Glial fibrillary acidic protein transcription responses to transforming growth factor-beta1 and interleukin-1beta are mediated by a nuclear factor-1-like site in the near-upstream promoter

Elevated expression of glial fibrillary acidic protein (GFAP) is associated with astrocyte activation during responses to injury in the CNS. Because transforming growth factor-beta1 (TGF-beta1) and interleukin-1beta (IL-1beta) are released during neural responses to injury and because these cytokines also modulate GFAP mRNA levels, it is of interest to define their role in GFAP transcription. The increases of GFAP mRNA in response to TGF-beta1 and decreases in response to IL-1beta were shown to be transcriptionally mediated in rat astrocytes transfected with a luciferase-reporter construct containing 1.9 kb of 5'-upstream rat genomic DNA. Constructs containing sequential deletions of the rat GFAP 5'-upstream promoter identified a short region proximal to the transcription start (-106 to -53 bp) that provides full responses to TGF-beta1 and IL-1beta. This region contains an unusual sequence motif with overlapping nuclear factor-1 (NF-1)- and nuclear factor-kappaB (NF-kappaB)-like binding sites and homology to known TGF-beta response elements. Mutagenesis (3-bp exchanges) in -70 to -68 bp blocked the induction of GFAP by TGF-beta1 and the repression by IL-1beta. Gel shift experiments showed that the DNA segment -85 to -63 bp was bound by a factor(s) in nuclear extracts from astrocytes. The concentrations of these DNA binding factors were increased by treatment of astrocytes with TGF-beta1 and decreased by IL-1beta. Binding of these nuclear factors was blocked by mutation of -70 to -68 bp. Despite homology to NF-1 or NF-kappaB binding sites in the GFAP promoter at segment -79 to -67 bp, anti-NF-kappaB or anti-NF-1 antibodies did not further retard the gel shift of the nuclear factors/DNA complex. Moreover, astrocytic nuclear proteins do not compete for the specific binding to NF-1 consensus sequence. Thus, nuclear factors from astrocytes that bind to the -85- to -63-bp promoter segment might be only distantly related to NF-1 or NF-kappaB. These findings are pertinent to the use of GFAP promoter constructs in transgenic animals, because cisacting elements in the GFAP promoter are sensitive to cytokines that may be elaborated in response to expression of transgene products.

Diffusible, nonfibrillar ligands derived from Abeta1-42 are potent central nervous system neurotoxins

Abeta1-42 is a self-associating peptide whose neurotoxic derivatives are thought to play a role in Alzheimer's pathogenesis. Neurotoxicity of amyloid beta protein (Abeta) has been attributed to its fibrillar forms, but experiments presented here characterize neurotoxins that assemble when fibril formation is inhibited. These neurotoxins comprise small diffusible Abeta oligomers (referred to as ADDLs, for Abeta-derived diffusible ligands), which were found to kill mature neurons in organotypic central nervous system cultures at nanomolar concentrations. At cell surfaces, ADDLs bound to trypsin-sensitive sites and surface-derived tryptic peptides blocked binding and afforded neuroprotection. Germ-line knockout of Fyn, a protein tyrosine kinase linked to apoptosis and elevated in Alzheimer's disease, also was neuroprotective. Remarkably, neurological dysfunction evoked by ADDLs occurred well in advance of cellular degeneration. Without lag, and despite retention of evoked action potentials, ADDLs inhibited hippocampal long-term potentiation, indicating an immediate impact on signal transduction. We hypothesize that impaired synaptic plasticity and associated memory dysfunction during early stage Alzheimer's disease and severe cellular degeneration and dementia during end stage could be caused by the biphasic impact of Abeta-derived diffusible ligands acting upon particular neural signal transduction pathways.

