While it has long been known that inflammation and infection reduce expression of hepatic cytochrome P450 (CYP) genes involved in xenobiotic metabolism and that exposure to xenobiotic chemicals can impair immune function, the molecular mechanisms underlying both of these phenomena have remained largely unknown. Here we show that activation of the nuclear steroid and xenobiotic receptor (SXR) by commonly used drugs in humans inhibits the activity of NF-κB, a key regulator of inflammation and the immune response. NF-κB target genes are upregulated and small bowel inflammation is significantly increased in mice lacking the SXR ortholog pregnane X receptor (PXR), thereby demonstrating a direct link between SXR and drug-mediated antagonism of NF-κB. Interestingly, NF-κB activation reciprocally inhibits SXR and its target genes whereas inhibition of NF-κB enhances SXR activity. This SXR/PXR-NF-κB axis provides a molecular explanation for the suppression of hepatic CYP mRNAs by inflammatory stimuli as well as the immunosuppressant effects of xenobiotics and SXR-responsive drugs. This mechanistic relationship has clinical consequences for individuals undergoing therapeutic exposure to the wide variety of drugs that are also SXR agonists.

Vitamin K 2 is a critical nutrient required for blood clotting that also plays an important role in bone formation. Vitamin K 2 supplementation up-regulates the expression of bone markers, increases bone density in vivo, and is used clinically in the management of osteoporosis. The mechanism of vitamin K 2 action in bone formation was thought to involve its normal role as an essential cofactor for ␥-carboxylation of bone matrix proteins. However, there is evidence that suggests vitamin K 2 also has a transcriptional regulatory function. Vitamin K 2 bound to and activated the orphan nuclear receptor SXR and induced expression of the SXR target gene, CYP3A4, identifying it as a bona fide SXR ligand. Vitamin K 2 treatment of osteosarcoma cells increased mRNA levels for the osteoblast markers bone alkaline phosphatase, osteoprotegerin, osteopontin, and matrix Gla protein. The known SXR activators rifampicin and hyperforin induced this panel of bone markers to an extent similar to vitamin K 2 . Vitamin K 2 was able to induce bone markers in primary osteocytes isolated from wild-type murine calvaria but not in cells isolated from mice deficient in the SXR ortholog PXR. We infer that vitamin K 2 is a transcriptional regulator of bonespecific genes that acts through SXR to favor the expression of osteoblastic markers. Thus, SXR has a novel role as a mediator of bone homeostasis in addition to its role as a xenobiotic sensor. An important implication of this work is that a subset of SXR activators may function as effective therapeutic agents for the management of osteoporosis.

The murine multidrug resistance gene mdr1b is highly induced in the endometrium during pregnancy. Evidence suggests that induction occurs mainly as a result of progesterone action. To study the molecular mechanisms involved in this induction, 5'-flanking sequences between -540 and +97 of the mdr1b gene were fused to the reporter gene, bacterial chloramphenicol acetyltransferase (p540CAT). Unlike most progesterone-responsive genes, mdr1b is preferentially activated by the A form of the progesterone receptor. We now report that activation is not observed with a DNA-binding domain mutant of progesterone receptor A (PRA) suggesting that induction occurs at the transcriptional level. Time course experiments demonstrated that induction was first observed 12 hr after hormone addition, suggestive of a secondary (or late) response gene. Sequence comparison highlighted the region M1 (-234 to -206), which contains a partially conserved progesterone response element. Its functional significance was evaluated by expression assays and gel shift analysis. Reporter plasmids with modifications of this element were transfected into HeLa cells. Constructs containing the native M1 element, or a mutated element (M1mt) that eliminated any similarity to a progesterone response element, were induced four-fold by progesterone whereas an element containing a consensus progesterone response element (M1PRE) was induced eight-fold. In addition, by gel shift analysis, the M1 element did not bind the progesterone receptor or any other factors. This suggested that the M1 region does not participate in the response to progesterone. 5' Nested deletion analysis, used to identify other regions of the upstream regulatory region that contributed to induction by progesterone, demonstrated that enhancer sequences between -122 and -65, which contain binding sites for C/EBPbeta and NF-Y, were important. Mutations in the binding sites for these factors decreased induction by progesterone. On the basis of our studies using 540 bp of upstream sequence, mdr1b is activated transcriptionally by progesterone, in an indirect manner dependent on basal factors.

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