Mechanical stretch increases secretion of parathyroid hormone-related protein by cultured bladder smooth muscle cells

Mechanotransduction as an Adaptation to Gravity

Gravity has played a critical role in the development of terrestrial life. A key event in evolution has been the development of mechanisms to sense and transduce gravitational force into biological signals. The objective of this manuscript is to review how living organisms on Earth use mechanotransduction as an adaptation to gravity. Certain cells have evolved specialized structures, such as otoliths in hair cells of the inner ear and statoliths in plants, to respond directly to the force of gravity. By conducting studies in the reduced gravity of spaceflight (microgravity) or simulating microgravity in the laboratory, we have gained insights into how gravity might have changed life on Earth. We review how microgravity affects prokaryotic and eukaryotic cells at the cellular and molecular levels. Genomic studies in yeast have identified changes in genes involved in budding, cell polarity, and cell separation regulated by Ras, PI3K, and TOR signaling pathways. Moreover, transcriptomic analysis of late pregnant rats have revealed that microgravity affects genes that regulate circadian clocks, activate mechanotransduction pathways, and induce changes in immune response, metabolism, and cells proliferation. Importantly, these studies identified genes that modify chromatin structure and methylation, suggesting that long-term adaptation to gravity may be mediated by epigenetic modifications. Given that gravity represents a modification in mechanical stresses encounter by the cells, the tensegrity model of cytoskeletal architecture provides an excellent paradigm to explain how changes in the balance of forces, which are transmitted across transmembrane receptors and cytoskeleton, can influence intracellular signaling pathways and gene expression.

Cyclic Mechanical Strain Regulates the PTHrP Expression in Cultured Chondrocytes via Activation of the Ca2+ Channel

The association between mechanical stimulation and chondrocyte homeostasis has been reported. However, the participation of PTHrP (parathyroid-hormone-related protein) in the mechano-regulation of chondrocyte metabolism remains unclear. We determined whether mechanical stimulation of chondrocytes induces the expression of PTHrP and, further, whether the mechano-modulation of PTHrP is dependent on the maturational status of chondrocytes. Cyclic mechanical strain was applied to rat growth plate chondrocytes at the proliferating, matrix-forming, and hypertrophic stages at 30 cycles/min. Cyclic mechanical strain significantly increased PTHrP mRNA levels in chondrocytes at the proliferating and matrix-forming stages only. The induction of PTHrP was dependent on loading magnitude at the proliferating stage. Using specific ion channel blockers, we determined that mechano-induction of PTHrP was inhibited by nifedipine, a Ca2+ channel blocker. These results suggest that mechanical induction of PTHrP possibly provides the environment for greater chondrocyte replication and matrix formation that would subsequently affect cartilage formation.

Expression of parathyroid hormone/parathyroid hormone-related peptide receptor 1 in normal and diseased bladder detrusor muscles: a clinico-pathological study

Background

To investigate the expression of parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor 1 (PTH1R) in clinical specimens of normal and diseased bladders. PTHrP is a unique stretch-induced endogenous detrusor relaxant that functions via PTH1R. We hypothesized that suppression of this axis could be involved in the pathogenesis of bladder disease.

Methods

PTH1R expression in clinical samples was examined by immunohistochemistry. Normal kidney tissue from a patient with renal cancer and bladder specimens from patients undergoing ureteral reimplantation for vesicoureteral reflux or partial cystectomy for urachal cyst were examined as normal control organs. These were compared with 13 diseased bladder specimens from patients undergoing bladder augmentation. The augmentation patients ranged from 8 to 31 years old (median 15 years), including 9 males and 4 females. Seven patients had spinal disorders, 3 had posterior urethral valves and 3 non-neurogenic neurogenic bladders (Hinman syndrome).

Results

Renal tubules, detrusor muscle and blood vessels in normal control bladders stained positive for PTH1R. According to preoperative urodynamic studies of augmentation patients, the median percent bladder capacity compared with the age-standard was 43.6% (range 1.5–86.6%), median intravesical pressure at maximal capacity was 30 cmH₂O (range 10–107 cmH₂O), and median compliance was 3.93 ml/cmH₂O (range 0.05–30.3 ml/cmH₂O). Detrusor overactivity was observed in five cases (38.5%). All augmented bladders showed negative stainings in PTH1R expression in the detrusor tissue, but positive staining of blood vessels in majority of the cases.

