The influence of high-density lipoprotein (HDL) and HDL subfractions on insulin secretion and cholesterol efflux in pancreatic derived β-cells

Pancreatic β cell models for screening insulin secretagogues and cytotoxicity

In the past 2-3 decades, numerous attempts have been made to create an insulin-secreting β cell line that maintains normal insulin secretion. However, primary β cell cultures have finite life and, therefore, cannot be used for long-term experiments. The most widely used insulin-secreting cell lines are Insulinoma-1, rat insulinoma cell line, hamster pancreatic β cell line, mouse insulinoma, and β tumor cell line. Insulinoma-derived cell lines show infinite growth in tissue culture but exhibit varying differences in their insulin responsiveness to glucose levels compared to normal β cells. Despite difficulties with β cell cultures, these cell lines have offered some useful insights in diabetes research concerning physiological functions and pathological investigations. In this review, we describe insulinoma cell lines used for drug screening, insulin secretion, cell viability, proliferation, and other relevant cellular functions. In addition, we have also incorporated recently developed human β cell lines. These cell lines have provided some helpful insights into physiological activities and pathology in diabetes research, despite challenges with β cell culturing. We propose that these cell lines could also be explored for screening Ayurvedic Rasayanas and homeopathy preparations for their cytotoxicity and insulin secretagogue activities to have evidence-based data on alternative and complementary medicines.

High-Density Lipoprotein Is Located Alongside Insulin in the Islets of Langerhans of Normal and Rodent Models of Diabetes

Recent studies have implicated pre-beta and beta lipoproteins (VLDL and LDL) in the etiopathogenesis of complications of diabetes mellitus (DM). In contrast, alpha lipoprotein (HDL) is protective of the beta cells of the pancreas. This study examined the distribution of HDL in the islets of Langerhans of murine models of type 1 diabetic rats (streptozotocin (STZ)-induced DM in Wistar rats) and type 2 models of DM rats (Goto-Kakizaki (GK), non-diabetic Zucker lean (ZL), and Zucker diabetic and fatty (ZDF)). The extent by which HDL co-localizes with insulin or glucagon in the islets of the pancreas was also investigated. Pancreatic tissues of Wistar non-diabetic, diabetic Wistar, GK, ZL, and ZDF rats were processed for immunohistochemistry. Pancreatic samples of GK rats fed with either a low-fat or a high-fat diet were prepared for transmission immune-electron microscopy (TIEM) to establish the cytoplasmic localization of HDL in islet cells. HDL was detected in the core and periphery of pancreatic islets of Wistar non-diabetic and diabetic, GK, ZL, and ZDF rats. The average total of islet cells immune positive for HDL was markedly (<0.05) reduced in GK and ZDF rats in comparison to Wistar controls. The number of islet cells containing HDL was also remarkably (p < 0.05) reduced in Wistar diabetic rats and GK models fed on high-fat food. The co-localization study using immunofluorescence and TIEM techniques showed that HDL is detected alongside insulin within the secretory granules of β-cells. HDL did not co-localize with glucagon. This observation implies that HDL may contribute to the metabolism of insulin.

Association of HDL Subfraction Profile with the Progression of Insulin Resistance

Type 2 diabetes mellitus (T2DM) is a major global public health problem, as it is associated with increased morbidity, mortality, and healthcare costs. Insulin resistance (IR) is a condition characterized by disturbances in carbohydrate and lipid metabolism that precedes T2DM. The aim of the present study was to investigate the association between HDL and its subfraction profile and the progression of IR, as assessed by the Homeostatic Model Assessment for IR (HOMA-IR) index, and to define cut-off values to identify an increased risk of IR. Individuals with a HOMA-IR greater than 3.63 were considered to have IR. The HDL subfractions were separated using the Lipoprint system, which identifies ten subfractions (HDL-1-10) in three subclasses as large (HDL-L), intermediate (HDL-I) and small (HDL-S). Analyses were performed on samples from 240 individuals without IR and 137 with IR from the Hungarian general and Roma populations. The HDL-1 to -6 subfractions and the HDL-L and -I classes showed a significant negative association with the progression and existence of IR. Among them, HDL-2 (B = -40.37, p = 2.08 × 10-11) and HDL-L (B = -14.85, p = 9.52 × 10-10) showed the strongest correlation. The optimal threshold was found to be 0.264 mmol/L for HDL-L and 0.102 mmol/L and above for HDL-2. Individuals with HDL-L levels below the reference value had a 5.1-fold higher risk of IR (p = 2.2 × 10-7), while those with HDL-2 levels had a 4.2-fold higher risk (p = 3.0 × 10-6). This study demonstrates that the HDL subfraction profile (especially the decrease in HDL-2 and -L) may be a useful marker for the early detection and intervention of atherogenic dyslipidemia in subjects with impaired glucose and insulin metabolism.

Association between high-density lipoprotein cholesterol to apolipoprotein A ratio and diabetic retinopathy: A cross-sectional study

AIMS

Our study is aimed to investigate the relationship between high-density lipoprotein cholesterol to apolipoprotein A ratio (HDL-C/ApoA) and diabetic retinopathy (DR) in subjects with type 2 diabetes mellitus (T2DM).

METHODS

We retrospect the consecutive medical files of 1058 subjects with T2DM and recorded their clinical information and laboratory findings. Subjects with T2DM were divided into DR group (n = 522) and non-DR group (n = 536). We compared the lipids values of the two groups. Meanwhile we also observed the prevalence of DR at different HDL-C/ApoA levels. Binary logistic regression was used to correct confounding factors. Smooth curve fitting model and subgroup analysis were used to determine the correlation, non-linear relationship and threshold effect between HDL/ApoA and DR.

