Interchange reconnection as the source of the fast solar wind within coronal holes.

The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called 'coronal holes'. The energy source responsible for accelerating the plasma is widely debated; however, there is evidence that it is ultimately magnetic in nature, with candidate mechanisms including wave heating1,2 and interchange reconnection3-5. The coronal magnetic field near the solar surface is structured on scales associated with 'supergranulation' convection cells, whereby descending flows create intense fields. The energy density in these 'network' magnetic field bundles is a candidate energy source for the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft6 that provide strong evidence for the interchange reconnection mechanism. We show that the supergranulation structure at the coronal base remains imprinted in the near-Sun solar wind, resulting in asymmetric patches of magnetic 'switchbacks'7,8 and bursty wind streams with power-law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data, including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, magnetic reconnection is continuous and the wind is driven by both the resulting plasma pressure and the radial Alfvénic flow bursts.

Alfvénic velocity spikes and rotational flows in the near-Sun solar wind.

The prediction of a supersonic solar wind1 was first confirmed by spacecraft near Earth2,3 and later by spacecraft at heliocentric distances as small as 62 solar radii4. These missions showed that plasma accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. Alfvénic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy5-7. Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams8. Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii9-11, well within the distance at which stream interactions become important. We find that Alfvén waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per second-considerably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age12-14.

Highly structured slow solar wind emerging from an equatorial coronal hole.

During the solar minimum, when the Sun is at its least active, the solar wind1,2 is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvénic rarefied stream of plasma originating from deep within coronal holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind3 of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain4; theories and observations suggest that they may originate at the tips of helmet streamers5,6, from interchange reconnection near coronal hole boundaries7,8, or within coronal holes with highly diverging magnetic fields9,10. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfvén-wave turbulence11,12, heating by reconnection in nanoflares13, ion cyclotron wave heating14 and acceleration by thermal gradients1. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe15 at 36 to 54 solar radii that show evidence of slow Alfvénic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities10,16 that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude coronal holes are a key source of the slow solar wind.

A genomic case study of mixed fibrolamellar hepatocellular carcinoma.

BACKGROUND: Mixed fibrolamellar hepatocellular carcinoma (mFL-HCC) is a rare liver tumor defined by the presence of both pure FL-HCC and conventional HCC components, represents up to 25% of cases of FL-HCC, and has been associated with worse prognosis. Recent genomic characterization of pure FL-HCC identified a highly recurrent transcript fusion (DNAJB1:PRKACA) not found in conventional HCC. PATIENTS AND METHODS: We performed exome and transcriptome sequencing of a case of mFL-HCC. A novel BAC-capture approach was developed to identify a 400 kb deletion as the underlying genomic mechanism for a DNAJB1:PRKACA fusion in this case. A sensitive Nanostring Elements assay was used to screen for this transcript fusion in a second case of mFL-HCC, 112 additional HCC samples and 44 adjacent non-tumor liver samples. RESULTS: We report the first comprehensive genomic analysis of a case of mFL-HCC. No common HCC-associated mutations were identified. The very low mutation rate of this case, large number of mostly single-copy, long-range copy number variants, and high expression of ERBB2 were more consistent with previous reports of pure FL-HCC than conventional HCC. In particular, the DNAJB1:PRKACA fusion transcript specifically associated with pure FL-HCC was detected at very high expression levels. Subsequent analysis revealed the presence of this fusion in all primary and metastatic samples, including those with mixed or conventional HCC pathology. A second case of mFL-HCC confirmed our finding that the fusion was detectable in conventional components. An expanded screen identified a third case of fusion-positive HCC, which upon review, also had both conventional and fibrolamellar features. This screen confirmed the absence of the fusion in all conventional HCC and adjacent non-tumor liver samples. CONCLUSION: These results indicate that mFL-HCC is similar to pure FL-HCC at the genomic level and the DNAJB1:PRKACA fusion can be used as a diagnostic tool for both pure and mFL-HCC.

Direct evidence for a three-dimensional magnetic flux rope flanked by two active magnetic reconnection X lines at Earth's magnetopause.

