Sortilin associates with transforming growth factor-beta family proteins to enhance lysosome-mediated degradation

Chordin-Like 2: A Possible Therapeutic Target for Gastric Cancer by Affecting Cell Cycle and Proliferation

PURPOSE

This study aimed to examine the role of chordin-like 2 (CHRDL2) in gastric cancer.

METHODS

The Gene Expression Omnibus (GEO) and the Cancer Genome Atlas (TCGA) datasets were screened and the differentially expressed gene CHRDL2 was identified. The CHRDL2 expression was examined in the Human Protein Atlas and TCGA. Clinical data on gastric cancer were evaluated for their association with CHRDL2 by using TCGA and KM-plotter databases. The possible relationship amongst CHRDL2, immune cells, and related genes was investigated via the TIMER database. Enrichment analysis was performed using GO and KEGG pathways to explore the mechanisms.

RESULTS

Screening of databases revealed that CHRDL2 was a differentially expressed gene. An increase in cytoplasmic CHRDL2 expression was found in cancer tissues compared with the surrounding normal tissues. The data, together with those from TCGA and the KM-plotter databases, showed that patients with gastric cancer with high level of CHRDL2 have worse prognosis than those with low expression. A strong correlation was found between CHRDL2 expression and T stage, race, pathological grade, and pathological type according to clinical data analysis. CHRDL2 expression is linked to immune infiltration, as shown by the TIMER database. The data suggested that CHRDL2 plays a pivotal role in the tumor microenvironment of gastric cancer and might help tumor cells evade the immune system. Gene set enrichment analysis showed that CHRDL2 is involved in the chemokine signaling route, the intestinal immune network, the MAPK pathway, cell cycle, and the PI3K-Akt signaling system that are associated with the pathological processes of gastric cancer.

CONCLUSION

Patients with gastric cancer with decreased CHRDL2 levels have dramatically improved OS, PFS, and PPS. CHRDL2 plays a pivotal role in enabling tumor cell immune evasion in tumor microenvironment, suggesting a function of this gene in the development of gastric cancer and its immune infiltration. Interfering with CHRDL2 may slow down the development of this malignancy by affecting cell cycle and apoptosis pathways.

Sorting through the extensive and confusing roles of sortilin in metabolic disease

Sortilin is a post-Golgi trafficking receptor homologous to the yeast vacuolar protein sorting receptor 10 (VPS10). The VPS10 motif on sortilin is a 10-bladed β-propeller structure capable of binding more than 50 proteins, covering a wide range of biological functions including lipid and lipoprotein metabolism, neuronal growth and death, inflammation, and lysosomal degradation. Sortilin has a complex cellular trafficking itinerary, where it functions as a receptor in the trans-Golgi network, endosomes, secretory vesicles, multivesicular bodies, and at the cell surface. In addition, sortilin is associated with hypercholesterolemia, Alzheimer's disease, prion diseases, Parkinson's disease, and inflammation syndromes. The 1p13.3 locus containing SORT1, the gene encoding sortilin, carries the strongest association with LDL-C of all loci in human genome-wide association studies. However, the mechanism by which sortilin influences LDL-C is unclear. Here, we review the role sortilin plays in cardiovascular and metabolic diseases and describe in detail the large and often contradictory literature on the role of sortilin in the regulation of LDL-C levels.

Tril dampens Nodal signaling through Pellino2- and Traf6-mediated activation of Nedd4l

Toll-like receptor 4 (Tlr) interactor with leucine-rich repeats (Tril) functions as a Tlr coreceptor to mediate innate immunity in adults. In Xenopus embryos, Tril triggers degradation of the transforming growth factor β (Tgf-ß) family inhibitor, Smad7. This enhances bone morphogenetic protein (Bmp) signaling to enable ventral mesoderm to commit to a blood fate. Here, we show that Tril simultaneously dampens Nodal signaling by catalytically activating the ubiquitin ligase NEDD4 Like (Nedd4l). Nedd4l then targets Nodal receptors for degradation. How Tril signals are transduced in a nonimmune context is unknown. We identify the ubiquitin ligase Pellino2 as a protein that binds to the cytoplasmic tail of Tril and subsequently forms a complex with Nedd4l and another E3 ligase, TNF-receptor associated factor 6 (Traf6). Pellino2 and Traf6 are essential for catalytic activation of Nedd4l, both in Xenopus and in mammalian cells. Traf6 ubiquitinates Nedd4l, which is then recruited to membrane compartments where activation occurs. Collectively, our findings reveal that Tril initiates a noncanonical Tlr-like signaling cascade to activate Nedd4l, thereby coordinately regulating the Bmp and Nodal arms of the Tgf-ß superfamily during vertebrate development.

