Glucose controls phosphoregulation of hydroxymethylglutaryl coenzyme A reductase through the protein phosphatase 2A-related phosphatase protein, Ppe1, and Insig in fission yeast

A secondary mechanism of action for triazole antifungals in Aspergillus fumigatus mediated by hmg1

Triazole antifungals function as ergosterol biosynthesis inhibitors and are frontline therapy for invasive fungal infections, such as invasive aspergillosis. The primary mechanism of action of triazoles is through the specific inhibition of a cytochrome P450 14-α-sterol demethylase enzyme, Cyp51A/B, resulting in depletion of cellular ergosterol. Here, we uncover a clinically relevant secondary mechanism of action for triazoles within the ergosterol biosynthesis pathway. We provide evidence that triazole-mediated inhibition of Cyp51A/B activity generates sterol intermediate perturbations that are likely decoded by the sterol sensing functions of HMG-CoA reductase and Insulin-Induced Gene orthologs as increased pathway activity. This, in turn, results in negative feedback regulation of HMG-CoA reductase, the rate-limiting step of sterol biosynthesis. We also provide evidence that HMG-CoA reductase sterol sensing domain mutations previously identified as generating resistance in clinical isolates of Aspergillus fumigatus partially disrupt this triazole-induced feedback. Therefore, our data point to a secondary mechanism of action for the triazoles: induction of HMG-CoA reductase negative feedback for downregulation of ergosterol biosynthesis pathway activity. Abrogation of this feedback through acquired mutations in the HMG-CoA reductase sterol sensing domain diminishes triazole antifungal activity against fungal pathogens and underpins HMG-CoA reductase-mediated resistance.

The potential of R. toruloides mevalonate pathway genes in increasing isoprenoid yields in S. cerevisiae: Evaluation of GGPPS and HMG-CoA reductase

The enzymes of the mevalonate pathway need to be improved to achieve high yields of isoprenoids in the yeast Saccharomyces cerevisiae. The red yeast Rhodosporidium toruloides produces high levels of carotenoids and may have evolved to carry a naturally high flux of isoprenoids. Enzymes from such yeasts are likely to be promising candidates for improvement. Towards this end, we have systematically investigated the various enzymes of the mevalonate pathway of R. toruloides and custom synthesized, expressed, and evaluated six key enzymes in S. cerevisiae. The two nodal enzymes geranyl pyrophosphate synthase (RtGGPPS) and truncated HMG-CoA reductase (RttHMG) of R. toruloides showed a significant advantage to the cells for isoprenoid production as seen by a visual carotenoid screen. These two were analyzed further, and attempts were also made at further improvement. RtGGPPS was confirmed to be superior to the S. cerevisiae enzyme, as seen from in vitro activity determinations and in vivo production of the heterologous diterpenoid sclareol. Four mutants were created through rational mutagenesis but were unable to improve the activity further. In the case of RttHMG, functional evaluation of the enzyme revealed that it was very unstable despite functioning very well in S. cerevisiae. We succeeded in stabilizing the enzyme through mutation of a conserved serine in the catalytic region, which did not alter the enzyme activity per se. In vivo evaluation of the mutant revealed that it could enable better sclareol yields. Therefore, these two enzymes from the red yeast are excellent candidates for heterologous isoprenoid production.

Emergence of W272C Substitution in Hmg1 in a Triazole-Resistant Isolate of Aspergillus fumigatus from a Chinese Patient with Chronic Cavitary Pulmonary Aspergillosis

Recently, mutations in the 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) reductase gene (hmg1) have been identified to be associated with triazole resistance in Aspergillus fumigatus. Here, we describe the first case of the G929C mutation in the hmg1 gene, leading to the W272C amino acid substitution, in a triazole-resistant isolate of A. fumigatus recovered from a chronic cavitary pulmonary aspergillosis patient who failed voriconazole therapy in China.

