Nicotinamide N-methyltransferase upregulation contributes to palmitate-elicited peroxisome proliferator-activated receptor transactivation in hepatocytes.

Peroxisome proliferator-activated receptor γ (PPARγ) plays a pivotal role in regulating lipid metabolism and hepatic PPARγ transactivation contributes to fatty liver development. Fatty acids (FAs) are well-known endogenous ligands for PPARγ. Palmitate, a 16-C saturated FA (SFA) and the most abundant SFA in human circulation, is a strong inducer of hepatic lipotoxicity, a central pathogenic factor for various fatty liver diseases. In this study, using both alpha mouse liver 12 (AML12) and primary mouse hepatocytes, we investigated the effects of palmitate on hepatic PPARγ transactivation and underlying mechanisms, as well as the role of PPARγ transactivation in palmitate-induced hepatic lipotoxicity, all of which remain ambiguous currently. Our data revealed that palmitate exposure was concomitant with both PPARγ transactivation and upregulation of nicotinamide N-methyltransferase (NNMT), a methyltransferase catalyzing the degradation of nicotinamide, the predominant precursor for cellular NAD+ biosynthesis. Importantly, we discovered that PPARγ transactivation by palmitate was blunted by NNMT inhibition, suggesting that NNMT upregulation plays a mechanistic role in PPARγ transactivation. Further investigations uncovered that palmitate exposure is associated with intracellular NAD+ decline and NAD+ replenishment with NAD+-enhancing agents, nicotinamide and nicotinamide riboside, obstructed palmitate-induced PPARγ transactivation, implying that cellular NAD+ decline resulted from NNMT upregulation represents a potential mechanism behind palmitate-elicited PPARγ transactivation. At last, our data showed that the PPARγ transactivation marginally ameliorated palmitate-induced intracellular triacylglycerol accumulation and cell death. Collectively, our data provided the first-line evidence supporting that NNMT upregulation plays a mechanistic role in palmitate-elicited PPARγ transactivation, potentially through reducing cellular NAD+ contents.NEW & NOTEWORTHY Hepatic PPARγ transactivation contributes to fatty liver development. Saturated fatty acids (SFAs) induce hepatic lipotoxicity. Here, we investigated whether and how palmitate, the most abundant SFA in the human blood, affects PPARγ transactivation in hepatocytes. We reported for the first time that upregulation of nicotinamide N-methyltransferase (NNMT), a methyltransferase catalyzing the degradation of nicotinamide, the predominant precursor for cellular NAD+ biosynthesis, plays a mechanistic role in regulating palmitate-elicited PPARγ transactivation through reducing intracellular NAD+ contents.

Discovery of a small-molecule inhibitor of the TRIP8b-HCN interaction with efficacy in neurons.

Major depressive disorder is a critical public health problem with a lifetime prevalence of nearly 17% in the United States. One potential therapeutic target is the interaction between hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and an auxiliary subunit of the channel named tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b). HCN channels regulate neuronal excitability in the mammalian hippocampus, and recent work has established that antagonizing HCN function rescues cognitive impairment caused by chronic stress. Here, we utilize a high-throughput virtual screen to find small molecules capable of disrupting the TRIP8b-HCN interaction. We found that the hit compound NUCC-0200590 disrupts the TRIP8b-HCN interaction in vitro and in vivo. These results provide a compelling strategy for developing new small molecules capable of disrupting the TRIP8b-HCN interaction.

Effects of Small Molecule Ligands on ACKR3 Receptors.

