Biomedical subjects
Publications and source records attributed to T C Lo.
We present a patient with neuroleptic malignant syndrome and intestinal pseudo-obstruction misdiagnosed as being secondary to septicaemia. The management of the patient is discussed with emphasis on the role of creatine kinase and liver function tests.
Records of 76 consecutive patients treated with radiotherapy for cerebral gliomas were reviewed. Eleven patients had no tissue diagnosis, and their outcome was comparable to that of patients with glioblastoma. Patients with other histologic diagnoses had much better results. Among the entire study population, the most important prognostic factors included age and histologic grade. Even with stratification by histologic grade and age, glucocorticoid (steroid) dependency was a reliable prognostic indicator in terms of survival. Fifteen of the 47 patients (32%) who were less steroid dependant are still alive with a median survival time of 29 months. Among the 29 patients who were heavily dependant on steroids during the course of radiotherapy, the median survival time was 5 months with only 2 patients (7%) still alive.
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We have recently demonstrated that two hexose-transport systems are present in undifferentiated rat L6 myoblasts: D-glucose and 2-deoxy-D-glucose are preferentially transported by the high-affinity system, whereas 3-O-methyl-D-glucose is transported primarily by the low-affinity system. Mutant D23 is found to be defective only in the high-affinity hexose-transport system. The low-affinity transport system is much more sensitive to inhibition by cytochalasin B (CB). The present study examines the identity, properties and regulation of the CB-binding sites by measuring CB binding to both whole cells and plasma membrane. Scatchard analysis of the binding data revealed the presence of two CB-binding sites, namely CBH and CBL. These two sites differ not only in their affinity for CB, but their levels can also be differentially altered by various biochemical, physiological and genetic manipulations. CBL resembles the high-affinity hexose-transport system in that it is absent in mutant D23 and is present in larger quantities in glucose-starved cells. Moreover, CB binding to this site is inhibited by D-glucose and 2-deoxy-D-glucose, the preferred substrates of the high-affinity hexose-transport system. On the other hand, CBH is found to be unaltered in mutant D23, which also retains the normal low-affinity hexose-transport system. CBH also resembles the low-affinity transport system in that it is not elevated in glucose-starved cells. Furthermore, binding of CB to this site can be inhibited by 3-O-methyl-D-glucose, the preferred substrate of the low-affinity transport system. It should be noted that 2-deoxy-D-glucose does not have much effect on CBH, and vice versa. Studies with purified membrane preparations indicate that both CB-binding sites are present in similar ratios in the plasma membrane and the low-density microsomal fraction. Plasma-membrane studies also reveal that D-glucose 6-phosphate, but not 2-deoxy-D-glucose 6-phosphate, is very effective in activating CB binding. Data presented suggest that CB binding may be regulated by sugar analogues in an allosteric manner.
A hexose-transport regulatory mutant (D1/S4) was isolated from L6 rat myoblasts on the basis of its resistance to detachment and cell lysis in the presence of antibody and complement. Growth studies indicated that D1/S4 cells had a slower doubling time (29 h) compared with the parental L6 cells (22 h). Furthermore, after 9 days growth, less than 1% cell fusion was observed with D1/S4 cells, whereas 95% cell fusion was observed with the L6 cells. When the parental L6 cells were starved of glucose or treated with anti-L6 antibody, a significant increase in the Vmax, of 2-deoxy-D-glucose (dGlc) and 3-O-methyl-D-glucose (MeGlc) transport was observed. Although glucose-grown D1/S4 cells possessed normal hexose-transport activity, the above treatments had no effect on dGlc and MeGlc transport in these cells. Electrophoresis and immunoblotting studies revealed that D1/S4 cells possessed decreased amounts of a 112 kDa plasma-membrane protein. It is conceivable that this protein may play a role in triggering the antibody- and glucose-starvation-mediated activation of hexose transport and in myogenic differentiation. Unlike D1/S4, mutant F72, a mutant defective in the high-affinity hexose-transport system, was found to possess normal amounts of the 112 kDa protein. Although glucose starvation has no effect on the hexose-transport activity in this mutant, its hexose transport activity can be increased by antibody treatment. These studies with mutants suggest the involvement of regulatory components in the activation of hexose transport.
From 1981 to 1986, 23 patients (24 hips) were treated with single-dose irradiation after hip surgery in an attempt to prevent heterotopic bone formation. All patients were at high risk for the development of heterotopic ossification because of the presence of heterotopic bone in either hip secondary to trauma or previous surgery, ankylosing spondylitis, or hypertrophic osteoarthritis. Thirteen patients (14 hips) underwent primary total-hip arthroplasty, and ten patients underwent revision total-hip arthroplasty or excision of heterotopic bone. The minimum follow-up period was 6 months. All patients were treated by means of a linear accelerator with a single dose of 700 cGy, calculated at midplane. Almost all treatments were given within 72 hours after surgery. Recurrent disease of Brooker grade II type developed in only one (4%) patient. This result is comparable with outcomes reported after fractionated courses of postoperative radiation therapy delivered over a period of 1 or 2 weeks. Postoperative hip irradiation with a single 700-cGy dose appears to be as effective as fractionated courses of radiation in the prevention of heterotopic bone formation in patients at high risk for the development of this complication.
A study of glucose, insulin, lipids and lipoproteins in myotonic dystrophy (MyD) has shown elevation of fasting plasma insulin, triglycerides and very low density lipoproteins (VLDL) but no significant difference from normal in the fasting plasma glucose, total cholesterol or low and high density lipoproteins. Elevation of the total triglyceride and VLDL levels showed a direct relationship to hyperinsulinaemia. Insulin binding to cultured MyD fibroblasts under optimal conditions was significantly reduced but there was no difference in receptor affinity between MyD and control cells. In contrast to insulin binding, LDL binding to MyD fibroblasts was normal although there was a tendency to reduced LDL binding at 37 degrees C that may reflect mildly reduced lipid metabolism. The alterations in lipids and insulin in MyD are compatible with insulin resistance. Laboratory and clinical findings in MyD were compared with other inherited insulin-resistant diseases. MyD showed marked similarity to a group of disorders that have mild insulin resistance and mildly elevated plasma insulin in contrast to others with severe hyperinsulinaemia and insulin resistance. It is suggested that at least some clinical features of MyD may be due to diminished overall effect of insulin or other trophic factors on cell metabolism.
Hexose transport in plasma membrane vesicles prepared from L6 rat myoblasts was shown to be stereospecific, activated by glucose starvation and occurred by both high and low affinity systems. Transport by the high affinity system was shown to occur by an active transport process. Furthermore, the high affinity system was shown to be defective in vesicles prepared from F72 cells (hexose transport mutant). These results indicate that the high affinity hexose transport system is retained in the plasma membrane vesicles. Thus plasma membrane vesicles could be of value in further characterization of the L6 high affinity hexose transport system, without interference from the various metabolic events occurring in whole cells.
Treatment of glucose-grown L6 rat myoblasts with rabbit or sheep anti-(L6-rat myoblast) antibody for 35 min or glucose starvation for at least 8 h results in a 2-fold increase in the Vmax. of 2-deoxy-D-glucose (dGlc) and 3-O-methyl-D-glucose uptake. In both cases, apparent transport affinities were not affected. Furthermore, once stimulation has occurred, further increases in hexose uptake could not be produced. Assays of antibody binding to whole cells suggested that the antibody is not internalized but remains bound on the cell surface. To elucidate the site and mechanism of antibody action, plasma-membrane vesicles from L6 cells were prepared. Anti-L6 antibody was found to cause a time- and dosage-dependent stimulation of dGlc transport in these vesicles. Maximum activation was achieved after 30 min exposure. This antibody-mediated activation could be inhibited by treatment of vesicles with various proteinase inhibitors. Treatment of vesicles with trypsin was also found to activate dGlc transport to levels observed with antibody. These results are virtually identical with those obtained with whole cells and suggest that antibody-mediated activation of hexose transport results from interaction of antibody with a specific membrane component(s).
