phosphopeptides obtained by in vivo phosphorylation with phosphopeptides derived from p34cdc2 or CDK2 obtained by in vitro phosphorylations. These studies indicate that protein R2 phosphorylation may play an important role in the regulation of ribonucleotide reduction, DNA synthesis, and cell cycle progression, and suggest a potentially important p34cdc2 and/or CDK2 regulation point in DNA replication.

Amino Acid Sequence↗
PubMed1992

Normocalcemic hyperparathyroidism. Biochemical and symptom profiles before and after surgery.

Patients with normocalcemic hyperparathyroidism represent a diagnostic and therapeutic challenge. It is unclear to what extent these patients benefit from surgery in terms of correction of their serum chemistry abnormalities and their symptoms. We studied 142 patients: 23 with normocalcemic hyperparathyroidism (serum calcium levels below 2.62 mmol/L), 35 with intermittent hypercalcemia, and 84 with hypercalcemic hyperparathyroidism. Serum chemistry analyses and a standardized questionnaire of symptoms were completed before and after surgery. Overall, patients in the normocalcemic group reported a similar frequency of preoperative symptoms; had a similar reduction in postoperative symptoms; and had a similar normalization of serum calcium, parathormone, and phosphate levels as those in the two control groups. This study indicates that factors other than elevated serum calcium levels are in large part responsible for the symptoms of hyperparathyroidism and that these patients benefit from operation.

Adenoma↗
PubMed1992

Segmental colonic dilatation in a neonate.

Segmental dilatation of the colon is rare, usually affecting older children, and may mimic Hirschsprung's disease. A neonate with Down's syndrome and malrotation presented with this condition and was cured by segmental resection.

Colon↗
PubMed1992

Cardioplegic arrest of the myocardium with calcium blocking agents.

This study was designed to compare the effects of the calcium slow channel blocking agents verapamil (0.15 mg/kg), diltiazem (0.15 mg/kg), and nifedipine (50 micrograms/kg) on the myocardium after global ischemia and reperfusion in the in situ canine model. Animals were subjected to 120-min normothermic global ischemia, followed by 45-min reperfusion. Cardioplegic arrest of the myocardium was achieved by administering one of the three calcium antagonists in a multidose fashion. Superior preservation (p less than 0.01) of left ventricular (LV) systolic function was achieved in group I (verapamil cardioplegia). dP/dt, at an intraventricular balloon volume of 25 cc, was 83% of control after reperfusion in group I. Group II (diltiazem) and group III (nifedipine) achieved only 55 and 63% of their preischemic dP/dt values. LV chamber stiffness was increased in hearts protected with nifedipine. The exponential constant m was increased from 0.04 +/- 0.01 to 0.08 +/- 0.01. Coronary blood flow after reperfusion increased from 120 to 184 cc/100 gr/min in group I (p less than 0.01). The hyperemic response in group III was negligible. The O2 consumption of the reperfused myocardium was not significantly altered in any of the treatment groups. Lactate metabolism during ischemia and after reperfusion was similar in all groups. ATP values were markedly reduced in all groups (p less than 0.05). Immediately after ischemia, ATP was 50, 28, and 44% of control in group I, II, and III, respectively. The excellent preservation of systolic function and a physiologic hyperemic response by verapamil could not be correlated with improved preservation of high-energy compounds or with significant changes in myocardial O2 consumption.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1992

Inhibition of mammalian ribonucleotide reductase by cis-diamminedichloroplatinum(II).