Kainic acid and decorticating lesions stimulate the synthesis of C1q protein in adult rat brain

The first component of the classic complement cascade, C1q, was increased in whole rat brain after lesioning by intraperitoneally injected kainic acid (KA) (20-fold, 3 days after KA) and in the striatum ipsilateral to unilateral decortication (fivefold, 10 days after decortication). C1q was measured after purification by chromatography and electrophoresis. De novo biosynthesis of C1q 3 days after KA was increased >10-fold, as measured by the incorporation of [35S]methionine into C1q after incubation of brain slices from KA-treated rats for 2 h. In parallel with these responses, KA induced fivefold increase of C1q bioactivity, as evaluated with C1q-dependent hemolysis. The contribution of C1q from entrapped cerebrovascular blood was evaluated by the effects of perfusion and was minor relative to the increases of C1q in response to KA lesioning. These findings support the hypothesis that the C1q protein detected by immunocytochemistry in senile plaques of Alzheimer brains and in the hippocampus after deafferenting lesions is synthesized by resident brain cells.

Clusterin (apoJ) alters the aggregation of amyloid beta-peptide (A beta 1-42) and forms slowly sedimenting A beta complexes that cause oxidative stress

Clusterin (apoJ), a multifunctional apolipoprotein made by cells in the brain and many other locations, is associated with aggregated amyloid beta-peptide (A beta) in senile and diffuse plaques of Alzheimer's disease (AD). We observed that purified human serum clusterin partially blocked the aggregation of synthetic A beta 1-42, as shown by centrifugal assays (14,000g x 10 min) and by atomic force (scanning probe) microscopy. Slowly sedimenting A beta complexes were formed in the presence of clusterin, which included aggregates > 200 kDa that resist dissociation by low concentrations of SDS. Clusterin enhanced the oxidative stress caused by A beta, as assayed by oxidative stress in PC12 cells with MTT, which is widely used to estimate neurotoxicity. These indications of enhanced neurotoxicity by the MTT assay were observed in the highly aggregated rapidly sedimenting fraction, but also in more slowly sedimenting "soluble" forms. This novel activity of slowly sedimenting A beta may enhance the neurotoxicity of A beta deposits in AD brains, because soluble complexes have a potential for diffusing to damage distal neurons.

Isotopomer studies of gluconeogenesis and the Krebs cycle with 13C-labeled lactate

Fasted rats were intragastrically infused with either [2,3-13C]lactate or [1,2,3-13C]lactate. The infusate also contained 14C-labeled lactate and [3-3H]glucose. Glucose, alanine, glutamate, and glutamine were isolated from liver and blood. There was near complete equilibration of lactate and alanine, and the relative specific activities and relative enrichments were the same in blood and liver. Glucose was cleaved enzymatically to lactate. The compounds were examined by gas chromatography-mass spectroscopy. From the mass isotopomer spectra of the lactate, glutamate, and glutamine and their cleavage fragments the positional isotopomer composition of these compounds was obtained. The enrichment and isotopomer pattern in the lactate from cleaved glucose represents that in phosphoenolpyruvate (PEP). When [1,2,3-13C]lactate was infused the mass isotopomer spectrum of glutamates consisted only of compounds containing either one, two, or three 13C carbons per molecule (m1, m2, and m3). There was little 13C in C-4 and C-5 of glutamate. The rate of pyruvate decarboxylation is low, and 3-4% of the acetyl-CoA flux in the Krebs cycle is contributed by lactate carbon. The major isotopomers in lactate, alanine, and PEP were m3 and m2 with 13C in C-2 and C-3. The predominant isotopomer in PEP from [2,3-13C]lactate was m2 with 13C in C-2 and C-3. There was much more of m1 isotopomer with 13C in C-3 and C-2 than the m1 isotopomer with 13C in C-1. There was very little m3, the isotopomer with 13C in all three carbons. Most of the 13C in C-3 and C-4 of glucose and C-1 of glutamate was introduced via 13CO2 fixation. From the isotopomer distribution and the rate of glucose turnover we deduced, applying the analysis described in the "Appendix," the absolute rates of gluconeogenic pathways, recycling of PEP and the Cori cycle, and flux in the Krebs cycle. The flux from oxaloacetate (OAA)-->PEP was seven times that of OAA-->citrate, and about half of PEP was recycled to pyruvate via pyruvate kinase. The mass isotopomer patterns in glutamate and glutamine were similar but differed from those of lactate and glucose. It appears that the glutamates are derived from alpha-ketoglutarate from a different Krebs cycle pool than PEP. The flux from OAA to PEP in this pool was two to three times that of OAA to citrate.(ABSTRACT TRUNCATED AT 400 WORDS)

