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Upregulation of the expression of the LDL receptor (LDLR) gene does not always lead to an increase in the LDLR protein. While the mRNA expression of the LDLR and the newly studied proprotein convertase PCSK9 are coordinately upregulated in absence of sterols, the latter proteinase apparently enhances the degradation of the LDLR protein.
Hypercholesterolemia is a major risk factor for coronary artery disease. Oxysterols are known to inhibit cholesterol biosynthesis and have been explored as potential antihypercholesterolemic agents. The ability of 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one (15-ketosterol) to lower non-HDL cholesterol has been demonstrated in rodent and primate models, but the mechanisms of action remain poorly understood. Here we show in a coactivator recruitment assay and cotransfection assays that the 15-ketosterol is a partial agonist for liver X receptor-alpha and -beta (LXRalpha and LXRbeta). The binding affinity for the LXRs was comparable to those of native oxysterols. In a macrophage cell line of human origin, the 15-ketosterol elevated ATP binding cassette transporter ABCA1 mRNA in a concentration-dependent fashion with a potency similar to those of other oxysterols. We further found that in human embryonic kidney HEK 293 cells, the 15-ketosterol suppressed sterol-responsive element binding protein processing activity and thus inhibited mRNA expression of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, LDL receptor, and PCSK9. Our data thus provide a molecular basis for the hypocholesterolemic activity of the 15-ketosterol and further suggest its potential antiatherosclerotic benefit as an LXR agonist.
BACKGROUND: Most drugs target proteins, and proteome-wide genetic analyses in diverse populations could discover potential novel and repurposed targets for improved prevention and treatment of ischemic heart disease (IHD) beyond statin therapy. OBJECTIVES: The purposes of this study were to use cis-acting single nucleotide polymorphisms (cis-pQTLs) identified for plasma proteins in East Asians and Europeans to discover and validate potential drug targets for IHD. METHODS: We measured plasma levels of 9,520 (Olink/SomaScan: 2,923/7,297) proteins in a case-cohort study of IHD (1,976 incident cases and 2,001 subcohort controls) in statin-free individuals in the prospective China Kadoorie Biobank (CKB). Genome-wide association studies identified 2,895 (Olink/SomaScan: 1,301/1,594) cis-pQTLs for these proteins in CKB. Two-sample Mendelian randomization (MR) and colocalization analyses assessed associations of all available cis-pQTLs for these proteins with IHD in East Asians (n = 29,319 cases), with further replication in Europeans (n = 181,522 cases) and comparison with findings in previous MR studies. RESULTS: In CKB observational analyses, a total of 959 (Olink/SomaScan: 426/533) proteins were associated at false discovery rate-corrected P < 0.05 with IHD after adjusting for major IHD risk factors. Two-sample MR analyses provided genetic support for 54 unique (Olink/SomaScan: 36/28) proteins in IHD etiology. Colocalization analyses confirmed shared gene-protein-IHD associations (posterior probability of hypothesis 4 [PPH4] ≥0.8) for 15 unique (Olink/SomaScan: 10/10) proteins, including 8 lipid-related, 3 inflammation-related, 1 blood pressure-related, and 3 alcohol-related proteins in East Asians. In Europeans, MR analyses of 12 non-alcohol-related proteins showed directionally concordant results for 8 proteins, with 5 having strong colocalization evidence of shared gene-protein-IHD associations (PPH4 ≥0.8), including 4 lipid-related (proprotein convertase subtilisin/kexin type 9, LPA, APOE, cadherin-1) and 1 systolic blood pressure-related (fibroblast growth factor 5) protein. However, 4 proteins showed directionally discordant MR results, including 2 lipid-related (APOA5, SORT1) and 1 inflammation-related (transforming growth factor beta 1) proteins with strong colocalization evidence of shared gene-protein-IHD associations (PPH4 ≥0.8). Comparison with previous MR studies revealed little consistency across studies in the number and identity of target proteins for IHD beyond well-established lipid-related (low-density lipoprotein cholesterol, lipoprotein(a), and triglycerides) or inflammation-related (interleukin-6) protein targets. CONCLUSIONS: The findings support a role for lipid-driven chronic inflammation in IHD etiology, and treatment strategies simultaneously targeting multiple lipid and inflammation pathways should be prioritized for further research to improve drug treatment of IHD beyond statin therapy.