Upregulation of Sortilin, a Lysosomal Sorting Receptor, Corresponds with Reduced Bioavailability of Latent TGFβ in Mucolipidosis II Cells

Mucolipidosis II (ML-II) is a lysosomal disease caused by defects in the carbohydrate-dependent sorting of soluble hydrolases to lysosomes. Altered growth factor signaling has been identified as a contributor to the phenotypes associated with ML-II and other lysosomal disorders but an understanding of how these signaling pathways are affected is still emerging. Here, we investigated transforming growth factor beta 1 (TGFβ1) signaling in the context of ML-II patient fibroblasts, observing decreased TGFβ1 signaling that was accompanied by impaired TGFβ1-dependent wound closure. We found increased intracellular latent TGFβ1 complexes, caused by reduced secretion and stable localization in detergent-resistant lysosomes. Sortilin, a sorting receptor for hydrolases and TGFβ-related cytokines, was upregulated in ML-II fibroblasts as well as GNPTAB-null HeLa cells, suggesting a mechanism for inappropriate lysosomal targeting of TGFβ. Co-expression of sortilin and TGFβ in HeLa cells resulted in reduced TGFβ1 secretion. Elevated sortilin levels correlated with normal levels of cathepsin D in ML-II cells, consistent with a compensatory role for this receptor in lysosomal hydrolase targeting. Collectively, these data support a model whereby sortilin upregulation in cells with lysosomal storage maintains hydrolase sorting but suppresses TGFβ1 secretion through increased lysosomal delivery. These findings highlight an unexpected link between impaired lysosomal sorting and altered growth factor bioavailability.

Long-term adenosine A1 receptor activation-induced sortilin expression promotes α-synuclein upregulation in dopaminergic neurons

Prolonged activation of adenosine A1 receptor likely leads to damage of dopaminergic neurons and subsequent development of neurodegenerative diseases. However, the pathogenesis underlying long-term adenosine A1 receptor activation-induced neurodegeneration remains unclear. In this study, rats were intraperitoneally injected with 5 mg/kg of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) for five weeks. The mobility of rats was evaluated by forced swimming test, while their cognitive capabilities were evaluated by Y-maze test. Expression of sortilin, α-synuclein, p-JUN, and c-JUN proteins in the substantia nigra were detected by western blot analysis. In addition, immunofluorescence staining of sortilin and α-synuclein was performed to detect expression in the substantia nigra. The results showed that, compared with adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (5 mg/kg) + CPA co-treated rats, motor and memory abilities were reduced, surface expression of sortin and α-synuclein in dopaminergic neurons was reduced, and total sortilin and total α-synuclein were increased in CPA-treated rats. MN9D cells were incubated with 500 nM CPA alone or in combination with 10 μM SP600125 (JNK inhibitor) for 48 hours. Quantitative real-time polymerase chain reaction analysis of sortilin and α-synuclein mRNA levels in MN9D cells revealed upregulated sortilin expression in MN9D cells cultured with CPA alone, but the combination of CPA and SP600125 could inhibit this expression. Predictions made using Jasper, PROMO, and Alibaba online databases identified a highly conserved sequence in the sortilin promoter that was predicted to bind JUN in both humans and rodents. A luciferase reporter assay of sortilin promoter plasmid-transfected HEK293T cells confirmed this prediction. After sortilin expression was inhibited by sh-SORT1, expression of p-JUN and c-JUN was detected by western blot analysis. Long-term adenosine A1 receptor activation levels upregulated α-synuclein expression at the post-transcriptional level by affecting sortilin expression. The online tool Raptor-X-Binding and Discovery Studio 4.5 prediction software predicted that sortilin can bind to α-synuclein. Co-immunoprecipitation revealed an interaction between sortilin and α-synuclein in MN9D cells. Our findings indicate that suppression of prolonged adenosine A1 receptor activation potently inhibited sortilin expression and α-synuclein accumulation, and dramatically improved host cognition and kineticism. This study was approved by the University Committee of Animal Care and Supply at the University of Saskatchewan (approval No. AUP#20070090) in March 2007 and the Animals Ethics Committee of University of South China (approval No. LL0387-USC) in June 2017.

