Liver Receptor Homologue-1 (LRH-1) Regulates Expression of Aromatase in Preadipocytes

Abstract

Estrogen biosynthesis from C 19 steroids is catalyzed by aromatase cytochrome P450. Aromatase is expressed in breast adipose tissue through the use of a distal, cytokine-responsive promoter (promoter I.4). Breast tumors, however, secrete soluble factors that stimulate aromatase expression through an alternative proximal promoter, promoter II. In other estrogenic tissues such as ovaries, transcription from promoter II requires the presence of the Ftz-F1 homologue steroidogenic factor-1 (SF-1); adipose tissue, however, does not express SF-1. We have explored the hypothesis that in adipose tissue, an alternative Ftz-F1 family member, liver receptor homologue-1 (LRH-1), substitutes for SF-1 in driving transcription from promoter II. In transient transfection assays using 3T3-L1 preadipocytes, promoter II reporter constructs were modestly (2-3-fold) stimulated by either treatment with activators of protein kinases A or C (PKA/C) or by cotransfection with LRH-1. In combination, these treatments synergistically activated promoter II (>30-fold). Induction by LRH-1 (but not by PKA/C) required an AGGTCA motif at ؊130 base pairs, to which LRH-1 bound in gel shift assays. Activity of GAL4-LRH-1 fusion proteins was not altered by activators of PKA or PKC. Quantitative real-time PCR revealed that LRH-1 (but not SF-1) is expressed in the preadipocyte fraction of human adipose tissue at levels comparable with that of liver. Differentiation of cultured human preadipocytes into mature adipocytes was associated with a time-dependent induction of peroxisome proliferator-activated receptor-␥ (PPAR␥), and rapid loss of LRH-1 and aromatase expression. We conclude that LRH-1 is a preadipocyte-specific nuclear receptor that regulates expression of aromatase in adipose tissue. Alterations in LRH-1 expression and/or activity in adipose tissue could therefore have considerable effects on local estrogen production and breast cancer development.

Aromatase expression in the ovary: Hormonal and molecular regulation

“…Exogenous expression of LRH-1 in bovine granulosa cells [104], mouse Leydig and Sertoli cell lines [117], or 3T3L1 mouse preadipocytes [118] increases aromatase promoter activity. However, overexpression of LRH-1 in rat granulosa cells does not enhance estrogen production or aromatase expression in either the presence or absence of FSH, although it did increase progesterone production [119].…”

The nuclear receptor liver receptor homolog-1 is an estrogen receptor target gene

“…This is of particular interest since cyclin D1, through its positive effect on ERa transcriptional activity, could exert a positive feedback loop on LRH-1 expression, finally resulting in an amplification of cell proliferation and tumor progression. Furthermore, it has been shown that local synthesis of aromatase, the cytochrome P450 enzyme responsible for the conversion of C 19 adrenal steroids into E2, is under the control of LRH-1 (Clyne et al, 2002), resulting in local accumulation of estrogens necessary for tumor progression. Therefore, LRH-1 could be a factor responsible for the local synthesis of E2, promoting epithelial breast tumor progression.…”

A Peroxisome Proliferator-response Element in the Murine mc2-r Promoter Regulates Its Transcriptional Activation during Differentiation of 3T3-L1 Adipocytes

“…However, although heterologous expression of SF-1 can induce mc2-r promoter activity in non-expressing, non-adrenal cell lines such as JEG3 and L cells (10,15), MC2-R is not expressed by all of the SF-1-positive tissues and, therefore, SF-1 is not sufficient for mc2-r gene expression. In contrast, 3T3-L1 adipocytes have been shown not to express SF-1 (16). In the absence of SF-1, mc2-r gene expression must be regulated via an alternative transcriptional mechanism in this cell line and the elucidation of such a mechanism would greatly improve our understanding of the tissue-specific expression of this receptor.…”