Induction of Bile Acid Synthesis in the Neonatal Liver

正常的儿童和成人有足够的胆汁酸形成胶束，从而有效地吸收饮食中的脂肪。其中一组人的管腔胆汁酸浓度低于形成胶束所需的浓度，这组人是非常小的早产儿。由于缺乏管腔胆汁酸，这些快速生长的婴儿的脂肪吸收和随后的生长受到损害。随着新生儿的成熟，胆汁酸的合成率最终会增加。调节新生儿胆汁酸合成率增加的事件(S)尚不清楚。我们推测，新生儿胆汁酸合成率的增加与人体胆汁酸合成的主要酶--7α-羟基酶(CYP7A1)的mRNA表达增加有关，也与肝脏中固醇平衡的改变有关。我们还假设活动的诱导是可调节的。由于人类和仓鼠在固醇和胆汁酸代谢方面的相似之处，这项研究将在新生仓鼠身上完成。为了解决这些假设，本文提出了三个具体目标。首先，我们将探索CYP7A1蛋白水平的调节机制；我们已经证明，Cyp7a1的蛋白水平和活性随着新生儿胆汁酸池的大小而增加。我们首先将测量Cyp7a1的mRNA水平。如果检测到差异，我们将检查已知的影响CYP7A1转录的核受体的mRNA水平和配体。其次，我们将确定用于新生儿胆汁酸合成的胆固醇的来源(S)。新合成的胆固醇或脂蛋白-胆固醇转化为胆汁酸的百分比将在不同的年龄进行测定。转化率将与Cyp7a1活性和脂蛋白清除率相关。第三，我们将检验这一假设，即CYP7A1活性的增加不是个体发育调节的，而是可以被外部因素操纵的。基于先前报道的研究和我们的初步结果，我们将试图通过操作1)已知的影响CYP7A1转录的核受体的配体和2)甾醇平衡来延迟或早熟表达Cyp7a1 mRNA和蛋白。此外，这些研究将检查在以前的目标中检测到的任何相关关系，以便我们能够描绘出酶活性变化的机制。从拟议的研究中获得的数据可能有助于找到新的方法来治疗非常小的早产儿，从而对他们的整体预后产生有益的影响。

Normal children and adults have sufficient bile acids to form micelles and thereby efficiently absorb dietary lipids. One group of individuals which has a lumenal bile acid concentration lower than that required to form micelles is the very small preterm infant. As a result of the lack of lumenal bile acids, lipid absorption and consequently growth is compromised in these rapidly growing infants. As the neonates mature, bile acid synthesis rates will eventually increase. The event(s) regulating the increase in bile acid synthesis rates in the neonate is(are) unknown. We hypothesize that the increase in bile acid synthesis rate in the neonate relates to an increase in the expression of mRNA for 7alpha- hydroxylase (CYP7A1), the major enzyme responsible for bile acid synthesis in humans, and from a change in the sterol balance across the liver. We also hypothesize that the induction of activity is regulable. The studies will be completed in the neonatal hamster due the similarities in sterol and bile acid metabolism between the human and hamster. To address these hypotheses, 3 specific aims are proposed. First, we will explore the mechanism of regulation of CYP7A1 protein levels; we have shown that protein levels and activities of Cyp7a1 increase as does the bile acid pool size in neonates. We initially will measure mRNA levels for Cyp7a1. If differences are detected, we will examine the mRNA levels and ligands of nuclear receptors known to affect CYP7A1 transcription. Second, we will identify the source(s) of cholesterol used for bile acid synthesis in the neonate. The percent conversion of newly synthesized cholesterol or lipoprotein-cholesterol to bile acids will be determined at different ages. The percent conversions will be correlated to Cyp7a1 activities and lipoprotein clearance rates. Third, we will test the hypothesis that the increase in CYP7A1 activity is not regulated ontogenically, but can be manipulated with exogenous factors. Based on studies reported previously and on our preliminary results, we will attempt to either delay or precociously express Cyp7a1 mRNA and protein by manipulating 1) the ligands for the nuclear receptors known to affect CYP7A1 transcription and 2) sterol balance. Additionally, these studies will examine any correlative relationships detected in previous aims so that we may be able to delineate the mechanisms responsible for changes in enzyme activity. Data obtained from the proposed studies may contribute towards new approaches to treat the very small premature infant and thereby have a beneficial impact upon their overall prognosis.