Yeast Kex2 and human furin are subtilisin-related proprotein convertases that function in the late secretory pathway and exhibit similar though distinguishable patterns of substrate recognition. Although both enzymes prefer Arg at P(1) and basic residues at P(2), the two differ in recognition of P(4) and P(6) residues. To probe P(4) and P(6) recognition by Kex2p, furin-like substitutions were made in the putative S(4) and S(6) subsites of Kex2. T252D and Q283E mutations were introduced to increase the preference for Arg at P(4) and P(6), respectively. Glu(255) was replaced with Ile to limit recognition of P(4) Arg. The effects of putative S(4) and S(6) mutations were determined by examining the cleavage by purified mutant enzymes of a series of fluorogenic substrates with systematic changes in P(4) and/or P(6). Whereas wild Kex2 exhibited little preference type for Arg at P(6), the T252D mutant and T252D/Q283E double mutant exhibited clear interactions with P(6) Arg. Moreover, the T252D and T252D/Q283E substitutions altered the influence of the P(6) residue on P(4) recognition. We infer that cross-talk between S(4) and S(6), not seen in furin, allows wild type and mutant forms of Kex2 to adapt their subsites for altered modes of recognition. This apparent plasticity may allow the subsites to rearrange their local environment to interact with different substrates in a productive manner. E255I-Kex2 exhibited significantly decreased recognition of P(4) Arg in a tetrapeptide substrate with Lys at P(1), although the general pattern of selectivity for aliphatic residues at P(4) remained unchanged.
Furin, a human subtilisin-related proprotein convertase (SPC), is emerging as an important pharmaceutical target because it processes vital proteins of many aggressive pathogens. Furin inhibitors reported as yet are peptide derivatives and proteins, with the exception of andrographolides, which are natural compounds. Here we report that the small and highly stable compounds M(chelate)Cl(2) (M is copper or zinc) inhibit furin and Kex2, with Cu(TTP)Cl(2) and Zn(TTP)Cl(2) as the most efficient inhibitors. (TTP is 4'-[p-tolyl]-2,2 ':6',2"-terpyridine.) Inhibition is irreversible, competitive with substrate, and affected by substituents on the chelate. The free chelates are not inhibitors. Solvated Zn(2+) is less potent than its complexes. This is true also for copper and Kex2. However, solvated Cu(2+) (k(on) of 25,000 +/- 2,500 s(-1)) is more potent than Cu(TTP)Cl(2) (k(on) = 140 +/- 13 s(-1) and allows recovery of furin activity prior to a second inhibition phase. A mechanism that involves coordination to the catalytic histidine is proposed for all inhibitors. Target specificity is indicated by the fact that these metal chelate inhibitors are much less potent toward Kex2, the yeast homologue of furin. For example, k(on) with Zn(TTP)Cl(2) is 120 +/- 20 s(-1) for furin, but only 1.2 +/- 0.1 s(-1) for Kex2.
