Fetal hypothalamic-pituitary-adrenal responses to hypoxi

胎儿下丘脑-垂体-肾上腺对缺氧的反应

• 批准号: 6875425
• 负责人: Charles A Ducsay
• 金额: $ 3.2万
• 依托单位: LOMA LINDA UNIVERSITY
• 依托单位国家: 美国
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-01-01 至 2009-12-31
• 项目状态: 已结题
• 来源: https://reporter.nih.gov/project-details/6875425
• 关键词: adrenocorticotropic hormone; altitude; arginine vasopressin; corticotropin releasing factor; cortisol; cyclic AMP; embryo /fetus; embryo /fetus hypoxia; environmental adaptation; hypothalamic pituitary adrenal axis; mitogen activated protein kinase; nitric oxide; physiologic stressor; protein kinase A; sheep

The fetus has the remarkable ability to adapt to conditions of chronic hypoxia over the course of gestation. One of the key factors mediating this adaptation is cortisol. Regulation of fetal cortisol is necessary for normal fetal growth and development and response to subsequent stressors. Understanding the mechanisms of fetal adaptation to chronic stress is also important in regard to effective use of antenatal glucocorticoid therapy. Our studies have shown that in late gestation, basal plasma cortisol levels are normal in the long-term hypoxic (LTH) sheep fetus, while expression of key adrenal steroidogenic enzymes (P450 cholesterol side chain cleavage [CYP11A] and P450 17alpha-hydroxylase [CYP17])is suppressed. Paradoxically, basal plasma adrenocorticotropin (ACTH) concentrations are elevated and, in response to a secondary stressor, cortisol production is enhanced compared to normoxic controls. Basal plasma concentrations of the ACTH precursors proopiomelanocortin (POMC) and 22 kDa proACTH (the major intermediate in processing of POMC to ACTH) are also elevated. We have further evidence that processing of POMC to ACTH in the anterior pituitary is enhanced during acute stress, leading to elevated plasma ACTH in LTH fetuses. Physiological concentrations of ACTH precursors have been demonstrated to be potent inhibitors of fetal cortisol production. Thus, it is apparent that the fetal hypothalamic-pituitary-adrenocortical (HPA) axis has acclimatized to LTH at all levels. However, the mechanisms driving this adaptation remain to be elucidated. The proposed studies will extend our previous observations on HPA function in the LTH fetus and examine specific mechanisms involved not only in maintaining basal glucocorticoid secretion but also in responding to secondary stressors. The proposed studies will test the general hypothesis that adaptive changes in the fetal HPA axis allow the fetus to acclimatize to the stress of long-term hypoxia while augmenting the ability to respond to a secondary, acute stressor. Specific Aim 1. To elucidate the mechanisms of enhanced hypothalamic drive (i.e., augmented corticotrope function) in LTH fetuses. These studies are designed to test the hypothesis that: LTH results in enhanced hypothalamic paraventricular nuclei (PVN) expression of cortiocotropin releasing factor (CRF) and/or arginine vasopressin (AVP) and/or increased pituitary responsiveness to these ACTH secretagogues resulting in the increased anterior pituitary corticotrope function observed in response to LTH. Specific Aim 2. To determine the molecular and cellular mechanisms by which the adrenal cortex has acclimatized to LTH that results in the observed suppressed adrenal expression of key rate limiting enzymes in glucocorticoid synthesis under basal conditions. Research to date supports that expression of CYP11A and CYP17 is regulated via interplay between the stimulatory ACTH-cAMP and inhibitory ERK pathways. Thus, these studies will test the hypothesis that LTH adrenals are more sensitive to inhibitory effects of the ACTH precursors POMC and 22kD pro ACTH, interfering with ACTH signaling, resulting in suppressed basal CYP expression. We also hypothesize that the extracellular signal-regulated kinase (ERK) pathway is enhanced in the adrenal cortex of the LTH fetus, leading to enhanced phosphorylation of steroidogenic factor-1 (SF-1) which, in turn, suppresses the expression of the key rate limiting genes regulating glucocorticoid production. Specific Aim 3. To elucidate the intracellular and molecular mechanisms for enhanced glucocorticoid secretion in response to an acute secondary stressor following LTH despite decreased CYP11A and CYP17 expression. These studies are designed to test the hypothesis that the enhanced cortisol output following a secondary stressor in LTH fetuses as compared to controls, is the result of enhanced ACTH stimulation of cAMP and protein kinase A, increasing activation of the steroidogenic acute regulatory protein (STAR). Specific Aim 4. To determine the mechanisms of potential changes in adrenal blood flow during a secondary stressor in LTH animals. These studies will test the hypothesis that: the enhanced cortisol output in LTH fetuses compared to controls following a secondary stressor is the result of enhanced adrenal blood flow and that such changes are mediated, at least in part, by nitric oxide.

