Peroxisome proliferators as inducers and substrates of UDP-glucuronosyltransferases

Summary— Peroxisome proliferators, despite their chemically unrelated structures, share the common property of being able to stimulate the glucuronidation of bilirubin in rodents and, probably, also in man. The aryloxycarboxylic acids (clofibric acid, fenofibrate, bezafibrate, ciprofibrate), tiadenol and probucol, all of which have hypolipidemic properties, as well as the fatty acid-like perfluorodecanoic acid all enhanced the expression of the UDP-glucuronosyltransferase (UGT) form involved in the conjugation of the pigment. This induction is manifested by an increase in the mRNA species encoding the protein with a subsequent increase in the neosynthesis of the corresponding protein in the endoplasmic reticulum. The induction process is concomitant with that of cytochrome P-450-IVA1 and cytosolic epoxide hydrolase, which, like bilirubin UGT, are mainly involved in the metabolism of endogenous substrates. With a series of carboxylic acids related to clofibric acid, it was possible to demonstrate that induction was mediated via specific interactions based on the physicochemical properties of the inducers. Until now, the molecular basis of induction of bilirubin UGT is not known. The peroxisome proliferators that possess a carboxyl group are good substrates of UGT, especially in man. The acylglucuronides formed are known for their instability and reactivity which could contribute to the toxicity encountered in some patients treated with the drugs. There is convincing evidence that UGT bilirubin does not catalyze the glucuronidation of these substances even if the two types of substrate form acylglucuronides.     

Friend or Foe? Implication of the autophagy-lysosome pathway in SARS-CoV-2 infection and COVID-19

There is increasing amount of evidence indicating the close interplays between the replication cycle of SARS-CoV-2 and the autophagy-lysosome pathway in the host cells. While autophagy machinery is known to either assist or inhibit the viral replication process, the reciprocal effects of the SARS-CoV-2 on the autophagy-lysosome pathway have also been increasingly appreciated. More importantly, despite the disappointing results from the clinical trials of chloroquine and hydroxychloroquine in treatment of COVID-19, there is still ongoing effort in discovering new therapeutics targeting the autophagy-lysosome pathway. In this review, we provide an update-to-date summary of the interplays between the autophagy-lysosome pathway in the host cells and the pathogen SARS-CoV-2 at the molecular level, to highlight the prognostic value of autophagy markers in COVID-19 patients and to discuss the potential of developing novel therapeutic strategies for COVID-19 by targeting the autophagy-lysosome pathway. Thus, understanding the nature of such interactions between SARS-CoV-2 and the autophagy-lysosome pathway in the host cells is expected to provide novel strategies in battling against this global pandemic.     

Heparan sulfate proteoglycans regulate autophagy in Drosophila

Heparan sulfate-modified proteoglycans (HSPGs) are important regulators of signaling and molecular recognition at the cell surface and in the extracellular space. Disruption of HSPG core proteins, HS-synthesis, or HS-degradation can have profound effects on growth, patterning, and cell survival. The Drosophila neuromuscular junction provides a tractable model for understanding the activities of HSPGs at a synapse that displays developmental and activity-dependent plasticity. Muscle cell-specific knockdown of HS biosynthesis disrupted the organization of a specialized postsynaptic membrane, the subsynaptic reticulum (SSR), and affected the number and morphology of mitochondria. We provide evidence that these changes result from a dysregulation of macroautophagy (hereafter referred to as autophagy). Cellular and molecular markers of autophagy are all consistent with an increase in the levels of autophagy in the absence of normal HS-chain biosynthesis and modification. HS production is also required for normal levels of autophagy in the fat body, the central energy storage and nutritional sensing organ in Drosophila. Genetic mosaic analysis indicates that HS-dependent regulation of autophagy occurs non-cell autonomously, consistent with HSPGs influencing this cellular process via signaling in the extracellular space. These findings demonstrate that HS biosynthesis has important regulatory effects on autophagy and that autophagy is critical for normal assembly of postsynaptic membrane specializations.     

Overcoming resistance to rapalogs in gliomas by combinatory therapies

Glioblastoma is the most common and aggressive brain tumor type, with a mean patient survival of approximately 1 year. Many previous analyses of the glioma kinome have identified key deregulated pathways that converge and activate mammalian target of rapamycin (mTOR). Following the identification and characterization of mTOR-promoting activity in gliomagenesis, data from preclinical studies suggested the targeting of mTOR by rapamycin or its analogs (rapalogs) as a promising therapeutic approach. However, clinical trials with rapalogs have shown very limited efficacy on glioma due to the development of resistance mechanisms. Analysis of rapalog-insensitive glioma cells has revealed increased activity of growth and survival pathways compensating for mTOR inhibition by rapalogs that are suitable for therapeutic intervention. In addition, recently developed mTOR inhibitors show high anti-glioma activity. In this review, we recapitulate the regulation of mTOR signaling and its involvement in gliomagenesis, discuss mechanisms resulting in resistance to rapalogs, and speculate on strategies to overcome resistance. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).     

