Measuring acetyl-CoA and acetylated histone turnover in vivo: Effect of a high fat diet

Cellular availability of acetyl-CoA, a central intermediate of metabolism, regulates histone acetylation. The impact of a high-fat diet (HFD) on the turnover rates of acetyl-CoA and acetylated histones is unknown. We developed a method for simultaneous measurement of acetyl-CoA and acetylated histones kinetics using a single ²H₂O tracer, and used it to examine effect of HFD-induced perturbations on hepatic histone acetylation in LDLR^-/- mice, a mouse model of non-alcoholic fatty liver disease (NAFLD). Mice were given ²H₂O in the drinking water and the kinetics of hepatic acetyl-CoA, histones, and acetylated histones were quantified based on their ²H-labeling. Consumption of a high fat Western-diet (WD) for twelve weeks led to decreased acetylation of hepatic histones (p< 0.05), as compared to a control diet. These changes were associated with 1.5-3-fold increased turnover rates of histones without any change in acetyl-CoA flux. Acetylation significantly reduced the stability of histones and the turnover rates of acetylated peptides were correlated with the number of acetyl groups in neighboring lysine sites. We conclude that ²H₂O-method can be used to study metabolically controlled histone acetylation and acetylated histone turnover in vivo.

Examining Targeted Protein Degradation from Physiological and Analytical Perspectives: Enabling Translation between Cells and Subjects

The ability to target specific proteins for degradation may open a new door toward developing therapeutics. Although effort in chemistry is essential for advancing this modality, i.e., one needs to generate proteolysis targeting chimeras (bifunctional molecules, also referred to as PROTACS) or "molecular glues" to accelerate protein degradation, we suspect that investigations could also benefit by directing attention toward physiological regulation surrounding protein homeostasis, including the methods that can be used to examine changes in protein kinetics. This perspective will first consider some metabolic scenarios that might be of importance when one aims to change protein abundance by increasing protein degradation. Specifically, could protein turnover impact the apparent outcome? We will then outline how to study protein dynamics by coupling stable isotope tracer methods with mass spectrometry-based detection; since the experimental conditions could have a dramatic effect on protein turnover, special attention is directed toward the application of methods for quantifying protein kinetics using in vitro and in vivo models. Our goal is to present key concepts that should enable mechanistically informed studies which test targeted protein degradation strategies.

Insulin: The Friend and the Foe in the Development of Type 2 Diabetes Mellitus

Insulin, a hormone produced by pancreatic β-cells, has a primary function of maintaining glucose homeostasis. Deficiencies in β-cell insulin secretion result in the development of type 1 and type 2 diabetes, metabolic disorders characterized by high levels of blood glucose. Type 2 diabetes mellitus (T2DM) is characterized by the presence of peripheral insulin resistance in tissues such as skeletal muscle, adipose tissue and liver and develops when β-cells fail to compensate for the peripheral insulin resistance. Insulin resistance triggers a rise in insulin demand and leads to β-cell compensation by increasing both β-cell mass and insulin secretion and leads to the development of hyperinsulinemia. In a vicious cycle, hyperinsulinemia exacerbates the metabolic dysregulations that lead to β-cell failure and the development of T2DM. Insulin and IGF-1 signaling pathways play critical roles in maintaining the differentiated phenotype of β-cells. The autocrine actions of secreted insulin on β-cells is still controversial; work by us and others has shown positive and negative actions by insulin on β-cells. We discuss findings that support the concept of an autocrine action of secreted insulin on β-cells. The hypothesis of whether, during the development of T2DM, secreted insulin initially acts as a friend and contributes to β-cell compensation and then, at a later stage, becomes a foe and contributes to β-cell decompensation will be discussed.