A method for the selection and isolation of hexose transport mutants in undifferentiated rat myoblast L6 cells is reported; 2-deoxy-D-glucose (2-DOG)-and 2-deoxy-2-fluoro-D-glucose (2FG)-resistant mutants were selected after mutagenization of L6 cells with ethyl methanesulfonate. Of these, D18 and D23 (selected with 0.1 mM 2-DOG) and F72 and F76 (selected with 0.1 mM 2FG) exhibited the lowest hexose transport activity. Uptake of 0.06 mM 2-DOG, 2FG, or 3-O-methyl-D-glucose (3-OMG) by mutants grown in fructose medium supplemented with 0.05 mM 2FG was about four- to five-fold lower than the parental L6 cells. These mutants contain normal levels of ATP and glycolytic enzyme activities. They also exhibit normal transport activities for alpha-aminoisobutyric acid and fructose. Furthermore, hexose transport was observed to be decreased in plasma membrane vesicles prepared from these mutants. Kinetic analysis of 2-DOG and 3-OMG transport in mutant F72 demonstrated that the Vmax for 2-DOG uptake was significantly reduced, whereas the Vmax for 3-OMG transport was not affected. In all cases, the affinity for these hexose analogues was unaffected. In addition mutant F72 was found to be only slightly affected by treatment with various energy inhibitors and sulfhydryl reagents. The results suggest that this mutant is defective in, or has low levels of, a plasma membrane component(s) involved in the high-affinity hexose transport system.
The hexose transport system of undifferentiated L6 rat myoblasts was investigated. 2-Deoxy-D-glucose (2-DOG) and 2-deoxy-2-fluoro-D-glucose (2FG) were used as analogues to investigate the rate-limiting step of hexose uptake into the cell. Virtually all of the 2-DOG or 2FG taken up into the cell was found to be in the phosphorylated form. No significant pool of intracellular free sugar could be detected. This demonstrates that hexose transport, not phosphorylation, is the rate-limiting step. The inhibitory effect of various glucose analogues on 2-DOG and 3-O-methyl-D-glucose (3-OMG) uptake revealed that these two sugars may be taken up into the cell by different carriers. In addition, kinetics analysis of the transport of both sugars also indicates that two hexose transport systems may be present in L6 cells. 2-DOG is transported by high and low affinity transport systems (Km 0.6 mM and 2.9 mM, respectively), whereas 3-OMG is transported by a low affinity system (Km 3.5 mM). Treatment of cells with ionophores or energy uncouplers results in inactivation of the high affinity system, but not the low affinity system.
The importance of sulfhydryl groups for hexose transport in undifferentiated L6 rat myoblasts was investigated. N-ethylmaleimide (NEM) and p-chloromercuribenzenesulfonic acid (pCMBS) inhibited 2-deoxy-D-glucose (2-DOG) transport in a time and concentration-dependent manner. The inhibition produced by both reagents was virtually complete within 5 min, although neither reagent inhibited transport more than 70-80% regardless of the concentrations or incubation times used. Furthermore, the inhibition of 2-DOG transport by pCMBS or NEM could not be prevented by simultaneous preincubation of cells with 20 mM D-glucose or 20 mM 2-DOG. This suggests that sulfhydryl groups required for transport are separate from the hexose binding and transport site. By comparing the effects of the membrane impermeant pCMBS to those of the membrane permeant NEM, cell surface sulfhydryl groups were shown to be essential for hexose binding and transport. In contrast to the inhibition of 2-DOG transport, pCMBS and NEM had much less of an effect on 3-O-methyl-D-glucose (3-OMG) transport. For example, 1 mM NEM inhibited 2-DOG transport by 66%, whereas 3-OMG transport was inhibited by only 7%. This supports the suggestion that these hexose analogues may be transported by different carriers. Kinetic analysis of transport shows that treatment of cells with 1 mM NEM or 1 pCMBS results in inactivation of the high affinity 2-DOG transport system, whereas the low affinity transport system is unaffected. 3-OMG is preferentially transported by the low affinity system.