Sortilin promotes macrophage cholesterol accumulation and aortic atherosclerosis through lysosomal degradation of ATP-binding cassette transporter A1 protein

Sortilin is closely associated with hyperlipidemia and the risk of atherosclerosis (AS). The role of sortilin and the underlying mechanism in peripheral macrophage are not fully understood. In this study, we investigated the effect of macrophage sortilin on ATP-binding cassette transporter A1 (ABCA1) expression, ABCA1-mediated cholesterol efflux, and aortic AS. Macrophage sortilin expression was upregulated by oxidized low-density lipoproteins (ox-LDLs) in both concentration- and time-dependent manners. Its expression reached the peak level when cells were incubated with 50 μg/ml ox-LDL for 24 h. Overexpression of sortilin in macrophage reduced cholesterol efflux, leading to an increase in intracellular total cholesterol, free cholesterol, and cholesterol ester. Sortilin was found to bind with ABCA1 protein and suppress macrophage ABCA1 expression, resulting in a decrease in cholesterol efflux from macrophages. The inhibitory effect of sortilin in cholesterol efflux was partially reversed by treatment with chloroquine, a lysosomal inhibitor. On the contrary, the ABCA1 protein level and ABCA1-mediated cholesterol efflux is increased by sortilin short hairpin RNA transfection. The fecal and biliary cholesterol 3H-sterol from cholesterol-laden mouse peritoneal macrophage was reduced by sortilin overexpression through lentivirus vector (LV)-sortilin in low-density lipoprotein receptor knockout mice, which was prevented by co-treatment with chloroquine. Treatment with LV-sortilin reduced plasma high-density lipoprotein and increased plasma ox-LDL levels. Accordingly, aortic lipid deposition and plaque area were exacerbated, and ABCA1 expression was reduced in mice in response to infection with LV-sortilin alone. These effects of LV-sortilin were partially reversed by chloroquine. Sortilin enhances lysosomal degradation of ABCA1 protein and suppresses ABCA1-mediated cholesterol efflux from macrophages, leading to foam cell formation and AS development.

Proteolytic Activation of Bmps: Analysis of Cleavage in Xenopus Oocytes and Embryos

Bone morphogenetic proteins (Bmps) are synthesized as inactive precursors that are cleaved to generate active ligands, along with prodomain fragments that can modulate growth factor activity. Here we provide three protocols that can be used to examine the process of proteolytic activation of Bmps. The first protocol describes how to generate radiolabeled Bmp precursor proteins in Xenopus oocytes and then analyze the time course of precursor cleavage by recombinant enzymes in vitro. The second protocol details how to analyze cleavage of radiolabeled precursor proteins in Xenopus oocytes over time using pulse-chase analysis and autoradiography. This protocol can also be used to analyze folding and cleavage of radiolabeled precursor proteins at steady state. Finally, the third protocol details methods for isolating Bmp cleavage products from the blastocoele of Xenopus embryos and then analyzing them on immunoblots.

Fibronectin type III and intracellular domains of Toll-like receptor 4 interactor with leucine-rich repeats (Tril) are required for developmental signaling

Toll-like receptor 4 interactor with leucine-rich repeats (Tril) functions as a coreceptor for Toll-like receptors (Tlrs) to mediate innate immune responses in adults. In embryos, Tril signals to promote degradation of the Bmp inhibitor, Smad7, to allow for blood formation. It is not known whether this function requires, or is independent of, Tlrs. In the current studies, we performed a structure-function analysis, which indicated that the fibronectin type III (FN) domain and the intracellular domain of Tril are required to trigger Smad7 degradation in Xenopus embryos. Furthermore, we found evidence suggesting that a Tril deletion mutant lacking the FN domain (Tril∆FN) can dominantly inhibit signaling by endogenous Tril when overexpressed. This finding raises the possibility that the FN domain functions to bind endogenous Tril ligands. We also show that Tril cycles between the cell surface and endosomes and that the Tril extracellular domain, as well as cadherin based cell-cell adhesion, are required for cell surface retention, while the intracellular domain is required for internalization in Xenopus ectodermal explants. Using a CHO cell aggregation assay, we show that, unlike other transmembrane proteins that contain leucine-rich repeats, Tril is not sufficient to mediate homophilic adhesion.