Genome-Wide Identification, Classification, and Expression Analysis of Autophagy-Associated Gene Homologues in Rice (Oryza sativa L.)

Autophagy is an intracellular degradation process for recycling macromolecules and organelles. It plays important roles in plant development and in response to nutritional demand, stress, and senescence. Organisms from yeast to plants contain many autophagy-associated genes (ATG). In this study, we found that a total of 33 ATG homologues exist in the rice [Oryza sativa L. (Os)] genome, which were classified into 13 ATG subfamilies. Six of them are alternatively spliced genes. Evolutional analysis showed that expansion of 10 OsATG homologues occurred via segmental duplication events and that the occurrence of these OsATG homologues within each subfamily was asynchronous. The Ka/Ks ratios suggested purifying selection for four duplicated OsATG homologues and positive selection for two. Calculating the dates of the duplication events indicated that all duplication events might have occurred after the origin of the grasses, from 21.43 to 66.77 million years ago. Semi-quantitative RT–PCR analysis and mining the digital expression database of rice showed that all 33 OsATG homologues could be detected in at least one cell type of the various tissues under normal or stress growth conditions, but their expression was tightly regulated. The 10 duplicated genes showed expression divergence. The expression of most OsATG homologues was regulated by at least one treatment, including hormones, abiotic and biotic stresses, and nutrient limitation. The identification of OsATG homologues showing constitutive expression or responses to environmental stimuli provides new insights for in-depth characterization of selected genes of importance in rice.     

GRASP55 regulates the unconventional secretion and aggregation of mutant huntingtin

Recent studies demonstrated that the Golgi reassembly stacking proteins (GRASPs), especially GRASP55, regulate Golgi-independent unconventional secretion of certain cytosolic and transmembrane cargoes; however, the underlying mechanism remains unknown. Here, we surveyed several neurodegenerative disease–related proteins, including mutant huntingtin (Htt-Q74), superoxide dismutase 1 (SOD1), tau, and TAR DNA–binding protein 43 (TDP-43), for unconventional secretion; our results show that Htt-Q74 is most robustly secreted in a GRASP55-dependent manner. Using Htt-Q74 as a model system, we demonstrate that unconventional secretion of Htt is GRASP55 and autophagy dependent and is enhanced under stress conditions such as starvation and endoplasmic reticulum stress. Mechanistically, we show that GRASP55 facilitates Htt secretion by tethering autophagosomes to lysosomes to promote autophagosome maturation and subsequent lysosome secretion and by stabilizing p23/TMED10, a channel for translocation of cytoplasmic proteins into the lumen of the endoplasmic reticulum–Golgi intermediate compartment. Moreover, we found that GRASP55 levels are upregulated by various stresses to facilitate unconventional secretion, whereas inhibition of Htt-Q74 secretion by GRASP55 KO enhances Htt aggregation and toxicity. Finally, comprehensive secretomic analysis identified novel cytosolic cargoes secreted by the same unconventional pathway, including transgelin (TAGLN), multifunctional protein ADE2 (PAICS), and peroxiredoxin-1 (PRDX1). In conclusion, this study defines the pathway of GRASP55-mediated unconventional protein secretion and provides important insights into the progression of Huntington’s disease.     

Cell structural changes in the needles of Norway spruce exposed to long-term ozone and drought

Effects of ozone and/or drought on Norway spruce needles werestudied using light microscopy and electron microscopy. Saplingswere exposed to ozone in open-top chambers during 1992–1995and also to drought in the late summers of 1993–1995.Samples from current and previous year needles were collectedfive times during 1995. Ozone increased the numbers of peroxisomesand mitochondria, which suggests that defence mechanisms againstoxidative stress were active. The results from peroxisomes suggestthat the oxidative stress was more pronounced in the upper sideof the needles, and those from mitochondria that defence wasmore active in the younger needle generation. Possibly due tothe good nitrogen status and the active defence, no ozone-specificchloroplast alterations were seen. At the end of the season,older needles from ozone treatments had smaller central vacuolescompared with other needles. Cytoplasmic vacuoles around thenucleus were increased by ozone in the beginning of the experiment,and did not increase towards the end of the season as in thecontrols. These results from vacuoles may indicate that ozoneaffected the osmotic properties of the cells. Decreased numberand underdevelopment of sclerenchyma cells and proliferationof tonoplast were related to nutrient imbalance, which was enhancedby drought. Larger vascular cylinders and more effective starchaccumulation before and after the drought periods compensatedfor the reduced water status. Numbers of peroxisomes and mitochondriawere increased in the drought-exposed needles before the onsetof drought treatments of the study year, i.e. these changeswere memory effects. Interactions between ozone and droughtwere few.