Prolonged Exposure to Insulin Inactivates Akt and Erk1/2 and Increases Pancreatic Islet and INS1E β-Cell Apoptosis

Chronic hyperinsulinemia, in vivo, increases the resistance of peripheral tissues to insulin by desensitizing insulin signaling. Insulin, in a heterologous manner, can also cause IGF-1 resistance. The aim of the current study was to investigate whether insulin-mediated insulin and IGF-1 resistance develops in pancreatic β-cells and whether this resistance results in β-cell decompensation. Chronic exposure of rat islets or INS1E β-cells to increasing concentrations of insulin decreased Akt^S473 phosphorylation in response to subsequent acute stimulation with 10 nM insulin or IGF-1. Prolonged exposure to high insulin levels not only inhibited Akt^S473 phosphorylation, but it also resulted in a significant inhibition of the phosphorylation of P70S6 kinase and Erk_1/2 phosphorylation in response to the acute stimulation by glucose, insulin, or IGF-1. Decreased activation of Akt, P70S6K, and Erk_1/2 was associated with decreased insulin receptor substrate 2 tyrosine phosphorylation and insulin receptor β-subunit abundance; neither IGF receptor β-subunit content nor its phosphorylation were affected. These signaling impairments were associated with decreased SERCA2 expression, perturbed plasma membrane calcium current and intracellular calcium handling, increased endoplasmic reticulum stress markers such as eIF2α^S51 phosphorylation and Bip (GRP78) expression, and increased islet and β-cell apoptosis. We demonstrate that prolonged exposure to high insulin levels induces not only insulin resistance, but in a heterologous manner causes resistance to IGF-1 in rat islets and insulinoma cells resulting in decreased cell survival. These findings suggest the possibility that chronic exposure to hyperinsulinemia may negatively affect β-cell mass by increasing β-cell apoptosis.

Turnover of histones and histone variants in postnatal rat brain: effects of alcohol exposure

BACKGROUND

Alcohol consumption during pregnancy is a significant public health problem and can result in a continuum of adverse outcomes to the fetus known as fetal alcohol spectrum disorders (FASD). Subjects with FASD show significant neurological deficits, ranging from microencephaly, neurobehavioral, and mental health problems to poor social adjustment and stress tolerance. Neurons are particularly sensitive to alcohol exposure. The neurotoxic action of alcohol, i.e., through ROS production, induces DNA damage and neuronal cell death by apoptosis. In addition, epigenetics, including DNA methylation, histone posttranslational modifications (PTMs), and non-coding RNA, play an important role in the neuropathology of FASD. However, little is known about the temporal dynamics and kinetics of histones and their PTMs in FASD.

RESULTS

We examined the effects of postnatal alcohol exposure (PAE), an animal model of human third-trimester equivalent, on the kinetics of various histone proteins in two distinct brain regions, the frontal cortex, and the hypothalamus, using in vivo ²H₂O-labeling combined with mass spectrometry-based proteomics. We show that histones have long half-lives that are in the order of days. We also show that H3.3 and H2Az histone variants have faster turnovers than canonical histones and that acetylated histones, in general, have a faster turnover than unmodified and methylated histones. Our work is the first to show that PAE induces a differential reduction in turnover rates of histones in both brain regions studied. These alterations in histone turnover were associated with increased DNA damage and decreased cell proliferation in postnatal rat brain.

CONCLUSION

Alterations in histone turnover might interfere with histone deposition and chromatin stability, resulting in deregulated cell-specific gene expression and therefore contribute to the development of the neurological disorders associated with FASD. Using in vivo ²H₂O-labeling and mass spectrometry-based proteomics might help in the understanding of histone turnover following alcohol exposure and could be of great importance in enabling researchers to identify novel targets and/or biomarkers for the prevention and management of fetal alcohol spectrum disorders.