Chemokines such as stromal derived factor 1 and their G protein coupled receptors are well-known regulators of the development and functions of numerous tissues. C-X-C motif chemokine ligand 12 (CXCL12) has two receptors: C-X-C chemokine motif receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3). ACKR3 has been described as an atypical "biased" receptor because it does not appear to signal through G proteins and, instead, signals solely through the ß-arrestin pathway. In support of this conclusion, we have shown that ACKR3 is unable to signal through any of the known mammalian G α isoforms and have generated a comprehensive map of the G α activation by CXCL12/CXCR4. We also synthesized a series of small molecule ligands which acted as selective agonists for ACKR3 as assessed by their ability to recruit ß-arrestin to the receptor. Using select point mutations, we studied the molecular characteristics that determine the ability of small molecules to activate ACKR3 receptors, revealing a key role for the deeper binding pocket composed of residues in the transmembrane domains of ACKR3. The development of more selective ACKR3 ligands should allow us to better appreciate the unique roles of ACKR3 in the CXCL12/CXCR4/ACKR3-signaling axis and better understand the structural determinants for ACKR3 activation. SIGNIFICANCE STATEMENT: We are interested in the signaling produced by the G protein coupled receptor atypical chemokine receptor 3 (ACKR3), which signals atypically. In this study, novel selective ligands for ACKR3 were discovered and the site of interactions between these small molecules and ACKR3 was defined. This work will help to better understand the unique signaling roles of ACKR3.

Structure-activity and structure-property relationship studies of spirocyclic chromanes with antimalarial activity.

Malaria is a prevalent and lethal disease. The fast emergence and spread of resistance to current therapies is a major concern and the development of a novel line of therapy that could overcome, the problem of drug resistance, is imperative. Screening of a set of compounds with drug/natural product-based sub-structural motifs led to the identification of spirocyclic chroman-4-one 1 with promising antimalarial activity against the chloroquine-resistant Dd2 and chloroquine-sensitive 3D7 strains of the parasite. Extensive structure-activity and structure-property relationship studies were conducted to identify the essential features necessary for its activity and properties.

Improved Cell-Potent and Selective Peptidomimetic Inhibitors of Protein N-Terminal Methyltransferase 1.

Protein N-terminal methyltransferase 1 (NTMT1) recognizes a unique N-terminal X-P-K/R motif (X represents any amino acid other than D/E) and transfers 1-3 methyl groups to the N-terminal region of its substrates. Guided by the co-crystal structures of NTMT1 in complex with the previously reported peptidomimetic inhibitor DC113, we designed and synthesized a series of new peptidomimetic inhibitors. Through a focused optimization of DC113, we discovered a new cell-potent peptidomimetic inhibitor GD562 (IC50 = 0.93 ± 0.04 µM). GD562 exhibited improved inhibition of the cellular N-terminal methylation levels of both the regulator of chromosome condensation 1 and the oncoprotein SET with an IC50 value of ~50 µM in human colorectal cancer HCT116 cells. Notably, the inhibitory activity of GD562 for the SET protein increased over 6-fold compared with the previously reported cell-potent inhibitor DC541. Furthermore, GD562 also exhibited over 100-fold selectivity for NTMT1 against several other methyltransferases. Thus, this study provides a valuable probe to investigate the biological functions of NTMT1.

Exploring Unconventional SAM Analogues To Build Cell-Potent Bisubstrate Inhibitors for Nicotinamide N-Methyltransferase.

Nicotinamide N-methyltransferase (NNMT) methylates nicotinamide and has been associated with various diseases. Herein, we report the first cell-potent NNMT bisubstrate inhibitor II399, demonstrating a Ki of 5.9 nM in a biochemical assay and a cellular IC50 value of 1.9 µM. The inhibition mechanism and cocrystal structure confirmed II399 engages both the substrate and cofactor binding pockets. Computational modeling and binding data reveal a balancing act between enthalpic and entropic components that lead to II399's low nM binding affinity. Notably, II399 is 1 000-fold more selective for NNMT than closely related methyltransferases. We expect that II399 would serve as a valuable probe to elucidate NNMT biology. Furthermore, this strategy provides the first case of introducing unconventional SAM mimics, which can be adopted to develop cell-potent inhibitors for other SAM-dependent methyltransferases.

