Macrodomain Mac1 of SARS-CoV-2 Nonstructural Protein 3 Hydrolyzes Diverse ADP-ribosylated Substrates.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for a global pandemic that resulted in more than 6-million deaths worldwide. The virus encodes several non-structural proteins (Nsps) that contain elements capable of disrupting cellular processes. Among these Nsp proteins, Nsp3 contains macrodomains, e.g., Mac1, Mac2, Mac3, with potential effects on host cells. Mac1 has been shown to increase SARS-CoV-2 virulence and disrupt ADP-ribosylation pathways in mammalian cells. ADP-ribosylation results from the transfer of the ADP-ribose moiety of NAD + to various acceptors, e.g., proteins, DNA, RNA, contributing on a cell's biological processes. ADP-ribosylation is the mechanism of action of bacterial toxins, e.g., Pseudomonas toxins, diphtheria toxin that disrupt protein biosynthetic and signaling pathways. On the other hand, some viral macrodomains cleavage ADP-ribose-acceptor bond, generating free ADP-ribose. By this reaction, the macrodomain-containing proteins interfere ADP-ribose homeostasis in host cells. Here, we examined potential hydrolytic activities of SARS-CoV-2 Mac1, 2, and 3 on substrates containing ADP-ribose. Mac1 cleaved α-NAD + , but not ß-NAD + , consistent with stereospecificity at the C-1" bond. In contrast to ARH1 and ARH3, Mac1 did not require Mg 2+ for optimal activity. Mac1 also hydrolyzed O -acetyl-ADP-ribose and ADP-ribose-1"-phosphat, but not Mac2 and Mac3. However, Mac1 did not cleave α-ADP-ribose-(arginine) and ADP-ribose-(serine)-histone H3 peptide, suggesting that Mac1 hydrolyzes ADP-ribose attached to O- and N-linked functional groups, with specificity at the catalytic site in the ADP-ribose moiety. We conclude that SARS-CoV-2 Mac1 may exert anti-viral activity by reversing host-mediated ADP-ribosylation. New insights on Nsp3 activities may shed light on potential SARS-CoV-2 therapeutic targets. IMPORTANCE: SARS-CoV-2, the virus responsible for COVID-19, encodes 3 macrodomain-containing proteins, e.g., Mac1, Mac2, Mac3, within non-structural proteins 3 (Nsp3). Mac1 was shown previously to hydrolyze ADP-ribose-phosphate. Inactivation of Mac1 reduced viral proliferation. Here we report that Mac1, but not Mac2 and Mac3, has multiple activities, i.e., Mac1 hydrolyzed. α-NAD + and O -acetyl-ADP-ribose. However, Mac1 did not hydrolyze ß-NAD + , ADP-ribose-serine on a histone 3 peptide (aa1-21), and ADP-ribose-arginine, exhibiting substrate selectivity. These data suggest that Mac1 may have multi-function as a α-NAD + consumer for viral replication and a disruptor of host-mediated ADP-ribosylation pathways. Understanding Mac1's mechanisms of action is important to provide possible therapeutic targets for COVID-19.

Molecular Mechanisms of Inhibitory Effects of Bovine Lactoferrin on Invasion of Oral Squamous Cell Carcinoma.

Lactoferrin (LF), an iron-binding glycoprotein, has been reported to have anticancer properties. However, the molecular mechanisms behind its anticancer effects on oral squamous cell carcinoma (OSCC) have not yet been elucidated. Therefore, we aimed to clarify the effects of LF on invasion of OSCC, and its underlying molecular mechanism. OSCC cell lines, HSC2 and HOC313, were treated with bovine LF (bLF). The effects of bLF on cell invasion were examined by a chamber migration assay, wound healing assay, and Boyden chamber method with a basement-membrane-analogue. Expression levels of MMP-1, MMP-3, and AP-1 were examined using RT-PCR, qRT-PCR, and western blotting. Roles of LRP1, a receptor of bLF, on cell invasion were analyzed using siLRP1 knockdown cells. Furthermore, to clarify the importance of LRP1 in invasion, the effects of bLF on tPA-induced invasion of OSCC cells were examined. The invasion assays showed that bLF suppressed invasion of the OSCC cells. Moreover, bLF down-regulated AP-1, and resulted in reductions of MMP-1 and MMP-3. With SiLRP1 knockdown, OSCC cells failed to induce their invasion, and bLF was not able to exert its effects on invasion. Furthermore, bLF remarkably inhibited tPA-induced cell invasion. These findings suggest the importance of LRP1 in bLF-suppressed invasion of OSCC cells via the reduction of AP-1 and MMP production.