Conclusions

Downregulation of PTH1R may be involved in the pathogenesis of human end-stage bladder disease requiring augmentation.

The preterm parturition syndrome

The implicit paradigm that has governed the study and clinical management of preterm labour is that term and preterm parturition are the same processes, except for the gestational age at which they occur. Indeed, both share a common pathway composed of uterine contractility, cervical dilatation and activation of the membranes/decidua. This review explores the concept that while term labour results from physiological activation of the components of the common pathway, preterm labour arises from pathological signalling and activation of one or more components of the common pathway of parturition. The term "great obstetrical syndromes" has been coined to reframe the concept of obstetrical disease. Such syndromes are characterised by: (1) multiple aetiology; (2) long preclinical stage; (3) frequent fetal involvement; (4) clinical manifestations that are often adaptive in nature; and (5) gene-environment interactions that may predispose to the syndromes. This article reviews the evidence indicating that the pathological processes implicated in the preterm parturition syndrome include: (1) intrauterine infection/inflammation; (2) uterine ischaemia; (3) uterine overdistension; (4) abnormal allograft reaction; (5) allergy; (6) cervical insufficiency; and (7) hormonal disorders (progesterone related and corticotrophin-releasing factor related). The implications of this conceptual framework for the prevention, diagnosis, and treatment of preterm labour are discussed.

Pharmacology of the lower urinary tract: basis for current and future treatments of urinary incontinence

The lower urinary tract constitutes a functional unit controlled by a complex interplay between the central and peripheral nervous systems and local regulatory factors. In the adult, micturition is controlled by a spinobulbospinal reflex, which is under suprapontine control. Several central nervous system transmitters can modulate voiding, as well as, potentially, drugs affecting voiding; for example, noradrenaline, GABA, or dopamine receptors and mechanisms may be therapeutically useful. Peripherally, lower urinary tract function is dependent on the concerted action of the smooth and striated muscles of the urinary bladder, urethra, and periurethral region. Various neurotransmitters, including acetylcholine, noradrenaline, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in this neural regulation. Muscarinic receptors mediate normal bladder contraction as well as at least the main part of contraction in the overactive bladder. Disorders of micturition can roughly be classified as disturbances of storage or disturbances of emptying. Failure to store urine may lead to various forms of incontinence, the main forms of which are urge and stress incontinence. The etiology and pathophysiology of these disorders remain incompletely known, which is reflected in the fact that current drug treatment includes a relatively small number of more or less well-documented alternatives. Antimuscarinics are the main-stay of pharmacological treatment of the overactive bladder syndrome, which is characterized by urgency, frequency, and urge incontinence. Accepted drug treatments of stress incontinence are currently scarce, but new alternatives are emerging. New targets for control of micturition are being defined, but further research is needed to advance the pharmacological treatment of micturition disorders.

Evaluation of the rat bladder-derived relaxant factor by coaxial bioassay system

The release of bladder-derived relaxant factor in a coaxial bioassay system and the effects of reactive oxygen species were studied. After precontraction with phenylephrine (10(-6)-3x10(-6)) or 50 mM K+, acetylcholine (10(-8)-10(-3) M) induced relaxation in rat anococcygeus muscle mounted within rat bladder in a tissue bath. This relaxation was not altered by the removal of the urothelium or incubation with tetrodotoxin (10(-6) M). However, bupivacaine (10(-4) M) and lidocaine (3 x 10(-4) M) inhibited this response after raising the pH of the nutrient solution to 7.8, and oxybuprocaine (10(-4) M) exerted inhibitory effect at both physiological pH (7.4) and at pH 7.8. Exposure to electrolysis-generated reactive oxygen species or incubation with hydrogen peroxide and pyrogallol did not alter the acetylcholine response. Present results indicate that the bladder-derived relaxant factor does not behave like endothelium-derived hyperpolarizing factor, but its release may be associated with tetrodotoxin-resistant Na+ channels, which are probably in the neurons of the bladder rather than in the urothelium or detrusor muscle. Furthermore, reactive oxygen species do not interact with this relaxing factor, the exact nature and the physiological importance of which, however, remains to be established.