Nicotinamide N-methyltransferase upregulation via the mTORC1-ATF4 pathway activation contributes to palmitate-induced lipotoxicity in hepatocytes.

Defined as the dysfunction and/or cell death caused by toxic lipids accumulation in hepatocytes, hepatic lipotoxicity plays a pathological role in nonalcoholic fatty liver disease. The cellular and molecular mechanisms underlying lipotoxicity remain to be elucidated. In this study, using AML12 cells, a nontransformed murine hepatocyte cell line, exposed to palmitate (a 16-C saturated fatty acid) as an experimental model, we investigated the role and mechanisms of nicotinamide N-methyltransferase (NNMT), a methyltransferase catalyzing nicotinamide methylation and degradation, in hepatic lipotoxicity. We initially identified activating transcription factor 4 (ATF4) as a major transcription factor for hepatic NNMT expression. Here, we demonstrated that palmitate upregulates NNMT expression via activating ATF4 in a mechanistic target of rapamycin complex 1 (mTORC1)-dependent mechanism in that mTORC1 inhibition by both Torin1 and rapamycin attenuated ATF4 activation and NNMT upregulation. We further demonstrated that the mTORC1-dependent ATF4 activation is an integral signaling event of unfolded protein response (UPR) as both ATF4 activation and NNMT upregulation by tunicamycin, a well-documented endoplasmic reticulum (ER) stress inducer, are blunted when hepatocytes were pretreated with Torin1. Importantly, our data uncovered that NNMT upregulation contributes to palmitate-induced hepatotoxicity as NNMT inhibition, via either pharmacological (NNMT inhibitors) or genetic approach (siRNA transfection), provided protection against palmitate lipotoxicity. Our further mechanistic exploration identified protein kinase A (PKA) activation to contribute, at least, partially to the protective effect of NNMT inhibition against lipotoxicity. Collectively, our data demonstrated that NNMT upregulation by the mTORC1-ATF4 pathway activation contributes to the development of lipotoxicity in hepatocytes.

Development of fluorescence polarization-based competition assay for nicotinamide N-methyltransferase.

Methylation-mediated pathways play important roles in the progression of various diseases. Thus, targeting methyltransferases has proven to be a promising strategy for developing novel therapies. Nicotinamide N-methyltransferase (NNMT) is a major metabolic enzyme involved in epigenetic regulation through catalysis of methyl transfer from the cofactor S-adenosyl-l-methionine onto nicotinamide and other pyridines. Accumulating evidence infers that NNMT is a novel therapeutic target for a variety of diseases such as cancer, diabetes, obesity, cardiovascular and neurodegenerative diseases. Therefore, there is an urgent need to discover potent and specific inhibitors for NNMT to assess its therapeutical potential. Herein, we reported the design and synthesis of a fluorescent probe II138, exhibiting a Kd value of 369 ± 14 nM for NNMT. We also established a fluorescence polarization (FP)-based competition assay for evaluation of NNMT inhibitors. Importantly, the unique feature of this FP competition assay is its capability to identify inhibitors that interfere with the interaction of the NNMT active site directly or allosterically. In addition, this assay performance is robust with a Z'factor of 0.76, indicating its applicability in high-throughput screening for NNMT inhibitors.

Discovery of a novel class of potent and selective tetrahydroindazole-based sigma-1 receptor ligands.

The sigma-1 and sigma-2 receptors have been shown to play important roles in CNS diseases, cancer, and other disorders. These findings suggest that targeting these proteins with small-molecule modulators may be of important therapeutic value. Here we report the development of a new class of tetrahydroindazoles that are highly potent and selective ligands for sigma-1. Molecular modeling was used to rationalize the observed structure-activity relationships and identify key interactions responsible for increased potency of the optimized compounds. Assays for solubility and microsomal stability showed this series possesses favorable characteristics and is amenable to further therapeutic development. The compounds described herein will be useful in the development of new chemical probes for sigma-1 and to aid in future work therapeutically targeting this protein.