TGF-beta is a putative mediator of immunosuppression associated with malignant glioma and other types of cancer. Subtilisin-like proprotein convertases such as furin are thought to mediate TGF-beta processing. Here we report that human malignant glioma cell lines express furin mRNA and protein, exhibit furin-like protease (FLP) activity, and release active furin into the cell culture supernatant. FLP activity is not modulated by exogenous TGF-beta or neutralizing TGF-beta Abs. Exposure of LN-18 and T98G glioma cell lines to the furin inhibitor, decanoyl-Arg-Val-Lys-Arg-chloromethylketone, inhibits processing of the TGF-beta1 and TGF-beta2 precursor molecules and, consequently, the release of mature bioactive TGF-beta molecules. Ectopic expression of PDX, a synthetic antitrypsin analog with antifurin activity, in the glioma cells inhibits FLP activity, TGF-beta processing, and TGF-beta release. Thus, subtilisin-like proprotein convertases may represent a novel target for the immunotherapy of malignant glioma and other cancers or pathological conditions characterized by enhanced TGF-beta bioactivity.
Here we developed small molecule inhibitors of SKI-1/S1P enzyme of the Proprotein Convertase family following two approaches. One involves the assembly of multi-branch peptides while the other utilizes the insertion of alkyloxy pseudo peptide bond at P1-P1' cleavage position. In first approach, 2 and 4-branch peptides were designed based on the human (h) SKI-1(128-137) sequence, located N-terminal to its secondary activation site (K(137) downward arrow L). The 4-branch peptide exhibited the highest SKI-1 inhibitory property (IC(50) = 0.9 microM) with approximately 8.6 and 1.3-fold more potency than the corresponding single and 2-branch peptides, respectively. In the second strategy, an oxymethylene containing unnatural amino acid such as aminooxy-acetic acid (Aoaa) or 8-amino-3, 6 dioxa-octanoic acid (Adoa) was introduced substituting P1, P1' or both residues of hSKI-1(183-190) and hSKI-1(178-190) segments. These domains contain the same primary hSKI-1 activation site L(186) downward arrow R. Among those tested, P7-Tyr mutant [(178)GRYSSRRL(Adoa)AIP(190)] exhibited higher SKI-1 inhibitory activity (K(i)in low microM). Circular dichroism (CD) spectra of SKI-1 inhibitors showed interactions of varying degrees between the enzyme and the inhibitor consistent with the observed inhibition profile. A 3D-homology model structure of SKI-1 catalytic domain indicated a broad catalytic pocket.
IMPORTANCE: Treatment to lower high levels of low-density lipoprotein cholesterol (LDL-C) reduces incident coronary artery disease (CAD) risk but modestly increases the risk for incident type 2 diabetes (T2D). The extent to which genetic factors across the cholesterol spectrum are associated with incident T2D is not well understood. OBJECTIVE: To investigate the association of genetic predisposition to increased LDL-C levels with incident T2D risk. DESIGN, SETTING, AND PARTICIPANTS: In this large prospective, population-based cohort study, UK Biobank participants who underwent whole-exome sequencing and genome-wide genotyping were included. Participants were separated into 7 groups with familial hypercholesterolemia (FH), predicted loss of function (pLOF) in APOB or PCSK9 variants, and LDL-C polygenic risk score (PRS) quintiles. Data were collected between 2006 and 2010, with a median follow-up of 13.7 (IQR, 12.9-14.5) years. Data were analyzed from March 1 to November 1, 2024. EXPOSURES: LDL-C level, LDL-C PRS, FH, or pLOF variant status. MAIN OUTCOMES AND MEASURES: Cox proportional hazards regression models adjusted for age, sex, genotyping array, lipid-lowering medication use, and the first 10 genetic principal components were fitted to assess the association between LDL-C genetic factors and incident T2D and CAD risks. RESULTS: Among the 361 082 participants, mean (SD) age was 56.8 (8.0) years, 194 751 (53.9%) were female, and mean (SD) baseline LDL-C level was 138.0 (33.6) mg/dL. During the follow-up period, 22 619 (6.3%) participants developed incident T2D and 17 966 (5.0%) developed incident CAD. The hazard ratio for incident T2D was lowest in the FH group (0.65; 95% CI, 0.54-0.77), while the highest risk was in the pLOF group (1.48; 95% CI, 1.18-1.86). The association between LDL-C PRS and incident T2D was 0.72 (95% CI, 0.66-0.79) for very high LDL-C PRS, 0.87 (95% CI, 0.84-0.90) for high LDL-C PRS, 1.13 (95% CI, 1.09-1.17) for low LDL-C PRS, and 1.26 (95% CI, 1.15-1.38) for very low LDL-C PRS. CAD risk increased directly with the LDL-C PRS. CONCLUSIONS AND RELEVANCE: In this cohort study, LDL-C and T2D risks were inversely associated across genetic mechanisms for LDL-C variation. Further elucidation of the mechanisms associating low LDL-C risk with increased risk of T2D is warranted.
