Human pregnane X receptor is expressed in breast carcinomas, potential heterodimers formation between hPXR and RXR-alpha

Breast samples from 99 patients including benign breast diseases, in situ and infiltrative carcinomas were processed for immunohistochemistry and Western-blot analysis.Cancer cells from patients that developed recurrent disease showed a high cytoplasmic location of both hPXR isoforms. Only the infiltrative carcinomas that relapsed before 48 months showed nuclear location of hPXR isoform 2. This location was associated with the nuclear immunoexpression of RXR-alpha.Breast cancer cells can express both variants 1 and 2 of hPXR. Infiltrative carcinomas that recurred showed a nuclear location of both hPXR and RXR-alpha; therefore, the overexpression and the subcellular location changes of hPXR could be considered as a potential new prognostic indicator.The human pregnane X receptor (hPXR, also known as SXR) is a member of the NR1I2 subfamily [1]. This receptor presents different isoforms that are differentially activated by a remarkably diverse collection of compounds including both xenobiotics and natural steroids [2]. PXR orthologs show marked differences in their activation profiles between species; thus, pregnenolone 16α-carbonitrile is an efficacious activator of mouse and rat PXR, but has much less activity on the human and rabbit receptors. Conversely, rifampicin activates the human and rabbit PXR but has no activity on the mouse or rat receptors [3].PXR is a needed partner of RXRs [4] to form heterodimers that induce transcription from ER6 [5] or IR6 [6] response elements present in steroid-inducible cytochrome P450 (CYP) gene promoters [7]. Cytochrome P450 constitutes a multigene family of hemoproteins responsible for the metabolism of numerous xenobiotics, including therapeutic drugs, environmental chemicals and dietary constituents, as well as endogenous compounds such as steroids and bile acids [8]. Kliewer et al. [3] demonstrated in mice that the strong activation of PXR evoked by the pregnane compounds seemed to be mediated by CYP3A induction; this effect