Interferon-gamma-mediated activation and ubiquitin-proteasome-dependent degradation of PPARgamma in adipocytes.

Z Elizabeth Floyd,Jacqueline M Stephens

doi:10.1074/jbc.m108473200

Z Elizabeth Floyd, Jacqueline M Stephens

Open Access

https://doi.org/10.1074/jbc.m108473200

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Abstract

Interferon-gamma (IFNgamma) treatment of adipocytes results in a down-regulation of the peroxisome proliferator-activated receptor gamma (PPARgamma). The decrease in PPARgamma expression is mediated by inhibition of PPARgamma synthesis and increased degradation of PPARgamma. In this study, we demonstrate that both PPARgamma1 and PPARgamma2 are targeted to the proteasome under basal conditions and that PPARgamma1 is more labile than PPARgamma2. The IFNgamma-induced increase in PPARgamma turnover is blocked by proteasome inhibition and is accompanied by an increase in PPARgamma-polyubiquitin conjugates. In addition, IFNgamma treatment results in the transcriptional activation of PPARgamma. Similar to ligand-dependent activation of PPARgamma, IFNgamma-induced activation was greater in the phosphorylation-deficient S112A form of PPARgamma when compared with wild-type PPARgamma. Moreover, the inhibition of ERKs 1 and 2 with a MEK inhibitor, U1026, lead to an inhibition in the decay of PPARgamma proteins, indicating that serine phosphorylation influences the degradation of PPARgamma in fat cells. Our results also demonstrate that the proteasome-dependent degradation of PPARgamma does not require nuclear export. Taken together, these results indicate that PPARgamma is targeted to the ubiquitin-proteasome pathway for degradation under basal conditions and that IFNgamma leads to an increased targeting of PPARgamma to the ubiquitin-proteasome system in a process that is affected by ERK-regulated serine phosphorylation of PPARgamma proteins.

Highlights

peroxisome proliferator-activated receptor ␥ (PPAR␥)1 is a member of the nuclear hormone receptor family, a group of transcription factors that are activated by small lipophilic ligands [1]
These results indicate that PPAR␥ is targeted to the ubiquitinproteasome pathway for degradation under basal conditions and that IFN␥ leads to an increased targeting of PPAR␥ to the ubiquitin-proteasome system in a process that is affected by ERK-regulated serine phosphorylation of PPAR␥ proteins
PPAR␥1 is a member of the nuclear hormone receptor family, a group of transcription factors that are activated by small lipophilic ligands [1]

Summary

The abbreviations used are

PPAR␥, peroxisome proliferator-activated receptor ␥; TZD, thiazolidinedione; IFN␥, interferon-␥; STAT, signal transducer and activator of transcription; HA, hemagglutinin; DMEM, Dulbecco’s modified Eagle’s medium; PBS, phosphate-buffered saline; MAPK, mitogen-activated protein kinase; ERK, extracellular signal-regulated kinase; MEK, MAPK/ERK kinase; LMB, leptomycin B. We have recently demonstrated that IFN␥ results in a substantial loss of PPAR␥ expression by regulating two cellular events: 1) targeting PPAR␥ to the proteasome for degradation, and 2) inhibiting the synthesis of PPAR␥ [13]. Our recent studies [13] have shown that acute IFN␥ treatment of 3T3-L1 adipocytes results in a repression of PPAR␥ transcription that is independent of new protein synthesis. We demonstrate that IFN␥ treatment is associated with an increase in the formation of polyubiquitin-PPAR␥ conjugates in 3T3-L1 adipocytes Together, these data indicate that IFN␥ signaling results in the increased targeting of PPAR␥ to the ubiquitin-proteasome system in adipocytes. Our results suggest that phosphorylation of PPAR␥2 at Ser112 contributes to the targeting of PPAR␥ to the ubiquitin-proteasome pathway These studies indicate that the IFN␥mediated ubiquitin-proteasome-dependent degradation of PPAR␥ occurs in the nucleus

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION