Activation of p44/p42 mitogen-activated protein kinase limits triiodothyronine-stimulated alkaline phosphatase activity in osteoblasts

Disruption of BMP Signaling Prevents Hyperthyroidism-Induced Bone Loss in Male Mice

Thyroid hormones (TH) are key regulators of bone health, and TH excess in mice causes high bone turnover-mediated bone loss. However, the underlying molecular mechanisms of TH actions on bone remain poorly defined. Here, we tested the hypothesis whether TH mediate their effects via the pro-osteogenic bone morphogenetic protein (BMP) signaling pathway in vitro and in vivo. Primary murine osteoblasts treated with 3,3',5-triiodo-L-thyronine (T₃ ) showed an enhanced differentiation potential, which was associated with activated canonical BMP/SMAD signaling reflected by SMAD1/5/8 phosphorylation. Blocking BMP signaling at the receptor (LDN193189) and ligand level (noggin, anti-BMP2/BMP4 neutralizing antibodies) inhibited T₃ -induced osteogenic differentiation. In vivo, TH excess over 4 weeks in male C57BL/6JRj mice led to severe trabecular bone loss with a high bone turnover that was completely prevented by treatment with the BMP ligand scavenger ALK3-Fc. Thus, TH activate the canonical BMP pathway in osteoblasts to promote their differentiation and function. Importantly, this study indicates that blocking the BMP pathway may be an effective strategy to treat hyperthyroidism-induced bone loss. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Role of thyroid hormones in the neoplastic process: an overview

Thyroid hormones (TH) are critical regulators of several physiological processes, which include development, differentiation and growth in virtually all tissues. In past decades, several studies have shown that changes in TH levels caused by thyroid dysfunction, disruption of deiodinases and/or thyroid hormone receptor (TR) expression in tumor cells, influence cell proliferation, differentiation, survival and invasion in a variety of neoplasms in a cell type-specific manner. The function of THs and TRs in neoplastic cell proliferation involves complex mechanisms that seem to be cell specific, exerting effects via genomic and nongenomic pathways, repressing or stimulating transcription factors, influencing angiogenesis and promoting invasiveness. Taken together, these observations indicate an important role of TH status in the pathogenesis and/or development of human neoplasia. Here, we aim to present an updated and comprehensive picture of the accumulated knowledge and the current understanding of the potential role of TH status on the different hallmarks of the neoplastic process.

Resveratrol suppresses thyroid hormone‑induced osteocalcin synthesis in osteoblasts

Resveratrol, a polyphenolic compound that is present in grape skins, berries and red wine, may be beneficial for human health through its anti‑inflammatory and anti‑oxidant effects. It has been previously demonstrated that resveratrol exerts its biological effects primarily via sirtuin 1 (SIRT1) activation. We previously reported that triiodothyronine (T3) induces osteocalcin synthesis in osteoblast‑like MC3T3‑E1 cells, and that p38 mitogen‑activated protein (MAP) kinase mediates the T3‑stimulated synthesis of osteocalcin. The present study investigated the effect of resveratrol on T3‑induced osteocalcin synthesis and its underlying mechanism in MC3T3‑E1 cells. Cultured cells were stimulated with T3, and osteocalcin release from MC3T3‑E1 cells was measured by ELISA and phosphorylation of p38 MAP kinase was analyzed by western blotting. Resveratrol significantly suppressed the release of osteocalcin stimulated by T3, and SRT1720, a SIRT1 activator, significantly reduced T3‑induced osteocalcin release. The expression level of osteocalcin mRNA stimulated by T3 was significantly attenuated by resveratrol and T3‑induced transactivation activity of the thyroid hormone‑responsive element was significantly diminished by resveratrol. However, only limited effects of resveratrol on the T3‑induced phosphorylation of p38 MAP kinase were observed. The results of the present study demonstrated that resveratrol suppresses T3‑stimulated osteocalcin synthesis at a point upstream of transcription in osteoblasts, and that the inhibitory effect of resveratrol is mediated, at least partially, through SIRT1 activation. These results indicate that there may be a novel role for the polyphenol in the modulation of bone metabolism.

Regulation of tenascin expression in bone

Tenascins regulate cell interaction with the surrounding pericellular matrix. Within bone, tenascins C and W influence osteoblast adhesion and differentiation, although little is known about the regulation of tenascin expression. In this study we examined the effect of osteogenic differentiation, bone morphogenetic protein (BMP) and Wnt growth factors, and mechanical loading on tenascin expression in osteogenic cells. Osteogenic differentiation increased tenascin C (TnC), and decreased tenascin W (TnW), expression. Both growth factors and mechanical loading increased both TnC and TnW expression, albeit via distinct signaling mechanisms. Both BMP-2 and Wnt5a induction of tenascin expression were mediated by MAP kinases. These data establish a role for BMP, Wnts, and mechanical loading in the regulation of tenascin expression in osteoblasts.

