RESUMO
Pro-opiomelanocortin (POMC) is a common precursor of melanocortin-related peptides in the pituitary and primarily regulated by corticotropin- releasing factor (CRF). Our results show that miR-375 is highly expressed in the mouse pituitary gland and located specifically in the intermediate lobe of pituitary. The functional studies show that the forced inhibition of endogenous miR-375 in AtT-20 mouse pituitary tumor cells and in the intermediate lobe of the pituitary gland significantly increases POMC expression, whereas miR-375 overexpression down-regulates POMC expression and ACTH secretion stimulated by CRF. This function of miR-375 is accomplished by its binding to the 3'-UTR of mitogen-activated protein kinase kinase kinase-8. Our results here have demonstrated that miR-375 acts as a negative regulating molecule mediating the signaling pathway of CRF and affecting POMC expression by targeting mitogen-activated protein kinase kinase kinase-8, which subsequently down-regulates ERK1/2 phosphorylation and nerve growth factor-induced clone B (NGFI-B) transcription activity. Taken together, our results show that miR-375 is a novel negative regulator of POMC expression and related hormone secretion.
Assuntos
Hormônio Liberador da Corticotropina/metabolismo , Regulação da Expressão Gênica/fisiologia , Sistema de Sinalização das MAP Quinases/fisiologia , MicroRNAs/metabolismo , Proteína Quinase 8 Ativada por Mitógeno/metabolismo , Hipófise/metabolismo , Pró-Opiomelanocortina/biossíntese , Animais , Linhagem Celular Tumoral , Hormônio Liberador da Corticotropina/genética , Feminino , Masculino , Camundongos , MicroRNAs/genética , Proteína Quinase 3 Ativada por Mitógeno/genética , Proteína Quinase 3 Ativada por Mitógeno/metabolismo , Proteína Quinase 8 Ativada por Mitógeno/genética , Membro 1 do Grupo A da Subfamília 4 de Receptores Nucleares/genética , Membro 1 do Grupo A da Subfamília 4 de Receptores Nucleares/metabolismo , Fosforilação/fisiologia , Neoplasias Hipofisárias/genética , Neoplasias Hipofisárias/metabolismo , Pró-Opiomelanocortina/genéticaRESUMO
In addition to the well known regulating effects of leptin on energy balance and glucose homeostasis through the central nervous system, circulating leptin has a direct effect on pancreatic islet and insulin secretion through its receptor (OBRb). The LIM-homeodomain transcription factor Isl-1 is expressed in all classes of pancreatic endocrine cells and is involved in regulating both islet development and insulin secretion. Both OBRb and Isl-1 mutations result in obesity-related diabetes. However, the interactions and physiological significance of leptin and Isl-1 in pancreatic islets remain to be established. Here, we show that most of leptin target cells in pancreatic islets and NIT beta cells express Isl-1. Both in vivo and in vitro results demonstrate that leptin suppresses Isl-1 expression and insulin secretion in islet in physiological and pathophysiological conditions, e.g. high fat diet. This effect of leptin on insulin secretion is lost in leptin receptor-defective db/db and Isl-1-inducible knock-out mice. We conclude that the action of leptin on insulin secretion is at least partly mediated by Isl-1. Another new finding of this study is that Isl-1 acts as a direct downstream target of leptin signaling molecule STAT3 to influence the effect of leptin on insulin secretion, whereas inversely, insulin has feedback regulating effects on Isl-1 expression through JAK-STAT3 pathway. These findings are crucial for understanding the mechanisms regulating insulin secretion and metabolism in related diseases, such as obesity and type 2 diabetes.
Assuntos
Regulação da Expressão Gênica , Células Secretoras de Insulina/metabolismo , Insulina/metabolismo , Proteínas com Homeodomínio LIM/biossíntese , Leptina/metabolismo , Fatores de Transcrição/biossíntese , Animais , Linhagem Celular , Diabetes Mellitus Tipo 2/genética , Diabetes Mellitus Tipo 2/metabolismo , Diabetes Mellitus Tipo 2/patologia , Insulina/genética , Secreção de Insulina , Células Secretoras de Insulina/patologia , Proteínas com Homeodomínio LIM/genética , Leptina/genética , Camundongos , Camundongos Knockout , Camundongos Obesos , Mutação , Obesidade/genética , Obesidade/metabolismo , Obesidade/patologia , Receptores para Leptina/genética , Receptores para Leptina/metabolismo , Fator de Transcrição STAT3/genética , Fator de Transcrição STAT3/metabolismo , Fatores de Transcrição/genéticaRESUMO
Estrogens are known to play a role in the feedback regulation of pituitary gonadotropin secretion in adults. However, it is still unknown whether estrogens are involved in promoting pituitary development. In this study, we selected chick embryo as the animal model and microinjected different doses of estradiol (E2) at stage E27-28, which was when endogenous E2 was not detected. First, the results demonstrated that E2 at different doses promoted pituitary cell proliferation and gonadotroph differentiation. Lower doses of E2 had a more significant effect on cell proliferation, while higher doses of E2 were required for luteinizing hormone (LH) secreting cell differentiation. Furthermore, the levels of early growth response protein 1 (Egr-1) and GATA2 mRNAs were also elevated with E2 treatment at a higher dose than that required to increase the level of proliferating cell nuclear antigen (PCNA) in vitro. To investigate whether estrogen receptors (ERs) mediate these effects of estradiol, the ER antagonist ICI 182,780 was added, and the results showed that ICI 182,780 did not modify the enhancing effects of E2 on cell proliferation; however, it inhibited the stimulatory effect of E2 on LH secreting cell differentiation. These results suggest that E2 at different doses promotes pituitary cell proliferation and gonadotroph differentiation with different mechanisms. Our results are important to further understanding of the physiological and pharmacological functions and related mechanisms of estrogens and their receptors, although the related mechanism need to be elucidated in future studies.