Structural insight into the mechanism of streptozotocin inhibition of O-GlcNAcase

Streptozotocin, 1982-2022: Forty Years from the FDA's Approval to Treat Pancreatic Neuroendocrine Tumors

In May 1982, the US Food and Drug Administration (FDA) approved the use of streptozotocin to treat pancreatic neuroendocrine tumors (panNETs). Thus, this year marks 40 years since that landmark date. This review of streptozotocin to treat panNETs is intended to commemorate this anniversary. A historical perspective of the chemical structure, pharmacokinetics, and mechanism of action of streptozotocin is followed by data from prospective and retrospective clinical studies. The last section of the review addresses the latest aspects and takes note of the prospects that lie ahead on the future horizon of the use of streptozotocin to treat panNETs, including ongoing clinical trials.

Challenges and issues with streptozotocin-induced diabetes - A clinically relevant animal model to understand the diabetes pathogenesis and evaluate therapeutics

Streptozotocin (STZ) has been extensively used over the last three decades to induce diabetes in various animal species and to help screen for hypoglycemic drugs. STZ induces clinical features in animals that resemble those associated with diabetes in humans. For this reason STZ treated animals have been used to study diabetogenic mechanisms and for preclinical evaluation of novel antidiabetic therapies. However, the physiochemical characteristics and associated toxicities of STZ are still major obstacles for researchers using STZ treated animals to investigate diabetes. Another major challenges in STZ-induced diabetes are sustaining uniformity, suitability, reproducibility and induction of diabetes with minimal animal lethality. Lack of appropriate use of STZ was found to be associated with increased mortality and animal suffering. During STZ use in animals, attention should be paid to several factors such as method of preparation of STZ, stability, suitable dose, route of administration, diet regimen, animal species with respect to age, body weight, gender and the target blood glucose level used to represent hyperglycemia. Therefore, protocol for STZ-induced diabetes in experimental animals must be meticulously planned. This review highlights specific skills and strategies involved in the execution of STZ-induced diabetes model. The present review aims to provide insight into diabetogenic mechanisms of STZ, specific toxicity of STZ with its significance and factors responsible for variations in diabetogenic effects of STZ. Further this review also addresses ways to minimize STZ-induced mortality, suggests methods to improve STZ-based experimental models and best utilize them for experimental studies purported to understand diabetes pathogenesis and preclinical evaluation of drugs.

Discovery of selective small-molecule activators of a bacterial glycoside hydrolase

Fragment-based approaches are used routinely to discover enzyme inhibitors as cellular tools and potential therapeutic agents. There have been few reports, however, of the discovery of small-molecule enzyme activators. Herein, we describe the discovery and characterization of small-molecule activators of a glycoside hydrolase (a bacterial O-GlcNAc hydrolase). A ligand-observed NMR screen of a library of commercially available fragments identified an enzyme activator which yielded an approximate 90 % increase in k_cat/K_M values (k_cat=catalytic rate constant; K_M=Michaelis constant). This compound binds to the enzyme in close proximity to the catalytic center. Evolution of the initial hits led to improved compounds that behave as nonessential activators effecting both K_M and V_max values (V_max=maximum rate of reaction). The compounds appear to stabilize an active “closed” form of the enzyme. Such activators could offer an orthogonal alternative to enzyme inhibitors for perturbation of enzyme activity in vivo, and could also be used for glycoside hydrolase activation in many industrial processes.

Standardized extract of Ficus deltoidea stimulates insulin secretion and blocks hepatic glucose production by regulating the expression of glucose-metabolic genes in streptozitocin-induced diabetic rats

BACKGROUND

Recently, there has been increasing interest in Ficus deltoidea Jack. (Moraceae) due to its chemical composition and the potential health benefits. The present study was undertaken to investigate the effect of extracts of F. deltoidea leaves on diabetes.

METHODS

The petroleum ether, chloroform and methanol extracts of F. deltoidea were prepared and subjected to standardization using preliminary phytochemical and HPLC analysis. Dose selection was made on the basis of acute oral toxicity study (50-5000 mg/kg b. w.) as per OECD guidelines. Diabetes mellitus was induced with streptozotocin and rats found diabetic were orally administered with the extract (250, 500 and 1000 mg/kg) for 14 days. Levels of blood glucose and insulin were measured in control as well as diabetic rats on 0, 7 and 14th day. In addition, glucose metabolism regulating gene expression was assessed using RT-PCR.