Pathophysiology of the Effects of Alcohol Abuse on the Endocrine System

Alcohol can permeate virtually every organ and tissue in the body, resulting in tissue injury and organ dysfunction. Considerable evidence indicates that alcohol abuse results in clinical abnormalities of one of the body's most important systems, the endocrine system. This system ensures proper communication between various organs, also interfacing with the immune and nervous systems, and is essential for maintaining a constant internal environment. The endocrine system includes the hypothalamic-pituitary-adrenal axis, the hypothalamic-pituitary-gonadal axis, the hypothalamic-pituitary-thyroid axis, the hypothalamic-pituitary-growth hormone/insulin-like growth factor-1 axis, and the hypothalamic-posterior pituitary axis, as well as other sources of hormones, such as the endocrine pancreas and endocrine adipose tissue. Alcohol abuse disrupts all of these systems and causes hormonal disturbances that may result in various disorders, such as stress intolerance, reproductive dysfunction, thyroid problems, immune abnormalities, and psychological and behavioral disorders. Studies in both humans and animal models have helped shed light on alcohol's effects on various components of the endocrine system and their consequences.

Transgenerational epigenetics and brain disorders

Neurobehavioral and psychiatric disorders are complex diseases with a strong heritable component; however, to date, genome-wide association studies failed to identify the genetic loci involved in the etiology of these brain disorders. Recently, transgenerational epigenetic inheritance has emerged as an important factor playing a pivotal role in the inheritance of brain disorders. This field of research provides evidence that environmentally induced epigenetic changes in the germline during embryonic development can be transmitted for multiple generations and may contribute to the etiology of brain disease heritability. In this review, we discuss some of the most recent findings on transgenerational epigenetic inheritance. We particularly discuss the findings on the epigenetic mechanisms involved in the heritability of alcohol-induced neurobehavioral disorders such as fetal alcohol spectrum disorders.

Effects of alcohol on the endocrine system

Chronic consumption of a large amount of alcohol disrupts the communication between nervous, endocrine, and immune system and causes hormonal disturbances that lead to profound and serious consequences at physiologic and behavioral levels. These alcohol-induced hormonal dysregulations affect the entire body and can result in various disorders such as stress abnormalities, reproductive deficits, body growth defect, thyroid problems, immune dysfunction, cancers, bone disease, and psychological and behavioral disorders. This review summarizes the findings from human and animal studies that provide consistent evidence on the various effects of alcohol abuse on the endocrine system.

An improved measurement of isotopic ratios by high resolution mass spectrometry

The study of protein kinetics requires an accurate measurement of isotopic ratios of peptides. Although Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometers yield accurate mass measurements of analytes, the isotopologue ratios are consistently lower than predicted. Recently, we demonstrated that the magnitude of the spectral error in the FT-ICR mass spectrometer is proportional to the scan duration of ions. Here, we present a novel isotopic ratio extrapolation (IRE) method for obtaining accurate isotopic ratio measurements. Accuracy is achieved by performing scans with different duration and extrapolation of the data to the initial moment of the ion rotation; IRE minimizes the absolute isotopic ratio error to ≤1%. We demonstrate the application of IRE in protein turnover studies using (2)H(2)O-metabolic labeling. Overall, this technique allows accurate measurements of the isotopic ratios of proteolytic peptides, a critical step for enabling routine studies of proteome dynamics.

Plasma proteome dynamics: analysis of lipoproteins and acute phase response proteins with 2H2O metabolic labeling

Understanding the pathologies related to the regulation of protein metabolism requires methods for studying the kinetics of individual proteins. We developed a (2)H(2)O metabolic labeling technique and software for protein kinetic studies in free living organisms. This approach for proteome dynamic studies requires the measurement of total body water enrichments by GC-MS, isotopic distribution of the tryptic peptide by LC-MS/MS, and estimation of the asymptotical number of deuterium incorporated into a peptide by software. We applied this technique to measure the synthesis rates of several plasma lipoproteins and acute phase response proteins in rats. Samples were collected at different time points, and proteins were separated by a gradient gel electrophoresis. (2)H labeling of tryptic peptides was analyzed by ion trap tandem mass spectrometry (LTQ MS/MS) for measurement of the fractional synthesis rates of plasma proteins. The high sensitivity of LTQ MS in zoom scan mode in combination with (2)H label amplification in proteolytic peptides allows detection of the changes in plasma protein synthesis related to animal nutritional status. Our results demonstrate that fasting has divergent effects on the rate of synthesis of plasma proteins, increasing synthesis of ApoB 100 but decreasing formation of albumin and fibrinogen. We conclude that this technique can effectively measure the synthesis of plasma proteins and can be used to study the regulation of protein homeostasis under physiological and pathological conditions.