AIMS/HYPOTHESIS: We examined a clinical model of ex vivo transdifferentiation of primary adult hepatocytes to insulin-secreting cells for the treatment of type 1 diabetes. MATERIALS AND METHODS: Isolated rat hepatocytes were transduced in primary culture with a human lentivirus containing pancreatic duodenal homeobox 1 (PDX1, now known as insulin promoter factor 1, homeodomain transcription factor [IPF1]). Insulin expression and secretion of the newly engineered cells were assessed in vitro by RT-PCR, in situ hybridisation, immunostaining and radioimmunoassay. PDX1-transduced hepatocytes were further studied in vivo by injecting them under the renal capsule of diabetic SCID mice. RESULTS: Isolated rat hepatocytes were efficiently transduced with the lentiviral vector, as assessed by green fluorescent reporter gene expression. The transduced cells exhibited insulin at both mRNA (RT-PCR, in situ hybridisation) and protein levels (immunostaining and radioimmunoassay). Moreover, insulin secretion by the engineered cells was dependent on glucose and sulfonylurea. Other beta cell genes, including those encoding solute carrier family 2 (facilitated glucose transporter), member 2 (Slc2a2), glucokinase (Gck), ATP-binding cassette, sub-family C (CFTR/MRP), member 8 (Abcc8), the potassium inwardly-rectifying channel, subfamily J, member 11 (Kcnj11) and proprotein convertase subtilisin/kexin type 1 (Pcsk1) were also expressed. The PDX1-transduced hepatocytes expressed several pancreatic transcription factors related to early pancreatic endocrine development (endogenous Pdx1, neurogenic differentiation factor 1 [Neurod1], and NK6 transcription factor related, locus 1 [Nkx6-1]) as well as the late-stage pancreatic transcription factors (paired box gene 4 [Pax4], paired box gene 6 [Pax6], and v-maf musculoaponeurotic fibrosarcoma oncogene homolog A [Mafa]). Transplantation of 3 x 10(6) transdifferentiated liver cells under the renal capsule of seven streptozotocin-induced diabetic SCID mice resulted in significant reduction of non-fasting blood glucose levels from 30.7 +/- 1.3 to 8.7 +/- 3.7 mmol/l (mean +/- SEM, p = 0.01), in 6 to 8 weeks. Removal of the graft resulted in severe hyperglycaemia. CONCLUSIONS/INTERPRETATION: Ex vivo lentiviral-mediated PDX1 expression in isolated adult liver cells represents a potential model for type 1 diabetes mellitus therapy.
The transporter associated with Ag processing (TAP) translocates antigenic peptides into the endoplasmic reticulum for binding onto MHC class I (MHC I) molecules. Tapasin organizes a peptide-loading complex (PLC) by recruiting MHC I and accessory chaperones to the N-terminal regions (N domains) of the TAP subunits TAP1 and TAP2. To investigate the function of the tapasin-docking sites of TAP in MHC I processing, we expressed N-terminally truncated variants of TAP1 and TAP2 in combination with wild-type chains, as fusion proteins or as single subunits. Strikingly, TAP variants lacking the N domain in TAP2, but not in TAP1, build PLCs that fail to generate stable MHC I-peptide complexes. This correlates with a substantially reduced recruitment of accessory chaperones into the PLC demonstrating their important role in the quality control of MHC I loading. However, stable surface expression of MHC I can be rescued in post-endoplasmic reticulum compartments by a proprotein convertase-dependent mechanism.