Bovine Lactoferrin Suppresses Tumor Angiogenesis through NF-κB Pathway Inhibition by Binding to TRAF6.

Tumor angiogenesis is essential for tumor progression. The inhibition of tumor angiogenesis is a promising therapy for tumors. Bovine lactoferrin (bLF) has been reported as an anti-tumor agent. However, bLF effects on tumor angiogenesis are not well demonstrated. This study evaluated the inhibitory effects of bLF on tumor angiogenesis in vivo and in vitro. Herein, tumor endothelial cells (TECs) and normal endothelial cells (NECs) were used. Proliferation, migration, tube formation assays, RT-PCR, flow cytometry, Western blotting, siRNA experiments and immunoprecipitation were conducted to clarify the mechanisms of bLF-induced effects. CD-31 immunoexpression was examined in tumor tissues of oral squamous cell carcinoma mouse models with or without Liposomal bLF (LbLF)-administration. We confirmed that bLF inhibited proliferation/migration/tube formation and increased apoptosis in TECs but not NECs. TNF receptor-associated factor 6 (TRAF6), p-p65, hypoxia inducible factor-α (HIF-1α) and vascular endothelial growth factor (VEGF) were highly expressed in TECs. In TECs, bLF markedly downregulated VEGF-A, VEGF receptor (VEGFR) and HIF-1α via the inhibition of p-p65 through binding with TRAF6. Since NECs slightly expressed p-p65, bLF-TRAF-6 binding could not induce detectable changes. Moreover, orally administrated LbLF decreased CD31-positive microvascular density only in TECs. Hence, bLF specifically suppressed tumor angiogenesis through p-p65 inhibition by binding to TRAF6 and suppressing HIF-1α activation followed by VEGF/VEGFR down-regulation. Collectively, bLF can be an anti-angiogenic agent for tumors.

ARH Family of ADP-Ribose-Acceptor Hydrolases.

The ARH family of ADP-ribose-acceptor hydrolases consists of three 39-kDa members (ARH1-3), with similarities in amino acid sequence. ARH1 was identified based on its ability to cleave ADP-ribosyl-arginine synthesized by cholera toxin. Mammalian ADP-ribosyltransferases (ARTCs) mimicked the toxin reaction, with ARTC1 catalyzing the synthesis of ADP-ribosyl-arginine. ADP-ribosylation of arginine was stereospecific, with ß-NAD+ as substrate and, α-anomeric ADP-ribose-arginine the reaction product. ARH1 hydrolyzed α-ADP-ribose-arginine, in addition to α-NAD+ and O-acetyl-ADP-ribose. Thus, ADP-ribose attached to oxygen-containing or nitrogen-containing functional groups was a substrate. Arh1 heterozygous and knockout (KO) mice developed tumors. Arh1-KO mice showed decreased cardiac contractility and developed myocardial fibrosis. In addition to Arh1-KO mice showed increased ADP-ribosylation of tripartite motif-containing protein 72 (TRIM72), a membrane-repair protein. ARH3 cleaved ADP-ribose from ends of the poly(ADP-ribose) (PAR) chain and released the terminal ADP-ribose attached to (serine)protein. ARH3 also hydrolyzed α-NAD+ and O-acetyl-ADP-ribose. Incubation of Arh3-KO cells with H2O2 resulted in activation of poly-ADP-ribose polymerase (PARP)-1, followed by increased nuclear PAR, increased cytoplasmic PAR, leading to release of Apoptosis Inducing Factor (AIF) from mitochondria. AIF, following nuclear translocation, stimulated endonucleases, resulting in cell death by Parthanatos. Human ARH3-deficiency is autosomal recessive, rare, and characterized by neurodegeneration and early death. Arh3-KO mice developed increased brain infarction following ischemia-reperfusion injury, which was reduced by PARP inhibitors. Similarly, PARP inhibitors improved survival of Arh3-KO cells treated with H2O2. ARH2 protein did not show activity in the in vitro assays described above for ARH1 and ARH3. ARH2 has a restricted tissue distribution, with primary involvement of cardiac and skeletal muscle. Overall, the ARH family has unique functions in biological processes and different enzymatic activities.

