Fluid shear stress induction of COX-2 protein and prostaglandin release in cultured MC3T3-E1 osteoblasts does not require intact microfilaments or microtubules

Brain regulates weight bearing bone through PGE2 skeletal interoception: implication of ankle osteoarthritis and pain

Bone is a mechanosensitive tissue and undergoes constant remodeling to adapt to the mechanical loading environment. However, it is unclear whether the signals of bone cells in response to mechanical stress are processed and interpreted in the brain. In this study, we found that the hypothalamus of the brain regulates bone remodeling and structure by perceiving bone prostaglandin E2 (PGE2) concentration in response to mechanical loading. Bone PGE2 levels are in proportion to their weight bearing. When weight bearing changes in the tail-suspension mice, the PGE2 concentrations in bones change in line with their weight bearing changes. Deletion of cyclooxygenase-2 (COX2) in the osteoblast lineage cells or knockout of receptor 4 (EP4) in sensory nerve blunts bone formation in response to mechanical loading. Moreover, knockout of TrkA in sensory nerve also significantly reduces mechanical load-induced bone formation. Moreover, mechanical loading induces cAMP-response element binding protein (CREB) phosphorylation in the hypothalamic arcuate nucleus (ARC) to inhibit sympathetic tyrosine hydroxylase (TH) expression in the paraventricular nucleus (PVN) for osteogenesis. Finally, we show that elevated PGE2 is associated with ankle osteoarthritis (AOA) and pain. Together, our data demonstrate that in response to mechanical loading, skeletal interoception occurs in the form of hypothalamic processing of PGE2-driven peripheral signaling to maintain physiologic bone homeostasis, while chronically elevated PGE2 can be sensed as pain during AOA and implication of potential treatment.

The cytoskeleton and connected elements in bone cell mechano-transduction

Bone is a mechano-responsive tissue that adapts to changes in its mechanical environment. Increases in strain lead to increased bone mass acquisition, whereas decreases in strain lead to a loss of bone mass. Given that mechanical stress is a regulator of bone mass and quality, it is important to understand how bone cells sense and transduce these mechanical cues into biological changes to identify druggable targets that can be exploited to restore bone cell mechano-sensitivity or to mimic mechanical load. Many studies have identified individual cytoskeletal components - microtubules, actin, and intermediate filaments - as mechano-sensors in bone. However, given the high interconnectedness and interaction between individual cytoskeletal components, and that they can assemble into multiple discreet cellular structures, it is likely that the cytoskeleton as a whole, rather than one specific component, is necessary for proper bone cell mechano-transduction. This review will examine the role of each cytoskeletal element in bone cell mechano-transduction and will present a unified view of how these elements interact and work together to create a mechano-sensor that is necessary to control bone formation following mechanical stress.

Transgelin induces dysfunction of fetal endothelial colony-forming cells from gestational diabetic pregnancies

Fetal exposure to gestational diabetes mellitus (GDM) predisposes children to future health complications including hypertension and cardiovascular disease. A key mechanism by which these complications occur is through the functional impairment of vascular progenitor cells, including endothelial colony-forming cells (ECFCs). Previously, we showed that fetal ECFCs exposed to GDM have decreased vasculogenic potential and altered gene expression. In this study, we evaluate whether transgelin (TAGLN), which is increased in GDM-exposed ECFCs, contributes to vasculogenic dysfunction. TAGLN is an actin-binding protein involved in the regulation of cytoskeletal rearrangement. We hypothesized that increased TAGLN expression in GDM-exposed fetal ECFCs decreases network formation by impairing cytoskeletal rearrangement resulting in reduced cell migration. To determine if TAGLN is required and/or sufficient to impair ECFC network formation, TAGLN was reduced and overexpressed in ECFCs from GDM and uncomplicated pregnancies, respectively. Decreasing TAGLN expression in GDM-exposed ECFCs improved network formation and stability as well as increased migration. In contrast, overexpressing TAGLN in ECFCs from uncomplicated pregnancies decreased network formation, network stability, migration, and alignment to laminar flow. Overall, these data suggest that increased TAGLN likely contributes to the vasculogenic dysfunction observed in GDM-exposed ECFCs, as it impairs ECFC migration, cell alignment, and network formation. Identifying the molecular mechanisms underlying fetal ECFC dysfunction following GDM exposure is key to ascertain mechanistically the basis for cardiovascular disease predisposition later in life.

The collective influence of 1, 25-dihydroxyvitamin D3 with physiological fluid shear stress on osteoblasts

1, 25-dihydroxyvitamin D₃ (1, 25 (OH)₂ D₃) and mechanical stimuli in physiological environment contributes greatly to osteoporosis pathogenesis. Wide investigations have been conducted on how 1, 25-dihydroxyvitamin D₃ and mechanical stimuli separately impact osteoblasts. This study reports the collective influences of 1, 25-dihydroxyvitamin D₃ and flow shear stress (FSS) on biological functions of osteoblasts. 1, 25 (OH)₂ D₃ were prepared in various kinds of concentrations (0, 1, 10, 100 nmmol/L), while physiological fluid shear stress (12 dynes/cm²) was produced by using a parallel-plate fluid flow system. 1, 25 (OH)₂ D₃ affects the responses of ROBs to FSS, including the inhibition of NO release and cell proliferation as well as the promotion of PGE₂ release and cell differentiation. These findings provide a possible mechanism by which 1, 25(OH)₂ D₃ influences osteoblasts' responses to FSS, thus most probably providing guidance for the selection of 1, 25(OH)₂ D₃ concentration and mechanical loading in order to produce functional bone tissues in vitro.

Biological responses of osteocytic connexin 43 hemichannels to simulated microgravity

Connexin 43 (Cx43) hemichannels and gap junctions in osteocytes are responsive to mechanical loading, which is important for bone formation and remodeling. However, the mechanism of these Cx43-forming channels in the process of mechanical unloading is still not very clear. In this study, unloading caused by weightlessness was simulated by using a random position machine (RPM). Osteocytic MLO-Y4 cells were subjected to 2 h of RPM treatment, and levels of Cx43 mRNA and total and cell surface expressed protein were determined by quantitative real-time PCR, western blotting, and biotinylation analysis. Although mRNA was elevated by RPM, total protein level of Cx43 was not altered; however, surface biotinylated Cx43 was significantly reduced. Interestingly, RPM promoted the retention of Cx43 in the Golgi apparatus detected by co-immunofluorescence with antibodies against Cx43 and 58 K Golgi marker protein. Dye uptake assay showed that hemichannels were induced open after RPM for 2 h. Consistently, prostaglandin E₂ release was increased and this increase was completely attenuated with the treatment of a Cx43 hemichannel blocking antibody. Together, this study demonstrates increased activity of Cx43 hemichannels to RPM, and active Cx43 hemichannels with prostaglandin E₂ release are likely to module biological function under simulated weightless conditions. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1195-1202, 2017.