Ribonucleotide reductase is a highly regulated, rate-limiting activity in the synthesis of DNA. A previous study has shown that the Escherichia coli enzyme is inhibited by the clinically important antitumor agent cis-diamminedichloroplatinum(II) (DDP), and this has led to the hypothesis that ribonucleotide reductase is an important site of action for this chemotherapeutic agent. This hypothesis has been directly tested in this investigation. We observed that DDP inhibits the mammalian ribonucleotide reductase, with 50% inhibition occurring at 0.3 mM. Unlike the E. coli enzyme where only one of the two protein components is targeted by DDP, we observed that both of the mammalian proteins (R1 and R2) were sites for the inhibitory activity of the drug. Colony-forming experiments, enzyme activity studies, and analyses of R1 and R2 message levels in mutant cell lines containing either high levels of ribonucleotide reductase activity or exhibiting resistance to the cytotoxic effects of DDP were used to further investigate the potential role of ribonucleotide reductase in DDP cytotoxic action and drug resistance. These studies did not support a hypothesis formulated in the earlier investigation that inhibition of ribonucleotide reductase is an important component of DDP cytotoxic activity or that it is a major participant in DDP resistance mechanisms. From a biological point of view, DDP is a very active drug, and in addition to its cytotoxic effects it is capable of inducing a variety of cellular changes. Whether or not the inhibition of mammalian ribonucleotide reductase activity that we have described in this study plays a role in mediating any of these other effects remains to be determined.

Animals↗
PubMed1992

CD4 lymphocyte count as an indicator of delay in seeking human immunodeficiency virus-related treatment.

BACKGROUND: As many as half of patients infected with the human immunodeficiency virus who are medically eligible for Pneumocystis prophylaxis and zidovudine treatment have not received these treatments. We used the CD4 lymphocyte count as an indicator of delay in seeking treatment among patients infected with human immunodeficiency virus and assessed whether insurance status was associated with the stage of illness when care is initiated. METHODS: Data from 96 patients who initiated medical care at a university acquired immunodeficiency syndrome clinic from August 1989 to January 1991 were retrospectively reviewed. RESULTS: Patients initiated care at a relatively late stage of illness (mean CD4 lymphocyte count, 0.37 x 10(9)/L [369/mm3]), and 29% were below the threshold for Pneumocystis prophylaxis. Patients with private insurance had significantly lower CD4 counts (mean, 0.27 x 10(9)/L) than did individuals with public insurance (mean, 0.46 x 10(9)/L). CD4 counts did not increase during the 18-month study period. CONCLUSIONS: The majority of patients infected with human immunodeficiency virus are eligible for medical therapy and could benefit by initiating care sooner. Private insurance was not associated with initiating early care, supporting anecdotal reports that some privately insured individuals may be reluctant to seek care for a human immunodeficiency virus-related condition.

Adult↗
PubMed1992

Antimycobacterial therapy for disseminated Mycobacterium avium complex infection in patients with acquired immunodeficiency syndrome.

BACKGROUND--Antimycobacterial therapy for disseminated Mycobacterium avium complex (DMAC) in patients with acquired immunodeficiency syndrome (AIDS) may ameliorate symptoms and decrease bacteremia. However, no studies have demonstrated improved survival in patients with AIDS treated for DMAC. We assessed the effects of treatment of DMAC on the survival of patients with AIDS. METHODS--We retrospectively reviewed records of patients with AIDS and DMAC seen at two San Francisco, Calif, hospitals between January 1, 1988, and January 1, 1990. The treatment group (N = 76) consisted of patients who received 2 weeks or more of antimycobacterial therapy with at least three agents. The untreated group (N = 74) received either no therapy or isoniazid alone. Patients in both groups lived a minimum of 2 weeks after the diagnosis of DMAC. RESULTS--The median survival in the treatment group was 191 days, compared with 80 days in the untreated group. In a multivariate proportional hazards model (N = 145), both treatment of DMAC (relative hazard = 0.34; 95% confidence interval, 0.23 to 0.51) and treatment with zidovudine (relative hazard = 0.54; 95% confidence interval, 0.36 to 0.82) were associated with improved survival. CONCLUSION--Patients with AIDS and DMAC who are treated with antimycobacterial drugs may survive longer than untreated patients. We recommend that a randomized trial be conducted to evaluate the optimal treatment of DMAC.

Acquired Immunodeficiency Syndrome↗
PubMed1991

Increasing sympathetic nerve terminal-dependent plasma extravasation correlates with decreased arthritic joint injury in rats.