Nongenomic actions of thyroxine modulate intermediate filament phosphorylation in cerebral cortex of rats

The developmental effects of thyroid hormones (TH) in mammalian brain are mainly mediated by nuclear receptors regulating gene expression. However, there are increasing evidences of nongenomic mechanisms of these hormones associated with kinase- and calcium-activated signaling pathways. In this context, the aim of the present work was to investigate the signaling pathways involved in the mechanism of action of TH on cytoskeletal phosphorylation in cerebral cortex of 15-day-old male rats. Results showed that L-thyroxine (L-T4) increased the intermediate filament (IF) phosphorylation independently of protein synthesis, without altering the total immunocontent of these proteins. Otherwise, neither 3,5,3'-triiodo-L-thyronine (L-T3) nor neurotransmitters (GABA, ATP, L-glutamate or epinephrine) acted on the IF-associated phosphorylation level. We also demonstrated that the mechanisms underlying the L-T4 effect on the cytoskeleton involve membrane initiated actions through Gi protein-coupled receptor. This evidence was reinforced by the inhibition of cyclic adenosine 5'-monophosphate (cAMP) levels. Moreover, we showed the participation of phospholipase C, protein kinase C, mitogen-activated protein kinase, calcium/calmodulin-dependent protein kinase II, intra- and extracellular Ca2+ mediating the effects of L-T4 on the cytoskeleton. Stimulation of 45Ca2+ uptake by L-T4 was also demonstrated. These findings demonstrate that L-T4 has important physiological roles modulating the cytoskeleton of neural cells during development.

Membrane receptors mediating thyroid hormone action

A recently identified thyroid hormone cell surface receptor on the extracellular domain of integrin alphaVbeta3 leads in human cell lines to activation of the mitogen-activated protein kinase (MAPK) signal transduction cascade. Examples of MAPK-dependent thyroid hormone actions are plasma membrane ion pump stimulation and specific nuclear events. These events include serine phosphorylation of the nuclear thyroid hormone receptor, leading to coactivator protein recruitment and complex tissue responses, such as thyroid hormone-induced angiogenesis or tumor cell growth. The existence of this cell surface receptor means that the activity of administered hormone could be limited through structural modification of the molecule to reproduce or inhibit only those hormone actions initiated at the cell surface. Examples of such modifications are provided.

Adenylyl cyclase-cAMP system inhibits thyroid hormone-stimulated osteocalcin synthesis in osteoblasts

It is generally recognized that thyroid hormone modulates osteoblast cell function. We have previously shown that triiodothyronine (T(3)) activates p38 mitogen-activated protein (MAP) kinase, resulting in the synthesis of osteocalcin in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effect of the adenylyl cyclase-cAMP system on thyroid hormone-stimulated osteocalcin synthesis in these cells. Dibutyryl-cAMP (DBcAMP) reduced the osteocalcin synthesis stimulated by T(3). Forskolin and cholera toxin suppressed the osteocalcin synthesis while dideoxyforskolin, a forskolin derivative that does not activate adenylyl cyclase, had little effect on the synthesis. KT5720, a selective inhibitor of protein kinase A, reversed the inhibitory effect of forskolin or DBcAMP. DBcAMP and forskolin markedly reduced the phosphorylation of p38 MAP stimulated by T(3). Pituitary adenylate cyclase-activating polypeptide (PACAP) significantly inhibited the T(3)-stimulated osteocalcin synthesis. These results strongly suggest that the adenylyl cyclase-cAMP system has an inhibitory role in thyroid hormone-stimulated osteocalcin synthesis via suppression of p38 MAP kinase activation in osteoblasts.

Activation of p38 mitogen-activated protein kinase mediates thyroid hormone-stimulated osteocalcin synthesis in osteoblasts

It is well known that thyroid hormone modulates osteoblast cell function. We have previously shown that triiodothyronine (T(3)) activates p44/p42 mitogen-activated protein (MAP) kinase, which limits T(3)-induced alkaline phosphatase activity in osteoblast-like MC3T3-E1 cells. In the present study, we investigated whether p44/p42 MAP kinase or p38 MAP kinase is involved in the thyroid hormone-stimulated osteocalcin synthesis in these cells. T(3) markedly induced the phosphorylation of p38 MAP kinase in addition to p44/p42 MAP kinase. PD98059 and U0126, inhibitors of the upstream kinase that activates p44/p42 MAP kinase, had little effect on the T(3)-induced synthesis of osteocalcin. On the contrary, the T(3)-induced osteocalcin synthesis was significantly reduced by SB203580 and PD169316, inhibitors of p38 MAP kinase. SB203580, PD169316 or PD98059 suppressed the T(3)-phosphorylation of myelin basic protein. T(3)-induced osteocalcin synthesis was significantly reduced by SB203580 or PD169316 also in primary cultured mouse osteoblasts. These results strongly suggest that p38 MAP kinase but not p44/p42 MAP kinase takes part in the thyroid hormone-stimulated osteocalcin synthesis in osteoblasts.