We report the direct detection by three THEMIS spacecraft of a magnetic flux rope flanked by two active X lines producing colliding plasma jets near the center of the flux rope. The observed density depletion and open magnetic field topology inside the flux rope reveal important three-dimensional effects. There was also evidence for nonthermal electron energization within the flux rope core where the fluxes of 1-4 keV superthermal electrons were higher than those in the converging reconnection jets. The observed ion and electron energizations differ from current theoretical predictions.

Isolation, ultrastructure, and protein composition of the flagellar rootlet of Naegleria gruberi.

Attached to the basal bodies of Naegleria gruberi flagellates is a striated rootlet or rhizoplast. The rootlet-basal body complex has been isolated by Triton X-100 lysis of deflagellated cells and differential centrifugation through a 25% glycerol medium. Rootlets isolated from mature flagellates are approximately 13 micrometers long but vary from 8 to 15 micrometers in length: they taper at both ends from a maximum width of approximately 0.25 micrometers in the vicinity of the basal bodies. They are highly stable during isolation but can be solubilized by urea, high salt, low pH, or detergent (Sarkosyl). Partial dissociation of rootlets with 1 M urea reveals that they are composed of filaments, approximately 5 nm diameter, associated in a linear fashion to yield the characteristic 21-nm cross-banded appearance. Differential solubilization of rootlets and their associated contaminants allowed identification of a major rootlet protein, comprising at least 50% of any purified rootlet preparation, with an apparent subunit molecular weight of 170,000. The localization of rootlets in situ by indirect immunofluorescence using a specific antibody directed against the purified rootlet protein demonstrated unequivocally that this 170,000-dalton protein is an organelle component.

Relationship between skeletal muscle lipoprotein lipase activity and 24-hour macronutrient oxidation.

A low ratio of whole-body 24-h fat/carbohydrate (CHO) oxidation has been shown to be a predictor of subsequent body weight gain. We tested the hypothesis that the variability of this ratio may be related to differences in skeletal muscle metabolism. Since lipoprotein lipase (LPL) plays a pivotal role in partitioning lipoprotein-borne triglycerides to adipose (storage) and skeletal muscle (mostly oxidation), we postulated that a low ratio of fat/CHO oxidation was associated with a low skeletal muscle LPL (SMLPL) activity. As an index of substrate oxidation, 24-h RQ was measured under sedentary and eucaloric conditions in 16 healthy nondiabetic Pima males. During a 6-h euglycemic, hyperinsulinemic clamp, muscle biopsies were obtained at baseline, 3, and 6 h. Heparin-elutable SMLPL activity was 2.92 +/- 0.56 nmol free fatty acids/g.min (mean +/- SD) at baseline, was unchanged (2.91 +/- 0.51) at the third hour, and increased significantly (P < 0.05) to 3.13 +/- 0.57 at the sixth hour of the clamp. The mean (of baseline and 3-h) SMLPL activity correlated inversely with 24-h RQ (r = 0.57, P < 0.03) but not with body size, body composition, or insulin-mediated glucose uptake. Since SMLPL activity is related to the ratio of whole body fat/CHO oxidation rate, a decreased muscle LPL activity may, therefore, predispose to obesity.

Reduced sympathetic nervous activity. A potential mechanism predisposing to body weight gain.

The sympathetic nervous system is recognized to play a role in the etiology of animal and possibly human obesity through its impact on energy expenditure and/or food intake. We, therefore, measured fasting muscle sympathetic nerve activity (MSNA) in the peroneal nerve and its relationship with energy expenditure and body composition in 25 relatively lean Pima Indian males (means +/- SD; 26 +/- 6 yr, 82 +/- 19 kg, 28 +/- 10% body fat) and 19 Caucasian males (29 +/- 5 yr, 81 +/- 13 kg, 24 +/- 9% body fat). 24-h energy expenditure, sleeping metabolic rate, and resting metabolic rate were measured in a respiratory chamber, whereas body composition was estimated by hydrodensitometry. Pima Indians had lower MSNA than Caucasians (23 +/- 6 vs 33 +/- 10 bursts/min, P = 0.0007). MSNA was significantly related to percent body fat in Caucasians (r = 0.55, P = 0.01) but not in Pimas. MSNA also correlated with energy expenditure adjusted for fat-free mass, fat mass, and age in Caucasians (r = 0.51, P = 0.03; r = 0.54, P = 0.02; and r = 0.53, P = 0.02 for adjusted 24-h energy expenditure, sleeping metabolic rate, and resting metabolic rate, respectively) but not in Pima Indians. In conclusion, the activity of the sympathetic nervous system is a determinant of energy expenditure in Caucasians. Individuals with low resting MSNA may be at risk for body weight gain resulting from a lower metabolic rate. A low resting MSNA and the lack of impact of MSNA on metabolic rate might play a role in the etiology of obesity in Pima Indians.

