The role of peroxisomes in hormone metabolism

Although peroxisomes carry out essential biological functions, cell type-specific features of this important organelle have only been superficially characterized. An intriguing new aspect of peroxisomal function was recently discovered by the group of Prof. Johannes Berger: for the first time a peroxisomal localization of the peptide hormones ß-lipotropin (ß-LPH) and ß-endorphin was demonstrated in various human tissues. This suggests a functional link between hormone metabolism and peroxisomes that has not yet been investigated. In contrast to peptide hormone metabolism, a role of peroxisomes in steroidogenesis has already been implied by abnormalities found in androgen metabolism of patients suffering from X-linked adrenoleukodystrophy (X-ALD), an inherited peroxisomal disorder caused by mutations in ABCD1 encoding the peroxisomal ATP-binding cassette (ABC) transporter ALDP. Further evidence for a link between steroids and peroxisomes is provided by our preliminary results showing the induction of the human ABCD1 promoter by steroidogenic factor 1 (SF-1), the master regulator of steroidogenesis. Together, a role of peroxisomes and ALDP in hormone metabolism is implied by several findings but this fundamental question in biology is a largely unexplored field that needs to be clarified. Our project aims to fill that gap in knowledge by deciphering the role of peroxisomes in ß-LPH and ß-endorphin peptide hormone metabolism. In addition, we will characterize the regulation of the adrenoleukodystrophy gene ABCD1 by SF-1 both in vitro and in vivo using tissue specific Sf-1 knock out mice, thus further linking peroxisomes to steroidogenesis. Finally, we aim to elucidate whether ALDP and peroxisomes have a role in androgen metabolism using cells lacking either ALDP or peroxisomal matrix proteins or peroxisomes. We will target these objectives by using state-of-the-art molecular biology techniques like in vivo cell imaging, chromatin immunoprecipitation (ChIP) assay or quantitative real time RT-PCR analysis. Insights from these exploratory analyses will deepen our knowledge of how peroxisomes participate in hormone metabolism in general and will also be of value for patients suffering from inherited peroxisomal disorders like X-ALD, which collectively represent an enormous burden not only for affected individuals but also for society. For most of these disorders no effective treatment is available. Thus, only the increased basic knowledge on peroxisomal functions can lead to the development of novel therapeutic strategies for these disorders.

Although peroxisomes carry out essential biological functions, cell type-specific features of this important organelle have only been superficially characterized. In the past, a role of peroxisomes in steroid hormone metabolism has been implied by several observations including abnormalities found in androgen metabolism of patients suffering from the peroxisomal disorder X-linked adrenoleukodystrophy (X-ALD), an inherited disease that affects the central nervous system white matter and is associated with very long-chain fatty acid (VLCFA) accumulation due to mutations in the ABCD1 gene. Within this project we focused on deciphering the role of the peroxisomal protein ABCD1 in steroid hormone metabolism and revealed that the transcription factor steroidogenic factor 1 (SF-1), the master regulator of steroidogenesis, has a role in regulating the ABCD1 gene both in vitro as shown by luciferase reporter assays as well as in vivo by using testes-specific Sf-1 knock out mice. Thus, these results closely link the control of steroidogenesis with the peroxisomal function of ABCD1. In addition, we could demonstrate that the level of VLCFA accumulation directly impacts steroid hormone metabolism in X-ALD fibroblasts by dysregulating the expression of two key enzymes in steroidogenesis, 5alpha-reductase isoform 2 catalyzing the conversion of testosterone into the highly active androgen dihydrotestosterone and 17- beta dehydrogenase type 2 that converts testosterone to androstenedione. This dysregulation was also reflected on hormone level by a concurrent increase of androstenedione in X-ALD fibroblasts. As androgens are proposed to possess immunosuppressive functions and X-ALD patients demonstrate alterations in androgen metabolism, we also investigated X-ALD patients derived monocytes/macrophages, which are the immune cell type most affected by X-ALD pathology and indeed observed that these cells present with impaired plasticity and pro-inflammatory skewing. Accordingly, the insights obtained from these exploratory analyses will provide a profound basis for future research addressing the role of ABCD1 and peroxisomal function in steroid hormone metabolism.