Measuring protein synthesis using metabolic ²H labeling, high-resolution mass spectrometry, and an algorithm

We recently developed a method for estimating protein dynamics in vivo with heavy water ((2)H(2)O) using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) [16], and we confirmed that (2)H labeling of many hepatic free amino acids rapidly equilibrated with body water. Although this is a reliable method, it required modest sample purification and necessitated the determination of tissue-specific amino acid labeling. Another approach for quantifying protein kinetics is to measure the (2)H enrichments of body water (precursor) and protein-bound amino acid or proteolytic peptide (product) and to estimate how many copies of deuterium are incorporated into a product. In the current study, we used nanospray linear trap Fourier transform ion cyclotron resonance mass spectrometry (LTQ FT-ICR MS) to simultaneously measure the isotopic enrichment of peptides and protein-bound amino acids. A mathematical algorithm was developed to aid the data processing. The most notable improvement centers on the fact that the precursor/product labeling ratio can be obtained by measuring the labeling of water and a protein (or peptide) of interest, thereby minimizing the need to measure the amino acid labeling. As a proof of principle, we demonstrate that this approach can detect the effect of nutritional status on albumin synthesis in rats given (2)H(2)O.

Measuring proteome dynamics in vivo: as easy as adding water?

Proteomics investigations typically yield information regarding static gene expression profiles. The central issues that limit the study of proteome dynamics include how to (i) administer a labeled amino acid in vivo, (ii) measure the isotopic labeling of a protein(s) (which may be low), and (iii) reliably interpret the precursor/product labeling relationships. In this study, we demonstrate the potential of quantifying proteome dynamics by coupling the administration of stable isotopes with mass spectrometric assays. Although the direct administration of a labeled amino acid(s) is typically used to measure protein synthesis, we explain the application of labeled water, comparing (2)H(2)O versus H(2)(18)O for measuring albumin biosynthesis in vivo. This application emphasizes two distinct advantages of using labeled water over a labeled amino acid(s). First, in long term studies (e.g. days or weeks), it is not practical to continuously administer a labeled amino acid(s); however, in the presence of labeled water, organisms will generate labeled amino acids. Second, to calculate rates of protein synthesis in short term studies (e.g. hours), one must utilize a precursor/product labeling ratio; when using labeled water it is possible to reliably identify and easily measure the precursor labeling (i.e. water). We demonstrate that labeled water permits studies of protein synthesis (e.g. albumin synthesis in mice) during metabolic "steady-state" or "non-steady-state" conditions, i.e. integrating transitions between the fed and fasted state or during an acute perturbation (e.g. following a meal), respectively. We expect that the use of labeled water is applicable to wide scale investigations of proteome dynamics and can therein be used to obtain a functional image of gene expression in vivo.