Lengthening primary cilia enhances cellular mechanosensitivity

The primary cilium is a mechanosensor in a variety of mammalian cell types, initiating and directing intracellular signalling cascades in response to external stimuli. When primary cilia formation is disrupted, cells have diminished mechanosensitivity and an abrogated response to mechanical stimulation. Due to this important role, we hypothesised that increasing primary cilia length would enhance the downstream response and therefore, mechanosensitivity. To test this hypothesis, we increased osteocyte primary cilia length with fenoldopam and lithium and found that cells with longer primary cilia were more mechanosensitive. Furthermore, fenoldopam treatment potentiated adenylyl cyclase activity and was able to recover primary cilia form and sensitivity in cells with impaired cilia. This work demonstrates that modulating the structure of the primary cilium directly impacts cellular mechanosensitivity. Our results implicate cilium length as a potential therapeutic target for combating numerous conditions characterised by impaired cilia function.

Biomechanical forces in the skeleton and their relevance to bone metastasis: biology and engineering considerations

Bone metastasis represents the leading cause of breast cancer related-deaths. However, the effect of skeleton-associated biomechanical signals on the initiation, progression, and therapy response of breast cancer bone metastasis is largely unknown. This review seeks to highlight possible functional connections between skeletal mechanical signals and breast cancer bone metastasis and their contribution to clinical outcome. It provides an introduction to the physical and biological signals underlying bone functional adaptation and discusses the modulatory roles of mechanical loading and breast cancer metastasis in this process. Following a definition of biophysical design criteria, in vitro and in vivo approaches from the fields of bone biomechanics and tissue engineering that may be suitable to investigate breast cancer bone metastasis as a function of varied mechano-signaling will be reviewed. Finally, an outlook of future opportunities and challenges associated with this newly emerging field will be provided.

P2Y2 receptors regulate osteoblast mechanosensitivity during fluid flow

Mechanical stimulation of osteoblasts activates many cellular mechanisms including the release of ATP. Binding of ATP to purinergic receptors is key to load-induced osteogenesis. Osteoblasts also respond to fluid shear stress (FSS) with increased actin stress fiber formation (ASFF) that we postulate is in response to activation of the P2Y2 receptor (P2Y2R). Furthermore, we predict that ASFF increases cell stiffness and reduces the sensitivity to further mechanical stimulation. We found that small interfering RNA (siRNA) suppression of P2Y2R attenuated ASFF in response to FSS and ATP treatment. In addition, RhoA GTPase was activated within 15 min after the onset of FSS or ATP treatment and mediated ASFF following P2Y2R activation via the Rho kinase (ROCK)1/LIM kinase 2/cofilin pathway. We also observed that ASFF in response to FSS or ATP treatment increased the cell stiffness and was prevented by knocking down P2Y2R. Finally, we confirmed that the enhanced cell stiffness and ASFF in response to RhoA GTPase activation during FSS drastically reduced the mechanosensitivity of the osteoblasts based on the intracellular Ca(2+) concentration ([Ca(2+)]i) response to consecutive bouts of FSS. These data suggest that osteoblasts can regulate their mechanosensitivity to continued load through P2Y2R activation of the RhoA GTPase signaling cascade, leading to ASFF and increased cell stiffness.

The optimal combination of substrate chemistry with physiological fluid shear stress

Osteoblasts on implanted biomaterials sense both substrate chemistry and mechanical stimulus. The effects of substrate chemistry alone and mechanical stimulus alone on osteoblasts have been widely studied. This study investigates the optimal combination of substrate chemistry and 12dyn/cm(2) physiological flow shear stress (FSS) by examining their influences on primary rat osteoblasts (ROBs), including the releases of ATP, nitric oxide (NO), and prostaglandin E2 (PGE2). Self-assembled monolayers (SAMs) on glass slides with -OH, -CH3, and -NH2 were employed to provide various substrate chemistries, whereas a parallel-plate fluid flow system produced the physiological FSS. Substrate chemistry alone exerted no observable effects on the releases of ATP, NO, and PGE2. Nevertheless, when ROBs were exposed to both substrate chemistry and FSS, the ATP releases of NH2 were upregulated about 12-fold compared to substrate chemistry alone, while the ATP releases of CH3 and OH was similarly increased 7-fold at the peak. Similar trends were observed for the releases of NO and PGE2. The expressions of ATP, NO, and PGE2 followed the pattern of NH2-FSS>Glass-FSS>CH3-FSS≈OH-FSS. ROBs on NH2 produced the optimal combination of substrate chemistry with the physiological FSS. The F-actin organization and focal adhesion (FA) formation of ROBs on various SAMs without FSS were examined. NH2 produced the best results whereas CH3 and OH produced the worst ones. Inhibition of FAs and/or disruption of F-actin significantly decreased the releases of FSS-induced PGE2, NO, and/or ATP. Consequently, a mechanism was proposed that the best F-actin organization and FA formation of ROBs on NH2 lead to the optimal combination of substrate chemistry with the 12dyn/cm(2) physiological FSS. This mechanism gives guidance for the design of implanted biomaterials and bioreactors for bone tissue engineering.

Calcium response in osteocytic networks under steady and oscillatory fluid flow

The fluid flow in the lacunar-canalicular system of bone is an essential mechanical stimulation on the osteocyte networks. Due to the complexity of human physical activities, the fluid shear stress on osteocyte bodies and processes consists of both steady and oscillatory components. In this study, we investigated and compared the intracellular calcium ([Ca(2+)](i)) responses of osteocytic networks under steady and oscillatory fluid flows. An in vitro osteocytic network was built with MLO-Y4 osteocyte-like cells using micro-patterning techniques to simulate the in vivo orderly organization of osteocyte networks. Sinusoidal oscillating fluid flow or unidirectional steady flow was applied on the cell surface with 2Pa peak shear stress. It was found that the osteocytic networks were significantly more responsive to steady flow than to oscillatory flow. The osteocytes can release more calcium peaks with higher magnitudes at a faster speed under steady flow stimulation. The [Ca(2+)](i) signaling transients under the steady and oscillatory flows have significantly different spatiotemporal characters, but a similar responsive percentage of cells. Further signaling pathway studies using inhibitors showed that endoplasmic reticulum (ER) calcium store, extracellular calcium source, ATP, PGE(2) and NO related pathways play similar roles in the [Ca(2+)](i) signaling of osteocytes under either steady or oscillating flow. The spatiotemporal characteristics of [Ca(2+)](i) transients under oscillating fluid flow are affected more profoundly by pharmacological treatments than under the steady flow. Our findings support the hypothesis that the [Ca(2+)](i) responses of osteocytic networks are significantly dependent on the profiles of fluid flow.