1alpha,25-dihydroxyvitamin D(3) and 24R,25-dihydroxyvitamin D(3) modulate growth plate chondrocyte physiology via protein kinase C-dependent phosphorylation of extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase

Membrane-mediated increases in protein kinase C (PKC) activity and PKC-dependent physiological responses of growth plate chondrocytes to vitamin D metabolites depend on the state of endochondral maturation; 1alpha,25-dihydroxyvitamin D(3) [1alpha,25-(OH)(2)D(3)] regulates growth zone (GC) cells, whereas 24R,25-(OH)(2)D(3) regulates resting zone (RC) cells. Different mechanisms, including protein kinase A signaling, mediate the effects of 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) on PKC, suggesting that different mechanisms may also regulate any MAPK involvement in the physiological responses. This study used confluent cultures of rat costochondral chondrocytes as a model. 1alpha,25-(OH)(2)D(3) stimulated MAPK specific activity in GC in a time- and dose-dependent manner, evident within 9 min. 24R,25-(OH)(2)D(3) stimulated MAPK in RC; increases were dose dependent, occurred after 9 min, and were greatest at 90 min. In both cells the effect was due to ERK1/2 activation (p42 > p44 in GC; p42 = p44 in RC). MAPK activation was dependent on PKC, but not protein kinase A. The effect of 1alpha,25-(OH)(2)D(3) required phospholipase C, and the effect of 24R,25-(OH)(2)D(3) required phospholipase D. Inhibition of cyclooxygenase activity reduced the effect of 1alpha,25-(OH)(2)D(3) on MAPK in GC and enhanced the effect of 24R,25-(OH)(2)D(3) in RC. Based on MAPK inhibition with PD98059, ERK1/2 MAPK mediated the effect of 24R,25-(OH)(2)D(3) on [(3)H]thymidine incorporation and [(35)S]sulfate incorporation by RC, but only partially mediated the effect of 1alpha,25-(OH)(2)D(3) on GC. ERK1/2 was not involved in the regulation of alkaline phosphatase specific activity by either metabolite. This paper supports the hypothesis that 1alpha,25-(OH)(2)D(3) regulates the physiology of GC via rapid membrane-mediated signaling pathways, and some, but not all, of the response to 1alpha,25-(OH)(2)D(3) is via the ERK family of MAPKs. In contrast, 24R,25-(OH)(2)D(3) exerts its effects on RC via PKC-dependent MAPK. Whereas 1alpha,25-(OH)(2)D(3) increases MAPK activity via phospholipase C and increased prostaglandin production, 24R,25-(OH)(2)D(3) increases MAPK via phospholipase D and decreased prostaglandin production. The cell specificity, metabolite stereospecificity, and the dependence on PKC argue for the participation of membrane receptors for 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) in the regulation of ERK1/2 in the growth plate.

Nongenomic actions of thyroid hormone on the heart

Extranuclear or nongenomic actions of thyroid hormone do not require formation of a nuclear complex between the hormone and its traditional 3,5,3'-triiodo-L-thyronine (T3) receptor (TR). Among nongenomic actions of iodothyronines that are relevant to the heart are those on membrane ion channels or pumps. These include stimulation of the sarcolemmal Na+ channel, inward-rectifying K+ channel, voltage-activated potassium channels, and calcium pump (Ca2+-adenosine triphosphatases [ATPases]) and have been shown in intact cells or isolated membranes. Because circulating levels of thyroid hormone are relatively stable, actions on channels or pumps may contribute to setting of basal activity of these transport functions. The mechanism of certain of these membrane effects may involve actions of the hormone on signal transducing protein kinases that modulate levels of activity of plasma membrane channels. Thyroid hormone nongenomically enhances myocardial contractility in isolated myocardial cells, in the isolated perfused rat heart and in human subjects. Iodothyronines also decrease vasomotor tone in a variety of models and in man by a mechanism independent of cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), or nitric oxide generation. Acutely increased myocardial mitochondrial respiration has been demonstrated in isolated organelles exposed to thyroid hormone. Genomic and nongenomic actions of thyroid hormone can interface, e.g., at the level of sarcoplasmic reticulum Ca2+-ATPase, where gene expression is regulated by the TR-T3 complex and activity of the enzyme can be modulated nongenomically. The relevance of nongenomic actions of thyroid hormone on the heart has been demonstrated in acute effects of the hormone on cardiac output and systemic vascular resistance in human subjects.