A comprehensive account of insulin and LDL receptor activity over the years: A highlight on their signaling and functional role.

Insulin receptor (IR) was discovered in 1970. Shortcomings in IR transcribed signals were found pro-diabetic, which could also inter-relate obesity and atherosclerosis in a time-dependent manner. Low-density lipoprotein receptor (LDLR) was discovered in 1974. Later studies showed that insulin could modulate LDLR expression and activity. Repression of LDLR transcription in the absence or inactivity of insulin showed a direct cause of atherosclerosis. Leptin receptor (OB-R) was found in 1995 and its resistance became responsible for developing obesity. The three interlinked pathologies namely, diabetes, atherosclerosis, and obesity were later on marked as metabolic syndrome-X (MSX). In 2012, the IR-LDLR inter-association was identified. In 2019, the proficiency of signal transmission from this IR-LDLR receptor complex was reported. LDLR was found to mimic IR-generated signaling path when it remains bound to IR in IR-DLR interlocked state. This was the first time LDLR was found sending messages besides its LDL-clearing activity from blood vessels.

Atherosclerosis and carcinoma: Two facets of dysfunctional cholesterol homeostasis.

Although cholesterol is an essential and necessary component for biological systems; inappropriate accumulation of cholesterol in blood vessels and intracellular territory is also detrimental to living things. On one hand, cholesterol is the acting precursor of many metabolic regulators, a component of the structural veracity and scaffold fluidity of biomembranes, an insulator of electrical transmission in nerves and many more; on the other hand, its deposition in blood vessels induces atherosclerotic plaque and cardiovascular complications with the consequences of heart attack and stroke. It is also an emerging fact that cholesterol is a prelate in the cell nucleus for cell proliferation and any oddity in this venture may be the cause of tumorigenesis. Hence, cholesterol homeostasis is a very crucial element in issues of health management. Cholesterol is now a global target for maintaining quality health, particularly to control the two giants of the present world health tragedy: atherosclerosis and carcinoma, which appear to be the two facets of dysfunctional cholesterol homeostasis.

LOX-1: Its cytotopographical variance and disease stress.

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is a canonical receptor for oxidized LDL (oxLDL) among the known modified LDL particles. Topographical variance on LOX-1 expression in different cell types and its influence on the atherogenic potential of the particular cell type is the main focus of this review. Characteristic features of LOX-1 on the atherogenic potential of aortic endothelial cells, macrophages, platelets, and vascular smooth muscle cells have been discussed. Nonspecificity of ligands, besides oxLDL, is also the highlight of this review to show the chameleon characteristics in the functional activity of the receptor protein. Induction of LOX-1 has been reported in diseases like atherosclerosis, diabetes, and hypertension, as well as in the inflammatory response of immune reactions. The expression of LOX-1 is upregulated by the vicious cycle of stimulatory response from proatherogenic molecules.

Differential response of T cells to an immunogen, a mitogen and a chemical carcinogen in a mouse model system.

In this study, we examined the relative immune response of T-lymphocytes and its intracellular cholesterol homeostasis, in a mouse model system, after treatment with immunogen, mitogen, and carcinogen. We studied the T-lymphocyte percentage, their LDL-receptor expression, along with the levels of serum interleukins (IL-2, IFNγ, IL-4, and IL-10) and intracellular cholesterol concentration (cytoplasmic and nuclear). The mitogen was found to be a better stimulator of T-cell marker expressions than the immunogen; though the immunogen was more effective on immunogenic response as was marked from interleukin levels. The chemical carcinogen benzo-α-pyrene at low concentration acted potentially like a mitogen but a reduced immune response was apparent at a carcinogenic dose. The findings in our study focus on the effect of carcinogenic dose of benzo-α-pyrene (BaP) on T-cell immunity. Benzo-α-pyrene causes immunosuppression through restriction of the T-cell population by targeting intracellular cholesterol.

LOX-1: A potential target for therapy in atherosclerosis; an in vitro study.

Pro-inflammatory signal generated from the interaction of oxLDL with its cognate receptor LOX-1 has been attenuated successfully by a novel combination siRNA (siLOX-1Ω) targeting unique regions of Homo sapien LOX-1 mRNA. Signalling via LOX-1R was studied in a potentially pro-atherogenic arena recreated in a metabolic, pulse-chase set up. An initial pulse of oxLDL (20µg/mL;5h) was chased (without oxLDL) on a temporal scale upto 72h. Our study shows that the pro-inflammatory signal generated via oxLDL-LOX-1R interaction was mediated in two rungs, an initial sustained increase in LOX-1R expression up to 12h, and a renewal after 48h. TNF-α acted as a primary mediator of LOX-1R signalling, presumably also stimulating CD40 and MMP-9. Both TNF-α and IL-6 were involved in the second rung of LOX-1R signalling; maximum secretion of both was detected at 48h. Our study suggests a temporal sustenance of LOX-1R signalling by pro-inflammatory cytokines even on withdrawal of oxLDL. Also, siLOX-1Ω successfully abated LOX-1R expression along with its signalling intermediates, NO and NF-kB. Overall, LOX-1 signalling and the crucial role of cytokines in sustaining it is reported. Attenuation of this receptor may be of therapeutic value in atherosclerosis.