Osteocytic network is more responsive in calcium signaling than osteoblastic network under fluid flow

Osteocytes, regarded as the mechanical sensor in bone, respond to mechanical stimulation by activating biochemical pathways and mediating the cellular activities of other bone cells. Little is known about how osteocytic networks respond to physiological mechanical stimuli. In this study, we compared the mechanical sensitivity of osteocytic and osteoblastic networks under physiological-related fluid shear stress (0.5 to 4 Pa). The intracellular calcium ([Ca(2+)](i)) responses in micropatterned in vitro osteoblastic or osteocytic networks were recorded and analyzed. Osteocytes in the network showed highly repetitive spikelike [Ca(2+)](i) peaks under fluid flow stimulation, which are dramatically different from those in the osteoblastic network. The number of responsive osteocytes in the network remained at a constant high percentage (>95%) regardless of the magnitude of shear stress, whereas the number of responsive osteoblasts in the network significantly depends on the strength of fluid flow. All spatiotemporal parameters of calcium signaling demonstrated that osteocytic networks are more sensitive and dynamic than osteoblastic networks, especially under low-level mechanical stimulations. Furthermore, pathway studies were performed to identify the molecular mechanisms responsible for the differences in [Ca(2+)](i) signaling between osteoblastic and osteocytic networks. The results suggested that the T-type voltage-gated calcium channels (VGCC) expressed on osteocytes may play an essential role in the unique kinetics of [Ca(2+)](i) signaling in osteocytic networks, whereas the L-type VGCC is critical for both types of cells to release multiple [Ca(2+)](i) peaks. The extracellular calcium source and intracellular calcium store in ER-, ATP-, PGE₂-, NO-, and caffeine-related pathways are found to play similar roles in the [Ca(2+)](i) signaling for both osteoblasts and osteocytes. The findings in this study proved that osteocytic networks possess unique characteristics in sensing and processing mechanical signals.

Effects of microgravity on osteoclast bone resorption and osteoblast cytoskeletal organization and adhesion

Exposure to microgravity has been associated with several physiological changes in astronauts, including an osteoporosis-like loss in bone mass. Despite many in vivo and in vitro studies in both microgravity and simulated microgravity conditions, the mechanism for bone loss is still not clear. The lack of weight-bearing forces makes microgravity an ideal physical stimulus to assess bone cell responses. In this work, we conduct a unique investigation of the effects of microgravity on bone-producing osteoblasts and, in parallel, on bone-resorbing osteoclasts. An increase in total number of discrete resorption pits is observed in osteoclasts that experienced microgravity versus ground controls. We further show that osteoblasts exposed to 5 days of microgravity have shorter and wavier microtubules (MTs), smaller and fewer focal adhesions, and thinner cortical actin and stress fibers. Space-flown osteoblasts present extended cell shapes as well as significantly more disrupted and often fragmented or condensed nuclei. The absence of gravitational forces therefore causes both an increase in bone resorption by osteoclasts, and a decrease in osteoblast cellular integrity. The observed effects on both major bone cell types likely accelerate bone loss in microgravity environments, and additionally offer a potential explanation to the development of disuse osteoporosis on Earth.

Non-overlapping functions for Pyk2 and FAK in osteoblasts during fluid shear stress-induced mechanotransduction

Mechanotransduction, the process by which cells convert external mechanical stimuli such as fluid shear stress (FSS) into biochemical changes, plays a critical role in maintenance of the skeleton. We have proposed that mechanical stimulation by FSS across the surfaces of bone cells results in formation of unique signaling complexes called mechanosomes that are launched from sites of adhesion with the extracellular matrix and with other bone cells [1]. Deformation of adhesion complexes at the cell membrane ultimately results in alteration of target gene expression. Recently, we reported that focal adhesion kinase (FAK) functions as a part of a mechanosome complex that is required for FSS-induced mechanotransduction in bone cells. This study extends this work to examine the role of a second member of the FAK family of non-receptor protein tyrosine kinases, proline-rich tyrosine kinase 2 (Pyk2), and determine its role during osteoblast mechanotransduction. We use osteoblasts harvested from mice as our model system in this study and compared the contributions of Pyk2 and FAK during FSS induced mechanotransduction in osteoblasts. We exposed Pyk2^+/+ and Pyk2^−/− primary calvarial osteoblasts to short period of oscillatory fluid flow and analyzed downstream activation of ERK1/2, and expression of c-fos, cyclooxygenase-2 and osteopontin. Unlike FAK, Pyk2 was not required for fluid flow-induced mechanotransduction as there was no significant difference in the response of Pyk2^+/+ and Pyk2^−/− osteoblasts to short periods of fluid flow (FF). In contrast, and as predicted, FAK^−/− osteoblasts were unable to respond to FF. These data indicate that FAK and Pyk2 have distinct, non-redundant functions in launching mechanical signals during osteoblast mechanotransduction. Additionally, we compared two methods of generating FF in both cell types, oscillatory pump method and another orbital platform method. We determined that both methods of generating FF induced similar responses in both primary calvarial osteoblasts and immortalized calvarial osteoblasts.

Modulus-driven differentiation of marrow stromal cells in 3D scaffolds that is independent of myosin-based cytoskeletal tension

Proliferation and differentiation of cells are known to be influenced by the physical properties of the extracellular environment. Previous studies examining biophysics underlying cell response to matrix stiffness utilized a two-dimensional (2D) culture format, which is not representative of the three-dimensional (3D) tissue environment in vivo. We report on the effect of 3D matrix modulus on human bone marrow stromal cell (hBMSC) differentiation. hBMSCs underwent osteogenic differentiation in poly(ethylene glycol) hydrogels of all modulus (300-fold modulus range, from 0.2 kPa to 59 kPa) in the absence of osteogenic differentiation supplements. This osteogenic differentiation was modulus-dependent and was enhanced in stiffer gels. Osteogenesis in these matrices required integrin-protein ligation since osteogenesis was inhibited by soluble Arginine-Glycine-Aspartate-Serine peptide, which blocks integrin receptors. Immunostained images revealed lack of well-defined actin filaments and microtubules in the encapsulated cells. Disruption of mechanosensing elements downstream of integrin binding that have been identified from 2D culture such as actin filaments, myosin II contraction, and RhoA kinase did not abrogate hBMSC material-driven osteogenic differentiation in 3D. These data show that increased hydrogel modulus enhanced osteogenic differentiation of hBMSCs in 3D scaffolds but that hBMSCs did not use the same mechanosensing pathways that have been identified in 2D culture.