Tril targets Smad7 for degradation to allow hematopoietic specification in Xenopus embryos

In Xenopus laevis, bone morphogenetic proteins (Bmps) induce expression of the transcription factor Gata2 during gastrulation, and Gata2 is required in both ectodermal and mesodermal cells to enable mesoderm to commit to a hematopoietic fate. Here, we identify tril as a Gata2 target gene that is required in both ectoderm and mesoderm for primitive hematopoiesis to occur. Tril is a transmembrane protein that functions as a co-receptor for Toll-like receptors to mediate innate immune responses in the adult brain, but developmental roles for this molecule have not been identified. We show that Tril function is required both upstream and downstream of Bmp receptor-mediated Smad1 phosphorylation for induction of Bmp target genes. Mechanistically, Tril triggers degradation of the Bmp inhibitor Smad7. Tril-dependent downregulation of Smad7 relieves repression of endogenous Bmp signaling during gastrulation and this enables mesodermal progenitors to commit to a blood fate. Thus, Tril is a novel component of a Bmp-Gata2 positive-feedback loop that plays an essential role in hematopoietic specification.

Sortilin is associated with the chlamydial inclusion and is modulated during infection

Chlamydia species are obligate intracellular pathogens that have a major impact on human health. The pathogen replicates within an intracellular niche called an inclusion and is thought to rely heavily on host-derived proteins and lipids, including ceramide. Sortilin is a transmembrane receptor implicated in the trafficking of acid sphingomyelinase, which is responsible for catalysing the breakdown of sphingomyelin to ceramide. In this study, we examined the role of sortilin in Chlamydia trachomatis L2 development. Western immunoblotting and immunocytochemistry analysis revealed that endogenous sortilin is not only associated with the inclusion, but that protein levels increase in infected cells. RNAi-mediated depletion of sortilin, however, had no detectable impact on ceramide delivery to the inclusion or the production of infectious progeny. This study demonstrates that whilst Chlamydia redirects sortilin trafficking to the chlamydial inclusion, RNAi knockdown of sortilin expression is insufficient to determine if this pathway is requisite for the development of the pathogen.

Summary: The acid sphingomyelinase trafficking protein sortilin is modulated by chlamydial infection. RNAi-mediated depletion of sortilin does not, however, perturb chlamydial infection.

Spatiotemporal patterns of sortilin and SorCS2 localization during organ development

BACKGROUND

Sortilin and SorCS2 are part of the Vps10p receptor family. They have both been studied in nervous tissue with several important functions revealed, while their expression and possible functions in developing peripheral tissue remain poorly understood. Here we deliver a thorough characterization of the prenatal localization of sortilin and SorCS2 in mouse peripheral tissue.

RESULTS

Sortilin is highly expressed in epithelial tissues of the developing lung, nasal cavity, kidney, pancreas, salivary gland and developing intrahepatic bile ducts. Furthermore tissues such as the thyroid gland, developing cartilage and ossifying bone also show high expression of sortilin together with cell types such as megakaryocytes in the liver. SorCS2 is primarily expressed in mesodermally derived tissues such as striated muscle, adipose tissue, ossifying bone and general connective tissue throughout the body, as well as in lung epithelia. Furthermore, the adrenal gland and liver show high expression of SorCS2 in embryos 13.5 days old.

CONCLUSIONS

The possible functions relating to the expression patterns of Sortilin and SorCS2 in development are numerous and hopefully this paper will help to generate new hypotheses to further our understanding of the Vps10p receptor family.

Sortilin and lipoprotein metabolism: making sense out of complexity

PURPOSE OF REVIEW

Genome-wide association studies have been used as an unbiased tool to identify novel genes that contribute to variations in LDL cholesterol levels in the hopes of uncovering new biology and new therapeutic targets for the treatment of atherosclerotic cardiovascular disease. The locus identified by genome-wide association studies with the strongest association with LDL cholesterol and atherosclerotic cardiovascular disease is the 1p13 sortilin-1 (SORT1) locus. Here, we review the identification and characterization of this locus, the initial physiological studies describing the role of SORT1 in lipoprotein metabolism, and recent work that has begun to sort out the complexity of this role.

RECENT FINDINGS

Studies by several groups support an important role for sortilin in lipoprotein metabolism; however, the directionality of the effect of sortilin on plasma cholesterol and its role in the secretion of hepatic lipoproteins remains controversial. Studies by several groups support a role for sortilin in inhibiting lipoprotein export, whereas other studies suggest that sortilin facilitates lipoprotein export.

SUMMARY

Understanding the mechanism by which sortilin affects LDL cholesterol will increase our understanding of the regulation of lipoprotein metabolism and hepatic lipoprotein export and may also allow us to harness the power of the 1p13 SORT1 locus for the treatment of atherosclerotic cardiovascular disease.