Further evidence for the classical pentose phosphate cycle in the liver

Isolated rat hepatocytes were incubated with [3-(14)C]xylitol or d-[3-(14)C]xylulose plus xylitol or glucose at substrate concentrations. The glucose formed was isolated and degraded to give the relative specific radioactivities in each carbon atom. C-4 of glucose had the highest specific radioactivity, followed by C-3, with half to one-fifth that of C-4. Only about 1% of the total radioactivity was in C-1. The data are compared with the predictions of the classical pentose phosphate cycle [Horecker, Gibbs, Klenow & Smyrniotis (1954) J. Biol. Chem.207, 393-403], and the proposed new version of the pentose phosphate cycle in liver [Longenecker & Williams (1980) Biochem. J.188, 847-857], which they denoted as the ;L-type pentose cycle'. The Williams pathway predicts that the specific radioactivity of C-1 of glucose should be half that of C-4 (after correction for approximately equal labelling on C-3 and C-4 of hexose phosphate in the pathway involving fructose 1,6-bisphosphatase). The actual labelling in C-1 is 20-350-fold less than this. When the hepatocytes are incubated with phenazine methosulphate, to stimulate the oxidative branch of the pentose phosphate cycle, the predicted relationship between (C-2/C-3) and (C-1/C-3) ratios of specific radio-activities is nearly exactly in accord with the classical pentose phosphate cycle. Glucose and glucose 6-phosphate were isolated and degraded from an incubation of hepatocytes from starved/re-fed rats with [3-(14)C]xylitol. Although the patterns were of the classical type, there was more randomization of (14)C into C-2 and C-1 in the glucose 6-phosphate isolated at the end of the incubation than in the glucose which was continuously produced.

Lipogenesis by hepatocytes of rats with hypothalamic obesity

Hepatocytes were prepared from livers of rats with bilateral lesions of the ventromedial hypothalamic nucleus (VMN lesioned) and sham-operated control rats. The incorporation of tritium labeled water and 14C glucose and lactate into lipids was studied. Endogenous tritium incorporation into fatty acids in controls ranged from 7-20 muatoms 3H per 100 mg liver protein per hour, and in cells from VMN lesioned rats 25 to 80 muatoms per 100 mg/hour. In both types of cells addition of glucose had no effect on tritium incorporation, but the addition of lactate, pyruvate or alanine stimulated it 30-60%. There was little difference in the labeling of glycerol by the two groups. In cells from fasted refed control or lesioned rats, tritium incorporation was greatly increased and was equal in both groups. Incorporation of glucose carbon in VMN lesioned rats was three to ten times that of controls, but the contribution of glucose carbon to fatty acid synthesis was small.

The metabolism of l-[3-3h]lactate by isolated hamster liver cells

The rate of tritium removal from L-[3-3H]lactate by hamster liver cells is faster than the analytical rate of lactate utilization, or the rate of 14C disappearance from 6-[U-14C, 3-3H]Llactate decrease. However, addition of low concentrations (0.1 to 1.0 mM) of L-cycloserine, a glutamate pyruvate transaminase inhibitor, nearly equalizes the rates of isotope utilization from L-[3-3H]lactate and L-[U-14C]lactate. The results suggest a very limited rate of recycling of phosphoenolpyruvate back to pyruvate during gluconeogenesis from lactate in fasted hamster liver cells.

Metabolism of [5-T]fructose by isolated liver cells

Glucose formed from [5-T]fructose in rat hepatocytes contains about 10 to 30% tritium. This does not appear to be due to fructose metabolism via hexokinase since neither the initial presence of glucose, nor wide variations in the original fructose concentration, have much effect on the relative labeling of glucose versus water from [5-T]fructose. Comparison of the T:14C ratios in glucose produced from [U-14C, 5-T]fructose and D-[U-14C, 2-T]glyceraldehyde indicate that there is tritium retention in the metabolism of fructose via the fructokinase-initiated pathway. The tritium retention can cause significant errors in the estimation of the futile cycle between fructose-1,6-P2 and fructose 6-P, by methods involving the use of [5-T]glucose.