The Load-Bearing Mechanosome Revisited

We introduced the mechanosome hypothesis in 2003 as a heuristic model for investigating mechanotransduction in bone (Pavalko et al., J Cell Biochem, 2003, 88(1):104-112). This model suggested specific approaches for investigating how mechanical information is conveyed from the membrane of the sensor bone cell to the target genes and how this transmitted information from the membrane is converted into changes in transcription. The key concepts underlying the mechanosome hypothesis are that load-induced deformation of bone deforms the sensor cell membrane; embedded in the membrane are the focal adhesion and cadherin-catenin complexes, which in turn are physically connected to the chromatin via a solid-state scaffold. The physical stimulation of the membrane launches multiprotein complexes (mechanosomes) from the adhesion platforms while concomitantly tugging target genes into position for contact with the incoming mechanosomes, the carriers of the mechanical information to the nucleus. The mechanosome is comprised of an adhesion-associated protein and a nucleocytoplasmic shuttling transcription factor. Upon arrival at the target gene, mechanosomes alter DNA conformation and thus influence the interactions between trans-acting proteins along the gene, changing gene activity. Here, we update significant progress related to the mechanosome concept since publication of our original hypothesis. The launching of adhesion- and cytoskeletal-associated proteins into the nucleus toward target genes appears to be a common mechanism for regulating cell response to changes in its mechanical microenvironment.

Stepwise increasing and decreasing fluid shear stresses differentially regulate the functions of osteoblasts

It is well accepted that osteoblasts respond to fluid shear stress (FSS) depending on the loading magnitude, rate, and temporal profiles. Although in vivo observations demonstrated that bone mineral density changes as the training intensity gradually increases/decreases, whether osteoblasts perceive such slow temporal changes in the strength of stimulation remains unclear. In this study, we hypothesized that osteoblasts can detect and respond differentially to the temporal gradients of FSS. In specific, we hypothesized that when the temporal FSS gradient is high enough, i) the increasing FSS inhibits the osteoblastic potential in supporting osteoclastogenesis and enhances the osteoblastic anabolic responses; ii) on the other hand, the deceasing FSS would have opposite effects on osteoclastogenesis and anabolic responses. To test the hypotheses, stepwise varying FSS was applied on primary osteoblasts and osteogenic and resorption markers were analyzed. The cells were subjected to FSS increasing from 5, 10, to 15 or decreasing from 15, 10, to 5 dyn/cm(2) at a step of 5 dyn/cm(2) for either 6 or 12 hours. In a subset experiment, the cells were stimulated with stepwise increasing or decreasing FSS at a higher step (10 dyn/cm(2)) for 12 hours. Our results showed that, with the step of 5 dyn/cm(2), the stepwise increasing FSS inhibited the osteoclastogenesis with a 3- to 4-fold decrease in RANKL/OPG gene expression versus static controls, while the stepwise decreasing FSS increased RANKL/OPG ratio by 2- to 2.5-fold versus static controls. Both increasing and decreasing FSS enhanced alkaline phosphatase expression and calcium deposition by 1.0- to 1.8 fold versus static controls. For a higher FSS temporal gradient (three steps of 10 dyn/cm(2) over 12 hour stimulation), the increasing FSS enhanced the expression of alkaline phosphatase expression and calcium deposition by 1.3 fold, while the decreasing FSS slightly inhibited them by -10% compared with static controls. Taken together, our results suggested that osteoblasts can detect the slow temporal gradients of FSS and respond differentially in a dose-dependent manner, which may account for the observed bone mineral density changes in response to the gradual increasing/decreasing exercise in vivo. The stepwise FSS can be a useful model to study bone cell responses to long-term mechanical usage or disuse. These studies will complement the short-term studies and provide additional clinically relevant insights on bone adaptation.

Effect of low-magnitude, high-frequency vibration on osteocytes in the regulation of osteoclasts

Osteocytes are well evidenced to be the major mechanosensor in bone, responsible for sending signals to the effector cells (osteoblasts and osteoclasts) that carry out bone formation and resorption. Consistent with this hypothesis, it has been shown that osteocytes release various soluble factors (e.g. transforming growth factor-beta, nitric oxide, and prostaglandins) that influence osteoblastic and osteoclastic activities when subjected to a variety of mechanical stimuli, including fluid flow, hydrostatic pressure, and mechanical stretching. Recently, low-magnitude, high-frequency (LMHF) vibration (e.g., acceleration less than <1 x g, where g=9.81m/s(2), at 20-90 Hz) has gained much interest as studies have shown that such mechanical stimulation can positively influence skeletal homeostasis in animals and humans. Although the anabolic and anti-resorptive potential of LMHF vibration is becoming apparent, the signaling pathways that mediate bone adaptation to LMHF vibration are unknown. We hypothesize that osteocytes are the mechanosensor responsible for detecting the vibration stimulation and producing soluble factors that modulate the activity of effector cells. Hence, we applied low-magnitude (0.3 x g) vibrations to osteocyte-like MLO-Y4 cells at various frequencies (30, 60, 90 Hz) for 1h. We found that osteocytes were sensitive to this vibration stimulus at the transcriptional level: COX-2 maximally increased by 344% at 90Hz, while RANKL decreased most significantly (-55%, p<0.01) at 60Hz. Conditioned medium collected from the vibrated MLO-Y4 cells attenuated the formation of large osteoclasts (> or =10 nuclei) by 36% (p<0.05) and the amount of osteoclastic resorption by 20% (p=0.07). The amount of soluble RANKL (sRANKL) in the conditioned medium was found to be 53% lower in the vibrated group (p<0.01), while PGE(2) release was also significantly decreased (-61%, p<0.01). We conclude that osteocytes are able to sense LMHF vibration and respond by producing soluble factors that inhibit osteoclast formation.