Gene expression analysis of children with acute hematogenous osteomyelitis caused by Methicillin-resistant Staphylococcus aureus: correlation with clinical severity of illness

Children with acute hematogenous osteomyelitis (AHO) demonstrate a broad spectrum of clinical manifestations, ranging from mild to severe. Several advances have been achieved in the study of host immune response to acute invasive Staphylococcus aureus infections through gene expression analysis. However, previous research has neither attempted to evaluate the response of children with AHO specific to Methicillin-resistant Staphylococcus aureus (MRSA) nor to correlate gene expression with clinical phenotype. Study objective was to correlate gene expression of children with AHO due to MRSA with clinical severity of illness. Whole blood samples were obtained in Tempus tubes from 12 children with osteomyelitis once cultures obtained directly from the site of infection confirmed to be positive for MRSA. Using an Illumina platform and a systems-wide modular analysis, microarray findings from ten of these children were compared to that of nine healthy (age, ethnicity and gender) matched controls and correlated with clinical severity of illness. Children with AHO from MRSA demonstrated over-expression of innate immunity with respect to neutrophil activity, coagulation, inflammatory response, and erythrocyte development. Concurrently, these children demonstrated under-expression of adaptive immunity with respect to lymphocyte activation and activity of T-cell, cytotoxic or NK cell, and B-cell lines. Three over-expressed genes, P2RX1, SORT1, and RETN, and two under-expressed genes, LOC641788 and STAT 4, were significantly correlated with severity of illness. STAT 4 showed the strongest correlation (R2 = –0.83). STAT4 downregulation could potentially explain under-expression of genes related to adaptive immunity in this cohort of patients with AHO. This study identified specific genes which correspond to disease severity during the early hospitalization of children with AHO from MRSA. Pattern recognition of this combination of genes could help to identify in the future severe clinical phenotypes before the disease is fully manifest and direct appropriate attention and resources to those children.

Regulation of TGFβ and related signals by precursor processing

Secreted cytokines of the TGFβ family are found in all multicellular organisms and implicated in regulating fundamental cell behaviors such as proliferation, differentiation, migration and survival. Signal transduction involves complexes of specific type I and II receptor kinases that induce the nuclear translocation of Smad transcription factors to regulate target genes. Ligands of the BMP and Nodal subgroups act at a distance to specify distinct cell fates in a concentration-dependent manner. These signaling gradients are shaped by multiple factors, including proteases of the proprotein convertase (PC) family that hydrolyze one or several peptide bonds between an N-terminal prodomain and the C-terminal domain that forms the mature ligand. This review summarizes information on the proteolytic processing of TGFβ and related precursors, and its spatiotemporal regulation by PCs during development and various diseases, including cancer. Available evidence suggests that the unmasking of receptor binding epitopes of TGFβ is only one (and in some cases a non-essential) function of precursor processing. Future studies should consider the impact of proteolytic maturation on protein localization, trafficking and turnover in cells and in the extracellular space.

Sortilins in neurotrophic factor signaling

The sortilin family of Vps10p-domain receptors includes sortilin, SorLA, and SorCS1-3. These type-I transmembrane receptors predominate in distinct neuronal tissues, but expression is also present in certain specialized non-neuronal cell populations including hepatocytes and cells of the immune system. The biology of sortilins is complex as they participate in both cell signaling and in intracellular protein sorting. Sortilins function physiologically in signaling by pro- and mature neurotrophins in neuronal viability and functionality. Recent genome-wide association studies have linked members to neurological disorders such as Alzheimer's disease and bipolar disorder and outside the nervous system to development of coronary artery disease and type-2 diabetes. Particularly well described are the receptor functions in neuronal signaling by pro- (proNT) and mature (NT) neurotrophins and in the processing/metabolism of amyloid precursor protein (APP).

Deciphering proneurotrophin actions

Like most growth factors, neurotrophins are initially synthesized as precursors that are cleaved to release C-terminal mature forms. The well-characterized mature neurotrophins bind to Trk receptors to initiate survival and differentiative responses. More recently, the precursor forms or proneurotrophins have been found to act as distinct ligands by binding to an unrelated receptor complex consisting of the p75 neurotrophin receptor (p75) and sortilin to initiate cell death. Induction of proNGF and p75 has been observed in preclinical injury models and in pathological states in the central nervous system, and strategies that block the proNGF/p75 interaction are effective in limiting neuronal apoptosis. In contrast, the mechanisms that regulate expression of other proneurotrophins, including proBDNF and proNT-3, are less well understood. Here, recent findings on the biological actions, regulation of expression, and pathophysiological effects of proneurotrophins will be reviewed.