Mechanical stretch is a highly selective regulator of gene expression in human bladder smooth muscle cells

Application of mechanical stimuli has been shown to alter gene expression in bladder smooth muscle cells (SMC). To date, only a limited number of "stretch-responsive" genes in this cell type have been reported. We employed oligonucleotide arrays to identify stretch-sensitive genes in primary culture human bladder SMC subjected to repetitive mechanical stimulation for 4 h. Differential gene expression between stretched and nonstretched cells was assessed using Significance Analysis of Microarrays (SAM). Expression of 20 out of 11,731 expressed genes ( approximately 0.17%) was altered >2-fold following stretch, with 19 genes induced and one gene (FGF-9) repressed. Using real-time RT-PCR, we tested independently the responsiveness of 15 genes to stretch and to platelet-derived growth factor-BB (PDGF-BB), another hypertrophic stimulus for bladder SMC. In response to both stimuli, expression of 13 genes increased, 1 gene (FGF-9) decreased, and 1 gene was unchanged. Six transcripts (HB-EGF, BMP-2, COX-2, LIF, PAR-2, and FGF-9) were evaluated using an ex vivo rat model of bladder distension. HB-EGF, BMP-2, COX-2, LIF, and PAR-2 increased with bladder stretch ex vivo, whereas FGF-9 decreased, consistent with expression changes observed in vitro. In silico analysis of microarray data using the FIRED algorithm identified c-jun, AP-1, ATF-2, and neurofibromin-1 (NF-1) as potential transcriptional mediators of stretch signals. Furthermore, the promoters of 9 of 13 stretch-responsive genes contained AP-1 binding sites. These observations identify stretch as a highly selective regulator of gene expression in bladder SMC. Moreover, they suggest that mechanical and growth factor signals converge on common transcriptional regulators that include members of the AP-1 family.

Bone growth stimulators. New tools for treating bone loss and mending fractures

In the new millennium, humans will be traveling to Mars and eventually beyond with skeletons that respond to microgravity by self-destructing. Meanwhile in Earth's aging populations growing numbers of men and many more women are suffering from crippling bone loss. During the first decade after menopause all women suffer an accelerating loss of bone, which in some of them is severe enough to result in "spontaneous" crushing of vertebrae and fracturing of hips by ordinary body movements. This is osteoporosis, which all too often requires prolonged and expensive care, the physical and mental stress of which may even kill the patient. Osteoporosis in postmenopausal women is caused by the loss of estrogen. The slower development of osteoporosis in aging men is also due at least in part to a loss of the estrogen made in ever smaller amounts in bone cells from the declining level of circulating testosterone and is needed for bone maintenance as it is in women. The loss of estrogen increases the generation, longevity, and activity of bone-resorbing osteoclasts. The destructive osteoclast surge can be blocked by estrogens and selective estrogen receptor modulators (SERMs) as well as antiosteoclast agents such as bisphosphonates and calcitonin. But these agents stimulate only a limited amount of bone growth as the unaffected osteoblasts fill in the holes that were dug by the now suppressed osteoclasts. They do not stimulate osteoblasts to make bone--they are antiresorptives not bone anabolic agents. (However, certain estrogen analogs and bisphosphates may stimulate bone growth to some extent by lengthening osteoblast working lives.) To grow new bone and restore bone strength lost in space and on Earth we must know what controls bone growth and destruction. Here we discuss the newest bone controllers and how they might operate. These include leptin from adipocytes and osteoblasts and the statins that are widely used to reduce blood cholesterol and cardiovascular damage. But the main focus of this article is necessarily the currently most promising of the anabolic agents, the potent parathyroid hormone (PTH) and certain of its 31- to 38-aminoacid fragments, which are either in or about to be in clinical trial or in the case of Lilly's Forteo [hPTH-(1-34)] tentatively approved by the Food and Drug Administration for treating osteoporosis and mending fractures.

Effects of dimethyl sulphoxide and heparin on stretch-activated ATP release by bladder urothelial cells from patients with interstitial cystitis

OBJECTIVE

To determine whether dimethyl sulphoxide (DMSO) and heparin reduce the greater stretch-activated ATP release in interstitial cystitis (IC), as ATP serves as a nocio-neurotransmitter in the bladder, and thus explain their beneficial effects in patients with IC (a disease characterized by hypersensory bladder symptoms).