Nmp4/CIZ inhibits mechanically induced beta-catenin signaling activity in osteoblasts

Cellular mechanotransduction, the process of converting mechanical signals into biochemical responses within cells, is a critical aspect of bone health. While the effects of mechanical loading on bone are well recognized, elucidating the specific molecular pathways involved in the processing of mechanical signals by bone cells represents a challenge and an opportunity to identify therapeutic strategies to combat bone loss. In this study we have for the first time examined the relationship between the nucleocytoplasmic shuttling transcription factor nuclear matrix protein-4/cas interacting zinc finger protein (Nmp4/CIZ) and beta-catenin signaling in response to a physiologic mechanical stimulation (oscillatory fluid shear stress, OFSS) in osteoblasts. Using calvaria-derived osteoblasts from Nmp4-deficient and wild-type mice, we found that the normal translocation of beta-catenin to the nucleus in osteoblasts that is induced by OFSS is enhanced when Nmp4/CIZ is absent. Furthermore, we found that other aspects of OFSS-induced mechanotransduction generally associated with the beta-catenin signaling pathway, including ERK, Akt, and GSK3beta activity, as well as expression of the beta-catenin-responsive protein cyclin D1 are also enhanced in cells lacking Nmp4/CIZ. Finally, we found that in the absence of Nmp4/CIZ, OFSS-induced cytoskeletal reorganization and the formation of focal adhesions between osteoblasts and the extracellular substrate is qualitatively enhanced, suggesting that Nmp4/CIZ may reduce the sensitivity of bone cells to mechanical stimuli. Together these results provide experimental support for the concept that Nmp4/CIZ plays an inhibitory role in the response of bone cells to mechanical stimulation induced by OFSS.

A Trabecular Bone Explant Model of Osteocyte-Osteoblast Co-Culture for Bone Mechanobiology

The osteocyte network is recognized as the major mechanical sensor in the bone remodeling process, and osteocyte-osteoblast communication acts as an important mediator in the coordination of bone formation and turnover. In this study, we developed a novel 3D trabecular bone explant co-culture model that allows live osteocytes situated in their native extracellular matrix environment to be interconnected with seeded osteoblasts on the bone surface. Using a low-level medium perfusion system, the viability of in situ osteocytes in bone explants was maintained for up to 4 weeks, and functional gap junction intercellular communication (GJIC) was successfully established between osteocytes and seeded primary osteoblasts. Using this novel co-culture model, the effects of dynamic deformational loading, GJIC, and prostaglandin E(2) (PGE(2)) release on functional bone adaptation were further investigated. The results showed that dynamical deformational loading can significantly increase the PGE(2) release by bone cells, bone formation, and the apparent elastic modulus of bone explants. However, the inhibition of gap junctions or the PGE(2) pathway dramatically attenuated the effects of mechanical loading. This 3D trabecular bone explant co-culture model has great potential to fill in the critical gap in knowledge regarding the role of osteocytes as a mechano-sensor and how osteocytes transmit signals to regulate osteoblasts function and skeletal integrity as reflected in its mechanical properties.

Focal adhesion kinase is important for fluid shear stress-induced mechanotransduction in osteoblasts

Mechanical loading of bone is important for maintenance of bone mass and structural stability of the skeleton. When bone is mechanically loaded, movement of fluid within the spaces surrounding bone cells generates fluid shear stress (FSS) that stimulates osteoblasts, resulting in enhanced anabolic activity. The mechanisms by which osteoblasts convert the external stimulation of FSS into biochemical changes, a process known as mechanotransduction, remain poorly understood. Focal adhesions are prime candidates for transducing external stimuli. Focal adhesion kinase (FAK), a nonreceptor tyrosine kinase found in focal adhesions, may play a key role in mechanotransduction, although its function has not been directly examined in osteoblasts. We examined the role of FAK in osteoblast mechanotransduction using short interfering RNA (siRNA), overexpression of a dominant negative FAK, and FAK(-/-) osteoblasts to disrupt FAK function in calvarial osteoblasts. Osteoblasts were subjected to varying periods oscillatory fluid flow (OFF) from 5 min to 4 h, and several physiologically important readouts of mechanotransduction were analyzed including: extracellular signal-related kinase 1/2 phosphorylation, upregulation of c-fos, cyclooxygenase-2, and osteopontin, and release of prostaglandin E(2). Osteoblasts with disrupted FAK signaling exhibited severely impaired mechanical responses in all endpoints examined. These data indicate the importance of FAK for both short and long periods of FSS-induced mechanotransduction in osteoblasts.

Cyclic Hydraulic Pressure and Fluid Flow Differentially Modulate Cytoskeleton Re-Organization in MC3T3 Osteoblasts

Mechanical loads are essential towards maintaining bone mass and skeletal integrity. Such loads generate various stimuli at the cellular level, including cyclic hydraulic pressure (CHP) and fluid shear stress (FSS). To gain insight into the anabolic responses of osteoblasts to CHP and FSS, we subjected MC3T3-E1 preosteoblasts to either FSS (12 dynes/cm(2)) or CHP varying from 0 to 68 kPa at 0.5 Hz. As with FSS, CHP produced a significant increase in ATP release over static controls within 5 min of onset. Cell stiffness examined by atomic force microscopy increased after 15 min of either CHP or FSS stimulation, which was attenuated when extracellular ATP was hydrolyzed with apyrase. As previously shown FSS induced polymerization of actins into stress fibers. However, the microtubule network was completely disrupted under FSS. In contrast, CHP appeared to maintain strong microtubule and f-actin networks. The purinergic signaling was found to be involved in the remodeling of f-actin, but not microtubule. Both CHP and FSS applied for 1 hour increased expression of COX-2. These data indicate that, while CHP and FSS produce similar anabolic responses, these stimuli have very different effects on the cytoskeleton remodeling and could contribute to loss of mechanosensitivity with extended loading.

Prostaglandin receptors mediate effects of substances released from ischaemic rat hearts on non-ischaemic cardiomyocytes

BACKGROUND

After ischaemia and during reperfusion, rat hearts release cardiodepressive substances that are putatively cyclooxygenase-2-dependent. The present study analyses the mechanisms by which these substances mediate their effect downstream of cyclooxygenase-2.

MATERIALS AND METHODS

After 10 min of global stop-flow ischaemia, isolated rat hearts were reperfused and post-ischaemic coronary effluent was collected over a period of 30 s. Non-ischaemic effluent collected before ischaemia was used as a control. We investigated the effect of the effluents on cell shortening and Ca(++)-metabolism, by application of fluorescence microscopy of field-stimulated adult rat cardiomyocytes incubated with fura-2. Cells were pre-incubated with inhibitors of protein kinase A and C and with antagonists of protein kinase A-dependent prostaglandin receptors. We examined the expression of prostaglandin receptors in cardiomyocytes by Western blotting.

