The Role of Endoplasmic Reticulum in the Differential Endurance against Redox Stress in Cortical and Spinal Astrocytes from the Newborn SOD1G93A Mouse Model of Amyotrophic Lateral Sclerosis

Recent studies reported that the uptake of [18F]-fluorodeoxyglucose (FDG) is increased in the spinal cord (SC) and decreased in the motor cortex (MC) of patients with ALS, suggesting that the disease might differently affect the two nervous districts with different time sequence or with different mechanisms. Here we show that MC and SC astrocytes harvested from newborn B6SJL-Tg (SOD1^G93A) 1Gur mice could play different roles in the pathogenesis of the disease. Spectrophotometric and cytofluorimetric analyses showed an increase in redox stress, a decrease in antioxidant capacity and a relative mitochondria respiratory uncoupling in MC SOD1^G93A astrocytes. By contrast, SC mutated cells showed a higher endurance against oxidative damage, through the increase in antioxidant defense, and a preserved respiratory function. FDG uptake reproduced the metabolic response observed in ALS patients: SOD1^G93A mutation caused a selective enhancement in tracer retention only in mutated SC astrocytes, matching the activity of the reticular pentose phosphate pathway and, thus, of hexose-6P dehydrogenase. Finally, both MC and SC mutated astrocytes were characterized by an impressive ultrastructural enlargement of the endoplasmic reticulum (ER) and impairment in ER–mitochondria networking, more evident in mutated MC than in SC cells. Thus, SOD1^G93A mutation differently impaired MC and SC astrocyte biology in a very early stage of life.

Obligatory role of endoplasmic reticulum in brain FDG uptake

PURPOSE

The endoplasmic reticulum (ER) contains hexose-6P-dehydrogenase (H6PD). This enzyme competes with glucose-6P-phosphatase for processing a variety of phosphorylated hexoses including 2DG-6P. The present study aimed to verify whether this ER glucose-processing machinery contributes to brain FDG uptake.

METHODS

Effect of the H6PD inhibitor metformin on brain 18F-FDG accumulation was studied, in vivo, by microPET imaging. These data were complemented with the in vitro estimation of the lumped constant (LC). Finally, reticular accumulation of the fluorescent 2DG analogue 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxyglucose (2NBDG) and its response to metformin was studied by confocal microscopy in cultured neurons and astrocytes.

RESULTS

Metformin halved brain 18F-FDG accumulation without altering whole body tracer clearance. Ex vivo, this same response faced the doubling of both glucose consumption and lactate release. The consequent fall in LC was not explained by any change in expression or activity of its theoretical determinants (GLUTs, hexokinases, glucose-6P-phosphatase), while it agreed with the drug-induced inhibition of H6PD function. In vitro, 2NBDG accumulation selectively involved the ER lumen and correlated with H6PD activity being higher in neurons than in astrocytes, despite a lower glucose consumption.

CONCLUSIONS

The activity of the reticular enzyme H6PD profoundly contributes to brain 18F-FDG uptake. These data challenge the current dogma linking 2DG/FDG uptake to the glycolytic rate and introduce a new model to explain the link between 18-FDG uptake and neuronal activity.

Proteome-wide Profiling of Clinical PARP Inhibitors Reveals Compound-Specific Secondary Targets

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) are a promising class of targeted cancer drugs, but their individual target profiles beyond the PARP family, which could result in differential clinical use or toxicity, are unknown. Using an unbiased, mass spectrometry-based chemical proteomics approach, we generated a comparative proteome-wide target map of the four clinical PARPi, olaparib, veliparib, niraparib, and rucaparib. PARPi as a class displayed high target selectivity. However, in addition to the canonical targets PARP1, PARP2, and several of their binding partners, we also identified hexose-6-phosphate dehydrogenase (H6PD) and deoxycytidine kinase (DCK) as previously unrecognized targets of rucaparib and niraparib, respectively. Subsequent functional validation suggested that inhibition of DCK by niraparib could have detrimental effects when combined with nucleoside analog pro-drugs. H6PD silencing can cause apoptosis and further sensitize cells to PARPi, suggesting that H6PD may be, in addition to its established role in metabolic disorders, a new anticancer target.

Genistein inhibits glucocorticoid amplification in adipose tissue by suppression of 11β-hydroxysteroid dehydrogenase type 1

Excess glucocorticoids promote visceral obesity, hyperlipidemia, and insulin resistance. The main regulator of intracellular glucocorticoid levels is 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), which converts inactive glucocorticoids into bioactive forms such as cortisol in humans and corticosterone in rodents. Hexose-6-phosphate dehydrogenase (H6PD), which is colocalized with 11β-HSD1 in the intralumen of the endoplasmic reticulum, supplies a crucial coenzyme, NADPH, for full reductase activity of 11β-HSD1. Therefore, it is possible that inhibition of 11β-HSD1 will become a considerable medical treatment for metabolic diseases including obesity and diabetes. Genistein, a soy isoflavone, has received attention for its therapeutic potential for obesity, diabetes, and cardiovascular disease, and has been proposed as a promising compound for the treatment of metabolic disorders. However, the mechanisms underlying the pleiotropic anti-obesity effects of genistein have not been fully clarified. Here, we demonstrate that genistein was able to inhibit 11β-HSD1 and H6PD activities within 10 or 20min, in dose- and time-dependent manners. Inhibition of 11β-HSD2 activity was not observed in rat kidney microsomes. The inhibition was not reversed by two estrogen receptor antagonists, tamoxifen and ICI182,780. A kinetic study revealed that genistein acted as a non-competitive inhibitor of 11β-HSD1, and its apparent Km value for 11-dehydrocorticosterone was 0.5μM. Genistein also acted as a non-competitive inhibitor of H6PD, and its apparent Km values for G6P and NADP were 0.9 and 3.3μM, respectively. These results suggest that genistein may exert its inhibitory effect by interacting with these enzymes.

Changing glucocorticoid action: 11β-hydroxysteroid dehydrogenase type 1 in acute and chronic inflammation

Since the discovery of cortisone in the 1940s and its early success in treatment of rheumatoid arthritis, glucocorticoids have remained the mainstay of anti-inflammatory therapies. However, cortisone itself is intrinsically inert. To be effective, it requires conversion to cortisol, the active glucocorticoid, by the enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). Despite the identification of 11β-HSD in liver in 1953 (which we now know to be 11β-HSD1), its physiological role has been little explored until recently. Over the past decade, however, it has become apparent that 11β-HSD1 plays an important role in shaping endogenous glucocorticoid action. Acute inflammation is more severe with 11β-HSD1-deficiency or inhibition, yet in some inflammatory settings such as obesity or diabetes, 11β-HSD1-deficiency/inhibition is beneficial, reducing inflammation. Current evidence suggests both beneficial and detrimental effects may result from 11β-HSD1 inhibition in chronic inflammatory disease. Here we review recent evidence pertaining to the role of 11β-HSD1 in inflammation. This article is part of a Special Issue entitled 'CSR 2013'.