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Biomedical subjects

Christa Buechler

Publications and source records attributed to Christa Buechler.

At least 19 recordsLinked to original sources

High molecular weight adiponectin reduces apolipoprotein B and E release in human hepatocytes.

Low circulating levels of high molecular weight adiponectin (HMW-Apm) have been linked to dyslipidaemia and systemic HMW-Apm negatively correlates with very low density lipoprotein (VLDL), apolipoprotein B (ApoB), and ApoE and is positively associated with ApoA-I. Therefore, it was investigated whether HMW-Apm alters the hepatic synthesis of ApoB, ApoE, and ApoA-I or the activity of the hepatic ATP-binding cassette transporter A1 (ABCA1), as the main determinant of plasma HDL. HMW-Apm reduces hepatic ApoB and ApoE release whereas ABCA1 protein, activity and ApoA-I were not altered. Global gene expression analysis revealed that hepatic nuclear factor 4-alpha (HNF4-alpha) and HNF4-alpha regulated genes like ApoB are downregulated by HMW-Apm and this was confirmed at the mRNA and protein level. Therefore it is concluded that HMW-adiponectin may ameliorate dyslipidaemia by reducing the hepatic release of ApoB and ApoE, whereas ABCA1 function and ApoA-I secretion are not influenced.

Adiponectin↗

Aldehyde oxidase 1 is highly abundant in hepatic steatosis and is downregulated by adiponectin and fenofibric acid in hepatocytes in vitro.

Adiponectin protects the liver from steatosis caused by obesity or alcohol and therefore the influence of adiponectin on human hepatocytes was analyzed. GeneChip experiments indicated that recombinant adiponectin downregulates aldehyde oxidase 1 (AOX1) expression and this was confirmed by real-time RT-PCR and immunoblot. AOX1 is a xenobiotic metabolizing protein and produces reactive oxygen species (ROS), that promote cell damage and fibrogenesis. Adiponectin and fenofibric acid activate peroxisome proliferator-activated receptor-alpha (PPAR-alpha) and both suppress AOX1 protein and this is blocked by the PPAR-alpha antagonist RU486. Obesity is associated with low adiponectin, reduced hepatic PPAR-alpha activity and fatty liver, and AOX1 was found induced in the liver of rats on a high-fat diet when compared to controls. Free fatty acids and leptin, that are elevated in obesity, failed to upregulate AOX1 in vitro. The current data indicate that adiponectin reduces AOX1 by activating PPAR-alpha whereas fatty liver disease is associated with elevated hepatic AOX1. High AOX1 may be associated with higher ROS well described to induce fibrogenesis in liver tissue but may also influence drug metabolism and activity.

Adiponectin↗

Adiponectin and its receptors in rodent models of fatty liver disease and liver cirrhosis.

AIM: To determine circulating and hepatic adiponectin in rodents with fatty liver disease or liver cirrhosis and investigate expression of the adiponectin receptors AdipoR1 on the mRNA and protein level and AdipoR2 on the mRNA level. METHODS: Fat fed rats were used as a model for fatty liver disease and bile duct ligation in mice to investigate cirrhotic liver. Expression of AdipoR1 and AdipoR2 mRNA was determined by real time RT-PCR. AdipoR1 protein was analysed by immunoblot. Adiponectin was measured by ELISA. RESULTS: Systemic adiponectin is reduced in fat-fed rats but is elevated in mice after bile duct ligation (BDL). Hepatic adiponectin protein is lower in steatotic liver but not in the liver of BDL-mice when compared to controls. Adiponectin mRNA was not detected in human liver samples or primary human hepatocytes nor in rat liver but recombinant adiponectin is taken up by isolated hepatocytes in-vitro. AdipoR1 mRNA and AdipoR1 protein levels are similar in the liver tissue of control and fat fed animals whereas AdipoR2 mRNA is induced. AdipoR2 mRNA and AdipoR1 mRNA and protein is suppressed in the liver of BDL-mice. CONCLUSION: Our studies show reduced circulating adiponectin in a rat model of fatty liver disease whereas circulating adiponectin is elevated in a mouse model of cirrhosis and similar findings have been described in humans. Diminished hepatic expression of adiponectin receptors was only found in liver cirrhosis.