Down-regulation of histone H3 and H4 gene transcription in differentiated L6 myotubes.

The core histone mRNA levels in terminally differentiated L6 myotubes decrease to less than 5% of the amount present in proliferating myoblasts in parallel with the cessation of DNA synthesis (Bird, RC., Jacobs, F.A., Stein, G., Stein, J. and Sells, B.H. (1985) Biochim. Biophys. Acta 824, 209-217). The role of gene transcription in the down-shift of histone mRNA levels was assessed using a cell-free system. The level of transcription from the differentiation-independent adenovirus major late promoter was directly related to the RNA polymerase II activity of myoblast and myotube nuclear extracts. In addition, both extracts actively transcribed the histone H4 gene template containing only the 5 proximal promoter region (-210 bp). In contrast, inclusion of the distal-proximal promoter region (-410 to -210 bp) in the template resulted in a 60% decrease in transcription by the myotube extract. A similar down-shift in transcription of the histone H3 gene template (containing 900 bp 5 of the initiation site) by myotube nuclear extracts was also observed. The decrease in histone mRNA levels in myotubes may therefore be controlled in part by a transcriptional mechanism involving a negative regulatory factor.

Histone H4 mRNA levels are down-regulated by 3' RNA processing during terminal differentiation of myoblasts.

The capacity for 3' processing of the histone H4 pre-mRNA is lost following differentiation of rat L6 myoblasts to myotubes. Nuclear extracts prepared from proliferating myoblasts, but not differentiated myotubes, actively process histone H4 pre-mRNA in vitro. The activity of two factors required for 3' processing, the heat-labile factor and U7 snRNP, also changes during the differentiation period, concurrent with the loss of 3' processing activity. During myotube formation, the activity of the heat-labile factor decreases significantly while the 5' sequences of the U7 snRNA become progressively resistant to micrococcal nuclease digestion. Thus, the dramatic down-shift in histone H4 mRNA levels which occurs during myoblast differentiation is controlled at both the transcriptional and posttranscriptional level.

The limited utility of fibrinogen I 125 leg scanning.

Using venography as the reference procedure, this study examined the utility of fibrinogen I 125 scanning for the detection or demonstration of deep venous thrombosis. The results demonstrate the inability of leg scanning to detect accurately the presence or absence of thrombi in the deep venous system. Most striking was the lack of sensitivity of this procedure in areas where the propensity for embolization is greatest. Sensitivity is extremely low in the anatomic areas where leg scanning demonstrates reasonable specificity. The results are nearly identical in the extremity not operated upon. The validity of all prior studies relying heavily or exclusively on 125I leg scans to determine the presence or absence of thrombi must be critically reassessed.

Thermic effect of food in humans: methods and results from use of a respiratory chamber.

During the past two decades, many investigators have measured the thermic effect of food (TEF) in humans and have speculated on its role in the development of obesity. In this study we compared different ways of computing TEF from daily energy expenditure measurements in a respiratory chamber, evaluated the determinants of TEF, and more importantly assessed for the first time the relation between TEF and change in body weight. In 471 subjects, TEF was 1697 +/- 857 kJ/d (mean +/- SD), ie, 18 +/- 9% of energy intake. In 114 subjects studied more than once, intraindividual TEF variability was very high (CV = 48%). TEF correlated positively with the level of spontaneous physical activity (SPA) and negatively with fasting plasma glucose and insulin concentrations. TEF correlated inversely with age (males only) and body weight, percent body fat, and waist-to-hip ratio (females only). The level of SPA and fasting plasma glucose concentration were the only significant determinants of TEF, explaining 15% of its variance. In 137 subjects in whom body weight was measured > or = 6 mo after TEF measurement (mean follow-up duration of 2.9 +/- 1.7 y), a low TEF was not predictive of body weight gain. We conclude that, despite the low reproducibility of TEF from use of a respiratory chamber, data in a large number of subjects suggest that TEF is increased by higher SPAs and that insulin resistance is associated with a low TEF. More important, longitudinal data indicate that the variability in TEF is not associated with changes in body weight.