Potent inhibitors of the Kex2/furin family precursor processing proteases were developed by randomizing adventitious contact sites and screening for optimized affinity using inhibition assays in 96-well format [1]. In this review, the binding interactions of the developed inhibitors will be examined in light of the three dimensional structures of Kex2 and furin [2-4].
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Mammalian cells express several transcription factors embedded in the endoplasmic reticulum (ER) as transmembrane proteins that are activated by proteolysis, and two types of these proteins have been extensively investigated. One type comprises the sterol regulatory element-binding proteins (SREBP-1 and SREBP-2). The other type comprises the activating transcription factors 6 (ATF6alpha and ATF6beta), which are activated in response to ER stress. It was shown previously that both SREBP and ATF6 are cleaved sequentially first by the Site-1 protease (serine protease) and then by the Site-2 protease (metalloprotease) (Ye, J., Rawson, R. B., Komuro, R., Chen, X., Dave, U. P., Prywes, R., Brown, M. S., and Goldstein, J. L. (2000) Mol. Cell 6, 1355-1364). In this study, we examined various protease inhibitors and found that 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF), a serine protease inhibitor, prevented ER stress-induced cleavage of ATF6alpha and ATF6beta, resulting in inhibition of transcriptional induction of ATF6-target genes. AEBSF also inhibited production of the mature form of SREBP-2 that was induced in response to sterol depletion, and appeared to directly prevent cleavage of ATF6alpha and ATF6beta by inhibiting Site-1 protease. As the Site-1 protease is localized in the Golgi apparatus, both SREBP and ATF6 must relocate to the Golgi apparatus to be cleaved. We showed here that AEBSF treatment had little effect on ER stress-induced translocation of ATF6 from the ER to the Golgi apparatus, but blocked nuclear localization of ATF6. These results indicate that the transport of ATF6 from the ER to the Golgi apparatus and that from the Golgi apparatus to the nucleus are distinct steps that can be distinguished by treatment with AEBSF.
We describe a permanent line of Chinese hamster ovary cells transfected with a cDNA encoding a truncated form of Site-1 protease (S1P) that is secreted into the culture medium in an enzymatically active form. S1P, a subtilisin-like protease, normally cleaves the luminal loop of sterol regulatory element-binding proteins (SREBPs). This cleavage initiates the two-step proteolytic process by which the NH(2)-terminal domains of SREBPs are released from cell membranes for translocation to the nucleus, where they activate transcription of genes involved in the biosynthesis and uptake of cholesterol and fatty acids. Truncated S1P (amino acids 1-983), produced by the transfected Chinese hamster ovary cells, lacks the COOH-terminal membrane anchor. Like native S1P, this truncated protein undergoes normal autocatalytic processing after residue 137 to release an NH(2)-terminal propeptide, thereby generating an active form, designated S1P-B. Prior to secretion, truncated S1P-B, like native S1P-B, is cleaved further after residue 186 to generate S1P-C, which is the only form that appears in the culture medium. The secreted enzyme, designated S1P(983)-C, cleaves a synthetic peptide that terminates in a 7-amino-4-methyl-coumarin fluorochrome. This peptide, RSLK-MCA, corresponds to the internal propeptide cleavage site that generates S1P-B as described in the accompanying paper (Espenshade, P. J., Cheng, D., Goldstein, J. L., and Brown, M. S. (1999), J. Biol. Chem. 274, 22795-22804). The secreted enzyme does not cleave RSVL-MCA, a peptide corresponding to the physiologic cleavage site in SREBP-2. However, S1P(983)-C does cleave after this leucine when the RSVL sequence is contained within a 16-residue peptide corresponding to the central portion of the SREBP-2 luminal loop. The catalytic activity of S1P(983)-C differs from that of furin/prohormone convertases, two related proteases, in its more alkaline pH optimum (pH 7-8), its relative resistance to calcium chelating agents, and its ability to cleave after lysine or leucine rather than arginine. These data provide direct biochemical evidence that S1P is the protease that cleaves SREBPs and thereby functions to control lipid biosynthesis and uptake in animal cells.
