Muscle Ring Finger‐1 (MuRF1) inhibits PPARα through mono‐ubiquitination of specific lysines adjacent to a novel nuclear export sequence (NES)

Abstract

MuRF1 inhibits fatty acid oxidation while increasing glucose metabolism by its specific inhibition of PPARα activity. Surprisingly, MuRF1 does not degrade PPARα through poly‐ubiquitination. Alternatively, we identified that MuRF1 mono‐ubiquitinates PPARα. To identify the underlying mechanism of MuRF1's inhibition of PPARα, we increased MuRF1 expression and determined PPARα's nuclear localization by confocal microscopy. Increasing MuRF1 cleared nuclear PPARα in 80% of the cells (vs. <0.1% of controls), leading to the hypothesis that MuRF1 enhances PPARα nuclear export. Using the nuclear export inhibitor leptomycin B, we prevented MuRF1‐dependent PPARα nuclear export, suggesting a role of the nuclear export machinery. We next identified two potential non‐canonical nuclear export signals in PPARα and mutated their leucines to confirm their role in nuclear export. Only the PPARα NES1 mutant was enriched in its nuclear localization. We next mutated the three adjacent lysines to PPARα's NES1 to determine their role in MuRF1's ubiquitination and activity. Mutating any one of the three lysines around NES1 prevented MuRF1‐dependent nuclear export of PPARα. These data suggest that the increased MuRF1 that occurs in cardiac stress may drive the mono‐ubiquitination of PPARα, resulting in enhanced nuclear export and inhibition of PPARα activity.