Adiponectin↗

Adiponectin-induced secretion of interleukin-6 (IL-6), monocyte chemotactic protein-1 (MCP-1, CCL2) and interleukin-8 (IL-8, CXCL8) is impaired in monocytes from patients with type I diabetes.

BACKGROUND: Systemic adiponectin is reduced in patients with cardiovascular disease (CVD) and low adiponectin may contribute to the pathogenesis of atherosclerosis. However, circulating adiponectin is elevated in type 1 diabetes (T1D) patients, who have also a higher incidence to develop CVD. Because monocytes play an important role in atherosclerosis, we analysed the influence of adiponectin on cytokine and chemokine release in monocytes from T1D patients and controls. METHODS: Systemic adiponectin was determined in the plasma and the high-molecular weight (HMW) form of adiponectin was analysed by immunoblot. Monocytes were isolated from T1D patients and controls and the adiponectin-stimulated release of interleukin-6 (IL-6), monocyte chemotactic protein-1 (MCP-1, CCL2) and interleukin-8 (IL-8, CXCL8) was analysed. RESULTS: Systemic adiponectin was higher in T1D patients. Immunoblot analysis of the plasma indicate abundance of HMW adiponectin in T1D patients and controls. IL-6, CCL2 and CXCL8 secretion in response to adiponectin were found induced in monocytes from controls whereas only IL-6 was upregulated in T1D cells. The induction of IL-6 by adiponectin was abrogated by an inhibitor of the NFkappaB pathway. CONCLUSION: These data indicate that adiponectin-mediated induction of IL-6, CCL2 and CXCL8 is disturbed in monocytes from T1D patients and therefore elevated systemic adiponectin in T1D patients may be less protective when compared to controls.

Adiponectin↗

Lipopolysaccharide regulated protein expression is only partly impaired in monocytes from patients with type I diabetes.

BACKGROUND: Monocytes play an important role in innate immunity and atherosclerosis. A disturbed secretion of cytokines in lipopolysaccharide (LPS) activated monocytes from type 1 diabetes (T1D) patients has been described and may contribute to the impaired inflammatory response in these individuals. In the present study the influence of LPS on five different proteins with a function in immunity and atherosclerosis was analyzed in monocytes from controls and T1D patients. METHODS: Monocytes were isolated from controls and T1D patients and the LPS-stimulated increase of IL-6, CXCL8, monocyte chemotactic protein 1 (CCL2, MCP-1) and superoxide dismutase (SOD 2), as well as the LPS-mediated decrease of apolipoprotein E (Apo E) in primary human monocytes from controls and T1D patients was determined. RESULTS: CCL2 and IL-6 secretion in response to LPS was found significantly reduced in monocytes from T1D patients when compared to controls whereas basal CCL2 release was similar in control and T1D cells. In contrast, CXCL8 and apolipoprotein E secretion and SOD 2 expression upon LPS stimulation is similar from T1D and control monocytes. CONCLUSION: These data indicate that LPS-mediated protein expression is only partly disturbed in monocytes from T1D patients. Reduced secretion of IL-6 and CCL2 in activated monocytes of these patients may contribute to an impaired inflammatory response and vascular disease.

Adult↗

Different effects of adiponectin isoforms in human monocytic cells.

Adiponectin (APM) is an adipocyte-derived adipokine with immunosuppressive, antidiabetic, and antiatherosclerotic properties. Low molecular weight (LMW)- and higher molecular weight (HMW)-APM circulate in the serum and activate different signaling pathways. We were interested to see whether LMW-APM exerts different effects on monocytic cells compared with the HMW isoform. Therefore, the effects of recombinant LMW-APM produced in insect cells and the APM from higher eukaryotic cells containing HMW forms on monocytic cells were investigated with respect to apoptosis and inflammation. LMW- and HMW-APM induce apoptosis in nondifferentiated THP-1 cells, reduce macrophage scavenger receptor (MSR) A mRNA expression, and stimulate phosphorylation of adenosine monophosphate-activated protein kinase (AMPK). However, HMW-APM induces the secretion of interleukin (IL)-6 in human monocytes and THP-1 cells but does not suppress lipopolysaccharide (LPS)-induced IL-6 secretion. In contrast, LMW-APM reduces LPS-mediated IL-6 release and furthermore, stimulates IL-10 secretion, most likely by reducing the abundance of inhibitor of nuclear factor (NF)-kappaB kinase beta, leading to a diminished nuclear translocation of NF-kappaB p65. Our data indicate that the different APM isoforms do share common effects on monocytic cells but also induce isoform-specific responses. Although apoptosis, the activation of AMPK, and the reduction of MSR are mediated by all APM isoforms, only LMW-APM displays anti-inflammatory properties.