This study compared the pharmacology of adrenergic agents that influence plasma extravasation in normal animals with those agents that influence tissue injury in an inflammatory disease model. Specifically we studied the effects of beta 2- and alpha 2-adrenergic receptor agonists and antagonists on bradykinin-induced plasma extravasation in normal Sprague-Dawley rats and on joint injury in rats with experimental arthritis. Plasma extravasation induced by infusion of bradykinin in the rat knee joint was attenuated by the beta 2-agonist salbutamol or by the alpha 2-antagonist yohimbine, and was enhanced by the beta 2-antagonist, ICI-118,551, or by the alpha 2-agonist, clonidine. In rats that had undergone chemical symphathectomy, bradykinin-induced plasma extravasation was markedly reduced, and there was no enhancement of bradykinin-induced plasma extravasation by either ICI-118,551 or clonidine. Although ICI-118,551 and clonidine enhanced bradykinin-induced plasma extravasation, these drugs significantly reduced joint injury in rats with adjuvant-induced arthritis. Neither salbutamol nor yohimbine, however, significantly increased joint injury in the arthritic rats, presumably because arthritis severity is already high in these animals. Consistent with this hypothesis, both salbutamol and yohimbine did significantly increase the joint injury associated with experimental arthritis in Wistar-Kyoto rats, a strain which develops a mild adjuvant arthritis. The fact that increased plasma extravasation is associated with decreased arthritis severity suggests that plasma extravasation, a major sign of acute inflammation, contributes to tissue reparative processes.

Animals↗
PubMed1990

Increased ferritin gene expression is associated with increased ribonucleotide reductase gene expression and the establishment of hydroxyurea resistance in mammalian cells.

In the present study, we show that hydroxyurea-inactivated ribonucleotide reductase protein M2 has a destabilized iron center, which readily releases iron. In addition, evidence is presented which indicates that single or multistep selection for hydroxyurea resistance, in a variety of mammalian cell lines, leads to alterations in the expression of the gene for the iron storage protein, ferritin. In all hydroxyurea-resistant cell lines examined, including human, hamster, rat, and mouse, there was an elevation in ferritin heavy (H)- and/or light (L)-mRNA levels, but no change in the corresponding gene copy number. A detailed analysis of ferritin expression in a hydroxyurea-resistant mouse L cell line showed that when compared to its wild type counterpart, there was an increase in H subunit concentration but no significant change in L subunit levels. The increased H/L subunit ratio was not brought about by specific changes in the rates of ferritin subunit biosynthesis, but rather resulted from changes in the post-translational stability of H subunits relative to L subunits in the resistant cell line compared to its parental wild type. Also, we show that treatment of cells with hydroxyurea results in an increased rate of ferritin biosynthesis in the absence of changes in H- or L-mRNA levels. These results indicate that the development of even low level hydroxyurea resistance in mammalian cells may require alterations in ferritin gene expression, and they show an interesting relationship between the expressions of two highly regulated activities, ribonucleotide reductase and ferritin.

Animals↗
PubMed1990

Regulation and drug resistance mechanisms of mammalian ribonucleotide reductase, and the significance to DNA synthesis.

Mammalian ribonucleotide reductase, which occupies a key position in the synthesis of DNA, is a highly controlled enzyme activity, because it is solely responsible for the de novo reduction of ribonucleoside diphosphates to their corresponding deoxyribonucleoside diphosphate forms, required for DNA synthesis. Ribonucleotide reductase consists of two dissimilar protein components often called M1 and M2, which are independently regulated during cell proliferation. The M1 component contains multiple effector binding sites and is responsible for the complex allosteric regulation of the enzyme, whereas the M2 protein contains nonheme iron and a unique tyrosyl-free radical required for ribonucleotide reduction. Since the reaction is rate limiting for DNA synthesis, ribonucleotide reductase plays an important role in regulating cell division, and hence, cell proliferation. There are many inhibitors of ribonucleotide reductase and perhaps the most valuable one from a cell biology, biochemistry, and clinical point of view is the hydroxamic acid, hydroxyurea. This drug has also been very useful as a selective agent for isolating a variety of mammalian mutant cell lines altered in ribonucleotide reductase gene expression. Regulatory, structural, and biological characteristics of ribonucleotide reductase are reviewed, including evidence that ribonucleotide reductase, particularly the M2 protein, has an important early role to play in tumor promotion. In addition, modifications in the expressions of genes altered in hydroxyurea-resistant mutants and cultured in the absence or presence of hydroxyurea are discussed, with emphasis on changes in M2 protein, M1 protein, and the iron-storage protein ferritin.(ABSTRACT TRUNCATED AT 250 WORDS)

Allosteric Regulation↗
PubMed1988

Molecular mechanisms responsible for the drug-induced posttranscriptional modulation of ribonucleotide reductase levels in a hydroxyurea-resistant mouse L cell line.

