Modulation of steroid receptor-mediated gene expression by vitamin B6.

Gene transcription mediated by steroid hormones has become one of the most extensively characterized model systems for studying the regulation of gene expression in eukaryotic cells. However, specific details of gene regulation by steroid hormones are often complex and may be unique in specific cell types. Diverse regulatory mechanisms leading to either activation or repression of particular genes frequently involve interactions between steroid hormone receptors and other ubiquitous and/or cell-specific transcription factors that act on the complex promoter of the regulated gene. Interplay between steroid receptor-mediated and other signal transduction pathways may also be involved. In addition, recent novel results indicate that moderate variations in the intracellular concentration of pyridoxal 5'-phosphate (PLP), the biologically active form of vitamin B6, can have pronounced modulatory effects on steroid-induced gene expression. Specifically, elevation of intracellular PLP levels leads to decreased transcriptional responses to glucocorticoid, progesterone, androgen, or estrogen hormones. Conversely, cells in a vitamin B6-deficient state exhibit enhanced responsiveness to steroid hormones. One aspect of the mechanism by which these transcriptional modulatory effects of PLP occur has recently been shown to involve interruption of functional interactions between steroid hormone receptors and the nuclear transcription factor NF1. These findings--that the vitamin B6 nutritional status of cells modulates their capacity to respond to steroid hormones--impose an additional level of cell-specific control over steroid hormone regulation of gene expression and will serve as the focal point for this review.