Marvin D. Siperstein

Regulation of cholesterol biosynthesis in normal and malignant tissues.

Publisher Summary This chapter discusses the major biochemical and subcellular characteristics of the systems that regulate cholesterol synthesis in the liver and in the intestine. The rate of cholesterol synthesis in the livers of higher animals is controlled primarily by a sensitive negative feedback system, and to a lesser extent, by caloric intake. Bile acids do not play a direct role in regulation of cholesterol synthesis in the liver. Cholesterol synthesis in the intestine of higher animals has been shown to be controlled by a similar feedback control mechanism; however, the feedback inhibitor consists of bile acids rather than cholesterol. Phylogenetic studies indicate that, in contrast to the case in higher animals, in nonmammalian vertebrates, cholesterol synthesis in the intestine is regulated by cholesterol itself. The most striking defect in the control of cholesterol synthesis so far observed involves the consistent loss of the cholesterol feedback system in all hepatomas.

Electron Microscopic Quantification of Diabetic Microangiopathy

1. Several methods of quantifying muscle capillary basement membrane width have been evaluated with the aim of determining whether variations in technic may influence the sensitivity of detecting significant capillary basement membrane thickening in diabetic subjects. 2. The results demonstrate that fixation of tissue in osmic acid followed by determination of mean basement membrane thickness reveals significant basement membrane thickening in over 90 per cent of diabetic subjects. 3. The error in estimation of mean basement membrane width due to oblique sectioning of diabetic capillaries is shown to be small, i.e. approximately 10 per cent. By contrast, measuring only the minimum basement membrane thickness of a capillary may underestimate diabetic basement membrane hypertrophy by from 20 to 40 per cent. 4. As compared to osmic acid fixation, glutaraldehyde leads to a very significant increase in the basement membrane width affecting normal capillaries relatively more than diabetic capillaries. 5. As a result, either fixation in glutaraldehyde or measurement of minimum basement membrane thickness may lead to a marked decrease in the sensitivity of the quantitative basement membrane technic. A procedure that employs both glutaraldehyde fixation and measurement of only minimum basement membrane width would appear to detect microangiopathy in only 45 per cent of diabetic subjects.

A Longitudinal Study of Adaptive Changes in Oxygen Transport and Body Composition

The effects of a 20-day period of bed rest followed by a 55-day period of physical training were studied in five male subjects, aged 19 to 21. Three of the subjects had previously been sedentary, and two of them had been physically active. The studies after bed rest and after physical training were both compared with the initial control studies. Effects of Bed Rest All five subjects responded quite similarly to the bed rest period. The total body weight remained constant; however, lean body mass, total body water, intracellular fluid volume, red cell mass, and plasma volume tended to decrease. Electron microscopic studies of quadriceps muscle biopsies showed no significant changes. There was no effect on total lung capacity, forced vital capacity, one-second expiratory volume, alveolar-arterial oxygen tension difference, or membrane diffusing capacity for carbon monoxide. Total diffusing capacity and pulmonary capillary blood volume were slightly lower after bed rest. These changes were related to changes in pulmonary blood flow. Resting total heart volume decreased from 860 to 770 ml. The maximal oxygen uptake fell from 3.3 in the control study to 2.4 L/min after bed rest. Cardiac output, stroke volume, and arterial pressure at rest in supine and sitting positions did not change significantly. The cardiac output during supine exercise at 600 kpm/min decreased from 14.4 to 12.4 L/min, and stroke volume fell from 116 to 88 ml. Heart rate increased from 129 to 154 beats/min. There was no change in arterial pressure. Cardiac output during upright exercise at submaximal loads decreased approximately 15% and stroke volume 30%. Calculated heart rate at an oxygen uptake of 2 L/min increased from 145 to 180 beats/min. Mean arterial pressures were 10 to 20 mm Hg lower, but there was no change in total peripheral resistance. The A-V 02 difference was higher for any given level of oxygen uptake. Cardiac output during maximal work fell from 20.0 to 14.8 L/min and stroke volume from 104 to 74 ml. Total peripheral resistance and A-V 02 difference did not change. The Frank lead electrocardiogram showed reduced T-wave amplitude at rest and during submaximal exercise in both supine and upright position but no change during maximal work. The fall in maximal oxygen uptake was due to a reduction of stroke volume and cardiac output. The decrease cannot exclusively be attributed to an impairment of venous return during upright exercise. Stroke volume and cardiac output were reduced also during supine exercise. A direct effect on myocardial function, therefore, cannot be excluded. Effects of Physical Training In all five subjects physical training had no effect on lung volumes, timed vitalometry, and membrane diffusing capacity as compared with control values obtained before bed rest. Pulmonary capillary blood volume and total diffusing capacity were increased proportional to the increase in blood flow. Alveolar-arterial oxygen tension differences during exercise were smaller after training, suggesting an improved distribution of pulmonary blood flow with respect to ventilation. Red cell mass increased in the previously sedentary subjects from 1.93 to 2.05 L, and the two active subjects showed no change. Maximal oxygen uptake increased from a control value of 2.52 obtained before bed rest to 3.41 L/min after physical training in the three previously sedentary (+33%) and from 4.48 to 4.65 L/min in the two previously active subjects (+4%). Cardiac output and oxygen uptake during submaximal work did not change, but the heart rate was lower and the stroke volume higher for any given oxygen uptake after training in the sedentary group. In the sedentary subjects cardiac output during maximal work increased from 17.2 L/min in the control study before bed rest to 20.0 L/min after training (+16.5%). Arterio-venous oxygen difference increased from 14.6 to 17.0 ml/100 ml (+16.5%). Maximal heart rate remained constant, and stroke volume increased from 90 to 105 (+17%). Resting total heart volumes were 740 ml in the control study before bed rest and 812 ml after training. In the previously active subjects changes in heart volume, maximal cardiac output, stroke volume, and arteriovenous oxygen difference were less marked. Previous studies have shown increases of only 10 to 15% in the maximal oxygen uptake of young sedentary male subjects after training. The greater increase of 33% in maximal oxygen uptake in the present study was due equally to an increase in stroke volume and arteriovenous oxygen difference. These more marked changes may be attributed to a low initial level of maximal oxygen uptake and to an extremely strenuous and closely supervised training program.

