The orphan receptor Nr4a1 (Nur77) is assumed to be constitutively active and primarily controlled via transcriptional regulation. Nr4a1 shows a heterogeneous expression profile in various metabolically active tissues and its function is closely related to energy demanding processes. Recently, we identified Nr4a1 as a candidate that might affect cardiac remodelling in isoproterenol induced heart failure. Moreover, our studies revealed two novel transcripts for Nr4a1 in the murine heart, which suggests transcriptional initiation from three distinct promoters.

Previous data show that Nr4a1 expression is influenced by cAMP and the circadian clock. To analyse, if distinct Nr4a1 transcripts are differentially regulated, we stimulated cardiac HL-1 cells with 8Br-cAMP in vitro and evaluated the in vivo situation by β-adrenergic stimulation with Isoproterenol. Further, the circadian expression pattern of Nr4a1 in the murine heart was investigated at 6 different time points.

Transcript specific analysis after 8Br-cAMP stimulation displayed an increased expression for two of the three Nr4a1 transcripts (2.1-fold and 5.3-fold). Comparable to HL-1 cells, the expression of Nr4a1 transcripts arose by β-adrenergic stimulation (3.3-fold to 6.4-fold) in vivo. Remarkably, the circadian expression also varied between different transcripts. Two of the three transcripts were up-regulated 2 hours after dawn (3.75- and 5-fold). One of these transcripts was also elevated 2.5-fold at +10 h. Noteworthy, the up-regulated transcripts displayed also the strongest response to cAMP.

In conclusion, expression of Nr4a1 in the murine heart is driven by at least three distinct promoters which may represent a basis for linking cardiac metabolism to cAMP-dependent and circadian regulation.