RESULTS

In contrast to non-ischaemic effluent, post-ischaemic effluent induced reduction of Ca(++) transient and cell shortening in the cardiomyocytes. In contrast to protein kinase C inhibitor Myr-PKC [19-27], the protein kinase A inhibitor Rp-cAMPS completely blocked the effect of post-ischaemic effluent. Furthermore, we determined a cyclic adenosine monophosphate increase in cardiomyocytes that were pre-incubated with post-ischaemic effluent. The antagonist of prostaglandin E-receptor EP2 AH6809 and the antagonist of receptor subtype EP4 AH23848 attenuated the effect of post-ischaemic effluent in contrast to other antagonists of prostaglandin D and I receptors, which did not influence the effect. In lysates of adherend cardiomyocytes, expression of prostaglandin D, E and I receptors was detected by Western blotting.

CONCLUSIONS

The effect of post-ischaemic effluent is mediated by the protein kinase A-dependent prostaglandin-receptor subtypes EP2 and EP4 downstream of cyclooxygenase-2.

Mechanical loading by fluid shear is sufficient to alter the cytoskeletal composition of osteoblastic cells

Many structural modifications have been observed as a part of the cellular response to mechanical loading in a variety of cell types. Although changes in morphology and cytoskeletal rearrangement have been widely reported, few studies have investigated the change in cytoskeletal composition. Measuring how the amounts of specific structural proteins in the cytoskeleton change in response to mechanical loading will help to elucidate cellular mechanisms of functional adaptation to the applied forces. Therefore, the overall hypothesis of this study was that osteoblasts would respond to fluid shear stress by altering the amount of specific cross-linking proteins in the composition of the cytoskeleton. Mouse osteoblats cell line MC3T3-E1 and human fetal osteoblasts (hFOB) were exposed to 2 Pa of steady fluid shear for 2 h in a parallel plate flow chamber, and then the amount of actin, vimentin, α-actinin, filamin, and talin in the cytoskeleton was measured using Western blot analyses. After mechanical loading, there was no change in the amount of actin monomers in the cytoskeleton, but the cross-linking proteins α-actinin and filamin that cofractionated with the cytoskeleton increased by 29% ( P < 0.01) and 18% ( P < 0.02), respectively. Localization of the cross-linking proteins by fluorescent microscopy revealed that they were more widely distributed throughout the cell after exposure to fluid shear. The amount of vimentin in the cytoskeleton also increased by 15% ( P < 0.01). These results indicate that osteoblasts responded to mechanical loading by altering the cytoskeletal composition, which included an increase in specific proteins that would likely enhance the mechanical resistance of the cytoskeleton.

Fluid Flow Induced Calcium Response in Bone Cell Network

In our previous work, bone cell networks with controlled spacing and functional intercellular gap junctions had been successfully established by using microcontact printing and self assembled monolayers technologies [Guo, X. E., E. Takai, X. Jiang, Q. Xu, G. M. Whitesides, J. T. Yardley, C. T. Hung, E. M. Chow, T. Hantschel, and K. D. Costa. Mol. Cell. Biomech. 3:95-107, 2006]. The present study investigated the calcium response and the underlying signaling pathways in patterned bone cell networks exposed to a steady fluid flow. The glass slides with cell networks were separated into eight groups for treatment with specific pharmacological agents that inhibit pathways significant in bone cell calcium signaling. The calcium transients of the network were recorded and quantitatively evaluated with a set of network parameters. The results showed that 18α-GA (gap junction blocker), suramin (ATP inhibitor), and thapsigargin (depleting intracellular calcium stores) significantly reduced the occurrence of multiple calcium peaks, which were visually obvious in the untreated group. The number of responsive peaks also decreased slightly yet significantly when either the COX-2/PGE(2) or the NOS/nitric oxide pathway was disrupted. Different from all other groups, cells treated with 18α-GA maintained a high concentration of intracellular calcium following the first peak. In the absence of calcium in the culture medium, the intracellular calcium concentration decreased slowly with fluid flow without any calcium transients observed. These findings have identified important factors in the flow mediated calcium signaling of bone cells within a patterned network.

COX-2-dependent and potentially cardioprotective effects of negative inotropic substances released after ischemia

During reperfusion, cardiodepressive factors are released from isolated rat hearts after ischemia. The present study analyzes the mechanisms by which these substances mediate their cardiodepressive effect. After 10 min of global stop-flow ischemia, rat hearts were reperfused and coronary effluent was collected over a period of 30 s. We tested the effect of this postischemic effluent on systolic cell shortening and Ca(2+) metabolism by application of fluorescence microscopy of field-stimulated rat cardiomyocytes stained with fura-2 AM. Cells were preincubated with various inhibitors, e.g., the cyclooxygenase (COX) inhibitor indomethacin, the COX-2 inhibitors NS-398 and lumiracoxib, the COX-1 inhibitor SC-560, and the potassium (ATP) channel blocker glibenclamide. Lysates of cardiomyocytes and extracts from whole rat hearts were tested for expression of COX-2 with Western blot analysis. As a result, in contrast to nonischemic effluent (control), postischemic effluent induced a reduction of Ca(2+) transient and systolic cell shortening in the rat cardiomyocytes (P < 0.001 vs. control). After preincubation of cells with indomethacin, NS-398, and lumiracoxib, the negative inotropic effect was attenuated. SC-560 did not influence the effect of postischemic effluent. The inducibly expressed COX-2 was detected in cardiomyocytes prepared for fluorescence microscopy. The effect of postischemic effluent was eliminated with applications of glibenclamide. Furthermore, postischemic effluent significantly reduced the intracellular diastolic and systolic Ca(2+) increase (P < 0.01 vs. control). In conclusion, the cardiodepressive effect of postischemic effluent is COX-2 dependent and protective against Ca(2+) overload in the cells.

