Inhibition of NF-κB transcriptional activity by the estrogen receptor - Insights into the molecular mechanism

Abstract

The steroid hormone estrogen plays a key role in the regulation of a wide variety of physiological processes, having profound effects on tissues throughout the human body. This becomes evident in postmenopausal women where estrogen deficiency is associated with increased risks for developing various diseases, including osteoporosis. Currently, it is established that estrogen replacement therapy (ERT) is an effective therapy for the treatment of postmenopausal osteoporosis. The protective effect of estrogen on the skeleton is due to inhibitory effect on bone resorption, which is attributed to suppressing the production of bone-resorbing cytokines. Unfortunately, ERT is associated with adverse effects, including an increased risk for breast and uterine cancer. An important alternative to ERT for the prevention of osteoporosis is the use of selective estrogen receptor modulators (SERMs), which are compounds with estrogen agonistic or antagonistic effects depending on the tissue type. The biological effects of estrogen are mediated by the estrogen receptor (ER) belonging to the superfamily of nuclear receptors, which is the largest known group of eukaryotic transcription factors. Gene regulation by ER can be accomplished through direct DNA binding via estrogen response elements (EREs) or by modulating the activity of other transcription factors. The research described in this thesis focuses on the mechanisms involved in ER-mediated inhibition of nuclear factor-kappaB (NF-kappaB) transcriptional activity. NF-kappaB is the major regulator of genes involved in immune and inflammatory responses, such as genes encoding cytokines. Therefore it is believed that cross-talk between ER and NF-kappaB lies at the basis of the anti-inflammatory, and thus bone protective, property of estrogen. ER exists as two different subtypes, ERalpha and ERbeta, and in this thesis it is shown that ERalpha, and not ERbeta, is the major ER through which transcription of NF-kappaB-regulated genes is inhibited (Chapter 2). Several indirect mechanisms, that may explain the inhibitory effect of ER on NF-kappaB activity, were excluded (Chapter 2 & 3). However, direct interactions between ER and NF-kappaB are described providing evidence for a direct mechanism (Chapter 4). The capacity to modulate NF-kappaB activity was assessed, not only for the natural ER-ligand 17beta-estradiol, but also for several SERMs (Chapter 2, 3 & 5). Through the use of specific mutants of ER it is shown that activation of ERE-dependent transcription and repression of NF-kappaB activity are two separate properties of ER that can occur independently of each other (Chapter 3). By using microarray analysis we determined the repressive effect of ERalpha on genes activated by TNFalpha, which is a cytokine that triggers the NF-kappaB pathway. Comparison of the expression profiles resulting from stimulation with 17beta-estradiol or SERMs revealed distinct effects of these ligands on TNFalpha-upregulated gene expression (Chapter 5). These findings may contribute to the development of estrogen-like compounds with anti-inflammatory properties for the treatment of diseases, such as osteoporosis and inflammatory diseases, without the adverse effects observed with current estrogen therapy.