Increased Lipid Oxidation Causes Oxidative Stress, Increased Peroxisome Proliferator-activated Receptor-γ Expression, and Diminished Pro-osteogenic Wnt Signaling in the Skeleton

Abstract

Loss of bone mass with advancing age in mice is because of decreased osteoblast number and is associated with increased oxidative stress and decreased canonical Wnt signaling. However, the underlying mechanisms are poorly understood. We report an age-related increase in the lipid oxidation product 4-hydroxynonenal (4-HNE) as well as increased expression of lipoxygenase and peroxisome proliferator-activated receptor-␥ (PPAR␥) in the murine skeleton. These changes together with decreased Wnt signaling are reproduced in 4-month-old mice bearing a high expressing allele of the lipoxygenase Alox15. The addition of 4-HNE to cultured osteoblastic cells increases oxidative stress, which in turn diverts ␤-catenin from T-cell-specific transcription factors to Forkhead box O (FoxO) transcription factors, thereby attenuating the suppressive effect of ␤-catenin on PPAR␥ gene expression. Oxidized lipids, acting as ligands of PPAR␥, promote binding of PPAR␥2 to ␤-catenin and reduce the levels of the latter, and they attenuate Wnt3a-stimulated proliferation and osteoblast differentiation. Furthermore, oxidized lipids and 4-HNE stimulate apoptosis of osteoblastic cells. In view of the role of oxidized lipids in atherogenesis, the adverse effects of lipoxygenase-mediated lipid oxidation on the differentiation and survival of osteoblasts may provide a mechanistic explanation for the link between atherosclerosis and osteoporosis.Age-related bone loss is primarily because of an insufficient number of osteoblasts (1, 2) attributed to exhaustion of multipotential mesenchymal stem cell progenitors (3, 4) and the diversion of these progenitors toward the adipocyte lineage (5-11). Increased osteoblast apoptosis may also be involved as we recently demonstrated that loss of bone mass and strength in C57BL/6 (B6) 3 mice with advancing age is associated with an increase in the prevalence of apoptotic osteoblasts and a corresponding decrease in osteoblast number and bone formation rate (12). Moreover, these changes are accompanied by increased oxidative stress and diminished canonical Wnt signaling, a critical regulator of bone formation. Wnts are secreted proteins that bind to a Frizzled receptor and a low density lipoprotein receptor-related protein 5 (LRP5) or LRP6 co-receptor, resulting in a rise in the level of ␤-catenin by preventing its degradation by the proteasome. Binding of ␤-catenin to members of TCF family converts them from repressors to activators of a variety of genes, including those involved in the differentiation and survival of osteoblasts (13-16). Importantly, however, ␤-catenin is also an essential partner of the FoxO family of transcription factors, which defend against oxidative stress by stimulating the transcription of oxidant scavenging enzymes such as manganese superoxide dismutase and catalase (17). In fact, we have elucidated that oxidative stress compromises ␤-catenin/TCF-mediated transcription and osteoblastogenesis because of competition between FoxO and TCF for a limited pool of ␤-catenin (18). Such competiti...

Effects of arachidonic acid on the concentration of hydroxyeicosatetraenoic acids in culture media of mesenchymal stromal cells differentiating into adipocytes or osteoblasts

“…Some HETE may modulate osteoblastogenesis. Thus, 12-and 15-HETE may inhibit differentiation and promote apoptosis of osteoblasts (Almeida et al 2009). Additionally, the 5-HETE may inhibit osteoblastogenesis (Traianedes et al 1998).…”

“…Thus, ß-catenin does not bind T cell-specific transcription factors to induce the MSC differentiation into osteoblasts, due to the increase in oxidative stress caused by such enzyme, but instead it binds to the forkhead box O (FoxO) transcription factors. This induces pparc2 gene and therefore triggers the MSC differentiation into adipocytes (Almeida et al 2009). …”

“…Although great attention has been focused on the COX-derived PG so far, emerging evidence suggests that LT as well as HETE LOX-catalyzed products may also exert profound biological effects on inflammation processes, being involved in the development and progression of specific human diseases, such as colorectal or pancreatic cancers (Comba and Pasqualini 2009;Di Mari et al 2007). In addition to their inflammatory role, 12-and 15-lipoxygenases have been reported as promoters of insulin resistance associated with obesity (Chakrabarti et al 2009) and osteoporosis (Almeida et al 2009). Interestingly, the same enzymes participating in the metabolism of omega-6 are involved in the metabolism of omega-3.…”

Hyperlipidemia induces resistance to PTH bone anabolism in mice via oxidized lipids

“…Since lipid oxidation products inhibit bone-forming cells in vitro, (6,8,10) we first tested the effects of an HFD on bone cell differentiation markers in Ldlr À/À mice on a C57BL/6 background. This HFD was previously shown by Towler and colleagues to increase atherogenic lipoproteins in this mouse model.…”

“…(4,5) Importantly, regardless of whether from a genetic or a dietary source, lipid oxidation products attenuate osteogenic differentiation in vitro. (6)(7)(8) In mice, hyperlipidemia induces bone loss (9,10) and impairs the anabolic effects (11) of intermittent parathyroid hormone (PTH), a regimen now used for the treatment of osteoporosis.…”