Maria Rosário Almeida

Oxidative Stress Antagonizes Wnt Signaling in Osteoblast Precursors by Diverting β-Catenin from T Cell Factor- to Forkhead Box O-mediated Transcription

We have elucidated that oxidative stress is a pivotal pathogenetic factor of age-related bone loss and strength in mice, leading to, among other changes, a decrease in osteoblast number and bone formation. To gain insight into the molecular mechanism by which oxidative stress exerts such adverse effects, we have tested the hypothesis that induction of the Forkhead box O (FoxO) transcription factors by reactive oxygen species may antagonize Wnt signaling, an essential stimulus for osteoblastogenesis. In support of this hypothesis, we report herein that the expression of FoxO target genes increases, whereas the expression of Wnt target genes decreases, with increasing age in C57BL/6 mice. Moreover, we show that in osteoblastic cell models, oxidative stress (exemplified by H2O2) promotes the association of FoxOs with β-catenin, β-catenin is required for the stimulation of FoxO target genes by H2O2, and H2O2 promotes FoxO-mediated transcription at the expense of Wnt-/T-cell factor-mediated transcription and osteoblast differentiation. Furthermore, β-catenin overexpression is sufficient to prevent FoxO-mediated suppression of T-cell factor transcription. These results demonstrate that diversion of the limited pool of β-catenin from T-cell factor- to FoxO-mediated transcription in osteoblastic cells may account, at least in part, for the attenuation of osteoblastogenesis and bone formation by the age-dependent increase in oxidative stress.

Oxidative stress antagonizes WNT signaling in osteoblast precursors by diverting B-catenin from TCF- to FOXO-mediated transcription

We have elucidated that oxidative stress is a pivotal pathogenetic factor of age-related bone loss and strength in mice, leading to, among other changes, a decrease in osteoblast number and bone formation. To gain insight into the molecular mechanism by which oxidative stress exerts such adverse effects, we have tested the hypothesis that induction of the Forkhead box O (FoxO) transcription factors by reactive oxygen species may antagonize Wnt signaling, an essential stimulus for osteoblastogenesis. In support of this hypothesis, we report herein that the expression of FoxO target genes increases, whereas the expression of Wnt target genes decreases, with increasing age in C57BL/6 mice. Moreover, we show that in osteoblastic cell models, oxidative stress (exemplified by H2O2) promotes the association of FoxOs with β-catenin, β-catenin is required for the stimulation of FoxO target genes by H2O2, and H2O2 promotes FoxO-mediated transcription at the expense of Wnt-/T-cell factor-mediated transcription and osteoblast differentiation. Furthermore, β-catenin overexpression is sufficient to prevent FoxO-mediated suppression of T-cell factor transcription. These results demonstrate that diversion of the limited pool of β-catenin from T-cell factor- to FoxO-mediated transcription in osteoblastic cells may account, at least in part, for the attenuation of osteoblastogenesis and bone formation by the age-dependent increase in oxidative stress.

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

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-γ (PPARγ) 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 β-catenin from T-cell-specific transcription factors to Forkhead box O (FoxO) transcription factors, thereby attenuating the suppressive effect of β-catenin on PPARγ gene expression. Oxidized lipids, acting as ligands of PPARγ, promote binding of PPARγ2 to β-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.

Receptor Activator of Nuclear Factor κB Ligand (RANKL) Protein Expression by B Lymphocytes Contributes to Ovariectomy-induced Bone Loss