RESULTS

HDL-C/ApoA value of DR group was significantly higher than non-DR group (0.88 ± 0.17 vs 0.84 ± 0.13, P < 0.05). The prevalence of DR significantly increased as HDL-C/ApoA level increased. There was association between HDL/ApoA levels and DR in the adjusted models (OR 1.55, 95%CI 0.60 to 4.02). After full adjustments for other relevant clinical covariates, patients with HDL/ApoA values in quartile 3 (Q3) had 1.50 times (95 % CI 1.00 to 2.17) and in Q4 had 2.39 times (95%CI 1.65 to 3.47) as high as the risk of DR compared with patients in Q1. HDL/ApoA showed a non-linear relationship with DR, with an inflection point value of 0.759. When HDL/ApoA>0.759, HDL/ApoA was significantly positively associated with DR (HR = 26.508, 95 % CI 7.623-92.174; P < 0.0001). Compared to patients with age < 60, HDL/ApoA was obviously associated with DR when age ≥ 60 (OR = 38.05, 95 % CI 8.06-179.69; P < 0.001).

CONCLUSIONS

HDL-C/ApoA was found to be associated with the incidence of DR in patients with T2DM. After adjusting potential related factors HDL-C/ApoA OR value was 1.55 (95%CI 0.60 to 4.02). A non-linear association between HDL/ApoA and DR was observed in T2DM. Subgroup analysis showed that age could alter the relationship between HDL/ApoA and DR.

HDL as Bidirectional Lipid Vectors: Time for New Paradigms

The anti-atherogenic properties of high-density lipoproteins (HDL) have been explained mainly by reverse cholesterol transport (RCT) from peripheral tissues to the liver. The RCT seems to agree with most of the negative epidemiological correlations between HDL cholesterol levels and coronary artery disease. However, therapies designed to increase HDL cholesterol failed to reduce cardiovascular risk, despite their capacity to improve cholesterol efflux, the first stage of RCT. Therefore, the cardioprotective role of HDL may not be explained by RCT, and it is time for new paradigms about the physiological function of these lipoproteins. It should be considered that the main HDL apolipoprotein, apo AI, has been highly conserved throughout evolution. Consequently, these lipoproteins play an essential physiological role beyond their capacity to protect against atherosclerosis. We propose HDL as bidirectional lipid vectors carrying lipids from and to tissues according to their local context. Lipid influx mediated by HDL appears to be particularly important for tissue repair right on site where the damage occurs, including arteries during the first stages of atherosclerosis. In contrast, the HDL-lipid efflux is relevant for secretory cells where the fusion of intracellular vesicles drastically enlarges the cytoplasmic membrane with the potential consequence of impairment of cell function. In such circumstances, HDL could deliver some functional lipids and pick up not only cholesterol but an integral part of the membrane in excess, restoring the viability of the secretory cells. This hypothesis is congruent with the beneficial effects of HDL against atherosclerosis as well as with their capacity to induce insulin secretion and merits experimental exploration.

HDL functionality in type 1 and type 2 diabetes: new insights

PURPOSE OF REVIEW

To critically appraise new insights into HDL structure and function in type 1 diabetes (T1DM) and type 2 diabetes (T2DM).

RECENT FINDINGS

In young T1DM patients with early renal impairment and a high inflammatory score, both HDL antioxidative activity and endothelial vasodilatory function were impaired, revealing a critical link between HDL dysfunction, subclinical vascular damage, systemic inflammation and end organ damage. HDL may inhibit development of T2DM by attenuating endoplasmic reticulum (ER) stress and apoptotic loss of pancreatic β-cells, an effect due in part to ABC transporter-mediated efflux of specific oxysterols with downstream activation of the hedghehog signalling receptor, Smoothened. The apoM-sphingosine-1-phosphate complex is critical to HDL antidiabetic activity, encompassing protection against insulin resistance, promotion of insulin secretion, enhanced β-cell survival and inhibition of hepatic glucose production. Structure-function studies of HDL in hyperglycemic, dyslipidemic T2DM patients revealed both gain and loss of lipidomic and proteomic components. Such changes attenuated both the optimal protective effects of HDL on mitochondrial function and its capacity to inhibit endothelial cell apoptosis. Distinct structural components associated with individual HDL functions.

SUMMARY

Extensive evidence indicates that both the proteome and lipidome of HDL are altered in T1DM and T2DM, with impairment of multiple functions.

High Density Lipoproteins and Diabetes

Epidemiological studies have established that a high plasma high density lipoprotein cholesterol (HDL-C) level is associated with reduced cardiovascular risk. However, recent randomised clinical trials of interventions that increase HDL-C levels have failed to establish a causal basis for this relationship. This has led to a shift in HDL research efforts towards developing strategies that improve the cardioprotective functions of HDLs, rather than simply increasing HDL-C levels. These efforts are also leading to the discovery of novel HDL functions that are unrelated to cardiovascular disease. One of the most recently identified functions of HDLs is their potent antidiabetic properties. The antidiabetic functions of HDLs, and recent key advances in this area are the subject of this review. Given that all forms of diabetes are increasing at an alarming rate globally, there is a clear unmet need to identify and develop new approaches that will complement existing therapies and reduce disease progression as well as reverse established disease. Exploration of a potential role for HDLs and their constituent lipids and apolipoproteins in this area is clearly warranted. This review highlights focus areas that have yet to be investigated and potential strategies for exploiting the antidiabetic functions of HDLs.