6-Fluoro-6-deoxy-D-glucose as a tracer of glucose transport

Glucose transport rates are estimated noninvasively in physiological and pathological states by kinetic imaging using PET. The glucose analog most often used is (18)F-labeled 2FDG. Compared with glucose, 2FDG is poorly transported by intestine and kidney. We examined the possible use of 6FDG as a tracer of glucose transport. Lacking a hydroxyl at its 6th position, 6FDG cannot be phosphorylated as 2FDG is. Prior studies have shown that 6FDG competes with glucose for transport in yeast and is actively transported by intestine. Its uptake by muscle has been reported to be unresponsive to insulin, but that study is suspect. We found that insulin stimulated 6FDG uptake 1.6-fold in 3T3-L1 adipocytes and azide stimulated the uptake 3.7-fold in Clone 9 cells. Stimulations of the uptake of 3OMG, commonly used in transport assays, were similar, and the uptakes were inhibited by cyclochalasin B. Glucose transport is by GLUT1 and GLUT4 transporters in 3T3-L1 adipocyte and by the GLUT1 transporter in Clone 9 cells. Cytochalasin B inhibits those transporters. Rats were also imaged in vivo by PET using 6(18)FDG. There was no excretion of (18)F into the urinary bladder unless phlorizin, an inhibitor of active renal transport, was also injected. (18)F activity in brain, liver, and heart over the time of scanning reached a constant level, in keeping with the 6FDG being distributed in body water. In contrast, (18)F from 2(18)FDG was excreted in relatively large amounts into the bladder, and (18)F activity rose with time in heart and brain in accord with accumulation of 2(18)FDG-6-P in those organs. We conclude that 6FDG is actively transported by kidney as well as intestine and is insulin responsive. In trace quantity, it appears to be distributed in body water unchanged. These results provide support for its use as a valid tracer of glucose transport.

Using 2H2O to study the influence of feeding on protein synthesis: effect of isotope equilibration in vivo vs. in cell culture

We previously reported that 2H2O can be used to measure rates of protein synthesis during prolonged steady-state conditions (Previs SF, Fatica R, Chandramouli V, Alexander JC, Brunengraber H, and Landau BR. Am J Physiol Endocrinol Metab 286: E665-E672, 2004). The underlying premise of our method is that following the administration of 2H2O, 2H atoms in body water rapidly equilibrate with free alanine before it is incorporated into newly synthesized proteins. We have now directly examined whether 2H2O can be used to measure the influence of a single meal on protein synthesis. In addition, we have compared the use of 2H2O for measuring rates of protein synthesis in vivo vs. in cell culture. Using a rat model, we observed rapid equilibration between 2H in body water and free alanine; therefore we were able to study the response of protein synthesis to a single meal. We observed that approximately 50% of the plasma albumin that is synthesized over the course of 24 h is made within approximately 5 h after eating (in rats trained to eat a complete 24-h ration of food in a single meal). Contrary to what we observed in vivo, feeding (the replenishment of cell culture medium) does influence the use of 2H2O for in vitro studies. In particular, since there can be slow equilibration of 2H between water and alanine in the cell culture medium, special consideration must be made to avoid underestimating the rate of protein synthesis in vitro.

Endothelin-1-stimulated glucose uptake is desensitized by tumor necrosis factor-alpha in 3T3-L1 adipocytes

Tumor necrosis factor-alpha (TNF-alpha) is a potent inducer of insulin resistance, and increased TNF-alpha expression is associated with impaired glucose disposal. Although insulin is the primary regulator of glucose transport in adipose, endothelin-1, a vasoconstrictor peptide that signals through the heterotrimeric G proteins Galphaq/11, potently stimulates glucose uptake in 3T3-L1 adipocytes by a mechanism independent of phosphatidylinositol (PI) 3-kinase. Here, we report that exposure of 3T3-L1 adipocytes to TNF-alpha for 48 h dose-dependently decreased endothelin-1-stimulated glucose uptake and translocation of GLUT4 to the plasma membrane. TNF-alpha exposure had no effect on endothelin-1 receptor number at the cell surface. In contrast, TNF-alpha treatment reduced the quantity of Galphaq/11 and proline-rich tyrosine kinase 2 (PYK2) and decreased endothelin-1-stimulated PYK2-Tyr402 tyrosine phosphorylation. Taken together, these results suggest that TNF-alpha-induced desensitization of endothelin-1-stimulated GLUT4 translocation and glucose uptake in 3T3-L1 adipocytes is due, at least in part, to a decreased expression of Galphaq/11, leading to a suppression in tyrosine phosphorylation of PYK2.

