Mecanismos moleculares que regulan la expresión de la shbg: implicaciones en la obesidad

La proteina transportadora de esteroides sexuales (SHBG) se sintetiza en el higado humano y es secretada a la sangre, donde une con gran afinidad androgenos y estrogenos, regulando su biodisponibilidad y accesibilidad a nivel de tejido. Los niveles plasmaticos de SHBG se encuentran bajos en personas obesas de todas las edades y se correlacionan negativamente con el indice de masa muscular. Ademas de la obesidad, los niveles plasmaticos de SHBG se encuentran bajos en otras patologias metabolicas como diabetes tipo 2 (DT2), higado graso no alcoholico (NAFLD) y predicen el riesgo de desarrollar enfermedad cardiovascular (EC). Durante esta tesis, nos hemos centrado por un lado en describir los mecanismos moleculares que regulan la expresion de la SHBG durante el desarrollo de la obesidad, mediante la generacion de un nuevo modelo de raton transgenico. Tambien hemos buscado nuevos factores que regulan la produccion de SHBG y hemos estudiado los mecanismos moleculares asociados. Finalmente, hemos estudiado si la SHBG juega un papel en el desarrollo y evolucion de la obesidad y NAFLD, una de sus patologias asociadas. En primer lugar, mediante el desarrollo de un raton doble transgenico diabetico y obeso que expresa SHBG (SHBG-db/db), hemos observado que a nivel hepatico el aumento de PPAR? y la disminucion de HNF-4? son responsables de la reduccion de los niveles de SHBG que ocurre durante el desarrollo de la obesidad. Por otro lado, mediante este modelo de raton SHBG-db/db, que presenta NAFLD, y otro modelo de NAFLD inducida con dieta rica en fructosa hemos demostrado que la sobreexpresion de SHBG disminuye la lipogenesis hepatica y les protege frente al desarrollo de higado graso. Utilizando celulas HepG2 como modelo in vitro, hemos comprobado que la modulacion de los niveles de SHBG afecta a la lipogenesis. Por tanto, la disminucion de SHBG en pacientes con NAFLD jugaria un papel en el desarrollo de esta enfermedad. Por otro lado, hemos encontrado nuevos factores que regulan la produccion de SHBG: la adiponectina como factor metabolico y el acido oleico como factor nutricional, y hemos descrito los mecanismos moleculares asociados a esta regulacion. En el caso de la adiponectina, dadas las correlaciones positivas existentes entre los niveles plasmaticos de adiponectina y de SHBG, y que ambas se encuentran disminuidas en pacientes obesos, hemos estudiado si esta relacion es causal. Utilizando celulas HepG2 hemos encontrado que la adiponectina aumenta la expresion hepatica de SHBG mediante la activacion de AMPK, que produce un aumento en la oxidacion de acidos grasos y una disminucion en la lipogenesis, lo que provoca un incremento en los niveles hepaticos de HNF-4? , activando la expresion de la SHBG. Ademas en biopsias hepaticas y plasma humano, se comprobo la correlacion existente entre los niveles de adiponectina con SHBG y HNF-4? Por su parte, el estudio de la regulacion de SHBG por acido oleico parte de que los niveles bajos de SHBG constituyen un riesgo independiente de sufrir EC, y el consumo de aceite de oliva, rico en acido oleico, protege frente al desarrollo de la misma. Por tanto, planteamos si el efecto cardioprotector del aceite de oliva viene mediado en parte por un aumento en la SHBG. En colaboracion con el grupo de Endocrinologia del hospital Carlos Haya (Malaga) en el marco del estudio PIZARRA, observamos que el consumo de aceite de oliva aumento los niveles plasmaticos de SHBG en hombres. En celulas HepG2, comprobamos que se produce este incremento y demostramos que es consecuencia de una disminucion del PPAR? hepatico inducida por el acido oleico. Sex Hormone-Binding Globulin (SHBG) is synthesized by human liver and secreted into the blood, where it binds with high affinity androgens and estrogens, regulating their bioavailability and accesibility at tissue level. SHBG levels are low in obese people of all age and are negatively correlated with body mass index. Besides obesity, plasma SHBG levels are low in other metabolic pathologies such as type 2 diabetes, non-alcoholic fatty liver disease (NAFLD) and predict the risk of cardiovascular disease (CVD). In this thesis, we have described the molecular mechanisms that regulate SHBG expression during obesity development through the generation of a new transgenic mouse model. Besides, we have searched for new factors that regulate SHBG production and the molecular mechanisms involved have been studied. Finally, we have studied whether SHBG plays a role in the development and progression of NAFLD, one of the obesity-related pathologies. In the first place, through the development of a double transgenic mouse model which is obese and diabetic and that expresses SHBG (SHBG-db/db mouse), we have observed that at hepatic level, PPAR? upregulation and HNF-4? downregulation are responsible for the decrease in SHBG levels during obesity development. Besides, by using this SHBG-db/db mouse model, which presents NAFLD, and a NAFLD model induced by high fructose diet, we have demonstrated that SHBG overexpression downregulates hepatic lipogenesis and protects them against fatty liver development. By the use of the cell line HepG2 as in vitro model, we have confirmed that the modulation of SHBG levels affects lipogenesis. We have concluded that SHBG reduction in NAFLD patients would play a role in the development of this disease. In the second place, we have found new factors that regulate SHBG production: adiponectin as metabolic factor and oleic acid as nutritional factor, and we have described the molecular mechanisms involved in such regulation. In the case of adiponectin, given the positive correlations between SHBG and adiponectin plasma levels, and that their levels are decreased in obese patients, we have studied whether this relation is causal. By the use of HepG2 cells, we have found that adiponectin increases SHBG hepatic expression through the activation of AMPK, which produces an increase in fatty acid oxidation and a decrease in lipogenesis, giving rise to an increase of HNF-4? hepatic levels, thus activating SHBG expression. Furthermore, in hepatic biopsies and human plasma, the correlation between adiponectin levels and SHBG and HNF-4? was confirmed. On the other hand, the study of SHBG regulation by oleic acid originates from the fact that low SHBG plasma levels constitute an independent indicator of CVD risk, and olive oil consumption, rich in oleic acid, protects against the development of CVD. Because of this, our objective was to investigate whether the cardioprotective effect of olive oil is mediated in part by SHBG increase. In colaboration with the Endocrinology and Nutrition Group from Carlos Haya Hospital (Malaga) in the frame of the study PIZARRA, we observed that olive oil consumption increased plasma SHBG levels in men. In HepG2 cells, this increment was confirmed and it was demonstrated that it is due to PPAR? downregulation induced by oleic acid.