Can LDL cholesterol be too low? Possible risks of extremely low levels

Following the continuous accumulation of evidence supporting the beneficial role of reducing low-density lipoprotein cholesterol (LDL-C) levels in the treatment and prevention of atherosclerotic cardiovascular disease and its complications, therapeutic possibilities now exist to lower LDL-C to very low levels, similar to or even lower than those seen in newborns and nonhuman species. In addition to the important task of evaluating potential side effects of such treatments, the question arises whether extremely low LDL-C levels per se may provoke adverse effects in humans. In this review, we summarize information from studies of human cellular and organ physiology, phenotypic characterization of rare genetic diseases of lipid metabolism, and experience from clinical trials. Specifically, we emphasize the importance of the robustness of the regulatory systems that maintain balanced fluxes and levels of cholesterol at both cellular and organismal levels. Even at extremely low LDL-C levels, critical capacities of steroid hormone and bile acid production are preserved, and the presence of a cholesterol blood-brain barrier protects cells in the central nervous system. Apparent relationships sometimes reported between less pronounced low LDL-C levels and disease states such as cancer, depression, infectious disease and others can generally be explained as secondary phenomena. Drug-related side effects including an increased propensity for development of type 2 diabetes occur during statin treatment, whilst further evaluation of more potent LDL-lowering treatments such as PCSK9 inhibitors is needed. Experience from the recently reported and ongoing large event-driven trials are of great interest, and further evaluation including careful analysis of cognitive functions will be important.

Long-term treatment with interleukin-1beta induces insulin resistance in murine and human adipocytes

AIMS/HYPOTHESIS

Adipose tissue inflammation has recently been implicated in the pathogenesis of insulin resistance and is probably linked to high local levels of cytokines. IL1B, a proinflammatory cytokine, may participate in this alteration.

MATERIALS AND METHODS

We evaluated the chronic effect (1-10 days) of IL1B (0.1-20 ng/ml) on insulin signalling in differentiating 3T3-F442A and differentiated 3T3-L1 murine adipocytes and in human adipocytes. We also assessed expression of the gene encoding IL1B in adipose tissue of wild-type and insulin-resistant mice (diet-induced and genetically obese ob/ob mice).

RESULTS

IL1B inhibited insulin-induced phosphorylation of the insulin receptor beta subunit, insulin receptor substrate 1, Akt/protein kinase B and extracellular regulated kinase 1/2 in murine and human adipocytes. Accordingly, IL1B suppressed insulin-induced glucose transport and lipogenesis. Long-term treatment of adipose cells with IL1B decreased cellular lipid content. This could result from enhanced lipolysis and/or decreased expression of genes involved in lipid metabolism (acetyl-CoA carboxylase, fatty acid synthase). Down-regulation of peroxisome proliferating-activated receptor gamma and CCAAT/enhancer-binding protein alpha in response to IL1B may have contributed to the altered phenotype of IL1B-treated adipocytes. Moreover, IL1B altered adipocyte differentiation status in long-term cultures. IL1B also decreased the production of adiponectin, an adipocyte-specific protein that plays a positive role in insulin sensitivity. Expression of the gene encoding IL1B was increased in epididymal adipose tissue of obese insulin-resistant mice.

CONCLUSIONS/INTERPRETATION

IL1B is upregulated in adipose tissue of obese and insulin-resistant mouse models and may play an important role in the development of insulin resistance in murine and human adipose cells.