Ribonucleotide reductase, which catalyzes the formation of deoxyribonucleotides from ribonucleoside diphosphate precursors, is the rate-limiting enzyme in DNA synthesis. The enzyme consists of two nonidentical subunits called M1 and M2, both of which are required for activity. Hydroxyurea, a specific inhibitor of DNA synthesis, acts by destroying the unique tyrosyl free radical of protein M2. Previously, we have described a mouse L cell line which exhibited a stable resistance to high concentrations of hydroxyurea. This mutant cell line contains elevated quantities of both proteins M1 and M2 as a result of corresponding increases in the levels of mRNAs for both subunits. Interestingly, both M1 and M2 protein levels were further elevated when mutant cells were cultured in the presence of hydroxyurea, and this elevation was not accompanied by increases in their corresponding mRNAs. These results indicated that hydroxyurea can modulate ribonucleotide reductase expression posttranscriptionally. In this report, we show that the level of both subunits of ribonucleotide reductase responds to hydroxyurea in a drug concentration dependent manner. Furthermore, results from kinetic studies indicate that protein M2 levels rise much more rapidly than protein M1. Pulse-chase experiments indicated that the half-lives of both the M1 and M2 polypeptides are increased by approximately 2-fold when the mutant cells are cultured in the presence of hydroxyurea. We also present evidence indicating that exposure of these cells to hydroxyurea leads to a relatively slow but specific increase in the rate of biosynthesis of both proteins M1 and M2, as assayed by pulse labeling.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Molecular mechanisms of drug resistance involving ribonucleotide reductase: hydroxyurea resistance in a series of clonally related mouse cell lines selected in the presence of increasing drug concentrations.

Mammalian ribonucleotide reductase is a highly regulated, rate-limiting activity responsible for converting ribonucleoside diphosphates to the deoxyribonucleotide precursors of DNA. The enzyme consists of two nonidentical proteins often called M1 and M2, both of which are required for activity. Hydroxyurea is an antitumor agent which inhibits ribonucleotide reductase by interacting with the M2 component specifically at a unique tyrosyl free radical. To obtain further information about drug resistance mechanisms, we have used M1 and M2 complementary DNAs and monoclonal antibodies to investigate the properties of a series of clonally related drug-resistant mouse cell lines, selected by a step-wise procedure for increasing levels of resistance to the cytotoxic effects of hydroxyurea. Several interesting mechanisms have been identified. Each successive drug selection step leading to the isolation of highly resistant cells was accompanied by stable elevations in cellular resistance and ribonucleotide reductase activities. The changes that occurred at each step involved the M2 component. A very early event, occurring at the first step in the selection process, was the amplification of the M2 gene accompanied by an increase in M2 messenger RNA. Although cellular resistance and M2 protein levels increased significantly during drug selection, only a modest change in M2 gene copy number was observed after the initial selection step. Analysis of wild type, moderately resistant, and highly resistant cells indicated that, in addition to M2 gene amplification, posttranscriptional modification also occurred during drug selection. This second mechanism was not due to alterations in protein M2 half-life, but involved an increase in translational efficiency. By increasing the rate of M2 synthesis, without altering degradation rates, resistant cells were able to accumulate high levels of this key regulatory protein. Cells selected for the ability to proliferate in concentrations of drug as high as 4 mM exhibited changes that involved M2, without detectable changes to M1. These results provide further evidence that M1 and M2 levels are controlled by different mechanisms in mammalian cells. Eventually, however, cells required an elevation in the M1 protein, as well as the M2 protein, to survive in a hydroxyurea concentration of 5 mM. These results illustrate the complexity of the drug-resistant phenotype and provide further information about the molecular processes that lead to the development of cells resistant to low, intermediate, and high concentrations of hydroxyurea.