The transferrin receptor (TfR) is a transmembrane protein that mediates cellular uptake of iron. Although the serum concentration of the soluble TfR (sTfR) is altered in several diseases and used for diagnostic purposes, the identity and regulation of the shedding protease is unknown. In this study we quantified sTfR release from microsomal membranes and leukocytic cell lines in the presence of numerous protease inhibitors and cell activating compounds. We show that sTfR release is mediated by an integral membrane metalloprotease and can be inhibited by matrix metalloproteinase inhibitor 2 and tumor necrosis factor alpha protease inhibitor-2 (TAPI-2). Cleavage is also inhibited by a specific furin inhibitor, indicating that the protease is activated by a furin-like proprotein convertase. Whereas stimulation of the cells by the ectodomain shedding activator phorbol 12-N-myristate 13-acetate did not alter sTfR release significantly, the phosphatase inhibitor pervanadate led to an increase of TfR shedding in several leukocytic cell lines. Our results suggest that TfR shedding is constitutively mediated by a member of the metalloprotease family known as ADAM (for a disintegrin and metalloprotease).
Site-1 protease (S1P) cleaves membrane-bound sterol regulatory element-binding proteins (SREBPs), allowing their transcription-stimulating domains to translocate to the nucleus where they activate genes governing lipid synthesis. S1P is a potential target for lipid-lowering drugs, but the effect of S1P blockade in animals is unknown. Here, we disrupt the S1P gene in mice. Homozygous germ-line disruptions of S1P were embryonically lethal. To disrupt the gene inducibly in liver, we generated mice homozygous for a floxed S1P allele and heterozygous for a transgene encoding Cre recombinase under control of the IFN-inducible MX1 promoter. When IFN was produced, 70-90% of S1P alleles in liver were inactivated, and S1P mRNA and protein were reduced. Nuclear SREBPs declined, as did mRNAs for SREBP target genes. Cholesterol and fatty acid biosynthesis in hepatocytes declined by 75%. Low density lipoprotein (LDL) receptor mRNA declined by 50%, as did the clearance of (125)I-labeled LDL from plasma, but plasma cholesterol fell, suggesting that LDL production was reduced. These data raise the possibility that S1P inhibitors may be effective lipid-lowering agents, but they suggest that nearly complete inhibition will be required.
Lipid homeostasis is transcriptionally regulated by three DNA-binding proteins, designated sterol regulatory element-binding protein (SREBP)-1a, -1c, and -2. Oligonucleotide arrays hybridized with RNA made from livers of transgenic SREBP-1a, transgenic SREBP-2, and SREBP cleavage-activating protein knockout mice recently identified 33 genes regulated by SREBPs in liver, four of which had no known connection to lipid metabolism. One of the four genes was PCSK9, which encodes proprotein convertase subtilisin/kexin type 9a, a protein that belongs to the proteinase K subfamily of subtilases. Mutations in PCSK9 are associated with an autosomal dominant form of hypercholesterolemia. Here, we demonstrate that hepatic overexpression of either wild-type or mutant PCSK9 in mice results in hypercholesterolemia. The hypercholesterolemia is due to a post-transcriptional event causing a reduction in low density lipoprotein (LDL) receptor protein prior to the internalization and recycling of the receptor. Overexpression of PCSK9 in primary hepatocytes and in mice lacking the LDL receptor does not alter apolipoprotein B secretion. These data are consistent with PCSK9 affecting plasma LDL cholesterol levels by altering LDL receptor protein levels via a post-transcriptional mechanism.