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Biomedical subjects

M G Thomas

Publications and source records attributed to M G Thomas.

At least 109 records · Page 6Linked to original sources
PubMed1995

Characterization of the exbBD operon of Escherichia coli and the role of ExbB and ExbD in TonB function and stability.

TonB protein appears to couple the electrochemical potential of the cytoplasmic membrane to active transport across the essentially unenergized outer membrane of gram-negative bacteria. ExbB protein has been identified as an auxiliary protein in this process. In this paper we show that ExbD protein, encoded by an adjacent gene in the exb cluster at 65', was also required for TonB-dependent energy transduction and, like ExbB, was required for the stability of TonB. The phenotypes of exbB exbD+ strains were essentially indistinguishable from the phenotypes of exbB+ exbD strains. Mutations in either gene resulted in the degradation of TonB protein and in decreased, but not entirely absent, sensitivities to colicins B and Ia and to bacteriophage phi 80. Evidence that the absence of ExbB or ExbD differentially affected the half-lives of newly synthesized and steady-state TonB was obtained. In the absence of ExbB or ExbD, newly synthesized TonB was degraded with a half-life of 5 to 10 min, while the half-life of TonB under steady-state conditions was significantly longer, approximately 30 min. These results were consistent with the idea that ExbB and ExbD play roles in the assembly of TonB into an energy-transducing complex. While interaction between TonB and ExbD was suggested by the effect of ExbD on TonB stability, interaction of ExbD with TonB was detected by neither in vivo cross-linking assays nor genetic tests for competition. Assays of a chromosomally encoded exbD::phoA fusion showed that exbB and exbD were transcribed as an operon, such that ExbD-PhoA levels in an exbB::Tn10 strain were reduced to 4% of the levels observed in an exbB+ strain under iron-limiting conditions. Residual ExbD-PhoA expression in an exbB::Tn10 strain was not iron regulated and may have originated from within the Tn10 element in exbB.

Alkaline Phosphatase↗
PubMed1995

Luminal and humoral influences on human rectal epithelial cytokinetics.

Multiple genetic and environmental steps may underpin the development of human colorectal neoplasia, and experimental evidence suggests that promoters of colorectal cancer also induce colorectal epithelial cell hyperplasia. In vitro crypt cell production rate (CCPR) was measured to determine the effect of calcium, epidermal growth factor (EGF), vitamin D3 metabolites and synthetic analogues on human rectal epithelial cell proliferation. In a double-blind trial of oral calcium supplementation, CCPR was reduced by 49% in patients with familial adenomatous polyposis (FAP), but there was no effect on established neoplasia. In control tissue, the active form of vitamin D3 (1,25(OH)2D3) reduced rectal CCPR by 57% at 1 microM, 55% at 10 nM and 45% at 100 pM. Likewise, in tissue taken from patients with FAP, 1,25(OH)2D3 reduced CCPR by 52%. Vitamin D3 has profound effects on calcium metabolism, but synthetic analogues can avoid these. The effects of a synthetic analogue (MC-903) on human rectal CCPR were therefore studied. MC-903 (10(-7) M) reduced CCPR in control tissue by 51%, and in FAP tissue by 52% at 10(-6) M and 51% at 10(-7) M. In addition, MC-903 and a related analogue, EB 1089, produced a clear-cut dose-dependent inhibition of both HT-29 and Caco2 colorectal cancer cells maintained in culture. Hence, vitamin D3 and its analogues can reduce the rate of cell proliferation in normal, premalignant and malignant colorectal epithelial cells and might therefore have future therapeutic uses as chemoprotective or chemotherapeutic agents. Lastly, EGF increases CCPR by 102% in FAP tissue that expresses the EGF receptor. Thus, human colorectal cell proliferation is influenced by a variety of luminal and humoral agents and a greater understanding of these actions should help plan future treatments.

Adenomatous Polyposis Coli↗
PubMed1994

Tetanus in Auckland: a preventable disease.

AIMS: To determine the clinical features, management and outcome for patients admitted to hospital with tetanus in Auckland. METHODS: Medical records of all patients with tetanus admitted to hospitals in Auckland between January 1982 and December 1992 were retrospectively reviewed. RESULTS: Fifteen patients were admitted to hospital during the study period. All patients were more than 40 years old and no patient had a history of more than one tetanus toxoid immunisation before the injury presumed responsible for tetanus. Failure to consider the diagnosis led to delay in starting appropriate treatment in most patients. Eleven patients required mechanical ventilation. The median duration of hospital stay was 44 days. All patients survived. CONCLUSIONS: Tetanus is rare in Auckland and almost exclusively affects those born before 1960 when universal childhood immunisation against tetanus was started. The disease causes significant morbidity which could be avoided by increased efforts to ensure that all persons have completed a full course of immunisation. Universal vaccination against tetanus with regular boosters to maintain immunity would also circumvent the problems encountered with postexposure tetanus prophylaxis.

