Endocrine modulation may occur through several
pathways, not only by compounds mimicking natural hormones. Endocrine response pathways are very com- plex, and involve synthesis, release and transport of signalling molecules (e.g. hormone) to target cells and interactions with cellular membrane or intracellular re- ceptors. The formation of hormone receptor complexes results in the induction ofa signalling cascade or directly interacts with specific genomic sequences and modulates gene transcription and translation, leading to an altered biochemical or physiological response. Homeostasis in cells is maintained through these complex signalling pathways, and unscheduled modulation ofany ofthese processes can lead to development ofendocrine modu- lation or toxicity at various levels. Furthermore, steroid hormone synthesis and metabolism are catalysed by a series ofenzymatic steps involving a number of different steroid hydroxylases, reductases and other enzymes that modify the parent cholesterol molecule. These steps take place in several different tissues and cell types, that all are possible target cells for an endocrine modulators.