AMP-Activated Protein Kinases↗

The role of 'adipotropins' and the clinical importance of a potential hypothalamic-pituitary-adipose axis.

Since adipocytes express specific receptors for pituitary hormones and hypothalamic releasing factors, adipose tissue has to be regarded as a fast-acting endocrine gland under the control of the brain. Expanding on this suggestion, the existence and clinical impact of a hypothalamic-pituitary-adipose axis is reviewed. The term 'adipotropins' is introduced in order to describe pituitary and hypothalamic hormones or releasing factors that directly target adipocytes by their specific receptors.

Adipose Tissue↗

Profiling adipocytokine secretion from creeping fat in Crohn's disease.

BACKGROUND: Adipose tissue is recognized as a compartment secreting highly active molecules. Creeping fat represents a characteristic feature of Crohn's disease (CD). Proinflammatory or anti-inflammatory adipose-derived secretory products, now generally called adipocytokines, may play a role in the pathogenesis of CD. MATERIALS AND METHODS: Adipose tissue specimens were obtained from creeping fat contiguous to the involved intestine of 10 patients with CD. Mesenteric adipose tissue specimens resected from 13 patients with colon cancer (CC) and from 7 patients with diverticulitis served as controls. Three fat tissue specimen per well and 6 to 8 wells per patient were incubated ex vivo for 24 h. The release of adiponectin, resistin, leptin, interleukin-6, macrophage colony-stimulating factor, monocyte chemotactic protein-1, and migration inhibitory factor was measured by ELISA. The expression of AdipoR1 and AdipoR2 receptors was investigated by real-time quantitative polymerase chain reaction in a subset of adipose tissues. RESULTS: The secretion of adiponectin and macrophage colony-stimulating factor, as well as leptin and migration inhibitory factor, was significantly upregulated in CD compared with CC and diverticulitis or CC only, respectively. Resistin, interleukin-6, and monocyte chemotactic protein-1 were not specifically induced in CD but were associated with unspecific inflammation. Adiponectin receptor expression was not different in CC, CD, or diverticulitis. Steroid treatment in CD patients had differential effects on the ex vivo secretion of adipocytokines. CONCLUSIONS: A specific secretion pattern of proinflammatory and anti-inflammatory adipocytokines indicates the significance of adipose tissue in intestinal inflammation in CD. Future investigations of this intestinal compartment may provide novel insights into the pathophysiological role of creeping fat and CD.

Adipose Tissue↗

The adiponectin paralog CORS-26 has anti-inflammatory properties and is produced by human monocytic cells.

The adiponectin paralog CORS-26 (collagenous repeat-containing sequence of 26kDa protein) is a member of the C1q/TNF-alpha molecular superfamily. CORS-26 is a secreted protein and baculovirus-produced CORS-26 released in the supernatant of insect cells forms stable trimers. Adiponectin exerts anti-inflammatory effects in LPS-treated monocytic cells and CORS-26 also reduces IL-6 and TNF-alpha secretion but does not increase IL-10. Suppression of NFkappaB signalling may explain the anti-inflammatory actions of CORS-26. Furthermore CORS-26 protein was detected in human monocytic and dendritic cells. The present data demonstrate for the first time that CORS-26 forms trimers, exerts anti-inflammatory properties and that it is expressed in monocytic cells. Therefore CORS-26 may provide a new target for pharmacological drugs in inflammatory diseases like the metabolic syndrome.

Active Transport, Cell Nucleus↗

Regulation of adiponectin receptor 1 in human hepatocytes by agonists of nuclear receptors.