Oral administration of bovine lactoferrin suppresses the progression of rheumatoid arthritis in an SKG mouse model.

Rheumatoid arthritis (RA) is an autoimmune disease characterized by inflammatory bone destruction in which tumor necrosis factor alpha (TNF-α) plays a key role. Bovine lactoferrin (bLF) is a multifunctional protein with anti-inflammatory and immunomodulatory properties. This study aimed to clarify the inhibitory effects of bLF on the pathological progression of RA. The mannan-induced arthritis model in SKG mice (genetic RA model) was used. Orally applied liposomal bLF (LbLF) markedly reduced ankle joint swelling and bone destruction. Histologically, pannus formation and osteoclastic bone destruction were prevented in the LbLF-treated animals. Moreover, orally administered LbLF improved the balance between Th17 cells and regulatory T cells isolated from the spleen of mannan-treated SKG mice. In an in vitro study, the anti-inflammatory effects of bLF on TNF-α-induced TNF-α production and downstream signaling pathways were analyzed in human synovial fibroblasts from RA patients (RASFs). bLF suppressed TNF-α production from RASFs by inhibiting the nuclear factor kappa B and mitogen-activated protein kinase pathways. The intracellular accumulation of bLF in RASFs increased in an applied bLF dose-dependent manner. Knockdown of the lipoprotein receptor-related protein-1 (LRP1) siRNA gene reduced bLF expression in RASFs, indicating that exogenously applied bLF was mainly internalized through LRP-1. Immunoprecipitated proteins with anti-TNF receptor-associated factor 2 (TRAF2; an adapter protein/ubiquitin ligase) included bLF, indicating that bLF binds directly to the TRAF2-TRADD-RIP complex. This indicates that LbLF may effectively prevent the pathological progression of RA by suppressing TNF-α production by binding to the TRAF2-TRADD-RIP complex from the RASFs in the pannus. Therefore, supplemental administration of LbLF may have a beneficial effect on preventive/therapeutic reagents for RA.

Barriers to early detection and management of oral cancer in the Asia Pacific region.

OBJECTIVE: Oral cancer is amenable to early detection but remains a prominent cause of mortality in the Asia Pacific region. This study aimed to identify barriers to early detection and management of oral cancer in the Asia Pacific region. METHODS: A mixed-methods approach was employed triangulating findings from a survey and focus groups. The survey was conducted among seven representative members of the Asia Pacific Oral Cancer Network (APOCNET) across six countries. Focus groups were conducted to gain deeper insights into the findings of the survey. RESULTS: The identified barriers were a lack of national cancer control strategies and cancer registries and the limited availability of trained health care professionals. Overcoming these challenges in the Asia Pacific region where resources are scarce will require collaborative partnerships in data collection and novel approaches for continuous professional training including eLearning. Further, to overcome the lack of trained health care professionals, innovative approaches to the management of oral potentially malignant lesions and oral cancer including telemedicine were suggested. CONCLUSION: The findings of this study should be taken into account when charting national cancer control plans for oral cancer and will form the basis for future collaborative studies in evaluating effective measures to improve oral cancer detection and management in low- and middle-income countries.

Anti-inflammatory effect of glycyrrhizin with Equisetum arvense extract.

Periodontal disease is the most prevalent infectious disease, and inflammatory mediators play critical roles in its progression. Therefore, controlling pro-inflammatory cytokine production, especially at initial disease stages, is essential to maintaining gingival and periodontal health. Glycyrrhizin (GL) has an anti-inflammatory effect and has been added to toothpaste and mouth rinse to prevent periodontal disease. However, there is a maximum dose for the use of GL. The aim of the present study is to screen plant extracts which can effectively enhance the effects of GL. The effects of extracts from six different plants on GL-suppressed TNF-α expression in Aggregatibacter actinomycetemcomitans (A.a.)-LPS-stimulated human oral keratinocytes (RT7) were examined. Results demonstrated that Equisetum arvense (EA) extract had the strongest additive effect on the suppression of TNF-α by GL at both mRNA and protein levels. In addition, GL downregulated the production of TNF-α by suppressing NF-κB p65 phosphorylation, but not JNK or p38 phosphorylation. In contrast, EA decreased JNK phosphorylation but not NF-κB p65 or p38 phosphorylation. The combination of GL and EA effectively attenuated A.a.-LPS-induced phosphorylation of NF-κB p65 and JNK. Furthermore, an LPS-induced periodontitis rat model showed that GL with EA supplementation significantly downregulated TNF-α mRNA in the gingival tissue. These results indicate that EA can suppress A.a.-LPS-induced pro-inflammatory cytokine production by inhibiting JNK activation and can promote the anti-inflammatory effects of GL. Our findings suggest that a combination of GL and EA may improve the development of new oral hygiene products aimed at enhancing periodontal health.