Adult↗
PubMed1994

Partial suppression of an Escherichia coli TonB transmembrane domain mutation (delta V17) by a missense mutation in ExbB.

Active transport of vitamin B12 and Fe(III)-siderophore complexes across the outer membrane of Escherichia coli appears to be dependent upon the ability of the TonB protein to couple cytoplasmic membrane-generated protonmotive force to outer membrane receptors. TonB is supported in this role by an auxiliary protein, ExbB, which, in addition to stabilizing TonB against the activities of endogenous envelope proteases, directly contributes to the energy transduction process. The topological partitioning of TonB and ExbB to either side of the cytoplasmic membrane restricts the sites of interaction between these proteins primarily to their transmembrane domains. In this study, deletion of valine 17 within the aminoterminal transmembrane anchor of TonB resulted in complete loss of TonB activity, as well as loss of detectable in vivo crosslinking into a 59 kDa complex believed to contain ExbB. The delta V17 mutation had no effect on TonB export. The loss of crosslinking appeared to reflect conformational changes in the TonB/ExbB pair rather than loss of interaction since ExbB was still required for some stabilization of TonB delta V17. Molecular modeling suggested that the delta V17 mutation caused a significant change in the predicted conserved face of the TonB amino-terminal membrane anchor. TonB delta V17 was unable to achieve the 23 kDa proteinase K-resistant form in lysed sphaeroplasts that is characteristic of active TonB. Wild-type TonB also failed to achieve the proteinase K-resistant configuration when ExbB was absent. Taken together these results suggested that the delta V17 mutation interrupted productive TonB-ExbB interactions. The apparent ability to crosslink to ExbB as well as a limited ability to transduce energy were restored by a second mutation (A39E) in or near the first predicted transmembrane domain of the ExbB protein. Consistent with the weak suppression, a 23 kDa proteinase K-resistant form of TonB delta V17 was not observed in the presence of ExbBA39E. Neither the ExbBA39E allele nor the absence of ExbB affected TonB or TonB delta V17 export. Unlike the tonB delta V17 mutation, the exbBA39E mutation did not greatly alter a modelled ExbB transmembrane domain structure. Furthermore, the suppressor ExbBA39E functioned normally with wild-type TonB, suggesting that the suppressor was not allele specific. Contrary to expectations, the TonB delta V17, ExbBA39E pair resulted in a TonB with a greatly reduced half-life (approximately 10 min). These results together with protease susceptibility studies suggest that ExbB functions by modulating the conformation of TonB.

Amino Acid Sequence↗
PubMed1994

Calcipotriol inhibits rectal epithelial cell proliferation in ulcerative proctocolitis.

Vitamin D3 reduces human rectal crypt cell production rate (CCPR) and may thereby protect against colorectal cancer. Cell turnover is increased in ulcerative proctocolitis, which might therefore respond to vitamin D3 metabolites. This study investigated the effect of calcipotriol, a synthetic vitamin D3 analogue that avoids hypercalcaemia, on human rectal CCPR in ulcerative proctocolitis. Paired rectal biopsy specimens from seven patients with severe disease were established in organ culture with or without calcipotriol (1 x 10(-6) M). After 15 hours, vincristine (0.6 microgram/ml) was added to induce metaphase arrest, and CCPR was determined by linear regression analysis of accumulated metaphases. Compared with values in 17 controls with incidental anal conditions, median rectal CCPR was 28% higher in ulcerative proctocolitis: 5.90 (5.00-9.50) v 4.80 (2.85-7.07) cells/crypt/hour (p < 0.01). Calcipotriol reduced CCPR by 62% in patients with ulcerative proctocolitis, from 5.90 (5.00-9.50) to 2.21 (0.81-3.22) cells/crypt/hour (median with range) p < 0.01. Thus calcipotriol can dampen the hyperproliferative state in ulcerative proctocolitis and could have a therapeutic role in the control of this inflammatory condition.

Adult↗
PubMed1994

Divergent effects of epidermal growth factor and calcipotriol on human rectal cell proliferation.