Chronic ethanol feeding impairs endothelin-1-stimulated glucose uptake via decreased G alpha 11 expression in rat adipocytes

Chronic ethanol feeding decreases insulin-stimulated glucose uptake in rat adipocytes. Here, we show that chronic ethanol also decreases endothelin-stimulated glucose uptake. Endothelin-1 increased uptake of 2-deoxyglucose 2.4-fold in adipocytes isolated from pair-fed rats. However, in adipocytes isolated from rats that had consumed a diet containing 35% ethanol for 4 wk, endothelin-1 did not increase glucose uptake. Although endothelin-1 increased GLUT4 quantity at the plasma membrane in adipocytes from pair-fed rats, there was no increase in GLUT4 after chronic ethanol feeding. Loss of endothelin-1-stimulated glucose uptake after ethanol feeding was associated with a specific decrease in the quantity of Galpha11 in plasma membranes, with no change in Galphaq quantity. Activation of proline-rich tyrosine kinase 2 (PYK2), a downstream target of Galphaq/11 that is required for endothelin-1-stimulated GLUT4 translocation in 3T3-L1 adipocytes, was also suppressed after chronic ethanol feeding. In contrast, activation of p38 MAPK by endothelin-1 was not affected by chronic ethanol exposure. These data demonstrate that chronic ethanol feeding suppresses endothelin-1-stimulated glucose uptake and suggest that decreased expression of Galpha11 coupled to impaired endothelin-1-dependent activation of PYK2 contributes to this response.

GLUT4 vesicle trafficking in rat adipocytes after ethanol feeding: regulation by heterotrimeric G-proteins

Long-term ethanol consumption decreases insulin-stimulated glucose uptake in isolated rat adipocytes. Here we investigate the mechanisms for this decrease. Male Wistar rats were fed for 4 weeks with a liquid diet containing 35% of the calories from ethanol and compared with pair-fed controls. Stimulation of 3-O-methylglucose transport in isolated adipocytes by insulin was decreased by 70% after ethanol feeding. However, stimulation by insulin of the tyrosine phosphorylation of the p85 subunit of phosphoinositide 3-kinase and the phosphorylation of Akt were not affected by ethanol feeding. GLUT4 was mobilized from intracellular light microsomes in response to insulin in both pair-fed and ethanol-fed rats, resulting in 4.3-fold and 3.3-fold increases in GLUT4 associated with plasma membrane in pair-fed and ethanol-fed rats respectively. Surface-accessible GLUT4, assessed by a trypsin cleavage assay or cell-surface labelling with bis-mannose photolabel, was increased 2.3-fold and 1.6-fold respectively, in pair-fed rats after treatment with insulin. In contrast, insulin did not increase surface-accessible GLUT4 in ethanol-fed rats. Treatment of adipocytes with R-phenylisopropyladenosine, an adenosine A1 receptor agonist, increased the transport of 3-O-methylglucose and trypsin-accessible GLUT4, in adipocytes from both pair-fed and ethanol-fed rats. These results demonstrate that whereas the insulin-mediated signalling and translocation of GLUT4 to the plasma membrane is maintained after ethanol feeding, the final fusion of GLUT4 vesicles to the plasma membrane is disrupted, preventing the stimulation of glucose uptake by insulin. Fusion of GLUT4 with the plasma membrane can be stimulated by the activation of adenosine A1 receptors.

Cyclic AMP regulates G(omicronalpha) protein and mRNA levels by modulating the transcriptional rate of G(omicronalpha) gene

In rat astroglial cells, four G(omicronalpha) transcripts were found: G(omicron2alpha) mRNA (5.7 kb) and three G(omicron1alpha) mRNAs (4.0, 3.0 and 2.3 kb). However, G(omicron2alpha) but little G(omicronalpha1) proteins were present in membrane-enriched fractions. Culturing astroglial cells with forskolin (10 microM) or isoproterenol (10 microM) a beta-adrenergic agonist increased transiently in a time-dependent manner the levels of G(omicronalpha) proteins. The degradation rate of G(omicronalpha) proteins was slightly decreased by the cAMP treatment. In parallel, forskolin (10 microM) treatment increased transiently the amounts of both G(omicron1alpha) and G(omicron2alpha) mRNAs. The relative transcription rate of G(omicronalpha) gene was increased by 1.7-fold in forskolin-treated cells whereas the half-lives of G(omicron1alpha) and G(omicron2alpha) mRNAs were not significantly changed. These results suggest that cAMP regulates the transcription rate of G(omicronalpha) gene and this is compatible with the existence of a cAMP responsive element in the promoter of the G(omicronalpha) gene.