This review discusses some of the approaches and general criteria that we have used to examine the properties of the hexose transport system in undifferentiated L6 rat myoblasts. These approaches include studying the kinetics of hexose transport in whole cells and plasma membrane vesicles, the effects of various inhibitors on hexose transport, the isolation and characterization of hexose transport mutants, and the use of cytochalasin B (CB) to identify the transport component(s). Transport kinetics indicated that two transport systems are present in these cells. 2-Deoxy-D-glucose is transported primarily by the high affinity system, whereas 3-O-methyl-D-glucose is transported by the low affinity system. Furthermore, these two transport systems are inactivated to different extents by CB. CB has a higher binding affinity for the low affinity hexose transport system. The inhibitory effect of various hexose analogues also revealed the presence of two hexose transport systems. The effects of various ionophores and energy uncouplers on hexose transport suggest that the high affinity system is an active transport process, whereas the low affinity system is of the facilitated diffusion type. The high affinity system is also sensitive to sulfhydryl reagents, whereas the low affinity system is not. Further evidence for the presence of two transport systems comes from the characterization of hexose transport mutants. Two of the mutants isolated are shown to be defective in the high affinity transport system, but not in the low affinity transport system. These mutants are also defective in the CB low affinity binding site. Based on our results a tentative working model for hexose transport in L6 rat myoblasts is presented.
Radiotherapy is a standard postoperative treatment for cerebral glioma. We have observed the onset of symptoms related to brain necrosis, as opposed to recurrent tumor, in surviving patients. This has been manifest as dementia with a computed tomographic pattern of low density in the frontal lobe uninvolved with tumor, but within the field of radiotherapy. Two patients presented with mass lesions also unrelated to recurrent tumor. We question the necessity of full brain irradiation and suggest that radiotherapy techniques be altered to target the tumor and not encompass the entire brain.
Rat eye lenses were prepared for ATP determination by homogenization in 8 M guanidinium chloride-0.01 M EDTA. Standards of ATP were made up in the same solution. ATP appears to be quite stable in this solution in both standards and lens homogenates whether storage is at room temperature, 4 degrees C, or -20 degrees C for up to several weeks. ATP was measured using a luciferin-luciferase preparation in Hepes buffer, pH 7.75. The photons of light produced were detected by a bioluminescence counter.
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Achievements in radiotherapy of head and neck cancer continue. During the past decade, bench marks have been established for certain early staged malignancies. Technical advances have occurred in higher energy accelerators with the development of dual photon and medium-range electron machines. Imaging technology has augmented precision tumor localization and staging, and its marriage to computerized treatment planning is leading the way toward an optimization of delivered radiation dose to the tumor volume. Neutron therapy is finding a definite place in moderately advanced head and neck cancers, and its wider use awaits further developments in compact neutron generators. Hyperthermia combined with photon radiation is undergoing serious clinical trials, and this combination appears to be most promising. The search for radiation sensitizers to enhance tumor destruction and for radioprotectors for normal tissue continues. Progress in the latter appears encouraging. Burgeoning radiobiologic data have led the way to several dose fractionation schemes, and the most promising clinically appears to be hyperfractionation. Combinations of surgery and radiotherapy have improved cure rates in moderately advanced malignancies, but a preference between preoperative and postoperative radiotherapy in specific sites is still controversial. The medical oncologist has become a valued member of the interdisciplinary team rather than remaining solely the recipient of failed radiotherapeutic and surgical patients. There is a keener awareness of the difficulties and intricacies involved in the construction and evaluation of clinical trials and results.
In our attempts to study alterations in cell behaviour by membrane perturbations, the effects of specific antisera on membrane functions were examined. The present communication reports the nature and kinetics of the changes in hexose transport as a result of treatment of L6 rat myoblast cells with specific antibodies. Exposure to complement-inactivated rabbit anti-myoblast antiserum resulted in an almost immediate two- to three-fold increase in hexose transport; morphological changes were observed only after prolonged incubation with the antiserum. Similar stimulation of hexose transport was observed upon exposure of cells to immunoglobulin G (IgG) from both sheep anti-myoblast and rabbit anti-myoblast plasma membrane sera. It seems that the interaction of specific IgG with cell surface components is sufficient to elicit this response. We have established that over 90% of 2-deoxyglucose is phosphorylated upon entrance into rat myoblasts and that exposure of cells to specific antibody does not cause changes in cell volume or general leakiness of the cell membrane. The antibody-stimulated hexose transport system resembles its normal counterpart in its substrate affinity and specificity; it differs from that of the control cells only in the transport capacity. This increase in hexose capacity is brought about by processes independent of protein synthesis and is not likely the result of extensive reorganization of membrane components.