Metabolism of Storage Lipids and the Role of Lipid Droplets in the Yeast Schizosaccharomyces pombe

Storage lipids, triacylglycerols (TAG), and steryl esters (SE), are predominant constituents of lipid droplets (LD) in fungi. In several yeast species, metabolism of TAG and SE is linked to various cellular processes, including cell division, sporulation, apoptosis, response to stress, and lipotoxicity. In addition, TAG are an important source for the generation of value-added lipids for industrial and biomedical applications. The fission yeast Schizosaccharomyces pombe is a widely used unicellular eukaryotic model organism. It is a powerful tractable system used to study various aspects of eukaryotic cellular and molecular biology. However, the knowledge of S. pombe neutral lipids metabolism is quite limited. In this review, we summarize and discuss the current knowledge of the homeostasis of storage lipids and of the role of LD in the fission yeast S. pombe with the aim to stimulate research of lipid metabolism and its connection with other essential cellular processes. We also discuss the advantages and disadvantages of fission yeast in lipid biotechnology and recent achievements in the use of S. pombe in the biotechnological production of valuable lipid compounds.

Phosphatases Generate Signal Specificity Downstream of Ssp1 Kinase in Fission Yeast

AMPK-related protein kinases (ARKs) coordinate cell growth, proliferation, and migration with environmental status. It is unclear how specific ARKs are activated at specific times. In the fission yeast Schizosaccharomyces pombe, the CaMKK-like protein kinase Ssp1 promotes cell cycle progression by activating the ARK Cdr2 according to cell growth signals. Here, we demonstrate that Ssp1 activates a second ARK, Ssp2/AMPKα, for cell proliferation in low environmental glucose. Ssp1 activates these two related targets by the same biochemical mechanism: direct phosphorylation of a conserved residue in the activation loop (Cdr2-T166 and Ssp2-T189). Despite a shared upstream kinase and similar phosphorylation sites, Cdr2 and Ssp2 have distinct regulatory input cues and distinct functional outputs. We investigated this specificity and found that distinct protein phosphatases counteract Ssp1 activity toward its different substrates. We identified the PP6 family phosphatase Ppe1 as the primary phosphatase for Ssp2-T189 dephosphorylation. The phosphatase inhibitor Sds23 acts upstream of PP6 to regulate Ssp2-T189 phosphorylation in a manner that depends on energy but not on the intact AMPK heterotrimer. In contrast, Cdr2-T166 phosphorylation is regulated by protein phosphatase 2A but not by the Sds23-PP6 pathway. Thus, our study provides a phosphatase-driven mechanism to induce specific physiological responses downstream of a master protein kinase.

Targeting prostate cancer cell metabolism: impact of hexokinase and CPT-1 enzymes

Glycolysis has been shown to be required for the cell growth and proliferation in several cancer cells. However, prostate cancer cells were accused of using more fatty acid than glucose to meet their bioenergetic demands. The present study was designed to evaluate the involvement of hexokinase and CPT-1 in the cell growth and proliferation of human prostate cancer cell lines, PC3, and LNCaP-FGC-10. Hexokinase and CPT-1 activities were examined in the presence of different concentrations of their inhibitors, lonidamine and etomoxir, to find the concentration of maximum inhibition ([I max]). To assess cell viability and proliferation, dimethylthiazol (MTT) assay was carried out using [I max] for 24, 48, and 72 h on PC3 and LNCaP cells. Apoptosis was determined using annexin-V, caspase-3 activity assay, Hoechst 33258 staining, and evaluation of mitochondrial membrane potential (MMP). Moreover, ATP levels were measured following lonidamine and etomoxir exposure. In addition, to define the impact of exogenous fatty acid on the cell growth and proliferation, CPT-1 activity was evaluated in the presence of palmitate (50 μM). Hexokinase and CPT-1 activities were significantly inhibited by lonidamine [600 μM] and etomoxir [100 μM] in both cell lines. Treatment of the cells with lonidamine [600 μM] resulted in a significant ATP reduction, cell viability and apoptosis, caspase-3 activity elevation, MMP reduction, and appearance of apoptosis-related morphological changes in the cells. In contrast, etomoxir [100 μM] just decreased ATP levels in both cell lines without significant cell death and apoptosis. Compared with glucose (2 g/L), palmitate intensified CPT-1 activity in both cell lines, especially in LNCaP cells. In addition, activity of CPT-1 was higher in LNCaP than PC3 cells. Our results suggest that prostate cancer cells may metabolize glucose as a source of bioenergetic pathways. ATP could also be produced by long-chain fatty acid oxidation. In addition, these data might suggest that LNCaP is more compatible with palmitate.