Studies on the feed-back regulation of cholesterol synthesis

Summary 1. By means of a newly developed gas-liquid Chromatographic technique it has been possible to measure the synthesis of both β -hydroxy- β -methylglutaric acid and mevalonic acid; this procedure has permitted the direct confirmation of our earlier conclusions that the site of the cholesterol negative feed-back system is specifically located at the point of conversion of β -hydroxy- β -methylglutaric acid to mevalonic acid. 2. The specific inhibitor of this reaction appears to be a cholesterol lipoprotein complex which is capable of causing inhibition of cholesterol synthesis within 2½ hr after parenteral injection. 3. An examination of the intracellular site of the cholesterol feed-back reaction has demonstrated that this system is localized to the microsome and more specifically to the membraneous portion of the microsome. 4. Finally, studies of three separate hepatomas derived from three animal species, including man, have demonstrated that the cholesterol feed-back system is absent from these hepatomas, a finding which provides experimental evidence for the suggestion that the deletion of feed-back control may be related in some manner to carcinogenesis.

Glycolytic Pathways: Their Relation to the Synthesis of Cholesterol and Fatty Acids

Following the suggestion by Lipmann and by Dickens in 1936 that the glucose molecule can be broken down within the cell by a process other than Embden-Meyerhof glycolysis, an extensive amount of research has been carried out in an attempt to elucidate the details of this alternate route of glucose metabolism. Largely through the studies of Horecker and of Racker the reactions involved in the pathway, now known as the pentose phosphate route, or hexosemonophosphate shunt, have been very well described. A summary of the more important of these reactions is shown in figure 1. Glucose-6-phosphate molecules, derived from glucose through the action of insulin, first lose one of their carbons as carbon dioxide to be-

Desmosterol Deposition in Human and Experimental Atherosclerosis

The role of desmosterol in the production of atherosclerotic plaques has been studied. By means of gas chromatographic analysis, it has been possible for the first time to show that in patients treated with triparanol, even for short periods of time, desmosterol does contribute to the sterol deposition. Furthermore, in the induced atherosclerotic plaques of rabbits desmosterol has been found to deposit at least as readily as does cholesterol. It is concluded from these observations that the replacement of serum cholesterol with desmosterol does not seem to offer a rational approach to the treatment of atherosclerosis.

Influence of Long Term Fat-Feeding on Excretion of Cholesterol-4-C14 Metabolites.

Summary Effects of long-term feeding of various types and amounts of fat on fecal cholesterol excretion are reported. Diets containing 30% corn oil produce marked acceleration in excretion of C14 as non-digitonin precipitable neutral sterols in rat feces, accounting in these animals for 17–25% of the administered C14. This non-digitonin precipitable neutral sterol does not contain a ketonic group and is presumably a 3-alphahydroxy sterol.