Cyclic AMP regulation of G(i alpha2) and G(i alpha3) mRNAs and proteins in astroglial cells

Culture of rat astrocytes in the presence of 10 microM forskolin, isoproterenol, a nonselective beta-adrenergic agonist, or 8-bromo-cyclic AMP increased transiently in a time-dependent manner the levels of G(i alpha2) and G(i alpha3) mRNAs as measured by northern blot analysis. G(i alpha1) mRNA was not expressed in these cells. After 6-9 h of culture with forskolin or isoproterenol the amounts of G(i alpha2) and G(i alpha3) mRNAs were maximal (150-300% over control). In cells challenged with 8-bromo-cyclic AMP, the level of G(i alpha2) mRNA level was maximal after 1 h of culture, while the maximal for G(i alpha3) mRNA was reached after 6 h of culture. For prolonged exposure times (24 h) to these agents the levels of G(i alpha2) and G(i alpha3) mRNAs decreased to the values observed in control cells. The forskolin-induced increase in G(i alpha2) protein expression measured by western blot analysis was similar to the increase in G(i alpha2) mRNA amount. In contrast, forskolin induced only a modest increase in the quantity of G(i alpha3) protein (150% over control) despite a large increase of G(i alpha3) mRNA content. Transcription rates and RNA stability were measured simultaneously after isolation of newly synthesized mRNA was performed. The half-lives of G(i alpha) mRNAs were approximately 0.7 and 3 h for G(i alpha2) and G(i alpha3), respectively. Culturing astrocytes in the presence of forskolin resulted in an increase in the half-lives of G(i alpha2) mRNA and G(i alpha3) mRNA by five- and twofold, respectively. The relative transcription rate of the G(i alpha3) gene was slightly increased by approximately 1.3-1.5-fold in forskolin-treated cells but not that of G(i alpha2) gene. These results suggest that cyclic AMP enhanced G(i alpha2) and G(i alpha3) mRNA expression by both transcriptional and posttranscriptional mechanisms, with an increase of G(i alpha) mRNA stability as one of the major checkpoints.

Long-term effect of forskolin on the activation of adenylyl cyclase in astrocytes

Long-term (48-h) forskolin treatment of rat astroglial cells led to a slight decrease (30-40%) in the response to isoproterenol, vasoactive-intestinal peptide, guanyl 5'-(beta gamma-imido)diphosphate, guanosine 5'-O-(3-thiotriphosphate) [GTP(S)], and AIF4- in crude membrane fractions. In contrast, the acute stimulatory effect of forskolin was increased by 1.25-1.5-fold. These two opposite effects of forskolin were mediated by a cyclic AMP-dependent mechanism. No changes in Gs alpha, Gi alpha, or G beta protein levels could be determined by immunoblotting using specific antisera. No significant differences were observed in the ability of G proteins extracted from control and forskolin-treated cells to reconstitute a full adenylyl cyclase activity in membranes from S49 cyc- cells, lacking Gs alpha protein. Gs alpha proteins were detected in two pools of membranes, one in the heavy sucrose fractions and the other in light sucrose fractions. Forskolin treatment of the cells shifted Gs alpha protein toward the light-density membranes. We did not find any significant change in the distribution of adenylyl cyclase. In contrast to the decreased stimulation of adenylyl cyclase activity by agonists acting via Gs alpha, observed in the crude membrane fraction, the responses of adenylyl cyclase to forskolin as well as to GTP(S) were increased in the purified plasma membrane fractions. These results may indicate that sensitization of the catalyst appears to be the dominant component in the astroglial cell response to long-term treatment by forskolin.