The adiponectin receptors AdipoR1 and AdipoR2 have been identified to mediate the insulin-sensitizing effects of adiponectin. Although AdipoR2 was suggested to be the main receptor for this adipokine in hepatocytes, AdipoR1 protein is highly abundant in primary human hepatocytes and hepatocytic cell lines. Nuclear receptors are main regulators of lipid metabolism and activation of peroxisome proliferator-activated receptor alpha and gamma, retinoid X receptor (RXR), and liver X receptor (LXR) by specific ligands may influence AdipoR1 abundance. AdipoR1 protein is neither altered by RXR or LXR agonists nor by pioglitazone. In contrast, fenofibric acid reduces AdipoR1 whereas hepatotoxic troglitazone upregulates AdipoR1 protein in HepG2 cells. Taken together this work shows for the first time that AdipoR1 protein is expressed in human hepatocytes but that it is not a direct target gene of nuclear receptors. Elevated AdipoR1 induced by hepatotoxic troglitazone may indicate a role of this receptor in adiponectin-mediated beneficial effects in liver damage.

Adiponectin↗

Association of ABCA1 with syntaxin 13 and flotillin-1 and enhanced phagocytosis in tangier cells.

The ATP-binding cassette transporter A1 (ABCA1) facilitates the cellular release of cholesterol and choline-phospholipids to apolipoprotein A-I (apoA-I) and several studies indicate that vesicular transport is associated with ABCA1 function. Syntaxins play a major role in vesicular fusion and have also been demonstrated to interact with members of the ABC-transporter family. Therefore, we focused on the identification of syntaxins that directly interact with ABCA1. The expression of syntaxins and ABCA1 in cultured human monocytes during M-CSF differentiation and cholesterol loading was investigated and syntaxins 3, 6, and 13 were found induced in foam cells together with ABCA1. Immunoprecipitation experiments revealed a direct association of syntaxin 13 and full-length ABCA1, whereas syntaxin 3 and 6 failed to interact with ABCA1. The colocalization of ABCA1 and syntaxin 13 was also shown by immunofluorescence microscopy. Silencing of syntaxin 13 by small interfering RNA (siRNA) led to reduced ABCA1 protein levels and hence to a significant decrease in apoA-I-dependent choline-phospholipid efflux. ABCA1 is localized in Lubrol WX-insoluble raft microdomains in macrophages and syntaxin 13 and flotillin-1 were also detected in these detergent resistant microdomains along with ABCA1. Syntaxin 13, flotillin-1, and ABCA1 were identified as phagosomal proteins, indicating the involvement of the phagosomal compartment in ABCA1-mediated lipid efflux. In addition, the uptake of latex phagobeads by fibroblasts with mutated ABCA1 was enhanced when compared with control cells and the recombinant expression of functional ABCA1 normalized the phagocytosis rate in Tangier fibroblasts. It is concluded that ABCA1 forms a complex with syntaxin 13 and flotillin-1, residing at the plasma membrane and in phagosomes that are partially located in raft microdomains.

ATP Binding Cassette Transporter 1↗

Expression of the lysophospholipid receptor family and investigation of lysophospholipid-mediated responses in human macrophages.

Some of the biological effects of lipoproteins have been attributed to their association with lysophosphatidic acid (LPA), lysophosphatidylcholine (LPC), sphingosine-1-phosphate (S1P) and sphingosylphosphorylcholine (SPC). These lysophospholipids mediate multiple biological responses via several G protein-coupled receptors (GPR). The expression of these receptors, however, has not been systematically investigated in primary human monocytes and macrophages as major cells involved in atherosclerosis. The mRNAs for all 15 receptors described so far were detected in monocytes, macrophages, foam cells and high density lipoprotein (HDL(3))-treated cells using real time RT-PCR. Immunoblots revealed that S1P(1), S1P(2), S1P(4), LPA(1), LPA(2) and GPR65 are expressed in monocytes and macrophages, while S1P(5) and LPA(3) have not been detected. S1P(3) was induced during differentiation but down-regulated by lipid-loading and HDL(3), whereas LPA(1) was down-regulated in differentiated macrophages. The influence of S1P on macrophages was investigated and the induction of CD32 indicates an enhanced phagocytic activity. Altogether, these data give insights into the expression and regulation of lysophospholipid receptors in primary human monocytes, macrophages and foam cells.

Adult↗

Screening for functional sequence variations and mutations in ABCA1.