Background: The contribution of B lymphocytes to the bone loss caused by estrogen deficiency is unclear. Results: Deletion of the cytokine receptor activator of NFκB ligand from B lymphocytes, but not T lymphocytes, blunted bone loss in ovariectomized mice. Conclusion: Cytokine production by B lymphocytes contributes to ovariectomy-induced bone loss. Significance: This mechanism may be relevant to the mechanisms responsible for postmenopausal osteoporosis. Production of the cytokine receptor activator of NFκB ligand (RANKL) by lymphocytes has been proposed as a mechanism by which sex steroid deficiency causes bone loss. However, there have been no studies that functionally link RANKL expression in lymphocytes with bone loss in this condition. Herein, we examined whether RANKL expression in either B or T lymphocytes contributes to ovariectomy-induced bone loss in mice. Mice harboring a conditional RANKL allele were crossed with CD19-Cre or Lck-Cre mice to delete RANKL in B or T lymphocytes, respectively. Deletion of RANKL from either cell type had no impact on bone mass in estrogen-replete mice up to 7 months of age. However, mice lacking RANKL in B lymphocytes were partially protected from the bone loss caused by ovariectomy. This protection occurred in cancellous, but not cortical, bone and was associated with a failure to increase osteoclast numbers in the conditional knock-out mice. Deletion of RANKL from T lymphocytes had no impact on ovariectomy-induced bone loss. These results demonstrate that lymphocyte RANKL is not involved in basal bone remodeling, but B cell RANKL does contribute to the increase in osteoclasts and cancellous bone loss that occurs after loss of estrogen.

Glucocorticoids and tumor necrosis factor α increase oxidative stress and suppress Wnt protein signaling in osteoblasts.

Background: Glucocorticoids and tumor necrosis factor (TNF) α decrease bone mass. Results: Oxidative stress is increased in bone-forming cells (osteoblasts) in response to glucocorticoids and TNFα. Conclusion: Glucocorticoids and TNFα decrease osteoblast numbers via oxidative stress-dependent and -independent mechanisms. Significance: This might help in finding treatments for osteoporosis. Endogenous glucocorticoids (GCs) and inflammatory cytokines contribute to the age-associated loss of bone mass and strength, but the molecular mechanisms responsible for their deleterious effects on the aging skeleton are unclear. Based on evidence that oxidative stress is a causal mechanism of the insulin resistance produced by either one of these two agents, we tested the hypothesis that their adverse skeletal effects also result from increased oxidative stress. We report that administration of prednisolone to mice increased reactive oxygen species (ROS) and the phosphorylation of p66shc (an amplifier of H2O2 generation in mitochondria) in bone. Dexamethasone (Dex) and TNFα had a similar effect on osteoblastic cells in vitro. The generation of ROS by Dex and TNFα required PKCβ/p66shc signaling and was responsible for the activation of JNK and induction of apoptosis by both agents. The activity of Forkhead box O (FoxO) transcription factors was also increased in response to ROS; however, FoxO activation opposed apoptosis induced by Dex and TNFα. In addition, both agents suppressed Akt phosphorylation as well as Wnt-induced proliferation and osteoblast differentiation. However, the inhibitory actions on Wnt signaling were independent of PKCβ/p66shc. Instead, they were mediated by inhibition of Akt and stimulation of FoxOs. These results demonstrate that ROS-induced activation of a PKCβ/p66shc/JNK signaling cascade is responsible for the pro-apoptotic effects of Dex and TNFα on osteoblastic cells. Moreover, modulation of Akt and FoxOs by GCs and TNFα are cell-autonomous mechanisms of Wnt/β-catenin antagonism contributing to the adverse effects of GC excess and inflammatory cytokines on bone alike.

Estrogen receptor-α signaling in osteoblast progenitors stimulates cortical bone accrual

The detection of estrogen receptor-α (ERα) in osteoblasts and osteoclasts over 20 years ago suggested that direct effects of estrogens on both of these cell types are responsible for their beneficial effects on the skeleton, but the role of ERα in osteoblast lineage cells has remained elusive. In addition, estrogen activation of ERα in osteoclasts can only account for the protective effect of estrogens on the cancellous, but not the cortical, bone compartment that represents 80% of the entire skeleton. Here, we deleted ERα at different stages of differentiation in murine osteoblast lineage cells. We found that ERα in osteoblast progenitors expressing Osterix1 (Osx1) potentiates Wnt/β-catenin signaling, thereby increasing proliferation and differentiation of periosteal cells. Further, this signaling pathway was required for optimal cortical bone accrual at the periosteum in mice. Notably, this function did not require estrogens. The osteoblast progenitor ERα mediated a protective effect of estrogens against endocortical, but not cancellous, bone resorption. ERα in mature osteoblasts or osteocytes did not influence cancellous or cortical bone mass. Hence, the ERα in both osteoblast progenitors and osteoclasts functions to optimize bone mass but at distinct bone compartments and in response to different cues.

