Introduction

Steroid hormones exert profound regulatory control over complex gene networks through direct interactions at the level of the cellular genome (O'Malley, 1990; Yamamoto, 1985). The products of these modulated genes serve to control processes essential to cellular growth and differentiation as well as to influence mechanisms integral to the maintenance of intracellular and extracellular homeostasis. The actions of these blood-borne signals following their diffusion into distant cells are mediated by unique intracellular receptors (Jensen et al., 1968; Jensen & De Sombre, 1972). Indeed, the presence of these receptors in cells and tissues represents the principal, although not the only, determinant of response to a particular hormone. These soluble signal-transducing proteins are members of a large gene family of latent transcription factors that acquire strong but unique gene-regulating capacities upon activation by their respective hormonal ligands (Evans, 1988). Recognition of the individual ligands is characterized by high affinity and specificity (Haussler, 1986). While hormone interactions have been well characterized, the events that follow association of the ligand with its receptor remain less well understood. These events include receptor activation, interaction with DNA, association with other transcription factors, and the eventual transactivation process itself. Despite the paucity of information about these mechanisms, the outcome of such events is modification of specific gene expression and in turn biological response.

The vitamin D hormone mechanism

The vitamin D hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2- D3) is believed to regulate a number of biological processes via a steroid hormone mechanism.