MATERIALS AND METHODS

Bladder epithelia in IC release more ATP in response to stretch than do control samples. Both DMSO and heparin are used intravesically to treat IC; such agents can modulate urothelial function because they directly contact bladder urothelium. Biopsies taken from patients with IC and from control subjects were grown in primary cultures using established cell-culture techniques. Cultured urothelial cells were stretched with the Flexcell device (Flexcell International Corp., McKeesport, PA, USA) and supernatant ATP was measured, using a luciferin-luciferase assay. DMSO (0.1%, 0.5% and 1%) or heparin (50, 200, 800 and 1600 U) was added to the cells at the start of the stretch experiments and the ATP released into the supernatant measured. Cell viability was also determined using Trypan Blue staining.

RESULTS

IC cells released significantly more ATP in response to stretch than did control cells. This increased release of ATP by stretched IC cells was significantly blocked by adding DMSO or heparin at all concentrations used. Heparin appeared to have a greater dose-dependent effect on ATP release than did DMSO.

CONCLUSIONS

These findings are consistent with the hypothesis that the urothelium provides sensory input via ATP release and that this process is increased in IC. Furthermore, stretch-activated ATP release was blocked by adding DMSO and heparin, both intravesical agents commonly used to treat the symptoms of IC. This study supports the notion that purinergic-targeted therapy is warranted in treating IC. Further studies are needed to determine the mechanisms of increased ATP release by IC urothelial cells.

Effect of doxazosin on stretch-activated adenosine triphosphate release in bladder urothelial cells from patients with benign prostatic hyperplasia

OBJECTIVES

Recent data suggest that the bladder urothelium may have a sensory function by way of release of adenosine triphosphate (ATP) during stretch, which then acts as a sensory neurotransmitter. Because benign prostatic hyperplasia (BPH) can give rise to irritative (hypersensory) voiding patterns, we questioned whether the bladder urothelium from patients with BPH released more ATP during in vitro stretch and whether doxazosin, an alpha(1)-adrenoceptor blocker, affects this purinergic mechanism.

METHODS

Bladder urothelial biopsies from patients with BPH (n = 4) and controls (n = 4) were cultured using established techniques. In vitro stretch was performed with a Flexcell 2000 device that uses vacuum to deform the cell growth surface to impart a stretch force. Doxazosin (5 microM and 20 microM) was added to cells, and supernatants were collected at various points for ATP assay. ATP was assayed using the luciferin-luciferase reaction. ATP data were normalized to the time 0 value and expressed as a percentage of the baseline value.

RESULTS

After 96 hours of stretch, the BPH urothelial cells released significantly more ATP than did the control urothelial cells (62.6% +/- 11.2% versus 24.2% +/- 5.4%, P = 0.005) and nonstretched BPH urothelial cells (62.6% +/- 11.2% versus 15.1% +/- 5.1%, P = 0.004). The augmented release of ATP by stretched BPH bladder urothelial cells was completely blocked by treatment with 20 microM doxazosin.

CONCLUSIONS

Irritative voiding secondary to BPH may arise from increased ATP release by bladder urothelium during stretch. Doxazosin inhibits ATP release by way of an unknown mechanism that may or may not involve the alpha1-adrenoreceptor. Treatment for hypersensory voiding symptoms secondary to BPH might also target the urothelial purinergic pathway.

Physiology of female micturition

Micturition is a dynamic physiologic process consisting of alternating storage and expulsion phases and is accomplished by complex interactions among innervation, smooth muscle, connective tissue, urothelium and supportive structures. Although our current understanding of the anatomy and physiology of the lower urinary tract is far from complete, intensive research over the last decade has dramatically improved our appreciation of the neural, biomechanical, biochemical, and morphologic properties of the bladder and urethra, as well as the hormonal influences and unique pelvic and perineal anatomy of women. Continued research related to the physiology of female micturition promises to offer new insights into the complex bladder-urethral interactions and to provide a basis for developing better management strategies for a variety of voiding dysfunctions in women.