Nongenotropic, Anti-Apoptotic Signaling of 1α,25(OH)2-Vitamin D3 and Analogs through the Ligand Binding Domain of the Vitamin D Receptor in Osteoblasts and Osteocytes MEDIATION BY Src, PHOSPHATIDYLINOSITOL 3-, AND JNK KINASES

Because sex steroids regulate the life span of bone cells by modulating cytoplasmic kinase activity via a nongenotropic action of their classical receptors, we have explored the possibility that the vitamin D nuclear receptor (VDR) might exhibit similar nongenotropic actions. We report that the conformationally flexible full VDR agonist, 1α,25(OH)2-vitamin D3 (1α,25(OH)2D3), and the 6-s-cis-locked 1α,25(OH)2-lumisterol3 (JN) analog, also acting through the VDR but with poor transcriptional activity, protected murine osteoblastic or osteocytic cells from apoptosis. This effect was reproduced in HeLa cells transiently transfected with either wild type VDR or a mutant consisting of only the VDR ligand binding domain. The VDR ligand binding domain bound [3H]1α,25(OH)2D3 as effectively as wild type VDR but did not induce vitamin D response element-mediated transcription. The anti-apoptotic effects of 1α,25(OH)2D3 and the 6-s-cis-locked 1α,25(OH)2-lumisterol3 analog in calvaria cells were blocked by three cytoplasmic kinase inhibitors: Src kinase inhibitor 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1), phosphatidylinositol 3 kinase inhibitor Wortmannin, and the JNK kinase inhibitor SP600125. However, inhibition of p38 with SB203580 or ERK with either U0126 or a transfected dominant negative MEK did not interfere with these anti-apoptotic actions. Further, 1α,25(OH)2D3 induced rapid (5 min) association of VDR with Src kinase in OB-6 cells. Finally, actinomycin D or cycloheximide prevented the anti-apoptotic effect of 1α,25(OH)2D3, indicating that transcriptional events are also required. These findings suggest that nongenotropic modulation of kinase activity is also a general property of the VDR and that ligands that activate nongenotropic signals, but lack transcriptional activity, display different biological profiles from the steroid hormone 1α,25(OH)2D3.

Natural polyphenols inhibit different steps of the process of transthyretin (TTR) amyloid fibril formation

TTR binds to TTR by electron microscopy (View interaction)

Thyroxine binding to transthyretin Met 119: Comparative studies of different heterozygotic carriers and structural analysis

The majority of the known transthyretin (TTR) variants are associated with amyloidosis, but there are also variants associated with euthyroid hyperthyroxinemia and others are apparently nonpathogenic. TTR Met 119 is a nonpathogenic variant found to be frequent in the Portuguese population. Previous studies on thyroxine (T4) binding to semi-purified TTR from heterozygotic carriers of TTR Met 119, reported by us and other groups, revealed different results. Therefore, to further characterize T4 binding to TTR Met 119 we performed T4-TTR binding studies in homotetrameric recombinant TTR Met 119 variant and normal TTR. We also studied T4 binding to TTR purified from serum of different heterozygotic carriers of TTR Met 119 including compound heterozygotic individuals carriers of a TTR mutation in the other allele. We observed an increased T4 binding affinity to TTR Met 119 from heterozygotic individuals and compound heterozygotes and this effect of increasing T4 binding affinity was consistent and independent from the mutation present in the other allele. Recombinant homotetrameric TTR Met 119 and heterotetrameric protein from heterozygotic carriers of TTR Met 119 presented similar T4 binding affinity demonstrating the increased T4 binding affinity of TTR Met 119. X-ray crystallography studies performed on the recombinant TTR Met 119 variant revealed structural alterations mainly at the level of residue Leu 110 allowing a closer contact between the hormone and the mutant protein. These results are consistent with the observed T4 binding results.

Binding of epigallocatechin‐3‐gallate to transthyretin modulates its amyloidogenicity

MINT‐7294529: TTR (uniprotkb:P02766) and TTR (uniprotkb:P02766) bind (MI:0407) by comigration in non‐denaturing gel electrophoresis (MI:0404)