Isoform‐Specific SCF^Fbw7 Ubiquitination Mediates Differential Regulation of PGC‐1α

Abstract

The E3 ubiquitin ligase and tumor suppressor SCFFbw7 exists as three isoforms that govern the degradation of a host of critical cell regulators, including c-Myc, cyclin E, and PGC-1α. Peroxisome proliferator activated receptor-gamma coactivator 1α (PGC-1α) is a transcriptional coactivator with broad effects on cellular energy metabolism. Cellular PGC-1α levels are tightly controlled in a dynamic state by the balance of synthesis and rapid degradation via the ubiquitin-proteasome system. Yet, isoform-specific functions of SCFFbw7 are yet to be determined. Here, we show that the E3 ubiquitin ligase, SCFFbw7, regulates cellular PGC-1α levels via two independent, isoform specific, mechanisms. The cytoplasmic isoform (SCFFbw7β) reduces cellular PGC-1α levels via accelerated ubiquitin-proteasome degradation. In contrast, the nuclear isoform (SCFFbw7α) increases cellular PGC-1α levels and protein stability via inhibition of ubiquitin-proteasomal degradation. When nuclear Fbw7α proteins are redirected to the cytoplasm, cellular PGC-1α protein levels are reduced through accelerated ubiquitin-proteasomal degradation. We find that SCFFbw7β catalyzes high molecular weight PGC-1α-ubiquitin conjugation, whereas SCFFbw7α produces low molecular weight PGC-1α-ubiquitin conjugates that are not effective degradation signals. Thus, selective ubiquitination by specific Fbw7 isoforms represents a novel mechanism that tightly regulates cellular PGC-1α levels. Fbw7 isoforms mediate degradation of a host of regulatory proteins. The E3 ubiquitin ligase, Fbw7, mediates PGC-1α levels via selective isoform-specific ubiquitination. Fbw7β reduces cellular PGC-1α via ubiquitin-mediated degradation, whereas Fbw7α increases cellular PGC-1α via ubiquitin-mediated stabilization.

Peroxisome Proliferator-activated Receptor γ Coactivator-1 α Isoforms Selectively Regulate Multiple Splicing Events on Target Genes

“…1F and Ref. 19), PGC-1␣4 showed greater stability with a remarkably long half-life, similar to what is described for yet another PGC-1␣ isoform, NT-PGC-1␣ (17).…”

“…Although PGC-1␣1 has been previously reported to be a short lived protein due to ubiquitin-proteasome degradation (17,18), not much is known about the protein stability of the other PGC-1␣ isoforms. To determine whether all PGC-1␣ variants are targeted to the same degradation pathway, myotubes expressing each of the PGC-1␣ isoforms were treated for 4 h with the proteasome inhibitor MG132.…”

RNF4-Dependent Oncogene Activation by Protein Stabilization

“…Protein stability in steady state and dynamic CHX chase and 35 S-Met pulse/chase experiments were performed as previously described (Abed et al, 2011; Trausch-Azar et al, 2015). Pulse labeling was 10 min for c-Myc and 30 min for NICD at 37°C.…”

“…Ubiquitylation and SUMOylation were performed as previously described (Abed et al, 2011; Trausch-Azar et al, 2015). Where indicated, cells were incubated with 40 µM MG132 for 4 hr and subjected to hot lysis, followed by immune precipitation.…”

The hitchhiker’s guide to PGC-1α isoform structure and biological functions

“…The acetylation state of PGC-1α1 can be modified by acetyltransferases such as GCN5 [25] or deacetylases like sirtuin 1 (SIRT1) [26,27]. Additionally, ubiquitylation by specific E3 ligases directly regulates PGC-1α protein stability [24,28,29]. Together, these multiple levels of regulation ensure that under any given condition the cellular PGC-1α activity is appropriate to regulate the gene programs necessary to adapt to the new biological environment.…”