Regulation of lipid metabolism: a tale of two yeasts

Eukaryotic cells synthesize multiple classes of lipids by distinct metabolic pathways in order to generate membranes with optimal physical and chemical properties. As a result, complex regulatory networks are required in all organisms to maintain lipid and membrane homeostasis as well as to rapidly and efficiently respond to cellular stress. The unicellular nature of yeast makes it particularly vulnerable to environmental stress and yeast has evolved elaborate signaling pathways to maintain lipid homeostasis. In this article we highlight the recent advances that have been made using the budding and fission yeasts and we discuss potential roles for the unfolded protein response (UPR) and the SREBP-Scap pathways in coordinate regulation of multiple lipid classes.

Phosphoproteome exploration reveals a reformatting of cellular processes in response to low sterol biosynthetic capacity in Arabidopsis

Sterols are membrane-bound isoprenoid lipids that are required for cell viability and growth. In plants, it is generally assumed that 3-hydroxy-3-methylglutaryl-CoA-reductase (HMGR) is a key element of their biosynthesis, but the molecular regulation of that pathway is largely unknown. In an attempt to identify regulators of the biosynthetic flux from acyl-CoA toward phytosterols, we compared the membrane phosphoproteome of wild-type Arabidopsis thaliana and of a mutant being deficient in HMGR1. We performed a N-terminal labeling of microsomal peptides with a trimethoxyphenyl phosphonium (TMPP) derivative, followed by a quantitative assessment of phosphopeptides with a spectral counting method. TMPP derivatization of peptides resulted in an improved LC-MS/MS detection due to increased hydrophobicity in chromatography and ionization efficiency in electrospray. The phosphoproteome coverage was 40% higher with this methodology. We further found that 31 proteins were in a different phosphorylation state in the hmgr1-1 mutant as compared with the wild-type. One-third of these proteins were identified based on novel phosphopeptides. This approach revealed that phosphorylation changes in the Arabidopsis membrane proteome targets major cellular processes such as transports, calcium homeostasis, photomorphogenesis, and carbohydrate synthesis. A reformatting of these processes appears to be a response of a genetically reduced sterol biosynthesis.

Regulation of HMG-CoA reductase in mammals and yeast

HMG-CoA reductase (HMGR), a highly conserved, membrane-bound enzyme, catalyzes a rate-limiting step in sterol and isoprenoid biosynthesis and is the primary target of hypocholesterolemic drug therapy. HMGR activity is tightly regulated to ensure maintenance of lipid homeostasis, disruption of which is a major cause of human morbidity and mortality. HMGR regulation takes place at the levels of transcription, translation, post-translational modification and degradation. In this review, we discuss regulation of mammalian, Saccharomyces cerevisiae and Schizosaccharomyces pombe HMGR and highlight recent advances in the field. We find that the general features of HMGR regulation, including a requirement for the HMGR-binding protein Insig, are remarkably conserved between mammals and ascomycetous fungi, including S. cerevisiae and S. pombe. However the specific details by which this regulation occurs differ in surprising ways, revealing the broad evolutionary themes underlying both HMGR regulation and Insig function.