Energy metabolism in weight-stable postobese individuals.

A low metabolic rate for a given body size and body composition and a low ratio of fat to carbohydrate oxidation predict body weight gain. Such metabolic traits could also explain, in part, the propensity of previously obese (postobese) individuals to regain weight after dieting. We studied 11 postobese volunteers (4 males, 7 females; aged 43 +/- 13 y, weighing 80.6 +/- 10.2 kg, with 30 +/- 7% body fat; x +/- SD) who lost 57 +/- 38 kg (23-139 kg) over 14 +/- 12 mo (6-48 mo) on various diet programs and had maintained this weight loss for > or = 2 mo (2-72 mo; 21 +/- 27 mo). After > or = 2 d of a weight-maintenance diet on a metabolic ward, 24-h energy expenditure and ratio of fat to carbohydrate oxidation were measured in a respiratory chamber. Compared with a control group (n = 110) with similar physical characteristics (aged 43 +/- 14 y, weighing 79.5 +/- 11.4 kg, with 30 +/- 12% body fat), [sequence: see text] postobese individuals had similar energy expenditures adjusted for fat-free mass, fat mass, age, and sex, but significantly higher respiratory quotients over 24 h (0.883 +/- 0.026 compared with 0.863 +/- 0.024, P < 0.01) and during sleep, 10 h after the last meal (0.894 +/- 0.063 compared with 0.845 +/- 0.055). These results suggest that postobese individuals have low rates of fat oxidation that may explain their propensity to regain weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Ad libitum food intake on a "cafeteria diet" in Native American women: relations with body composition and 24-h energy expenditure.

Epidemiologic studies consistently report associations between obesity and dietary fat but not total energy intake. We measured ad libitum food intake in a laboratory setting and evaluated its relation to body weight and composition, energy expenditure, and macronutrient utilization in 28 women of Pima-Papago heritage (aged 27 +/- 7 y, 85.3 +/- 19.0 kg, 44 +/- 6% body fat; means +/- SD). All women were studied during the follicular phase of the menstrual cycle. After a 4-d weight-maintenance period, the volunteers selected their food for 5 d from computerized vending machines offering a variety of familiar and preferred foods, ie, a "cafeteria diet". Twenty-four-hour energy expenditure and substrate oxidation were measured in a respiratory chamber on the 4th d o weight maintenance and the 5th d of ad libitum intake. Average ad libitum intake was 13,732 +/- 4238 kJ/d (11 +/- 1% protein, 40 +/- 1% fat, 49 +/- 4% carbohydrate), ie, moderate overeating by 27 +/- 37% above weight maintenance requirements (range: -27% to 124%). Percent body fat correlated with daily energy intake (r = 0.53, P < 0.01), the degree of overeating (r = 0.41, P < 0.05), and the selection of a diet higher in fat and lower in carbohydrate (r = 0.70 and r = -0.63, respectively, P < 0.001). Excess carbohydrate intake caused an increase in carbohydrate oxidation (r = 0.51, P < 0.01), whereas excess fat intake resulted in a decrease in fat oxidation (r = -0.53, P < 0.01) and thus a positive fat balance of 85 +/- 65 g/d. The positive relations among degrees of obesity, dietary fat intake and overeating, and the fact that dietary fat does not induce fat oxidation, support the hypothesis that dietary fat promotes obesity in women.

Ad libitum food intake in humans after manipulation of glycogen stores.