Mutations in the ATP-binding cassette 1 transporter gene (ABCA1) are responsible for the genetic HDL-deficiency syndromes, which are characterized by severely diminished plasma HDL-C levels and a predisposition to cardiovascular disease and splenomegaly. The ABCA1 gene contains 50 exons and codes for a 2261-amino acid long membrane protein that facilitates phospholipid and cholesterol transport. Several mutations have been identified so far as responsible either for Tangier disease or for reduced HDL levels. We have selectively looked for additional polymorphisms in functionally relevant regions of the gene in cohorts constituted of individuals with altered HDL levels as well as healthy blood donors and octogenarians, and screened for mutations in the complete coding region of selected individuals with extremely aberrant HDL levels. In the promoter region, which is important for regulation of gene expression, we have identified several polymorphisms including one VNTR polymorphism, located at a putative ZNF202 binding site, which displayed different binding of ZNF202 in an electromobility shift assay. Three novel SNPs were discovered in the promoter region (G1047C, C1152T and C1440T). The prevalence of exchange G1047C (G-395C) was found significantly increased in probands with low HDL compared to probands with high HDL. Exchanges C1152T (C-290T) and C1440T (C-7T) were significantly more frequent in the cohort with low HDL compared to healthy blood donors and octogenarians. In the C-terminal part of ABCA1, known to interact with other proteins, two novel sequence variations (F2163S and V2244I) have been found in one phenotype related to cardiovascular disease, but none in the aforementioned cohorts. In one individual with extremely high HDL levels, the V771M polymorphism was found in a homozygous state. In patients with HDL deficiency, three novel mutations have been identified (W590L, W840R and R1068C). To facilitate further research in ABCA1 sequence variations and expand our understanding of their effects, we are introducing a webpage archive (http://www.abca1-mutants.all.at) containing all sequence variations reported in ABCA1 so far. This webpage provides a more recent and detailed summary of sequence variations and mutations in ABCA1 than existing databases and should also be of interest for molecular diagnosis of ABCA1-related HDL deficiency.

ATP Binding Cassette Transporter 1↗

Expression pattern and raft association of NIPSNAP3 and NIPSNAP4, highly homologous proteins encoded by genes in close proximity to the ATP-binding cassette transporter A1.

The highly homologous genes NIPSNAP3 and NIPSNAP4, with 87% amino acid identity, are members of the NIPSNAP family with putative roles in vesicular trafficking. NIPSNAP3 mRNA and NIPSNAP4 mRNA and protein were detected in multiple tissues and cells at varying degrees. Interestingly, NIPSNAP3 is most highly expressed in skeletal muscle, where NIPSNAP4 has a low mRNA abundance. NIPSNAP4 was found associated with membranes and partly localized in rafts. The ubiquitous expression of the highly conserved NIPSNAPs and their association with membranes further support an important cellular function of these proteins probably linked to vesicular trafficking. The NIPSNAP3 and NIPSNAP4 genes are located in close proximity to the 3' end of the ATP-binding cassette transporter A1 (ABCA1), whose mutations cause familial high-density lipoprotein deficiency syndromes. The adjacent genomic location and the finding that ABCA1 is a regulator of vesicular trafficking may indicate a functional relation of these proteins, even though NIPSNAP4 does not interact directly with ABCA1 nor is its expression altered in cells with mutated ABCA1.

ATP Binding Cassette Transporter 1↗

Lipoprotein (a) downregulates lysosomal acid lipase and induces interleukin-6 in human blood monocytes.

The association of elevated lipoprotein (a) (Lp(a)) with an increased risk for coronary events is clearly established. This increased risk may in part be due to the activation of monocytes as major cells involved in atherogenesis. High concentrations of plasma Lp(a) were shown to influence the gene expression of human blood monocytes and in the present study we demonstrate a reduced abundance of the lysosomal acid lipase (LAL) mRNA in monocytes of patients with coronary disease and selective Lp(a) hyperlipidemia. This is also supported by in vitro studies where purified Lp(a) but not low-density lipoprotein (LDL) was shown to downregulate mRNA levels of the LAL in control monocytes. A correlation of Lp(a) serum levels and the proinflammatory cytokine IL-6 was recently also described. Therefore, we investigated whether Lp(a) is capable to enhance the release of this acute phase cytokine from human blood monocytes. Purified Lp(a) led to an increased secretion of IL-6, but not TNF-alpha arguing against a general activation of these cells. The association of reduced LAL activity with the premature development of coronary artery disease has been demonstrated in patients with hypercholesterolemia, and in the present study we show for the first time that LAL expression is suppressed in monocytes from patients with Lp(a) hyperlipidemia and by purified Lp(a). In addition, increased levels of IL-6 also predict future cardiovascular events and IL-6 secretion was also induced by purified Lp(a).