Differential pro-inflammatory responses of TNF-α receptors (TNFR1 and TNFR2) on LOX-1 signalling.

TNF-α potently induces LOX-1 expression in THP-1 macrophages at concentrations between 1.25-50 ng/mL. The interplay between the two TNF receptors (TNFR1 and TNFR2) was apparent in the expression pattern of LOX-1 in response to TNF-α. Interestingly, R1 signal abrogation depleted both TNFR2 as well as LOX-1 transcript expression, suggesting that TNFR1 holds priority in the relative signaling mechanism between TNFR1 and TNFR2. TNF-α was also found to abrogate the oxidized-LDL (ox-LDL) mediated increase in intracellular pool of NO, a known downstream intermediate of LOX-1 pro-inflammatory signaling cascade. At the level of ox-LDL clearance, TNF-α inhibited the uptake (scavenging) of ox-LDL via LOX-1. Our study demonstrates the ability of TNF-α to enhance the signaling propensity of LOX-1 by increasing its expression and inhibiting its scavenging property.

Role of leptin on the expression of low density lipoprotein receptor.

BACKGROUND & OBJECTIVES: Leptin resistance oriented hyperleptinaemia is a common problem in obese subjects in association with hypercholesterolaemia. The most common target for hypercholesterolaemia is impaired low density lipoprotein receptor (LDLR). This study was carried out to investigate whether any alteration in LDLR expression could explain the occurrence of hypercholesterolaemia in the event of hyperleptinaemia. METHODS: Expression of LDLR and SREBP2 (sterol regulatory element binding protein 2) were examined in HepG2 cells by RT-PCR and Western blotting. JAK2 inhibitor II was used to verify the effect of JAK-STAT (Janus Kinase-Signal Transducer and Activator of Transcription) pathway (common mediator for cytokine signaling). Co-localization of LDLR and insulin receptor (IR) was examined by confocal microscopy. RESULTS: Leptin was found to reduce the expression of LDLR and its transcription factor SREBP2. On the other hand, a weak signal for stimulation of LDLR by leptin was noted to be mediated by JAK2 pathway. But the joint effect of the two signaling pathways kept LDLR only in depressed mode in presence of leptin. Confocal microscopy showed that LDLR made an intensively co-localized complex with insulin receptor in presence of leptin. INTERPRETATION & CONCLUSIONS: Our results show that though leptin stimulates LDLR expression very weakly through JAK-STAT signaling pathway, it mainly imposes inhibition on LDLR expression by inhibiting transcription factor SREBP2. The inter-association between LDLR and IR may be a reason to render LDLR functionally inactive in presence of leptin.

Effect of IL-10 on LOX-1 expression, signalling and functional activity: an atheroprotective response.

The lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) has gained attention for its pro-inflammatory potential in atherogenesis. This study evaluates LOX-1 receptor modulation in the presence of an atheroprotective cytokine, interleukin-10 (IL-10). Both oxidized low-density lipoprotein (oxLDL) and IL-10 stimulated LOX-1 cell surface expression on THP-1 macrophages. However, our study demonstrates differential roles of oxLDL and IL-10 on LOX-1 functionality. Seemingly, oxLDL-induced LOX-1 promoted pro-inflammatory signalling by increasing intracellular NO, a substrate for pro-inflammatory peroxynitrite. In contrast, IL-10-induced LOX-1 facilitated scavenging of extracellular oxLDL without any effect on pro-inflammatory signalling. The atheroprotective effects of IL-10 were demonstrated by both facilitation of cellular oxLDL uptake and expression of LOXIN, an atheroprotective haplotype of the LOX-1 gene. Thus, increased expression of IL-10 may help to attenuate the risk of atherosclerosis developed by pro-inflammatory signal(s) generated through the interaction of oxLDL with its cognate receptor LOX-1 on macrophages.

The association between insulin and low-density lipoprotein receptors.

The insulin receptor (IR) and low-density lipoprotein receptor (LDLR) maintain glucose and lipid metabolism, respectively. Diabetes is associated with increased blood glucose, dyslipidaemia and increased risk of atherosclerosis. We hypothesise that interactions between IR and LDLR play a role in the atherosclerotic process in subjects with diabetes. Therefore, in this work we studied potential interactions between these two receptors. Our data show an intracellular and surface membrane-bound co-association of IR and LDLR. The co-association makes LDLR functionally poor in clearing extra-cellular LDL particles. A short 10 min exposure of cells to insulin disrupts the association between the two receptors and generates LDLR with higher LDL clearing activity without any change in protein expression. This co-association of LDLR with IR and their dissociation by insulin may be an important part of the regulatory mechanism of the normal physiological receptor function in a biological system. Modulation of receptor co-association is potentially a therapeutic target to reduce cardiovascular risk, and further studies are needed to investigate this possibility.