title: Map4k4 negatively regulates peroxisome proliferator-activated receptor (PPAR) gamma protein translation by suppressing the mammalian target of rapamycin (mTOR) signaling pathway in cultured adipocytes.
abstract: The receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is considered a master regulator of adipocyte differentiation and promotes glucose and lipid metabolism in mature adipocytes. We recently identified the yeast Sterile 20 (Ste20) protein kinase ortholog, Map4k4, in an RNA interference-based screen as an inhibitor of PPARgamma expression in cultured adipocytes. Here, we show that RNA interference-mediated silencing of Map4k4 elevates the levels of both PPARgamma1 and PPARgamma2 proteins in 3T3-L1 adipocytes without affecting PPARgamma mRNA levels, suggesting that Map4k4 regulates PPARgamma at a post-transcriptional step. PPARgamma degradation rates are remarkably rapid as measured in the presence of cycloheximide (t(1/2) = 2 h), but silencing Map4k4 had no effect on PPARgamma degradation. However, depletion of Map4k4 significantly enhances [(35)S]methionine/cysteine incorporation into proteins, suggesting that Map4k4 signaling decreases protein translation. We show a function of Map4k4 is to inhibit rapamycin-sensitive mammalian target of rapamycin (mTOR) activity, decreasing 4E-BP1 phosphorylation. In addition, our results show mTOR and 4E-BP1 are required for the increased PPARgamma protein expression upon Map4k4 knockdown. Consistent with this concept, adenovirus-mediated expression of Map4k4 decreased PPARgamma protein levels and mTOR phosphorylation. These data show that Map4k4 negatively regulates PPARgamma post-transcriptionally, by attenuating mTOR signaling and a 4E-BP1-dependent mechanism.
authors.affiliation: Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, Massachusetts 01605, USA.
authors.forename: Kalyani V P
authors.initials: KV
authors.lastname: Guntur
authors.forename: Adilson
authors.initials: A
authors.lastname: Guilherme
authors.forename: Liting
authors.initials: L
authors.lastname: Xue
authors.forename: Anil
authors.initials: A
authors.lastname: Chawla
authors.forename: Michael P
authors.initials: MP
authors.lastname: Czech
mesh_terms: D041721:3T3-L1 Cells; D048868:Adaptor Proteins, Signal Transducing; D017667:Adipocytes / Q000166:cytology / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D018797:Cell Cycle Proteins; D039642:Eukaryotic Initiation Factors; D005786:Gene Expression Regulation; D047908:Intracellular Signaling Peptides and Proteins / Q000037:antagonists & inhibitors*; D051379:Mice; D047495:PPAR gamma / Q000037:antagonists & inhibitors* / Q000096:biosynthesis; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors* / Q000502:physiology*; D055550:Protein Stability; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases; D000097782:NF-kappaB-Inducing Kinase
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D039642:Eukaryotic Initiation Factors; D047908:Intracellular Signaling Peptides and Proteins; D047495:PPAR gamma; D010750:Phosphoproteins; D017346:Protein Serine-Threonine Kinases; D058570:TOR Serine-Threonine Kinases; D000097782:NF-kappaB-Inducing Kinase; C499527:Eif4ebp1 protein, mouse; C546843:mTOR protein, mouse