RESULTS

HPLC analysis revealed that the methanol extract is enriched with C-glycosylflavones particularly, vitexin and isovitexin. In oral glucose tolerance test, oral administration of the methanol extract increased the glucose tolerance. The methanol extract showed significant (P < 0.01) antidiabetic activity. The extract treatment caused significant reduction (p < 0.01) in elevated fasting blood glucose level in streptozotocin-induced diabetic rats. The streptozotocin-related weight loss in rats was noticeably reversed by the extract treatment. Finally, RT-PCR analysis revealed a novel mechanisms for the anti-diabetic action of methanol extract of F. deltoidea. The extract exerted its effect via an increase of insulin secretion which impeded the hepatic glucose production, via down-regulation of phosphoenolpyruvate carboxykinase and glucose-6-phosphatase genes expression on one hand, and up-regulation of hepatic GK and PPARγ genes expression on the other hand. The extract caused an increased expression of GLUT-4 gene expression in skeletal muscles which leads to normalize the hyperglycemia. The extract also nullified the toxic effects of streptozitocin by blocking its entry into the islet β-cells through reducing the expression of GLUT-2 gene.

CONCLUSION

It can be concluded that, F. deltoidea could potentially inhibits the streptozitocin-induced hyperglycemia in rats. Further the herb can be utilized as useful remedy for alleviation of diabetes complications.

Defining the structural origin of the substrate sequence independence of O-GlcNAcase using a combination of molecular docking and dynamics simulation

Protein glycosylation with O-linked N-acetylglucosamine (O-GlcNAc) is a post-translational modification of serine/threonine residues in nucleocytoplasmic proteins. O-GlcNAc has been shown to play a role in many different cellular processes and O-GlcNAcylation is often found at sites that are also known to be phosphorylated. Unlike phosphorylation, O-GlcNAc levels are regulated by only two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAc hydrolase (O-GlcNAcase or OGA). So far, no obvious consensus sequence has been found for sites of O-GlcNAcylation. Additionally, O-GlcNAcase recognizes and cleaves all O-GlcNAcylated proteins, independent of their sequence. In this work, we generate and analyze five models of O-GlcNAcylated peptides in complex with a bacterial OGA. Each of the five glycopeptides bind to OGA in a similar fashion, with OGA-peptide interactions primarily, but not exclusively, involving the peptide backbone atoms, thus explaining the lack of sensitivity to peptide sequence. Nonetheless, differences in peptide sequences, particularly at the -1 to -4 positions, lead to variations in predicted affinity, consistent with observed experimental variations in enzyme kinetics. The potential exists, therefore, to employ the present analysis to guide the development glycopeptide-specific inhibitors, or conversely, the conversion of OGA into a reagent that could target specific O-GlcNAcylated peptide sequences.

Urinary angiotensinogen as a novel early biomarker of intrarenal renin-angiotensin system activation in experimental type 1 diabetes

Urinary excretion of albumin (UAlb) is used clinically as a marker of diabetic nephropathy (DN). Although DN was thought to be a unidirectional process, recent studies demonstrated that a large proportion of patients diagnosed with DN reverted to normoalbuminuria. Moreover, despite the normoalbuminuria, one-third of them exhibited reduced renal function even during the microalbuminuric stage. This study was performed to investigate whether urinary angiotensinogen (UAGT) level may serve as a useful marker of the early stage of experimental type 1 diabetes (T1DM). T1DM was induced by a single intraperitoneal injection of streptozotocin. Control mice were injected with citrate buffer. Two days after streptozotocin injection, half of the mice received continuous insulin treatment. Our data showed that UAlb excretion was increased 6 days after streptozotocin injection compared to controls, whereas UAGT excretion was increased at an earlier time point. These increases were reversed by insulin treatment. The UAGT to UAlb ratio was increased in diabetic mice compared to control mice. Furthermore, the increased AGT expression in the kidneys was observed in diabetic mice. These data suggest that UAGT might be useful as a novel early biomarker of activation of the renin-angiotensin system in experimental type 1 diabetes.

