Nuclear Hormone Receptors: Epidermal Liposensors
Nuclear hormone receptors are a large family of transcription factors. Currently, 48 nuclear hormone receptors have been identiﬁed in humans, making them one of the largest known families of transcription factors (1,2). The nuclear receptors can be divided into several classes. Class I receptors form homodimers and represent targets of the ﬁve classical steroid hormones (glucocorticoids, mineralocorticoids, androgen, estrogen, and progesterone). Class II receptors form heterodimers with retinoid X receptors (RXR). This class includes the thyroid receptor (TR), vitamin D receptor (VDR), retinoic acid receptor (RAR), peroxisome proliferator-activated receptors (PPAR), liver X receptor (LXR), farnesol X receptor (FXR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR) (Fig. 1, Table 1). The role of this class of receptors in epidermal homeostasis and barrier function will be described in this chapter in more detail. Class III receptors lack identiﬁcation of cognate ligands and are therefore called orphan receptors. Class IV receptors bind as monomers to a single hexameric core recognition motif ﬂanked by additional sequences upstream of this motif. In this chapter, we will focus on recently described class II receptors (PPAR, LXR) that are powerful modulators of epidermal homeostasis and barrier function.