Synthesis of a novel fused pyrrolodiazepine-based library with anti-cancer activity.

Development of drugs for new and persistent diseases will increasingly rely on the expansion of accessible chemical space to allow exploration of novel molecular targets. Here we report the synthesis of a library of novel fused heterobicyclic small molecules based on the 1,4-diazepine and 2,4-pyrrolidinedione scaffolds. Key chemical transformations included a Mannich-type condensation and chemoselective N-acylation reactions. Screening shows anti-cancer activity of several library compounds which suggests translational potential of this novel chemical scaffold.

Menoctone Resistance in Malaria Parasites Is Conferred by M133I Mutations in Cytochrome b That Are Transmissible through Mosquitoes.

Malaria-related mortality has slowly decreased over the past decade; however, eradication of malaria requires the development of new antimalarial chemotherapies that target liver stages of the parasite and combat the emergence of drug resistance. The diminishing arsenal of anti-liver-stage compounds sparked our interest in reviving the old and previously abandoned compound menoctone. In support of these studies, we developed a new convergent synthesis method that was facile, required fewer steps, produced better yields, and utilized less expensive reagents than the previously published method. Menoctone proved to be highly potent against liver stages of Plasmodium berghei (50 percent inhibitory concentration [IC50] = 0.41 nM) and erythrocytic stages of Plasmodium falciparum (113 nM). We selected for resistance to menoctone and found M133I mutations in cytochrome b of both P. falciparum and P. berghei The same mutation has been observed previously in atovaquone resistance, and we confirmed cross-resistance between menoctone and atovaquone in vitro (for P. falciparum) and in vivo (for P. berghei). Finally, we assessed the transmission potential of menoctone-resistant P. berghei and found that the M133I mutant parasites were readily transmitted from mouse to mosquitoes and back to mice. In each step, the M133I mutation in cytochrome b, inducing menoctone resistance, was confirmed. In summary, this study is the first to show the mechanism of resistance to menoctone and that menoctone and atovaquone resistance is transmissible through mosquitoes.

Comparative Analysis of Two NNMT Bisubstrate Inhibitors through Chemoproteomic Studies: Uncovering the Role of Unconventional SAM Analogue Moiety for Improved Selectivity.

Unconventional S-adenosyl-L-methionine (SAM) mimics with enhanced hydrophobicity are an adaptable building block to develop cell-potent inhibitors for SAM-dependent methyltransferases as targeted therapeutics. We recently discovered cell-potent bisubstrate inhibitors for nicotinamide N-methyltransferase (NNMT) by using an unconventional SAM mimic. To delve into the selectivity implications of the unconventional SAM mimic, we employed a chemoproteomic approach to assess two potent NNMT inhibitors LL320 (Ki, app = 6.8 nM) and II399 (containing an unconventional SAM mimic, Ki, app = 5.9 nM) within endogenous proteomes. Our work began with the rational design and synthesis of immobilized probes 1 and 2, utilizing LL320 and II399 as parent compounds. Systematic analysis of protein networks associated with these probes revealed a comprehensive landscape. Notably, NNMT emerged as the top-ranking hit, substantiating the high selectivity of both inhibitors. Meanwhile, we identified additional interacting proteins for LL320 (38) and II399 (17), showcasing the intricate selectivity profiles associated with these compounds. Subsequent experiments confirmed LL320's interactions with RNMT, DPH5, and SAHH, while II399 exhibited interactions with SHMT2 and MEPCE. Importantly, incorporating the unconventional SAM mimic in II399 led to improved selectivity compared to LL320. Our findings underscore the importance of selectivity profiling and validate the utilization of the unconventional SAM mimic as a viable strategy to create highly selective and cell-permeable inhibitors for SAM-dependent methyltransferases.

Structure-Activity Relationship Studies on Cell-Potent Nicotinamide N-Methyltransferase Bisubstrate Inhibitors.

