Evidence for a glucocorticoid receptor beta splice variant in the rat and its physiological regulation in liver

Glucocorticoids are important regulators of metabolism and immune function. Synthetic glucocorticoids are extensively used for immunosuppression/anti-inflammatory therapy. Since the glucocorticoid receptor (GR) is central to most hormone effects; its in vivo regulation will influence hormone/drug action. An alternative splice variant, GR beta, is present in humans and may function as a dominant negative regulator of GR transcriptional activity. Recently, a similar splice variant was reported in mouse, although the mechanism of alternative splicing differs from that in humans. We present evidence that a splice variant of GR with an alternative C-terminus also occurs in the rat by a mechanism of intron inclusion. A highly quantitative qRT-PCR assay for the simultaneous measurement of both splice variants in a single sample was developed in order to accurately measure their regulation. We used this assay to assess the tissue specific expression of both mRNAs, and demonstrate that GR alpha is predominant in all tissues. In addition, the regulation of both GR alpha and GR beta mRNA by various physiological factors in rat liver was assessed. GR alpha showed a robust circadian rhythm, which was entrained with the circadian oscillation of the endogenous hormone. Time series experiments showed that both corticosteroids and LPS but not insulin dosing resulted in the transient down-regulation of GR alpha mRNA. LPS treatment also resulted in down-regulation of GR beta expression. A modest up-regulation in GR beta expression was observed only in animals having chronically elevated plasma insulin concentrations. However the expression of GR beta was significantly lower than that of GR alpha, in all cases. (C) 2012 Elsevier Inc. All rights reserved.