Osteoblasts and osteocytes respond differently to oscillatory and unidirectional fluid flow profiles

Bone cells subjected to mechanical loading by fluid shear stress undergo significant architectural and biochemical changes. The models of shear stress used to analyze the effects of loading bone cells in vitro include both oscillatory and unidirectional fluid shear profiles. Although the fluid flow profile experienced by cells within bone is most likely oscillatory in nature, to date there have been few direct comparisons of how bone cells respond to these two fluid flow profiles. In this study we evaluated morphologic and biochemical responses to a time course of unidirectional and oscillatory fluid flow in two commonly used bone cell lines, MC3T3-E1 osteoblasts and MLO-Y4 osteocytes. We determined that stress fibers formed and aligned within osteoblasts after 1 h of unidirectional fluid flow, but this response was not observed until greater than 5 h of oscillatory fluid flow. Despite the delay in stress fiber formation, oscillatory and unidirectional fluid flow profiles elicited similar temporal effects on the induction of both cyclooxygenase-2 (Cox-2) and osteopontin protein expression in osteoblasts. Interestingly, MLO-Y4 osteocytes formed organized stress fibers after exposure to 24 h of unidirectional shear stress, while the number of dendritic processes per cell increased along with Cox-2 protein levels after 24 h of oscillatory shear stress. Despite these differences, both flow profiles significantly altered osteopontin levels in MLO-Y4 osteocytes. Together these results demonstrate that the profile of fluid shear can induce significantly different responses from osteoblasts and osteocytes.

The role of endothelial glycocalyx components in mechanotransduction of fluid shear stress

The surface of endothelial cells is decorated with a wide variety of membrane-bound macromolecules that constitute the glycocalyx. These include glycoproteins bearing acidic oligosaccharides with terminal sialic acids (SA), and proteoglycans with their associated glycosaminoglycan that include: heparan sulfate (HS), chondroitin sulfate (CS), and hyaluronic acid (HA). In this study, enzymes were used to selectively degrade glycocalyx components from the surface of bovine aortic endothelial cells and the effects of these alterations on fluid shear-induced nitric oxide (NO) and prostacyclin (PGI(2)) production were determined. Depletion of HS, HA, and SA, but not CS, blocked shear-induced NO production. Surprisingly, the same enzyme depletions that blocked NO production had no influence on shear-induced PGI(2) production. The results may be interpreted in terms of a glypican-caveolae-eNOS mechanism for shear-induced NO transduction, with PGI(2) being transduced in basal adhesion plaques that sense the same reaction stress whether the glycocalyx is intact or not.

The role of actin cytoskeleton in oscillatory fluid flow-induced signaling in MC3T3-E1 osteoblasts

Fluid flow due to loading in bone is a potent mechanical signal that may play an important role in bone adaptation to its mechanical environment. Previous in vitro studies of osteoblastic cells revealed that the upregulation of cyclooxygenase-2 (COX-2) and c-fos induced by steady fluid flow depends on a change in actin polymerization dynamics and the formation of actin stress fibers. Exposing cells to dynamic oscillatory fluid flow, the temporal flow pattern that results from normal physical activity, is also known to result in increased COX-2 expression and PGE(2) release. The purpose of this study was to determine whether dynamic fluid flow results in changes in actin dynamics similar to steady flow and to determine whether alterations in actin dynamics are required for PGE(2) release. We found that exposure to oscillatory fluid flow did not result in the development of F-actin stress fibers in MC3T3-E1 osteoblastic cells and that inhibition of actin polymerization with cytochalasin D did not inhibit intracellular calcium mobilization or PGE(2) release. In fact, PGE(2) release was increased threefold in the polymerization inhibited cells and this PGE(2) release was dependent on calcium release from the endoplasmic reticulum. This was in contrast to the PGE(2) release that occurs in normal cells, which is independent of calcium flux from endoplasmic reticulum stores. We suggest that this increased PGE(2) release involves a different molecular mechanism perhaps involving increased deformation due to the compromised cytoskeleton.

PTH-induced actin depolymerization increases mechanosensitive channel activity to enhance mechanically stimulated Ca2+ signaling in osteoblasts

Disruption of the actin cytoskeleton with cytochalasin D enhanced the mechanically induced increase in intracellular Ca(2+) ([Ca(2+)](i)) in osteoblasts in a manner similar to that of PTH. Stabilization of actin with phalloidin prevented the PTH enhanced [Ca(2+)](i) response to shear. Patch-clamp analyses show that the MSCC is directly influenced by alterations in actin integrity.

INTRODUCTION

PTH significantly enhances the fluid shear-induced increase in [Ca(2+)](i) in osteoblasts, in part, through increased activation of both the mechanosensitive, cation-selective channel (MSCC) and L-type voltage-sensitive Ca(2+) channel (L-VSCC). Both stimuli have been shown to produce dynamic changes in the organization of the actin cytoskeleton. In this study, we examined the effects of alterations in actin polymerization on [Ca(2+)](i) and MSCC activity in MC3T3-E1 and UMR-106.01 osteoblasts in response to shear +/- PTH pretreatment.

MATERIALS AND METHODS

MC3T3-E1 or UMR-106.01 cells were plated onto type I collagen-coated quartz slides, allowed to proliferate to 60% confluency, and mounted on a modified parallel plate chamber and subjected to 12 dynes/cm(2). For patch-clamp studies, cells were plated on collagen-coated glass coverslips, mounted on the patch chamber, and subjected to pipette suction. Modulators of actin cytoskeleton polymerization were added 30 minutes before the experiments, whereas channel inhibitors were added 10 minutes before mechanical stimulation. All drugs were maintained in the flow medium for the duration of the experiment.

RESULTS AND CONCLUSIONS

Depolymerization of actin with 1-5 microM cytochalasin D (cyto D) augmented the peak [Ca(2+)](i) response and increased the number of cells responding to shear, similar to the increased responses induced by pretreatment with 50 nM PTH. Stabilization of actin with phalloidin prevented the PTH enhanced [Ca(2+)](i) response to shear. Inhibition of the MSCC with Gd(3+) significantly blocked both the peak Ca(2+) response and the number of cells responding to shear in cells pretreated with either PTH or cyto D. Inhibition of the L-VSCC reduced the peak [Ca(2+)](i) response to shear in cells pretreated with PTH, but not with cyto D. Patch-clamp analyses found that addition of PTH or cyto D significantly increased the MSCC open probability in response to mechanical stimulation, whereas phalloidin significantly attenuated the PTH-enhanced MSCC activation. These data indicate that actin reorganization increases MSCC activity in a manner similar to PTH and may be one mechanism through which PTH may reduce the mechanical threshold of osteoblasts.