Nicotinamide N-methyltransferase (NNMT) is a metabolic enzyme implicated in multiple diseases, making it a promising therapeutic target. Building upon our recently reported NNMT inhibitor II399, we systematically investigate the structure-activity relationship by designing and synthesizing a series of analogues. Among them, two top inhibitors II559 (Ki = 1.2 nM) and II802 (Ki = 1.6 nM) displayed over 5000-fold selectivity for NNMT over closely related methyltransferases. Moreover, II559 and II802 showed enhanced cellular inhibition, with a cellular IC50 value of approximately 150 nM, making them the most cell-potent bisubstrate inhibitors reported to date. Furthermore, both inhibitors reduced the cell viability with a GI50 value of ∼10 µM and suppressed the migration of aggressive clear cell renal cancer cell carcinoma cell lines. Overall, II559 and II802 would serve as valuable probes to investigate the enzymatic function of NNMT in health and diseases.

A unique binding pocket induced by a noncanonical SAH mimic to develop potent and selective PRMT inhibitors.

Protein arginine methyltransferases (PRMTs) are attractive targets for developing therapeutic agents, but selective PRMT inhibitors targeting the cofactor SAM binding site are limited. Herein, we report the discovery of a noncanonical but less polar SAH surrogate YD1113 by replacing the benzyl guanidine of a pan-PRMT inhibitor with a benzyl urea, potently and selectively inhibiting PRMT3/4/5. Importantly, crystal structures reveal that the benzyl urea moiety of YD1113 induces a unique and novel hydrophobic binding pocket in PRMT3/4, providing a structural basis for the selectivity. In addition, YD1113 can be modified by introducing a substrate mimic to form a "T-shaped" bisubstrate analogue YD1290 to engage both the SAM and substrate binding pockets, exhibiting potent and selective inhibition to type I PRMTs (IC50 < 5 nmol/L). In summary, we demonstrated the promise of YD1113 as a general SAH mimic to build potent and selective PRMT inhibitors.

Going beyond Binary: Rapid Identification of Protein-Protein Interaction Modulators Using a Multifragment Kinetic Target-Guided Synthesis Approach.

Kinetic target-guided synthesis (KTGS) is a powerful screening approach that enables identification of small molecule modulators for biomolecules. While many KTGS variants have emerged, a majority of the examples suffer from limited throughput and a poor signal/noise ratio, hampering reliable hit detection. Herein, we present our optimized multifragment KTGS screening strategy that tackles these limitations. This approach utilizes selected reaction monitoring liquid chromatography tandem mass spectrometry for hit detection, enabling the incubation of 190 fragment combinations per screening well. Consequentially, our fragment library was expanded from 81 possible combinations to 1710, representing the largest KTGS screening library assembled to date. The expanded library was screened against Mcl-1, leading to the discovery of 24 inhibitors. This work unveils the true potential of KTGS with respect to the rapid and reliable identification of hits, further highlighting its utility as a complement to the existing repertoire of screening methods used in drug discovery.

Comparative Study of Adenosine Analogs as Inhibitors of Protein Arginine Methyltransferases and a Clostridioides difficile-Specific DNA Adenine Methyltransferase.

S-Adenosyl-l-methionine (SAM) analogs are adaptable tools for studying and therapeutically inhibiting SAM-dependent methyltransferases (MTases). Some MTases play significant roles in host-pathogen interactions, one of which is Clostridioides difficile-specific DNA adenine MTase (CamA). CamA is needed for efficient sporulation and alters persistence in the colon. To discover potent and selective CamA inhibitors, we explored modifications of the solvent-exposed edge of the SAM adenosine moiety. Starting from the two parental compounds (6e and 7), we designed an adenosine analog (11a) carrying a 3-phenylpropyl moiety at the adenine N6-amino group, and a 3-(cyclohexylmethyl guanidine)-ethyl moiety at the sulfur atom off the ribose ring. Compound 11a (IC50 = 0.15 µM) is 10× and 5× more potent against CamA than 6e and 7, respectively. The structure of the CamA-DNA-inhibitor complex revealed that 11a adopts a U-shaped conformation, with the two branches folded toward each other, and the aliphatic and aromatic rings at the two ends interacting with one another. 11a occupies the entire hydrophobic surface (apparently unique to CamA) next to the adenosine binding site. Our work presents a hybrid knowledge-based and fragment-based approach to generating CamA inhibitors that would be chemical agents to examine the mechanism(s) of action and therapeutic potentials of CamA in C. difficile infection.

