Role of the tumour suppressor pathway p53‐p21 in the regulation of metabolism

Abstract

Changes in diet and life style caused an alarming increase in the incidence of obesity, which is considered a major risk factor for the onset of the metabolic syndrome. One of the initial events in obesity is adipocyte hypertrophy, a stressful condition that triggers cellular responses like inflammation that eventually lead to insulin resistance (IR) and hyperinsulinemia, which may be the link between obesity and other obesity-associated disorders such as T2D, cardiovascular diseases, atherosclerosis and cancer. The major goal of this thesis was to unravel novel molecular mechanisms underlying the development of obesity and the associated IR and hyperinsulinemia. We pursued this goal performing two different studies:

1. Analysis of the role of the p53/p21 tumour suppressor pathway in obesity. 2. Analysis of the effect of JNK activation in pancreatic beta-cells in obesity-associated systemic IR and hyperinsulinemia.

p21 gene and protein expression was upregulated in the adipose tissue (AT) of different models for obesity: the leptin-deficient (ob/ob) and the diet-induced obese (DIO) mice, whereas p21-deficiency protected from the development of adiposity and IR induced by a high fat diet (HFD).

Functional genomics analysis showed that p21 deficiency did not affect adipogenic genes but highlighted an inverse correlation between p21 and leptin receptor (LepR) expression in the AT of p21-knockout (KO) compared to wild type (WT) mice. Furthermore, mice KO for both p21 and leptin developed obesity to the same extent as the ob/ob mice. Overall, these results indicated that the mechanism by which p21-deficiency protects from obesity depends on a properly working leptin/LepR signalling.

Furthermore, LepR protein levels were also increased in the AT of p53 KO mice. So, our hypothesis to explain at the molecular level the obesity-protective phenotype of the p53 and p21 KO mice on HFD conditions, was that these animals have an increased leptin sensitivity due to increased LepR levels in the AT, considering that the overexpression of LepR specifically in AT protects from diet-induced obesity. In vivo experiments to investigate leptin sensitivity in p21 KO animals showed a slightly greater percentage of weight decrease compared to WT, with no differences in food intake.

In order to establish a mechanistic link between LepR expression and the p53/p21 pathway, we performed an in silico analysis to identify putative binding sites for the E2F transcription factor in the LepR gene. p21 can modulate gene transcription through the regulation of pRB, which in turn negatively regulates the activity of E2F. Supporting this idea, pRB deletion in AT also protects from DIO. Moreover, E2F has a role in metabolism independently of its function in cell cycle: E2F may regulate metabolic genes and we propose that the LepR gene would be one of them.

Regarding our second study, JNK activation induces IR locally but no systemically in MKK7D mice on HFD. At the same time obesity and obesity-induced inflammation, generated at the AT, did not suffice to induce systemic IR. Preserving skeletal muscle sensitivity to insulin seemed to be sufficient to prevent systemic IR. In addition, our data in the MKK7D mice on HFD suggest that a chronic exposure to increased plasma insulin levels are required for the development of systemic IR.

Cambios en dieta y estilos de vida han causado un incremento en la incidencia de la obesidad, uno de los factores de riesgo para el síndrome metabólico. Uno evento inicial en la obesidad es la hipertrofia de los adipocitos, una condición de estrés que desencadena respuestas celulares como la inflamación que pueden conducir al desarrollo de resistencia a insulina (IR) e hiperinsulinemia, que constituyen la principal conexión de la obesidad a otras patologías asociadas a obesidad como la T2D, enfermedades cardiovasculares, aterosclerosis y cáncer.

El principal objetivo de esta tesis fue desvelar nuevos mecanismos moleculares a la base del desarrollo de obesidad y de la IR e híperinsulinemia a esa asociadas. Perseguimos este objetivo a través del:

1. Análisis del papel de la ruta supresora tumoral p53/p21 en obesidad. 2. Análisis del efecto de la activación de JNK en células beta- pancreáticas en la IR sistémica e híperinsulinemia asociadas a obesidad.

El déficit de expresión de p21 en ratones (sujetos a dieta grasa) genera resistencia al incremento de peso corporal y grasa, y a la alteración de la homeóstasis glucídica inducidos por obesidad. La dieta grasa desencadena un mecanismo de respuesta relacionado al déficit de p21, conectado a la señalización de leptina: la ausencia de p21 provoca un incremento significativo de los niveles del receptor de leptina. Además, la simple ausencia de p21 no es suficiente para el mejoramiento del fenotipo obeso y tiene que será acompañada por una funcional señalización de leptina.

La activación de JNK induce IR a nivel local pero efectos contrarios en la sensibilidad sistémica a insulina. Al mismo tiempo la obesidad y la inflamación inducida por obesidad, generada a nivel periférico, no son suficientes para inducir IR sistémica. Preservar la sensibilidad a insulina en músculo esquelético es suficiente para prevenir la IR sistémica. Así que es posible que para la IR sistémica se requiera la exposición crónica a incrementados niveles de insulina plasmática.