Role and regulation of CDK2 and p27 in adipose tissue and obesity

Abstract

Worldwide obesity prevalence has exponentially increase over the past decades. new targets and therapeutic strategies are needed to combat this current epidemic. Dysfunctional adipose tissue play a key role in the physiopathology of obesity, aging and associated metabolic disorders. In this sense, cell cycle regulators have emerged as possible regulators of adipose tissue metabolism and glucose homeostasis. Cell cycle progression is regulated by complexes formed by cyclin dependent kinases (CDKs) and cyclins, which can be inhibited by CDK inhibitors (CDKIs). In this sense, CDK2 regulates the transition from phase G1 to phase S and is repressed by p27. Functions of these regulators in adipose tissue and whole-body metabolism have been rarely assessed. Therefore, the aim of the current study was to unravel the function and regulation of CDK2 and p27 in adipose tissue in a context of obesity and aging. Different mouse models were used in this thesis project. The effects of aging and obesity were assessed in young, adult, and aged female mice fed with control or high fat diet (HFD) up 18 months of age. A whole-body knockout model of p27 (p27 ko) was also used and, to study the role of CDK2 in adipose tissue (cdk2ATKO), a specific knockout of this CDK in adipose tissue was generated. p27 ko and cdk2ATKO mice were subjected to a normal control diet or to a high fat diet. The phenotype of the different models was examined by different techniques of in vivo analysis such as, body composition by magnetic resonance, glucose and insulin tolerance test, indirect calorimetry, and BAT activity assesement by microPET. Protein and gene expression levels in adipose tissue were assessed by western blot or real time PCR respectively. A transcriptomic study of adipose tissue by RNAseq was also carried out Additionally, human visceral and subcutaneous white adipose tissues from individuals with different body mass index were examined by real time PCR. p27, exhibits a lower expression in subcutaneous than in visceral white adipose tissue (vWAT) in mice and humans. p27 is drastically downregulated by aging in subcutaneous WAT, but not in gonadal WAT, of female mice. Obesity upregulates p27 and cdk2 expression in scWAT, but not in other fat depots of aged mice. In humans, a significant upregulation of p27 was observed in visceral WAT of subjects with obesity. p27 ko mice are more prone to develop obesity and insulin resistance, exhibiting increased size of all fat depots. p27 ko mice tend to have lower oxygen consumption and energy expenditure together with a higher respiratory exchange ratio. Interscapular brown adipose tissue (iBAT) presents larger adipocytes in p27 ko HFD mice, accompanied by downregulation of both Glut1 and UCP1 and in parallel with a defective insulin signaling in iBAT. Moreover, p27 ko HFD mice exhibit an impaired response to cold exposure characterized by a reduced iBAT 18F-fluorodeoxyglucose uptake and a difficulty to maintain body temperature when exposed to cold as compared to WT HFD mice, suggesting reduced thermogenic capacity. The lack of cdk2 in adipose tissue confers resistance to diet-induced obesity together with an improvement in serum metabolic parameters in mice. Smaller adipocyte size is detected in the gWAT of mice lacking cdk2 in adipose tissue fed with HFD (cdk2ATKO HFD) which is accompanied by a differential expression of genes related to lipid storage, biosynthesis and localization processes, as well as to inflammatory response and extracellular matrix. These results show a differential adipose depot-dependent regulation of p27 and cdk2 in aging and obesity, suggesting that p27 and CDK2 could contribute to the adipose-tissue depot s metabolic differences. Studies in p27 deficient mice suggest that p27 could play a role in BAT activation and in the susceptibility to develop obesity and insulin resistance. Adipose tissue-specific cdk2 deficiency modifies the expression of genes involved in lipid accumulation.