Cellular fluid mechanics and mechanotransduction

Mechanotransduction, the transformation of an applied mechanical force into a cellular biomolecular response, is briefly reviewed focusing on fluid shear stress and endothelial cells. Particular emphasis is placed on recent studies of the surface proteoglycan layer (glycocalyx) as a primary sensor of fluid shear stress that can transmit force to apical structures such as the plasma membrane or the actin cortical web where transduction can take place or to more remote regions of the cell such as intercellular junctions and basal adhesion plaques where transduction can also occur. All of these possibilities are reviewed from an integrated perspective.

The effect of cytoskeletal disruption on pulsatile fluid flow-induced nitric oxide and prostaglandin E2 release in osteocytes and osteoblasts

Fluid flowing through the bone porosity might be a primary stimulus for functional adaptation of bone. Osteoblasts, and osteocytes in particular, respond to fluid flow in vitro with enhanced nitric oxide (NO) and prostaglandin E(2) (PGE(2)) release; both of these signaling molecules mediate mechanically-induced bone formation. Because the cell cytoskeleton is involved in signal transduction, we hypothesized that the pulsatile fluid flow-induced release of NO and PGE(2) in both osteoblastic and osteocytic cells involves the actin and microtubule cytoskeleton. In testing this hypothesis we found that fluid flow-induced NO response in osteoblasts was accompanied by parallel alignment of stress fibers, whereas PGE(2) response was related to fluid flow stimulation of focal adhesions formed after cytoskeletal disruption. Fluid flow-induced PGE(2) response in osteocytes was inhibited by cytoskeletal disruption, whereas in osteoblasts it was enhanced. These opposite PGE(2) responses are likely related to differences in cytoskeletal composition (osteocyte structure was more dependent on actin), but may occur via cytoskeletal modulation of shear/stretch-sensitive ion channels that are known to be dominant in osteocyte (and not osteoblast) response to mechanical loading.

Influence of increased mechanical loading by hypergravity on the microtubule cytoskeleton and prostaglandin E2 release in primary osteoblasts

Cells respond to a wide range of mechanical stimuli such as fluid shear and strain, although the contribution of gravity to cell structure and function is not understood. We hypothesized that bone-forming osteoblasts are sensitive to increased mechanical loading by hypergravity. A centrifuge suitable for cell culture was developed and validated, and then primary cultures of fetal rat calvarial osteoblasts at various stages of differentiation were mechanically loaded using hypergravity. We measured microtubule network morphology as well as release of the paracrine factor prostaglandin E2 (PGE2). In immature osteoblasts, a stimulus of 10x gravity (10 g) for 3 h increased PGE2 2.5-fold and decreased microtubule network height 1.12-fold without affecting cell viability. Hypergravity (3 h) caused dose-dependent (5-50 g) increases in PGE2 (5.3-fold at 50 g) and decreases (1.26-fold at 50 g) in microtubule network height. PGE2 release depended on duration but not orientation of the hypergravity load. As osteoblasts differentiated, sensitivity to hypergravity declined. We conclude that primary osteoblasts demonstrate dose- and duration-dependent sensitivity to gravitational loading, which appears to be blunted in mature osteoblasts.

Integrin-mediated Adhesion and Soluble Ligand Binding Stabilize COX-2 Protein Levels in Endothelial Cells by Inducing Expression and Preventing Degradation

Cyclooxygenase-2 (COX-2), a key enzyme in prostaglandin synthesis, is highly expressed during inflammation and cellular transformation and promotes tumor progression and angiogenesis. We have previously demonstrated that endothelial cell COX-2 is required for integrin alphaVbeta3-dependent activation of Rac-1 and Cdc-42 and for endothelial cell spreading, migration, and angiogenesis (Dormond, O., Foletti, A., Paroz, C., and Ruegg, C. (2001) Nat. Med. 7, 1041-1047; Dormond, O., Bezzi, M., Mariotti, A., and Ruegg, C. (2002) J. Biol. Chem. 277, 45838-45846). In this study, we addressed the question of whether integrin-mediated cell adhesion may regulate COX-2 expression in endothelial cells. We report that cell detachment from the substrate caused rapid degradation of COX-2 protein in human umbilical vein endothelial cells (HUVEC) independent of serum stimulation. This effect was prevented by broad inhibition of cellular proteinases and by neutralizing lysosomal activity but not by inhibiting the proteasome. HUVEC adhesion to laminin, collagen I, fibronectin, or vitronectin induced rapid COX-2 protein expression with peak levels reached within 2 h and increased COX-2-dependent prostaglandin E2 production. In contrast, nonspecific adhesion to poly-L-lysine was ineffective in inducing COX-2 expression. Furthermore, the addition of matrix proteins in solution promoted COX-2 protein expression in suspended or poly-L-lysine-attached HUVEC. Adhesion-induced COX-2 expression was strongly suppressed by pharmacological inhibition of c-Src, phosphatidylinositol 3-kinase, p38, extracellular-regulated kinase 1/2, and, to a lesser extent, protein kinase C and by the inhibition of mRNA or protein synthesis. In conclusion, this work demonstrates that integrin-mediated cell adhesion and soluble integrin ligands contribute to maintaining COX-2 steady-state levels in endothelial cells by the combined prevention of lysosomal-dependent degradation and the stimulation of mRNA synthesis involving multiple signaling pathways.

Cells in fluidic environments are sensitive to flow frequency

Virtually all cells accommodate to their mechanical environment. In particular, cells subject to flow respond to rapid changes in fluid shear stress (SS), cyclic stretch (CS), and pressure. Recent studies have focused on the effect of pulsatility on cellular behavior. Since cells of many different tissue beds are constantly exposed to fluid flows over a narrow range of frequencies, we hypothesized that an intrinsic flow frequency that is optimal for determining cell phenotype exists. We report here that cells from various tissue beds (bovine aortic endothelial cells (BAEC), rat small intestine epithelial cells (RSIEC), and rat lung epithelial cells (RLEC)) proliferate maximally when cultured in a perfusion bioreactor under pulsatile conditions at a specific frequency, independent of the applied SS. Vascular endothelial and pulmonary epithelial cell proliferation peaked under 1 Hz pulsatile flow. In contrast, proliferation of gastrointestinal cells, which in their physiological context are subject to no flow or higher wavelength signal, was maximum at 0.125 Hz or under no flow. Moreover, exposure of BAEC to pulsatile flow of varying frequency influenced their nitric oxide synthase activity and prostacyclin production, which reached maximum values at 1 Hz. Notably, the "optimal" frequencies for the cell types examined correspond to the physiologic operating range of the organs from where they were initially derived. These findings suggest that frequency, independent of shear, is an essential determinant of cell response in pulsatile environments.