It is controversial whether food intake in humans is under day-to-day regulation to maintain constant body glycogen stores. In eight white males with a mean (+/-SD) age of 30 +/- 4 y, body weight of 82 +/- 20 kg, and percentage body fat of 22 +/- 5%, exercise and diets were used to produce either high (HG) or low glycogen (LG) stores in a randomized crossover design. After each treatment a vastus lateralis muscle biopsy was obtained. Subsequent ad libitum food intake was measured with an automated food-selection system during 2 d in a respiratory chamber. Despite a 46 +/- 21% difference in muscle glycogen between the two treatments, ad libitum 2-d food intakes (energy, weight, or macronutrients) were similar between treatments (HG: 23.80 +/- 4.67 MJ/d; LG: 21.20 +/- 6.73 MJ/d). However, energy intake on the second day of ad libitum feeding was negatively correlated with carbohydrate balance on the first day, adjusted for the effect of total energy intake and treatment. Adjusted carbohydrate balance on day 1 only explained 9% of the variance in energy intake on day 2. The 24-h respiratory quotient on the first day after treatment was higher after the HG than after the LG treatment: 0.94 +/- 0.04 and 0.88 +/- 0.07 (P < 0.001). The findings suggest that 1) body glycogen stores play at most a minor role in short-term food intake regulation, and 2) in the short term, imbalances in glycogen stores are corrected by adjustments of macronutrient oxidation rates.

Fat intake and adiposity in children of lean and obese parents.

We examined the relations between obesity in parents and fat intake in their children, and the effect of fat intake on fat mass in these children. Our heterogenous sample (-x+/-SD: 20.2+/-3.4 kg; 3.2+/-1.3kg fat mass) consisted of 56 white and 15 Mohawk children 4-7 y of age (35 girls and 36 boys). Dietary intake was assessed with the Willett food-frequency questionnaire revised for children. Body composition was measured by bioelectrical resistance and subscapular and triceps skinfold thicknesses. Physical-activity energy expenditure was estimated by the difference between total energy expenditure (measured over 14 d by the doubly labeled water method) and postprandial resting energy expenditure (measured by indirect calorimetry). Before statistical analysis, fat mass was adjusted for fat-free mass, and fat intake was adjusted for nonfat intake. There was no effect of sex or ethnicity on fat intake and no effect of ethnicity on the relation between fat intake and fat mass. Adjusted mean (+/-SE) fat intakes for the groups of children, based on parental obesity status, were as follows: 1.65+/-0.09 MJ/d (nonobese mother and father), 2.58+/-0.10 MJ/d (obese father, nonobese mother), and 2.79+/-0.10 MJ/d (obese mother and father). We found an influence of maternal obesity on dietary fat intake in children (P=0.052) and a significant correlation between fat mass and fat intake in boys (r=0.48, P<0.01) but not in girls after adjustment for physical-activity energy expenditure. Our data suggest that 1)mothers may contribute to the development of obesity in children by influencing their dietary fat intake, and 2) dietary fat intake contributes to obesity in boys, independent of physical-activity energy expenditure.

Energy expenditure and free-living physical activity in black and white women: comparison before and after weight loss.

BACKGROUND: The prevalence of obesity is higher in black than in white women. Differences in energy economy and physical activity may contribute to this difference. OBJECTIVE: The objective of this study was to compare free-living energy expenditure and physical activity in black and white women before and after weight loss. DESIGN: Participants were 18 white and 14 black women with body mass indexes (in kg/m(2)) between 27 and 30. Diet, without exercise, was used to achieve a weight loss of >/=10 kg and a body mass index <25. After 4 wk of energy balance in overweight and normal-weight states, body composition was assessed by using a 4-compartment model, sleeping and resting energy expenditures were assessed by using a chamber calorimeter, physiologic stress of exercise and exercise economy were measured by using standardized exercise tasks, and daily energy expenditure was assessed by using doubly labeled water. RESULTS: Weight loss averaged 12.8 kg. Sleeping and resting energy expenditures decreased in proportion to changes in body composition. Weight reduction significantly improved physiologic capacity for exercise in both groups of women, making it easier for them to be physically active. Black women had lower body composition-adjusted energy requirements than did white women-both before and after weight loss-during sleep (9% lower, 519 kJ/d; P < 0.001), at rest (14% lower, 879 kJ/d; P < 0.001), during exercise (6% lower; P < 0. 05), and as a daily total (9% lower, 862 kJ/d; P < 0.06). By contrast, free-living physical activity was similar between the groups. CONCLUSIONS: Weight-reduced women had metabolic rates appropriate for their body sizes. Black women had lower resting and nonresting energy requirements in both overweight and normal-weight states than did white women and did not compensate with greater physical activity, potentially predisposing them to greater weight regain.

