No abstract

Mechanical stimulation is crucial for bone growth/remodeling, and fluid shear stress promotes anabolic responses in osteoblasts through multiple second messengers, including nitric oxide (NO), but the underlying mechanisms are not well understood. Here we demonstrate that the NO/cGMP/PKG signaling pathway activates Src in mechanically-stimulated osteoblasts, initiating a proliferative response. PKG II is necessary for Src activation, which also requires Src docking to β3 integrins and Src dephosphorylation by a Shp-1/2 phosphatase complex. PKG II directly phosphorylates and stimulates Shp-1 activity, and fluid shear stress triggers PKG II, Src, and Shp recruitment to a mechanosome containing β3 integrins. PKG II-null mice show defective osteoblast Src/Erk signaling, and decreased Erk-dependent gene expression in bone. Our findings reveal crosstalk between NO/cGMP/PKG and integrin signaling and establish a new mechanism of Src activation. Since Src controls Erk, which is key to osteoblast growth and survival, these results support use of PKG-activating drugs as mechano-mimetics for treating osteoporosis.

Thyroid hormone (TH) is essential for vertebrate development and the homeostasis of most adult tissues, including bone. TH stimulates target gene expression through the nuclear thyroid receptors TRα and TRβ; however, TH also has rapid, transcription-independent (nongenomic) effects. We found a previously uncharacterized plasma membrane-bound receptor that was necessary and sufficient for nongenomic TH signaling in several cell types. We determined that this receptor is generated by translation initiation from an internal methionine of TRα, which produces a transcriptionally incompetent protein that is palmitoylated and associates with caveolin-containing plasma membrane domains. TH signaling through this receptor stimulated a pro-proliferative and pro-survival program by increasing the intracellular concentrations of calcium, nitric oxide (NO), and cyclic guanosine monophosphate (cGMP), which led to the sequential activation of protein kinase G II (PKGII), the tyrosine kinase Src, and extracellular signal-regulated kinase (ERK) and Akt signaling. Hypothyroid mice exhibited a cGMP-deficient state with impaired bone formation and increased apoptosis of osteocytes, which was rescued by a direct stimulator of guanylate cyclase. Our results link nongenomic TH signaling to a previously uncharacterized membrane-bound receptor, and identify NO synthase, guanylate cyclase, and PKGII as TH effectors that activate kinase cascades to regulate cell survival and proliferation.

No abstract

Continuous bone remodeling in response to mechanical loading is critical for skeletal integrity, and interstitial fluid flow is an important stimulus for osteoblast/osteocyte growth and differentiation. However, the biochemical signals mediating osteoblast anabolic responses to mechanical stimulation are incompletely understood. In primary human osteoblasts and murine MC3T3-E1 cells, we found that fluid shear stress induced rapid expression of c-fos, fra-1, fra-2, and fosB/⌬fosB mRNAs; these genes encode transcriptional regulators that maintain skeletal integrity. Fluid shear stress increased osteoblast nitric oxide (NO) synthesis, leading to activation of cGMP-dependent protein kinase (PKG). Pharmacological inhibition of the NO/cGMP/PKG signaling pathway blocked shear-induced expression of all four fos family genes. Induction of these genes required signaling through MEK/Erk, and Erk activation was NO/cGMP/PKG-dependent. Treating cells with a membranepermeable cGMP analog partly mimicked the effects of fluid shear stress on Erk activity and fos family gene expression. In cells transfected with small interfering RNAs (siRNA) specific for membrane-bound PKG II, shear-and cGMP-induced Erk activation and fos family gene expression was nearly abolished and could be restored by transducing cells with a virus encoding an siRNA-resistant form of PKG II; in contrast, siRNA-mediated repression of the more abundant cytosolic PKG I isoform was without effect. Thus, we report a novel function for PKG II in osteoblast mechanotransduction, and we propose a model whereby NO/cGMP/PKG II-mediated Erk activation and induction of c-fos, fra-1, fra-2, and fosB/⌬fosB play a key role in the osteoblast anabolic response to mechanical stimulation.Mechanical stress is a primary determinant of bone growth and remodeling; the strength of bone increases with weight bearing and muscular activity and decreases with unloading and disuse (1, 2). Weight bearing and locomotion stimulate interstitial fluid flow through the bone canalicular system, and the resultant shear stress is thought to be a major mechanism whereby mechanical forces stimulate bone growth (1-4). Fluid shear stress activates various signal transduction pathways and initiates an anabolic response in osteocytes and osteoblasts, leading to changes in gene expression and increased cell proliferation and differentiation (1, 5). As part of this response, a rapid and transient increase in intracellular calcium, nitric oxide (NO), 2 and prostaglandin E 2 occurs, and transcription of genes such as c-fos, cox-2, and igf-1/2 is induced (1, 2, 5).The transcription factor complex AP1, composed of Fos and Jun proteins, plays an essential role in bone development and post-natal skeletal homeostasis (6). De-regulated c-fos expression in mice interferes with normal bone development and induces osteosarcomas, whereas c-Fos-deficient mice develop osteopetrosis because of an early arrest in osteoclast differentiation (7, 8). Mice overexpressing Fra-1, Fra-2, or ⌬FosB (a splice variant of FosB) ex...