Odontogenic infection by Porphyromonas gingivalis exacerbates fibrosis in NASH via hepatic stellate cell activation.

Odontogenic infection of Porphyromonas gingivalis (P.g.), a major periodontal pathogen, exacerbates pathological progression of non-alcoholic steatohepatitis (NASH). In this study, we aimed to clarify the detailed mechanism in which P.g. induced hepatic stellate cells (HSCs; key effector cells in liver fibrosis) activation. In the liver of high fat diet-induced NASH mouse model with P.g. odontogenic infection, immunolocalization of P.g. was detected. The number of hepatic crown-like structure, which was macrophage aggregation and related to liver fibrosis, was drastically increased and fibrosis area was also increased through upregulating immunoexpression of Phosphorylated Smad2 (key signaling molecule of TGF-ß1) and Galectin-3. P.g.-secreted trypsin-like enzyme [gingipain; an activator of protease-activated receptor 2 (PAR2)] stimulated HSC proliferation and differentiation through Smad and ERK signaling induced by TGF-ß1 produced from HSCs with P.g.-infection. Further, Galectin-3 produced from HSCs with P.g. infection and P.g.-derived LPS/lipoprotein stimulation stabilized TGFß-receptor II resulting in increasing sensitivity for TGF-ß1, finally leading to HSC differentiation via activating Smad and ERK signaling. In addition to them, hepatocytes (main component cells of liver) contributed to HSC activation through TGF-ß1 and Galectin-3 production in paracrine manner. Collectively, P.g.-odontogenic infection exacerbates fibrosis of NASH by HSC activation through TGF-ß1 and Gal-3 production from HSCs and hepatocytes.

The transition of tissue inhibitor of metalloproteinases from -4 to -1 induces aggressive behavior and poor patient survival in dedifferentiated liposarcoma via YAP/TAZ activation.

Liposarcoma (LS) is the most common soft-tissue sarcoma. Dedifferentiated liposarcoma (DDLS) shows more aggressive biological behavior than that of well-differentiated liposarcoma (WDLS), so advanced therapeutic agents based on molecular mechanism are urgently needed. Here we show that tissue inhibitors of metalloproteinases (TIMPs) from TIMP-1 to TIMP-4 are differently expressed and regulate yes-associated protein (YAP)/transcriptional co-activator with PDZ binding motif (TAZ) in LS. Database analysis showed high TIMP-1 expression in DDLS patients correlating with poor prognosis, but high TIMP-4 expression in WDLS patients with better prognosis. Stable TIMP-1 knockdown inactivated YAP/TAZ and inhibited proliferation, colony formation and migration in DDLS cells, which was rescued by a constitutive active YAP. However, stable overexpression of TIMP-1 showed the opposite in WDLS cells. Stable TIMP-4 knockdown activated YAP/TAZ and promoted proliferation and migration in WDLS cells, which was suppressed by YAP/TAZ inhibitor (verteporfin) or knockdown of YAP/TAZ. Recombinant TIMP-4 showed opposite results in DDLS cells. These results indicate that dedifferentiation in LS shifts the expression of TIMPs from type 4 to type 1, inducing more aggressive behavior and poor prognosis through YAP/TAZ activation, which can be prognostic markers and therapeutic targets for LS patients.

Molecular mechanisms underlying the inhibitory effects of bovine lactoferrin on osteosarcoma.

