Introduction

Steroid hormones have a common mechanism of action: they enter the target tissue and become bound to nuclear protein receptors that interact with chromatin to change gene expression. This property is also shared with the thyroid hormones and morphogens such as retinoic acid. In this chapter, the hormones which will be discussed are the oestrogens, progesterone, androgens, glucocorticoids and dihydroxycalciferol (Fig. 10.1). Dihydroxycalciferol differs from the rest in that it is not synthesised in an endocrine gland but is derived from calciferol (vitamin D, see Section 2.4.2) by successive hydroxylations in the liver and kidney. However, its mechanism of action is similar to the others. Table 10.1 summarises the site of synthesis of the steroid hormones, their principal target tissues and the genes expressed as a result of their interaction. These hormones regulate gene expression by affecting transcription of specific genes, catalysed by RNA polymerase II. The increased amount of mRNA generally results in a corresponding increase in protein synthesis. The most extensively studied avian proteins that are hormonally controlled by gene expression are egg-white proteins and the egg-yolk proteins. The former are synthesised in the oviduct and the latter in the liver. The control of protein synthesis in hen oviduct is often regarded as a model for understanding hormonal gene expression in general. The general features of steroid hormone gene expression are first described, and this is followed by a more detailed consideration of each different system.