Animals↗
PubMed1987

Elevated expression of M1 and M2 components and drug-induced posttranscriptional modulation of ribonucleotide reductase in a hydroxyurea-resistant mouse cell line.

Ribonucleotide reductase, a rate-limiting enzyme in the synthesis of DNA, consists of two nonidentical subunits, proteins M1 and M2. Hydroxyurea, a specific inhibitor of DNA synthesis, acts by destroying the unique tyrosyl free radical of protein M2. In the past, we have described a mouse L cell line which exhibited a stable resistance to high concentrations of hydroxyurea [McClarty, G. A., Chan, A., & Wright, J.A. (1986) Somat. Cell Mol. Genet. 12, 121-131]. When this line was grown in the absence of hydroxyurea, the cells contained a modest but stable elevation in ribonucleotide reductase activity. However, the activity was further increased on the addition of drug to the culture medium. This was accompanied by an increase in protein M2 activity as shown by activity titration experiments. Likewise, removal of hydroxyurea resulted in a decrease in M2 activity. In the present study, we make use of recently isolated cDNAs and monoclonal antibodies for both the M1 and M2 proteins to further our understanding of the mechanism of hydroxyurea resistance at the molecular level in a subclone of this cell line. Our results indicated that protein M1 levels were elevated 2-3-fold and protein M2 levels were increased about 50-fold in the mutant cells when they were grown in the absence of hydroxyurea, compared to wild-type cells. These protein increases were accompanied by corresponding elevations in the levels of mRNAs for both subunits and increased rates of transcription of both genes. There was a 6-fold amplification in the gene copy number for protein M2.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Reversion of hydroxyurea resistance, decline in ribonucleotide reductase activity, and loss of M2 gene amplification.

A key rate-limiting reaction in the synthesis of DNA is catalyzed by ribonucleotide reductase, the enzyme which reduces ribonucleotides to provide the deoxyribonucleotide precursors of DNA. The antitumor agent, hydroxyurea, is a specific inhibitor of this enzyme and has been used in the selection of drug resistant mammalian cell lines altered in ribonucleotide reductase activity. An unstable hydroxyurea resistant population of mammalian cells with elevated ribonucleotide reductase activity has been used to isolate three stable subclones with varying sensitivities to hydroxyurea cytotoxicity and levels of ribonucleotide reductase activities. These subclones have been analyzed at the molecular level with cDNA probes encoding the two nonidentical subunits of ribonucleotide reductase (M1 and M2). Although no significant differences in M1 mRNA levels or gene copy numbers were detected between the three cell lines, a strong correlation between cellular resistance, enzyme activity, M2 mRNA and M2 gene copies was observed. This is the first demonstration that reversion of hydroxyurea resistance is directly linked to a decrease in M2 mRNA levels and M2 gene copy number, and strongly supports the concept that M2 gene amplification is an important mechanism for achieving resistance to this antitumor agent through elevations in ribonucleotide reductase.

Animals↗
PubMed1986

Characterization of a mouse cell line selected for hydroxyurea resistance by a stepwise procedure: drug-dependent overproduction of ribonucleotide reductase activity.

Hydroxyurea was used as a selective agent in culture, to isolate by a stepwise procedure, a unique mouse L cell line called LHF which exhibited a stable resistance to high concentrations of drug (5 mM). LHF cells contained an elevation in ribonucleotide reductase activity which depended upon whether cells were previously cultured in the presence or absence of hydroxyurea. M1 immunoprecipitation and M2 titration experiments indicated that both ribonucleotide reductase subunits were elevated in drug-resistant cells. Interestingly, a very large drug-dependent change in the M2 activity (about a 100-fold) was observed. Studies on enzyme activity with cycloheximide and actinomycin D indicated that the hydroxyurea-dependent increase in activity required de novo protein synthesis and transcriptional activity. These results are different from other ribonucleotide reductase overproducing cell lines previously described, and indicate that hydroxyurea modulates enzyme activity by an interesting mechanism.

Animals↗