Mechanisms and inhibitors of nicotinamide N-methyltransferase.

Nicotinamide N-methyltransferase (NNMT) plays an important role in diverse biological processes by regulating methylation potential and the degradation of nicotinamide. Meanwhile, the aberrant expression of NNMT has been implicated in multiple cancers, metabolic and liver diseases. Therefore, there has been an emerging interest in assessing NNMT as a potential therapeutic target and discovering NNMT inhibitors over the past 5 years. Herein, we focus on the recognition, mechanism, and inhibitors of NNMT with emphasis on key advancements in the field. We also discuss future directions for the development of NNMT inhibitors.

A Pan-Inhibitor for Protein Arginine Methyltransferase Family Enzymes.

Protein arginine methyltransferases (PRMTs) play important roles in transcription, splicing, DNA damage repair, RNA biology, and cellular metabolism. Thus, PRMTs have been attractive targets for various diseases. In this study, we reported the design and synthesis of a potent pan-inhibitor for PRMTs that tethers a thioadenosine and various substituted guanidino groups through a propyl linker. Compound II757 exhibits a half-maximal inhibition concentration (IC50) value of 5 to 555 nM for eight tested PRMTs, with the highest inhibition for PRMT4 (IC50 = 5 nM). The kinetic study demonstrated that II757 competitively binds at the SAM binding site of PRMT1. Notably, II757 is selective for PRMTs over a panel of other methyltransferases, which can serve as a general probe for PRMTs and a lead for further optimization to increase the selectivity for individual PRMT.

Development of Tetrahydroindazole-Based Potent and Selective Sigma-2 Receptor Ligands.

The sigma-2 receptor has been shown to play important roles in a number of important diseases, including central nervous system (CNS) disorders and cancer. However, mechanisms by which sigma-2 contributes to these diseases remain unclear. The development of new sigma-2 ligands that can be used to probe the function of this protein and potentially as drug discovery leads is therefore of great importance. Herein we report the development of a series of tetrahydroindazole compounds that are highly potent and selective for sigma-2. Structure-activity relationship data were used to generate a pharmacophore model that summarizes the common features present in the potent ligands. Assays for solubility and microsomal stability showed that several members of this compound series possess promising characteristics for further development of useful chemical probes or drug discovery leads.

Novel Propargyl-Linked Bisubstrate Analogues as Tight-Binding Inhibitors for Nicotinamide N-Methyltransferase.

Nicotinamide N-methyltransferase (NNMT) catalyzes the methyl transfer from the cofactor S-adenosylmethionine to nicotinamide and other pyridine-containing compounds. NNMT is an important regulator for nicotinamide metabolism and methylation potential. Aberrant expression levels of NNMT have been implicated in cancer, metabolic, and neurodegenerative diseases, which makes NNMT a potential therapeutic target. Therefore, potent and selective NNMT inhibitors can serve as valuable tools to investigate the roles of NNMT in its mediated diseases. Here, we applied a rational strategy to design and synthesize the tight-binding bisubstrate inhibitor LL320 through a novel propargyl linker. LL320 demonstrates a Ki value of 1.6 ± 0.3 nM, which is the most potent inhibitor to date. The cocrystal structure of LL320 confirms its interaction with both the substrate and cofactor binding sites on NNMT. Importantly, this is the first example of using the propargyl linker to construct potent methyltransferase inhibitors, which will expand our understanding of the transition state of methyl transfer.