The intracellular domain of sortilin interacts with amyloid precursor protein and regulates its lysosomal and lipid raft trafficking

The processing of Amyloid precursor protein (APP) is multifaceted, comprising of protein transport, internalization and sequential proteolysis. However, the exact mechanism of APP intracellular trafficking and distribution remains unclear. To determine the interaction between sortilin and APP and the effect of sortilin on APP trafficking and processing, we studied the binding site and its function by mapping experiments, colocalization, coimmunoprecipitation and sucrose gradient fractionation. We identified for the first time that sortilin interacts with APP at both N- and C-terminal regions. The sortilin-FLVHRY (residues 787–792) and APP-NPTYKFFE (residues 759–766) motifs are crucial for the C-terminal interaction. We also found that lack of the FLVHRY motif reduces APP lysosomal targeting and increases APP distribution in lipid rafts in co-transfected HEK293 cells. These results are consistent with our in vivo data where sortilin knockout mice showed a decrease of APP lysosomal distribution and an increase of APP in lipid rafts. We further confirmed that overexpression of sortilin-FLVHRY mutants failed to rescue the lysosomal degradation of APP. Thus, our data suggests that sortilin is implicated in APP lysosomal and lipid raft targeting via its carboxyl-terminal F/YXXXXF/Y motif. Our study provides new molecular insights into APP trafficking and processing.

Hepatic sortilin regulates both apolipoprotein B secretion and LDL catabolism

Genome-wide association studies (GWAS) have identified a genetic variant at a locus on chromosome 1p13 that is associated with reduced risk of myocardial infarction, reduced plasma levels of LDL cholesterol (LDL-C), and markedly increased expression of the gene sortilin-1 (SORT1) in liver. Sortilin is a lysosomal sorting protein that binds ligands both in the Golgi apparatus and at the plasma membrane and traffics them to the lysosome. We previously reported that increased hepatic sortilin expression in mice reduced plasma LDL-C levels. Here we show that increased hepatic sortilin not only reduced hepatic apolipoprotein B (APOB) secretion, but also increased LDL catabolism, and that both effects were dependent on intact lysosomal targeting. Loss-of-function studies demonstrated that sortilin serves as a bona fide receptor for LDL in vivo in mice. Our data are consistent with a model in which increased hepatic sortilin binds intracellular APOB-containing particles in the Golgi apparatus as well as extracellular LDL at the plasma membrane and traffics them to the lysosome for degradation. We thus provide functional evidence that genetically increased hepatic sortilin expression both reduces hepatic APOB secretion and increases LDL catabolism, providing dual mechanisms for the very strong association between increased hepatic sortilin expression and reduced plasma LDL-C levels in humans.

The multi-level action of fatty acids on adiponectin production by fat cells

Current epidemics of diabetes mellitus is largely caused by wide spread obesity. The best-established connection between obesity and insulin resistance is the elevated and/or dysregulated levels of circulating free fatty acids that cause and aggravate insulin resistance, type 2 diabetes, cardiovascular disease and other hazardous metabolic conditions. Here, we investigated the effect of a major dietary saturated fatty acid, palmitate, on the insulin-sensitizing adipokine adiponectin produced by cultured adipocytes. We have found that palmitate rapidly inhibits transcription of the adiponectin gene and the release of adiponectin from adipocytes. Adiponectin gene expression is controlled primarily by PPARγ and C/EBPα. Using mouse embryonic fibroblasts from C/EBPα-null mice, we have determined that the latter transcription factor may not solely mediate the inhibitory effect of palmitate on adiponectin transcription leaving PPARγ as a likely target of palmitate. In agreement with this model, palmitate increases phosphorylation of PPARγ on Ser273, and substitution of PPARγ for the unphosphorylated mutant Ser273Ala blocks the effect of palmitate on adiponectin transcription. The inhibitory effect of palmitate on adiponectin gene expression requires its intracellular metabolism via the acyl-CoA synthetase 1-mediated pathway. In addition, we found that palmitate stimulates degradation of intracellular adiponectin by lysosomes, and the lysosomal inhibitor, chloroquine, suppressed the effect of palmitate on adiponectin release from adipocytes. We present evidence suggesting that the intracellular sorting receptor, sortilin, plays an important role in targeting of adiponectin to lysosomes. Thus, palmitate not only decreases adiponectin expression at the level of transcription but may also stimulate lysosomal degradation of newly synthesized adiponectin.