Stretch-stimulated surfactant synthesis is coordinated by the paracrine actions of PTHrP and leptin

Intrauterine lung development, culminating in physiological pulmonary surfactant production by epithelial type II (TII) cells, is driven by fluid distension through unknown mechanisms. Differentiation of alveolar epithelial and mesenchymal cells is mediated by soluble factors like parathyroid hormone-related protein (PTHrP), a stretch-sensitive TII cell product. PTHrP stimulates pulmonary surfactant production by a paracrine feedback loop mediated by leptin, a soluble product of the mature lipofibroblast (LF). When LFs and TIIs are stretched in coculture, there is a fivefold increase in surfactant phospholipid synthesis that can be "neutralized" by inhibitors of PTHrP or leptin, implicating a paracrine feedback loop in this mechanism. Stretching LFs stimulates PTHrP binding (2.5-fold) and downstream stimulation of triglyceride uptake quantitatively (15-25%) due to upregulation of adipose differentiation-related protein expression. Stretching TII cells increases leptin stimulation of their surfactant phospholipid synthesis threefold, suggesting that retrograde signaling by leptin to TII cells is also stretch sensitive. We conclude that the effect of stretch on alveolar LF and TII differentiation is coordinated by PTHrP, leptin, and their receptors.

Stretch-regulated exocytosis/endocytosis in bladder umbrella cells

The epithelium of the urinary bladder must maintain a highly impermeable barrier despite large variations in urine volume during bladder filling and voiding. To study how the epithelium accommodates these volume changes, we mounted bladder tissue in modified Ussing chambers and subjected the tissue to mechanical stretch. Stretching the tissue for 5 h resulted in a 50% increase in lumenal surface area (from approximately 2900 to 4300 microm(2)), exocytosis of a population of discoidal vesicles located in the apical cytoplasm of the superficial umbrella cells, and release of secretory proteins. Surprisingly, stretch also induced endocytosis of apical membrane and 100% of biotin-labeled membrane was internalized within 5 min after stretch. The endocytosed membrane was delivered to lysosomes and degraded by a leupeptin-sensitive pathway. Last, we show that the exocytic events were mediated, in part, by a cyclic adenosine monophosphate, protein kinase A-dependent process. Our results indicate that stretch modulates mucosal surface area by coordinating both exocytosis and endocytosis at the apical membrane of umbrella cells and provide insight into the mechanism of how mechanical forces regulate membrane traffic in non-excitable cells.

Modulation of membrane traffic by mechanical stimuli

All cells experience and respond to mechanical stimuli, such as changes in plasma membrane tension, shear stress, hydrostatic pressure, and compression. This review is an examination of the changes in membrane traffic that occur in response to mechanical forces. The plasma membrane has an associated tension that modulates both exocytosis and endocytosis. As membrane tension increases, exocytosis is stimulated, which acts to decrease membrane tension. In contrast, increased membrane tension slows endocytosis, whereas decreased tension stimulates internalization. In most cases, secretion is stimulated by external mechanical stimuli. However, in some cells mechanical forces block secretion. External stimuli also enhance membrane and fluid endocytosis in several cell types. Transduction of mechanical stimuli into changes in exocytosis/endocytosis may involve the cytoskeleton, stretch-activated channels, integrins, phospholipases, tyrosine kinases, and cAMP.

Prolonged hydrodistention of the bladder for symptomatic treatment of interstitial cystitis: efficacy at 6 months and 1 year

OBJECTIVE

To determine the efficacy of hydrodistention of the bladder for symptomatic treatment of interstitial cystitis after 6 months and 1 year of follow-up and to identify a predictive factor.

METHODS

The study included 65 consecutive patients (a first retrospective series of 33 and a second prospective series of 32) treated by hydrodistention of the bladder for urinary symptoms attributed to interstitial cystitis. All experienced pain on bladder filling, which was relieved by micturition or bladder voiding, and had more than two nocturias. Glomerulations were detected at short hydrodistention during cystoscopy. No patients were subject to NIH exclusion criteria. Hydrodistention was performed continuously for 3 h without rest intervals under epidural anesthesia using a balloon with a pressure equal to the patient's mean arterial pressure. Efficacy was defined as the disappearance of pain on bladder filling or the persistence of moderate, non-disabling pain for which the patient did not request treatment, and a low frequency of nocturia (zero to two times). The efficacy period was estimated according to Kaplan-Meier methods for survival curves. The second series was used to verify the analytic results of the first series.

RESULTS

Treatment efficacy was 12/32 (37.7% CI: 20.7-54.3) at 6 months and 7/32 (21.9% CI: 7.6-36.2) at 1 year for the first series, and 18/30 (60.0% CI: 45.0-75.0) at 6 months and 13/30 (43.3% CI: 25.6-61.1) at 1 year for the second series. In both series, results were better for the subgroup of patients with a bladder capacity > or = 150 ml during cystometry before distention.

