Insulin resistance (IR) is a state in which target cells fail to uptake glucose at ordinary levels of plasma insulin. Hence, higher than normal insulin concentrations are needed to maintain euglycemia. IR is associated with various pathological conditions, e. g. metabolic syndrome, polycystic ovary syndrome (PCOS), type 2 diabetes and lipodystrophies. IR and hyperinsulinemia finally lead to serious cardiovascular complications.
Central obesity with accumulation of visceral adipose tissue has been recognized as the most common cause of IR. One of the main mechanisms is increased free fatty-acid release from adipose tissues. Additionally, altered adipokine secretion is involved. Free fatty acid release is regulated by two fundamental processes: lipogenesis and lipolysis. In the present doctoral dissertation, the PCOS patients as a model disease group were used. The genes for some important enzymes involved in lipogenesis - triglyceride synthesis were studied: lipoprotein lipase (LPL) and lipin 1 (LPIN1) along with the genes associated with lipolysis: the gene for hormone-sensitive lipase
(LIPE) and the gene for glucocorticoid reactivating enzyme 11β-hydroksisteroid dehydrogenase type 1 (HSD11B1), as glucocorticoids augment lipolysis and act as insulin antagonists. Further, the expression of peroxisome-proliferator acivated receptor γ gene (PPARG) was studied due to its role in adipogenesis and triglyceride synthesis. Another two genes involved in providing insulin sensitivity were analyzed: adiponectin gene (a protective factor from adipocytes) and glucose transporter 4 (GLUT4) gene. As central obesity is particularly associated with IR, the expression of genes in visceral and subcutaneous adipose tissues was explored. We enrolled 85 PCOS patients according to the National Institutes of Health criteria and 44 healthy women for subcutaneous adipose tissue biopsy and 30 patients and 44 controls for visceral adipose tissue biopsy. In all study subjects, anthropometric and metabolic markers were measured. Thirty-four PCOS patients underwent 6-month insulin sensitizing therapy with either PPARγ agonist rosiglitazone or metformin. In patients and controls the basal levels of gene expression in association with metabolic state and, in the aforementioned patients, the changes after the therapy were measured. Gene expression was measured by real time PCR and two housekeeping genes were usedfor normalization. In PCOS patients, decreased LPIN1 expression was found as compared to healthy controls, in visceral and subcutaneous adipose tissue. This could decrease lipogenesis in adipose tissues shifting lipids to liver and muscle and contributing to IR. HSD11B1 expression was increased in both types of adipose tissue. In visceral fat, increased amounts of glucocorticoids induce LPL, PPARG and LIPE genes, which stimulate the development of central obesity and free fatty acid release into the circulation. In subcutaneous fat, adiponectin expression is attenuated by glucocorticoids. Thus, IR is exaggerated by glucocorticoids in both
types of adipose tissue. There is inhibited expression of lipid accumulation genes in subcutaneous adipose tissue in PCOS patients: LPL and PPARG, which additionally shifts lipids to less safe visceral depot. Additionally, the expression of lipolysis LIPE gene is diminished in subcutaneous fat, which could represent an adaptive mechanism in obesity to repress free fatty acid release. PPARγ agonist augments LPL
expression and reduces HSD11B1 expression in subcutaneous depot. In this way lipogenesis is increased in subcutaneous fat and the negative impact of glucocorticoids is reversed. The mechanisms of lipogenesis and lipolysis differ qualitatively between the two types of adipose tissue. Only LPIN1 and HSD11B1 show correlation between expression in visceral and subcutaneous adipose tissue, all other genes need to be analyzed in particular type of fat.The genotype analysis showed that mutated -681C>G and Pro12Ala genotypes of PPARG gene favor a
leaner phenotype. Mutated genotypes of -681C>G and Pro12Ala polimorphisms augment the beneficial effects of rosiglitazone treatment. SNP1 and SNP17 polimorphisms in LPIN1 have mutually opposite effects on IR markers. Heterozygotes of IVS+53_54insA polymorphism in HSD11B1 gene have the lowest HSD11B1 expression in subcutaneous adipose tissue and the most favorable metabolic profile. We can conclude that HSD11B1 and PPARG genes which modulate the action of glucocorticoids and PPARγ transcription factors play a crucial role in modifying IR by regulating lipogenesis and lipolysis. The mechanisms of this regulation in visceral and subcutaneous adipose tissue are considerably different.
|