Osteosarcoma (OS) is one the most common primary malignancies of the bone in children and young adults with high metastasis. The use of non-toxic naturally derived compounds is one of present strategies in OS therapy to reduce secondary effects and chemo-resistance. Lactoferrin (LF), a transferrin protein derived from milk, currently appears to be an anticancer agent. However, its suppressive effects on OS have not been fully investigated. Therefore, we aimed to examine the molecular mechanism underlying the inhibitory effects of bovine LF (bLF) on OS. OS cell lines (NOS1, U2OS, MG63, and 143B) and an osteoblastic (ST2) were treated with bLF. Effects of bLF on OS-cell proliferation and migration were examined by proliferation and wound-healing assays. Expression levels of low-density-lipoprotein-receptor-related protein 1 (LRP1) and cytokines including interleukin-1 beta (IL-1ß), IL-6, and receptor-activator of nuclear factor kappa-Β ligand (RANKL) were measured using western blotting. Osteoclast formation was examined by co-culture of 143B, ST2, and bone marrow cells. We found that bLF down-regulated IL-1ß, IL-6, and RANKL expression and suppressed phosphorylation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) p65 in 143B cells; bLF also drastically suppressed 143B-activated RANKL production in ST2 cells. This may have contributed to the reduction in the number of differentiated osteoclasts. Taken together, these data reveal that bLF down-regulates NF-κB to attenuate proliferation, migration, and bone resorption in OS and the OS-microenvironment. This study provides new findings and the precise underlying mechanisms of the inhibitory effects of bLF on OS. bLF can be a possible therapeutic agent for OS patients.

Bovine lactoferrin reverses programming of epithelial-to-mesenchymal transition to mesenchymal-to-epithelial transition in oral squamous cell carcinoma.

Epithelial-to-mesenchymal transition (EMT) is a biological process of invasion and metastasis in cancers, including in oral squamous cell carcinoma (OSCC). However, an effective anticancer drug that directly targets EMT has not yet been discovered. Therefore, we aimed to investigate the repressive effects of bovine lactoferrin (bLF) on EMT to achieve mesenchymal-to-epithelial transition (MET) in OSCC. OSCC cell lines, HOC313 (EMT-induced) and SCCVII (without EMT induction), were treated with bLF. The effects of bLF on EMT in OSCC were identified histologically by haematoxylin and eosin staining and observed morphologically and immunohistochemically using an anti-E-cadherin antibody. Expression levels of E-cadherin and vimentin were investigated using RT-PCR and western blotting. Immuno-expression of E-cadherin was examined in vivo in tumour tissues of C3H/HeN mice, transplanted with SCCVII cells, with or without bLF administration. We found that bLF changed the spindle-like mesenchymal cells to cuboidal-like epithelial cells and enhanced the affinity of membrane-bound E-cadherin in HOC313 cells. The transformation of EMT-MET in HOC313 cells was confirmed by the upregulation of E-cadherin and suppression of vimentin. Moreover, bLF suppressed TWIST expression through downregulation of ERK1/2 phosphorylation. Additionally, the inhibition tumour cell infiltration and increase in E-cadherin expression were observed in xenografts of the mice orally administered with bLF. Thus, based on the results from in vitro and in vivo studies, we concluded that bLF caused the restoration of epithelial properties through MET. Importantly, this finding is novel and is the first report indicating that bLF inhibited EMT and induced MET in OSCC, suggesting that bLF may provide a novel therapeutic strategy in OSCC.

Interaction between N-cadherin and decoy receptor-2 regulates apoptosis in head and neck cancer.

N-cadherin is a neural cell adhesion molecule that aberrantly occurs in head and neck cancers to promote cancer cell growth. However, the underlying mechanisms remain unclear. Here we report that N-cadherin increases cancer cell growth by inhibiting apoptosis. Apoptosis eliminates old, unnecessary, and unhealthy cells. However, tumor cells have the ability of avoiding apoptosis that increases cancer cell growth. Recent studies have found that tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) selectively induces apoptosis in tumor cells by reacting with four distinct cell surface receptors: TRAIL-R1 (DR-4), TRAIL-R2 (DR-5), TRAIL-R3 (DcR-1), and TRAIL-R4 (DcR-2). Among these TRAIL receptors, the death receptors DR-4 and DR-5 transmit apoptotic signals owing to the death domain in the intracellular portion. Conversely, the decoy receptors DcR-1 and DcR-2 lack a complete intracellular portion, so neither can transmit apoptotic signals. DcR-1 or DcR-2 overexpression suppresses TRAIL-induced apoptosis. In this study, N-cadherin overexpression increased DcR-2 expression and decreased DR-5 expression. In contrast, knockdown of N-cadherin expression upregulated DR-5 expression and downregulated DcR-2 expression. A significantly positive relationship between N-cadherin and DcR-2 expression was also found in HNSCC specimens. Those specimens with a lower apoptotic index showed a higher expression of N-cadherin and/or DcR-2. In addition, we demonstrated that N-cadherin interacts directly with DcR-2. Notably, DcR-2 induces cancer cell survival through the cleavage of caspases and PARP by activating MAPK/ERK pathway and suppressing NF-kB/ p65 phosphorylation, which has a very important role in resistance to chemotherapy.

