Nuclear Receptors of the Adrenal Cortex: Genomic Targets of Steroidogenic Factor 1 and Evidence of Adrenal Failure in DAX1-Deficient Mice.

Abstract

Steroidogenic Factor 1 (SF1) and DAX1 are nuclear receptors that play an integral role in the adrenal cortex. SF1 is a transcription factor that regulates expression of myriad genes, including those involved in steroidogenesis, production of various endocrine hormones, and development of the adrenals and gonads. DAX1 is an orphan nuclear receptor that inhibits SF1 and, when mutated, is responsible for X-linked Adrenal Hypoplasia Congenita (AHC). In the adrenal cortex, adrenocorticotropic hormone (ACTH) activates SF1-mediated transcription of the genes involved in cholesterol transport and steroid production. However, SF1 is also implicated in adrenal proliferation and differentiation; therefore, it is likely that several different signaling pathways regulate SF1-dependent transcription by altering cofactor availability or by feedback mechanisms that modulate signaling molecules that impinge on SF1 transactivation. This thesis describes three genomic targets of SF1 that provide feedback upon and affect subsequent SF1 transcriptional activity. First, I describe the identification of Edg5, the sphingosine 1-phosphate receptor, as a target of SF1 using chromatin immunoprecipitation-chip microarray (ChIP-chip). Activation of Edg5 may alter levels of intracellular sphingosine 1-phosphate, a ligand of SF1 that activates steroidogenesis and stimulates release of adrenal hormones. I also describe association of SF1 on the adenylyl cyclase 4 (Adcy4) promoter. Though usually an activator, SF1 represses transcription of Adcy4, which may alter cAMP-dependent activation of SF1. Finally, I describe an interaction between SF1 and GR to activate expression of Dax1, which is abrogated by ACTH. In all three cases, the genomic target of SF1 encodes a factor that can modulate SF1-dependent transcription. While loss of DAX1 in humans results in AHC, Dax1 deficient-mice have a mild adrenal phenotype. To reconcile these disparate phenotypes, I analyzed Dax1-deficient mice across their lifespan. Young Dax1-deficent mice have increased steroidogenic capacity, expression of steroidogenic enzymes, and proliferation. However, after 60 weeks all three measures are dramatically reduced, suggesting that loss of Dax1 may result in premature differentiation resulting in ultimate exhaustion of the adrenal cortex, which has implications for AHC. Overall, this dissertation contributes to the understanding of both SF1-dependent gene regulation and the significance of DAX1 in homeostasis of the adrenal cortex.Ph.D.Cellular & Molecular BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/78891/1/jscheys_1.pd