CONCLUSIONS

This study showed good but transient efficacy in the least developed or least severe forms of the disease.

Parathyroid hormone-related protein and its receptors: nuclear functions and roles in the renal and cardiovascular systems, the placental trophoblasts and the pancreatic islets

The cloning of the so-called 'parathyroid hormone-related protein' (PTHrP) in 1987 was the result of a long quest for the factor which, by mimicking the actions of PTH in bone and kidney, is responsible for the hypercalcemic paraneoplastic syndrome, humoral calcemia of malignancy. PTHrP is distinct from PTH in a number of ways. First, PTHrP is the product of a separate gene. Second, with the exception of a short N-terminal region, the structure of PTHrP is not closely related to that of PTH. Third, in contrast to PTH, PTHrP is a paracrine factor expressed throughout the body. Finally, most of the functions of PTHrP have nothing in common with those of PTH. PTHrP is a poly-hormone which comprises a family of distinct peptide hormones arising from post-translational endoproteolytic cleavage of the initial PTHrP translation products. Mature N-terminal, mid-region and C-terminal secretory forms of PTHrP are thus generated, each of them having their own physiologic functions and probably their own receptors. The type 1 PTHrP receptor, binding both PTH(1-34) and PTHrP(1-36), is the only cloned receptor so far. PTHrP is a PTH-like calciotropic hormone, a myorelaxant, a growth factor and a developmental regulatory molecule. The present review reports recent aspects of PTHrP pharmacology and physiology, including: (a) the identification of new peptides and receptors of the PTH/PTHrP system; (b) the recently discovered nuclear functions of PTHrP and the role of PTHrP as an intracrine regulator of cell growth and cell death; (c) the physiological and developmental actions of PTHrP in the cardiovascular and the renal glomerulo-vascular systems; (d) the role of PTHrP as a regulator of pancreatic beta cell growth and functions, and, (e) the interactions of PTHrP and calcium-sensing receptors for the control of the growth of placental trophoblasts. These new advances have contributed to a better understanding of the pathophysiological role of PTHrP, and will help to identify its therapeutic potential in a number of diseases.

Activation of the transcription factors nuclear factor-kappaB and activator protein-1 in bladder smooth muscle exposed to outlet obstruction and mechanical stretching

PURPOSE

Transcriptional control of bladder genes in response to outlet obstruction, growth factors and mechanical force is poorly understood. We analyzed the effects of bladder obstruction, mechanical stretching and platelet derived growth factor on the activation of the major growth controlling transcription factors nuclear factor-kappaB and activator protein-1.

MATERIALS AND METHODS

Complete outlet obstruction was created in female rats by proximal urethral ligation and bladders were harvested 3, 6 and 24 hours later, respectively. Bladder cells were grown in culture and stimulated with 10 ng./ml. platelet derived growth factor or 10 cycles per minute of mechanical stretching for 0.5 to 4 hours. Nuclear proteins were high salt extracted and incubated with 32phosphorus double strand oligonucleotides containing a consensus binding sequence for activator protein-1 or nuclear factor-kappaB. The resulting DNA protein complexes were analyzed by electrophoretic mobility shift assay.

RESULTS

Nuclear extract isolated from obstructed bladders showed intense activator protein-1 binding activity 3, 6 and 24 hours after obstruction as well as increased nuclear factor-kappaB binding activity after 6 and 24 hours. Binding activity was absent or minimal in sham operated rats. Cultured cells exposed to mechanical stretching for 2 and 4 hours showed increased activator protein-1 and nuclear factor-kappaB DNA binding compared with unstretched cells. Likewise stimulation with platelet derived growth factor caused a consistent increase in activator protein-1 and nuclear factor-kappaB binding activity. The binding of nuclear proteins was abolished by a 40-fold excess of an unlabeled specific oligonucleotide but not by excess irrelevant oligonucleotide. Thus, the assays were specific for the factors involved.

CONCLUSIONS

Bladder obstruction and mechanical stretching cause the formation of activator protein-1 and nuclear factor-kappaB DNA complexes, consistent with a role of these transcription factors in the control of hypertrophy associated gene activation.