Galectin-3 Plays an Important Role in Preterm Birth Caused by Dental Infection of Porphyromonas gingivalis.

Dental infection is risk for preterm birth (PTB) through unclear mechanisms. We established a dental infection-induced PTB mouse model, in which Porphyromonas gingivalis (P.g.) induced PTB by 2 days. We analysed pathogenic factors contributing to PTB and their effects on trophoblasts in vitro. TNF-α, IL-8, and COX-2 were upregulated in P.g.-infected placenta. Galectin-3 (Gal-3), an immune regulator, was significantly upregulated in placenta, amniotic fluid, and serum. In vitro, P.g.-lipopolysaccharide (P.g.-LPS) increased TNF-α and Gal-3 in trophoblasts via NF-κB/MAPK signalling. Gal-3 inhibition significantly downregulated P.g.-LPS-induced TNF-α production. TNF-α upregulated Gal-3. Gal-3 also increased cytokines and Gal-3 through NF-κB/MAPK signalling. Moreover, Gal-3 suppressed CD-66a expression at the maternal-foetal interface. Co-stimulation with Gal-3 and P.g.-LPS upregulated cytokine levels, while Gal-3 plus Aggregatibacter actinomycetemcomitans (A.a.)- or Escherichia coli (E. coli)-LPS treatment downregulated them, indicating the critical role of Gal-3 especially in P.g. dental infection-induced PTB. P.g.-dental infection induced PTB, which was associated with Gal-3-dependent cytokine production. New therapies and/or diagnostic systems targeting Gal-3 may reduce PTB.

Molecular mechanism of inhibitory effects of bovine lactoferrin on the growth of oral squamous cell carcinoma.

BACKGROUND: Lactoferrin (LF), a member of the transferrin family, recently has been demonstrated to have anticancer effects on various cancers including oral squamous cell carcinoma (OSCC). However, little is known about the underlying mechanisms of its effects on OSCC. Therefore, we aimed to investigate the mechanism of the suppressive effects of bovine LF (bLF) on the growth of OSCC cells. METHODS: In the current study, HSC2, HSC3, HSC4 and normal human oral keratinocytes (RT7) cell lines were tested with bLF 1, 10, and 100 µg/ml. The effects and detail mechanisms of bLF on proliferation and apoptosis of cells were investigated using flow cytometry and western blotting. RESULTS: We found that bLF (1, 10, and 100 µg/ml) induced activation of p53, a tumor suppressor gene, is associated with the induction of cell cycle arrest in G1/S phase and apoptosis in OSCC. Moreover, bLF downregulated the phosphorylation of Akt and activated suppressor of cytokine signaling 3 (SOCS3), thereby attenuating multiple signaling pathways including mTOR/S6K and JAK/STAT3. Interestingly, we revealed that bLF exerted its effect selectively against HSC3 but not on RT7 via different effects on the phosphorylation status of NF-κB and Akt. CONCLUSION: This is the first report showing that bLF selectively suppresses proliferation through mTOR/S6K and JAK/STAT3 pathways and induction of apoptosis in OSCC. This study provides important new findings, which might be useful in the prevention and treatment of OSCC.

Loss of Stearoyl-CoA Desaturase-1 Activity Induced Leptin Resistance in Neuronal Cells.

The lack of response to leptin's actions in the brain, "leptin resistance," is one of the main causes of the pathogenesis of obesity. However, although high-fat diets affect sensitivity to leptin, the underlying mechanisms of leptin resistance are still an enigma. Here we examined the effect of excess saturated fatty acids (SFAs) on leptin signaling in human neuronal cells. Palmitate, the principle source of SFAs in diet, induced leptin resistance in a human neuroblastoma cell line stably transfected with the Ob-Rb leptin receptor (SH-SY5Y-ObRb). We next investigated the function of stearoyl-CoA desaturase-1 (SCD1), an enzyme which converts SFAs into monounsaturated fatty acids (MUFAs), on leptin-induced signaling. We found that reduction of SCD1 activity, through SCD1 inhibition and knockdown, impairs leptin-induced signal transducer and activator of transcription 3 (STAT3) phosphorylation in human neuronal cells. Our findings suggested that SCD1 plays a key role in the pathophysiology of leptin resistance in neuronal cells associated with obesity.