Dietary fat in relation to body fat and intraabdominal adipose tissue: a cross-sectional analysis.

Numerous studies report positive links between dietary fat and adiposity. However, the relation between fat intake and intraabdominal adipose tissue (IAAT), a risk factor for cardiovascular disease and diabetes, is not known. We therefore evaluated the association between dietary fat and adipose tissue stores in 135 white men aged 44 +/- 10 y (mean+/- SD: weight, 86 +/- 14 kg; body fat, 23 +/- 8%) and in 214 white women aged 45 +/- 14 y (weight, 64 +/- 12 kg; body fat, 33 +/- 10%). Dietary intake was estimated from 3-d food records, body composition from hydrostatic weighing, IAAT and subcutaneous abdominal adipose tissue (SCAAT) by computed tomography, and physical activity by using the Baecke Questionnaire. After adjustment for fat-free mass, sex, age, physical activity, and nonfat energy intake, fat intake was weakly correlated with fat mass, explaining only 2% of the variance (partial R2 = 0.018, P < 0.01). In a separate model that evaluated type of fat, saturated fat was positively related (partial R2 = 0.025, P < 0.01) to fat mass after adjustment for fat-free mass, sex, age, physical activity, and nonfat energy intake whereas polyunsaturated fat intake was negatively related (partial R2 = 0.007, P = 0.056). On the basis of partial correlation analyses, dietary fat was also associated with SCAAT adjusted for nonfat energy intake and IAAT (partial R2 = 0.014, P < 0.01), but not IAAT adjusted for nonfat energy intake and SCAAT. However, the association between dietary fat and adjusted SCAAT was not significant after further adjustment for sex, age, and physical activity. Thus, results of this cross-sectional analysis suggest that dietary fat independently plays a very minor role in increasing overall adiposity and does not specifically influence fat accretion in the intraabdominal region.

Dexamethasone stimulates ribosomal protein L32 gene transcription in rat myoblasts.

Incubation of rat L6 myoblasts for 24 h with 10(-7) M dexamethasone, a glucocorticoid analogue, resulted in a 2.5-fold increase in the rate of ribosomal protein L32 (rpL32) gene transcription with a corresponding increase in the level of rpL32 mRNA. The increased rate of transcription was accompanied by a dramatic enhancement in binding of the delta, but not beta and gamma, factors to the rpL32 gene promoter as measured by gel mobility shift assays. This increased binding reflects a change in the activity of the delta factor since its level is unchanged by dexamethasone treatment. The presence of the glucocorticoid analogue RU38486 reversed the stimulating effect of dexamethasone on rpL32 gene transcription and binding of the delta factor to the delta element. These results suggest that the mechanism which enhances rpL32 gene transcription in dexamethasone-treated rat L6 myoblasts involves glucocorticoid-receptor mediated changes in the activity of the delta factor.

Dexamethasone stimulates rRNA gene transcription in rat myoblasts.

The glucocorticoid analogue, dexamethasone, stimulated RNA synthesis more than two-fold in rat L6 myoblasts, without affecting the rate of cell proliferation. Treatment of myoblasts for 24 h with 10(-7) M dexamethasone resulted in a 30% increase in the cellular RNA level. More than a two-fold stimulation of pre-rRNA gene transcription by dexamethasone, as measured in isolated nuclei and by cell-free transcription, was accompanied by a corresponding increase in pre-rRNA levels. Co-incubation of myoblasts with cycloheximide and dexamethasone did not affect the enhanced pre-rRNA gene transcription demonstrating that de novo protein synthesis was unnecessary to manifest the dexamethasone effect on rDNA transcription. Support for this conclusion is provided by the finding that the levels of UBF1 and UBF2, rDNA upstream binding transcription factors, remain unchanged. The glucocorticoid antagonist RU38486 [11 beta-(4-dimethylaminophenyl)17 beta-hydroxy-17 alpha-(prop-1-ynyl)estra- 4,9-dien-3-one] inhibited the dexamethasone-stimulated rRNA gene transcription suggesting that the glucocorticoid receptor is involved in the response mechanism.