Structural determinants of an insect beta-N-Acetyl-D-hexosaminidase specialized as a chitinolytic enzyme

β-N-acetyl-D-hexosaminidase has been postulated to have a specialized function. However, the structural basis of this specialization is not yet established. OfHex1, the enzyme from the Asian corn borer Ostrinia furnacalis (one of the most destructive pests) has previously been reported to function merely in chitin degradation. Here the vital role of OfHex1 during the pupation of O. furnacalis was revealed by RNA interference, and the crystal structures of OfHex1 and OfHex1 complexed with TMG-chitotriomycin were determined at 2.1 Å. The mechanism of selective inhibition by TMG-chitotriomycin was related to the existence of the +1 subsite at the active pocket of OfHex1 and a key residue, Trp(490), at this site. Mutation of Trp(490) to Ala led to a 2,277-fold decrease in sensitivity toward TMG-chitotriomycin as well as an 18-fold decrease in binding affinity for the substrate (GlcNAc)(2). Although the overall topology of the catalytic domain of OfHex1 shows a high similarity with the human and bacterial enzymes, OfHex1 is distinguished from these enzymes by large conformational changes linked to an "open-close" mechanism at the entrance of the active site, which is characterized by the "lid" residue, Trp(448). Mutation of Trp(448) to Ala or Phe resulted in a more than 1,000-fold loss in enzyme activity, due mainly to the effect on k(cat). The current work has increased our understanding of the structure-function relationship of OfHex1, shedding light on the structural basis that accounts for the specialized function of β-N-acetyl-D-hexosaminidase as well as making the development of species-specific pesticides a likely reality.

Inhibition of a bacterial O-GlcNAcase homologue by lactone and lactam derivatives: structural, kinetic and thermodynamic analyses

The dynamic, intracellular, O-GlcNAc modification is of continuing interest and one whose study through targeted "chemical genetics" approaches is set to increase. Of particular importance is the inhibition of the O-GlcNAc hydrolase, O-GlcNAcase (OGA), since this provides a route to elevate cellular O-GlcNAc levels, and subsequent phenotypic evaluation. Such a small molecule approach complements other methods and potentially avoids changes in protein-protein interactions that manifest themselves in molecular biological approaches to O-GlcNAc transferase knockout or over-expression. Here we describe the kinetic, thermodynamic and three-dimensional structural analysis of a bacterial OGA analogue from Bacteroides thetaiotaomicron, BtGH84, in complex with a lactone oxime (LOGNAc) and a lactam form of N-acetylglucosamine and compare their binding signatures with that of the more potent inhibitor O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino N-phenyl carbamate (PUGNAc). We show that both LOGNAc and the N-acetyl gluconolactam are significantly poorer inhibitors than PUGNAc, which may reflect poorer mimicry of transition state geometry and steric clashes with the enzyme upon binding; drawbacks that the phenyl carbamate adornment of PUGNAc helps mitigate. Implications for the design of future generations of inhibitors are discussed.

Development of GlcNAc-inspired iminocyclitiols as potent and selective N-acetyl-beta-hexosaminidase inhibitors

Human N-acetyl-beta-hexosaminidase (Hex) isozymes are considered to be important targets for drug discovery. They are directly linked to osteoarthritis because Hex is the predominant glycosidase released by chondrocytes to degrade glycosaminoglycan. Hex is also associated with lysosomal storage disorders. We report the discovery of GlcNAc-type iminocyclitiols as potent and selective Hex inhibitors, likely contributed by the gain of extra electrostatic and hydrophobic interactions. The most potent inhibitor had a K(i) of 0.69 nM against human Hex B and was 2.5 x 10(5) times more selective for Hex B than for a similar human enzyme O-GlcNAcase. These glycosidase inhibitors were shown to modulate intracellular levels of glycolipids, including ganglioside-GM2 and asialoganglioside-GM2.

Structural analyses of enzymes involved in the O-GlcNAc modification

In order to study the O-GlcNAc modification in vivo, it is evident that a range of specific small molecule inhibitors would be a valuable asset. One strategy for the design of such compounds would be to utilise 3-D structural information in tandem with knowledge of catalytic mechanism. The last few years has seen major breakthroughs in our understanding of the 3-D structure of the enzymes involved in the O-GlcNAc modification notably from the study of the tetratricopeptide repeat (TPR) domain of the human O-GlcNAc transferase, of the bacterial homologs of the O-GlcNAc hydrolase and more latterly bacterial homologs of the O-GlcNAc transferase itself. Of particular note are the bacterial O-GlcNAc hydrolase homologs that provide near identical active centres to the human enzyme. These have informed the design and/or subsequent analysis of inhibitors of this enzyme which have found great use in the chemical dissection of the O-GlcNAc in vivo, as described by Macauley and Vocadlo elsewhere in this issue.