胎儿有显著的能力适应慢性缺氧的条件下，在整个过程中， 怀孕调节这种适应的关键因素之一是皮质醇。胎儿皮质醇的调节对于正常的胎儿生长发育和对随后的应激反应是必要的。了解胎儿适应慢性应激的机制对于有效使用产前糖皮质激素治疗也很重要。我们的研究表明，在妊娠晚期，基础血浆皮质醇水平在长期缺氧（LTH）羊胎儿是正常的，而关键的肾上腺类固醇生成酶（P450胆固醇侧链裂解[CYP 11 A]和P450 17 α-羟化酶[CYP 17]）的表达受到抑制。巧合的是， 基础血浆促肾上腺皮质激素（ACTH）浓度升高，并且与含氧量正常的对照相比，响应于次级应激，皮质醇产生增强。ACTH前体阿黑皮素原（POMC）和22 kDa促肾上腺皮质激素原（POMC加工为ACTH的主要中间体）的基础血浆浓度也升高。我们有进一步的证据表明，在急性应激过程中，垂体前叶中POMC向ACTH的加工增强，导致LTH胎儿血浆ACTH升高。生理浓度的ACTH前体已被证明是胎儿皮质醇产生的有效抑制剂。因此，很明显，胎儿下丘脑-垂体-肾上腺皮质（HPA）轴具有 在各个层面上适应LTH。然而，驱动这种适应的机制仍有待阐明。拟议中的研究将扩展我们以前的观察HPA功能在LTH胎儿和检查的具体机制不仅涉及维持基础糖皮质激素分泌，但也在响应二次应激。拟议的研究将测试胎儿HPA轴的适应性变化允许胎儿适应长期缺氧的压力，同时增强对次级急性应激源的反应能力的一般假设。具体目标1。为了阐明增强下丘脑驱动的机制（即，增强的促肾上腺皮质激素功能）。这些研究旨在验证以下假设：LTH导致下丘脑室旁核（PVN）促皮质激素释放因子（CRF）和/或精氨酸加压素（AVP）表达增强和/或垂体对这些ACTH促分泌素的反应性增加，导致垂体前叶促肾上腺皮质激素功能增加。具体目标2。为了确定肾上腺皮质适应LTH的分子和细胞机制，导致观察到的肾上腺关键限速酶表达抑制， 基础条件下糖皮质激素的合成。迄今为止的研究支持CYP 11 A和CYP 17的表达通过刺激性ACTH-cAMP和抑制性ERK途径之间的相互作用来调节。因此，这些研究将检验以下假设：LTH肾上腺对ACTH前体POMC和22 kD促ACTH原的抑制作用更敏感，干扰ACTH信号传导，导致基础ACTH表达受抑制。我们还假设，细胞外信号调节激酶（ERK）途径增强， LTH胎儿的肾上腺皮质，导致类固醇生成因子-1（SF-1）的磷酸化增强，进而抑制调节糖皮质激素产生的关键限速基因的表达。具体目标3。阐明尽管CYP 11 A和CYP 17表达降低，但LTH后对急性继发性应激刺激的糖皮质激素分泌增加的细胞内和分子机制。这些研究旨在检验以下假设，即与对照组相比，LTH胎儿中继发性应激源后皮质醇输出增强是cAMP和蛋白激酶A的ACTH刺激增强的结果，增加了类固醇生成急性调节蛋白（星星）的活化。 具体目标4。确定LTH动物在次级应激期间肾上腺血流的潜在变化机制。这些研究将测试的假设，即：增强皮质醇输出在LTH胎儿相比，控制以下的二级应激源是增强肾上腺血流量的结果，这种变化是介导的，至少在一定程度上，由一氧化氮。