Cells, Cultured↗

Molecular and functional interaction of the ATP-binding cassette transporter A1 with Fas-associated death domain protein.

ATP-binding cassette transporter A1 (ABCA1) is a major regulator of cellular cholesterol and phospholipid homeostasis. Its function has not been fully characterized and may depend on the association with additional proteins. To identify ABCA1-interacting proteins a human liver yeast two-hybrid library was screened with the 144 C-terminal amino acids of ABCA1. Fas-associated death domain protein (FADD) was identified to bind to ABCA1, and this interaction was confirmed by pull-down assays and co-immunoprecipitations. Recombinant expression of a dominant negative form of FADD or the C terminus of ABCA1 in the human hepatoma cell line HepG2 markedly reduced the transfer of phospholipids to apoA-I. This indicates that the binding of additional proteins, one of them being full-length FADD, is required for ABCA1 function. The association of FADD with ABCA1 provides an unexpected link between high density lipoprotein metabolism and an adaptor molecule mainly described in death receptor signal transduction.

ATP Binding Cassette Transporter 1↗

The carboxyterminus of the ATP-binding cassette transporter A1 interacts with a beta2-syntrophin/utrophin complex.

Recent work identified ABCA1 as the major regulator of plasma HDL-cholesterol responsible for the removal of excess choline-phospholipids and cholesterol from peripheral cells and tissues. ABCA1 function may depend on the association with heteromeric proteins and to identify these candidates a human liver yeast two-hybrid library was screened with the carboxyterminal 144 amino acids of ABCA1. Beta2-syntrophin was found to interact with ABCA1 and the C-terminal five amino acids of ABCA1 proned to represent a perfect tail for binding to syntrophin PDZ domains. Immunoprecipitation further confirmed the association of ABCA1 and beta2-syntrophin and in addition utrophin, known to couple beta2-syntrophin and its PDZ ligands to the F-actin cytoskeleton, was identified as a constituent of this complex. ABCA1 in the plasmamembrane of human macrophages was found to be partially associated with Lubrol rafts and effluxed choline-phospholipids involve these microdomains. Beta2-syntrophin does not colocalize in these rafts indicating that beta2-syntrophin may participate in the retaining of ABCA1 in cytoplasmic vesicles and for the targeting of ABCA1 to plasmamembrane microdomains when ABCA1 is released from beta2-syntrophin.

ATP Binding Cassette Transporter 1↗

Cloning and characterization of a novel apolipoprotein A-I binding protein, AI-BP, secreted by cells of the kidney proximal tubules in response to HDL or ApoA-I.

Apolipoprotein A-I (apoA-I) is the major apolipoprotein of high-density lipoproteins (HDL) and has an important role in the regulation of the stability, lipid transport, and metabolism of HDL particles. To identify novel proteins that are involved in HDL metabolism, we used mature apoA-I (amino acids 25-267) as a bait for the screening of a human liver two-hybrid cDNA library. Among the identified genes, several encoded known proteins, including serum amyloid A(2a) (SAA(2a)), apoC-I, and phosphodiesterase HCAM1 (PDE1A), found to interact with apoA-I. In addition, we have cloned a novel 29 kDa apoA-I interacting protein, which we named AI-BP (apoA-I binding protein). The AI-BP encoding gene, APOA1BP, which is located on chromosome 1q21, is composed of six exons and five introns and spans 2.5 kb. Northern blot analysis demonstrated ubiquitous expression of the APOA1BP mRNA with the highest expression in kidney, heart, liver, thyroid gland, adrenal gland, and testis. AI-BP protein is not detectable in serum of healthy probands, but serum samples of patients with septic syndromes may contain elevated levels of AI-BP. Significant amounts of AI-BP protein are found in cerebrospinal fluid and urine of healthy probands. The stimulation of cells derived from the kidney proximal tubules with apoA-I or HDL induces a concentration-dependent secretion of AI-BP indicating an important role for AI-BP, in the renal tubular degradation or resorption of apoA-I.

Amino Acid Sequence↗