Vitamin D may protect against colorectal cancer by reducing cell proliferation and inducing differentiation. By contrast, epidermal growth factor (EGF) stimulates cell proliferation and may encourage gastrointestinal mucosal healing. This study investigated the effect of a synthetic vitamin D analogue, calcipotriol, and EGF on human rectal epithelial cell proliferation in patients with familial adenomatous polyposis (FAP). In addition, a new technique to measure the cell cycle time is described. Sigmoidoscopic biopsy specimens were obtained from 14 patients with FAP. Tissue was established in organ culture, with or without the addition of EGF (n = 8), or calcipotriol (n = 6). Proliferation was determined using (a) metaphase arrest to measure the crypt cell production rate, (b) native mitotic index, and (c) the growth fraction using PC10 antibody. EGF receptor expression was shown using a polyclonal antibody AP12E. Calcipotriol reduced crypt cell production rate by 52% from mean (SEM) 5.29 (1.18) to 2.56 (0.80) cells/crypt/hour (p < 0.01) and EGF increased crypt cell production rate by 102% from 3.62 (0.59) to 7.33 (0.90) cells/crypt/hour (p < 0.05), and this tissue expressed the EGF receptor. The growth fraction was 48.40 (4.0)%, and the native mitotic index 1.08 (0.14)%. The cell cycle time was estimated as 94.5 hours and the time for mitosis as one hour. Thus, calcipotriol and EGF have divergent effects on human rectal mucosal proliferation.

Adenomatous Polyposis Coli↗
PubMed1993

Outcome of adults with acquired immunodeficiency syndrome in Auckland 1983-9.

AIMS: To assess the survival of Auckland residents diagnosed with AIDS between 1983 and 1989. METHODS: Retrospective case review RESULTS: Sixty nine Auckland residents were diagnosed with AIDS between 1983 and the end of 1989, and were cared for by the infectious disease unit, Auckland Hospital. The overall median survival was 289 days with 48% and 20% surviving at one and two years respectively. Survival was longer among those who were infected through homosexual contact, were younger, and not known to the infectious disease unit prior to the diagnosis of AIDS. Although survival was longer for those diagnosed in 1988 compared to earlier years, it was shorter again for those diagnosed in 1989. Pneumocystis carinii pneumonia, the AIDS defining condition in 63.8% of people, was found less frequently in 1989, and overall was associated with a longer survival than most of the other AIDS-defining diagnoses. A quarter of the people were not known to be infected with HIV prior to the AIDS-defining condition. CONCLUSION: In New Zealand, as in other countries, changes in treatment strategies of people with HIV infection may impinge on the survival of people with AIDS.

Acquired Immunodeficiency Syndrome↗
PubMed1993

Oral calcium inhibits rectal epithelial proliferation in familial adenomatous polyposis.

Calcium reduces colorectal cell turnover and might therefore protect against neoplasia. The inhibitory effects of dietary calcium were tested in a double-blind controlled trial in patients with familial adenomatous polyposis who had undergone previous abdominal colectomy and ileorectal anastomosis. Patients received supplemental calcium carbonate (1500 mg/day) or placebo tablets for 6 months; sigmoidoscopy was performed before and after treatment. Rectal biopsies were maintained in short-term organ culture, and crypt cell production rate (CCPR) was measured stathmokinetically. A total of 25 patients completed the trial; polyp counts were obtained before and after treatment in all and CCPR values in 16. Calcium treatment reduced the mean (s.e.m.) CCPR from 4.72 (0.48) to 2.42 (0.48) cells per crypt per h (P < 0.05), while values for placebo were unchanged (5.46 (1.21) versus 5.08 (1.17) cells per crypt per h). Calcium had no demonstrable effect on the number, size or distribution of rectal polyps. The ability of oral calcium supplementation to suppress rectal epithelial proliferation supports its potential to prevent development of colorectal carcinoma in high-risk individuals.

Adenomatous Polyposis Coli↗
PubMed1992

Upper gastrointestinal endoscopy in patients with human immunodeficiency virus infection: is it worthwhile?

AIM: To determine the diagnostic accuracy of upper gastrointestinal endoscopy in a selected group of patients with HIV infection, who had severe symptoms unresponsive to empirical treatment. METHOD: We reviewed all patients with HIV infection, who had undergone upper gastrointestinal endoscopy at Auckland Hospital. Specific diagnoses were based on the endoscopic appearances and on the histological and microbiological examination of endoscopic biopsies taken from the oesophagus, stomach and duodenum. RESULTS: A definitive diagnosis was reached in 16 of the 21 patients endoscoped, of whom 13 had a good clinical response to treatment. The median survival was three months. The most common diagnosis was oesophageal candidiasis seen in six patients, all of whom responded to treatment with a median survival of six months. Invasive cytomegalovirus gastrointestinal disease was seen in three patients, none of whom had positive blood cultures for cytomegalovirus. All responded to intravenous ganciclovir. Three patients with severe diarrhoea had opportunistic infection of the small bowel. All three had advanced AIDS with a median survival of less than one month. CONCLUSION: Upper gastrointestinal endoscopy has a high diagnostic yield in patients with HIV infection with oesophageal symptoms which fail to respond to antifungal therapy, or with severe weight loss and diarrhoea and it should be considered in these patients because excellent palliation may be possible.

AIDS-Related Opportunistic Infections↗