Estrogen-related receptor α regulates osteoblast differentiation via Wnt/ß-catenin signaling.

Based on its homology to the estrogen receptor and its roles in osteoblast and chondrocyte differentiation, the orphan nuclear receptor estrogen-related receptor α (ERRα (ESRRA)) is an intriguing therapeutic target for osteoporosis and other bone diseases. The objective of this study was to better characterize the molecular mechanisms by which ERRα modulates osteoblastogenesis. Experiments from multiple systems demonstrated that ERRα modulates Wnt signaling, a crucial pathway for proper regulation of bone development. This was validated using a Wnt-luciferase reporter, where ERRα showed co-activator-dependent (peroxisome proliferator-activated receptor gamma co-activator 1α, PGC-1α) stimulatory effects. Interestingly, knockdown of ERRα expression also enhanced WNT signaling. In combination, these data indicated that ERRα could serve to either activate or repress Wnt signaling depending on the presence or absence of its co-activator PGC-1α. The observed Wnt pathway modulation was cell intrinsic and did not alter ß-catenin nuclear translocation but was dependent on DNA binding of ERRα. We also found that expression of active ERRα correlated with Wnt pathway effects on osteoblastic differentiation in two cell types, consistent with a role for ERRα in modulating the Wnt pathway. In conclusion, this work identifies ERRα, in conjunction with co-activators such as PGC-1α, as a new regulator of the Wnt-signaling pathway during osteoblast differentiation, through a cell-intrinsic mechanism not affecting ß-catenin nuclear translocation.

The conserved ATPase Get3/Arr4 modulates the activity of membrane-associated proteins in Saccharomyces cerevisiae.

The regulation of cellular membrane dynamics is crucial for maintaining proper cell growth and division. The Cdc48-Npl4-Ufd1 complex is required for several regulated membrane-associated processes as part of the ubiquitin-proteasome system, including ER-associated degradation and the control of lipid composition in yeast. In this study we report the results of a genetic screen in Saccharomyces cerevisiae for extragenic suppressors of a temperature-sensitive npl4 allele and the subsequent analysis of one suppressor, GET3/ARR4. The GET3 gene encodes an ATPase with homology to the regulatory component of the bacterial arsenic pump. Mutants of GET3 rescue several phenotypes of the npl4 mutant and transcription of GET3 is coregulated with the proteasome, illustrating a functional relationship between GET3 and NPL4 in the ubiquitin-proteasome system. We have further found that Get3 biochemically interacts with the trans-membrane domain proteins Get1/Mdm39 and Get2/Rmd7 and that Deltaget3 is able to suppress phenotypes of get1 and get2 mutants, including sporulation defects. In combination, our characterization of GET3 genetic and biochemical interactions with NPL4, GET1, and GET2 implicates Get3 in multiple membrane-dependent pathways.

Transcriptional regulation by the proteasome as a mechanism for cellular protein homeostasis.

Protein degradation mediated by the ubiquitin-proteasome system is a vitally important means of regulation for many cellular processes. An increasing body of evidence implicates the proteasome in the regulation of gene transcription through a variety of mechanisms, including transcription factor processing and proteasome-chromatin association. Recently a genomic approach was used to elucidate the transcriptional effects of the proteasome in budding yeast. Results indicate a positive role for proteasome activity in the transcription of several functional gene classes, including the ribosomal protein genes. In addition, proteasome activity was found to be required for the expression of Spt23 target genes, independent of the proteasomal processing of this transcription factor, suggesting cooperativity between two forms of transcriptional regulation by the proteasome. Here we discuss several implications of these findings, including a possible feedback mechanism between protein synthesis and protein degradation via the transcriptional regulation of ribosomal protein genes by the proteasome.

Genomic association of the proteasome demonstrates overlapping gene regulatory activity with transcription factor substrates.

The proteasome can regulate transcription through proteolytic processing of transcription factors and via gene locus binding, but few targets of proteasomal regulation have been identified. Using genome-wide location analysis and transcriptional profiling in Saccharomyces cerevisiae, we have established which genes are bound and regulated by the proteasome and by Spt23 and Mga2, transcription factors activated by the proteasome. We observed proteasome association with gene sets that are highly transcribed, controlled by the mating type loci, and involved in lipid metabolism. At ribosomal protein (RP) genes, proteasome and RNA polymerase II (RNA Pol II) binding was enriched in a proteasome mutant, indicating a role for the proteasome in dissociating elongation complexes. The genomic occupancies of Spt23 and Mga2 overlapped significantly with the genes bound by the proteasome. Finally, the proteasome acts in two distinct ways, one dependent and one independent of Spt23/Mga2 cleavage, providing evidence for cooperative gene regulation by the proteasome and its substrates.