Ryan P Kopp | PubMed Health

PubMed2006

HMGB1 expression and release by bone cells.

Immune and bone cells are functionally coupled by pro-inflammatory cytokine intercellular signaling networks common to both tissues and their crosstalk may contribute to the etiologies of some immune-associated bone pathologies. For example, the receptor activator of NF-kappaB ligand (RANKL)/osteoprotegerin (OPG)/receptor activator of NF-kappaB (RANK) signaling axis plays a critical role in dendritic cell (DC) function as well as bone remodeling. The expression of RANKL by immune cells may contribute to bone loss in periodontitis, arthritis, and multiple myeloma. A recent discovery reveals that DCs release the chromatin protein high mobility group box 1 (HMGB1) as a potent immunomodulatory cytokine mediating the interaction between DCs and T-cells, via HMGB1 binding to the membrane receptor for advanced glycation end products (RAGE). To determine whether osteoblasts or osteoclasts express and/or release HMGB1 into the bone microenvironment, we analyzed tissue, cells, and culture media for the presence of this molecule. Our immunohistochemical and immunocytochemical analyses demonstrate HMGB1 expression in primary osteoblasts and osteoclasts and that both cells express RAGE. HMGB1 is recoverable in the media of primary osteoblast cultures and cultures of isolated osteoclast precursors and osteoclasts. Parathyroid hormone (PTH), a regulator of bone remodeling, attenuates HMGB1 release in cultures of primary osteoblasts and MC3T3-E1 osteoblast-like cells but augments this release in the rat osteosarcoma cell line UMR 106-01, both responses primarily via activation of adenylyl cyclase. PTH-induced HMGB1 discharge by UMR cells exhibits similar release kinetics as reported for activated macrophages. These data confirm the presence of the HMGB1/RAGE signaling axis in bone.

Alkaline Phosphatase↗

PubMed2002

Peroxisome proliferator-activated receptor coactivator-1alpha (PGC-1alpha) coactivates the cardiac-enriched nuclear receptors estrogen-related receptor-alpha and -gamma. Identification of novel leucine-rich interaction motif within PGC-1alpha.

The transcriptional coactivator PPARgamma coactivator-1alpha (PGC-1alpha) has been characterized as a broad regulator of cellular energy metabolism. Although PGC-1alpha functions through many transcription factors, the PGC-1alpha partners identified to date are unlikely to account for all of its biologic actions. The orphan nuclear receptor estrogen-related receptor alpha (ERRalpha) was identified in a yeast two-hybrid screen of a cardiac cDNA library as a novel PGC-1alpha-binding protein. ERRalpha was implicated previously in regulating the gene encoding medium-chain acyl-CoA dehydrogenase (MCAD), which catalyzes the initial step in mitochondrial fatty acid oxidation. The cardiac perinatal expression pattern of ERRalpha paralleled that of PGC-1alpha and MCAD. Adenoviral-mediated ERRalpha overexpression in primary neonatal cardiac mycoytes induced endogenous MCAD expression. Furthermore, PGC-1alpha enhanced the transactivation of reporter plasmids containing an estrogen response element or the MCAD gene promoter by ERRalpha and the related isoform ERRgamma. In vitro binding experiments demonstrated that ERRalpha interacts with PGC-1alpha via its activation function-2 homology region. Mutagenesis studies revealed that the LXXLL motif at amino acid position 142-146 of PGC-1alpha (L2), necessary for PGC-1alpha interactions with other nuclear receptors, is not required for the PGC-1alpha.ERRalpha interaction. Rather, ERRalpha binds PGC-1alpha primarily through a Leu-rich motif at amino acids 209-213 (Leu-3) and utilizes additional LXXLL-containing domains as accessory binding sites. Thus, the PGC-1alpha.ERRalpha interaction is distinct from that of other nuclear receptor PGC-1alpha partners, including PPARalpha, hepatocyte nuclear factor-4alpha, and estrogen receptor alpha. These results identify ERRalpha and ERRgamma as novel PGC-1alpha interacting proteins, implicate ERR isoforms in the regulation of mitochondrial energy metabolism, and suggest a potential mechanism whereby PGC-1alpha selectively binds transcription factor partners.

Acyl-CoA Dehydrogenase↗