Inactivation of atrial natriuretic factor-stimulated cyclic guanosine 3',5'-monophosphate (cGMP) in UMR-106 osteoblast-like cells

Therapeutic potential of phosphodiesterase inhibitors in the treatment of osteoporosis: Scopes for therapeutic repurposing and discovery of new oral osteoanabolic drugs

Cyclic nucleotide phosphodiesterases (PDEs) are ubiquitously expressed enzymes that hydrolyze phosphodiester bond in the second messenger molecules including cAMP and cGMP. A wide range of drugs blocks one or more PDEs thereby preventing the inactivation of cAMP/cGMP. PDEs are differentially expressed in bone cells including osteoblasts, osteoclasts and chondrocytes. Intracellular increases in cAMP/cGMP levels in osteoblasts result in osteogenic response. Acting via the type 1 PTH receptor, teriparatide and abaloparatide increase intracellular cAMP and induce osteoanabolic effect, and many PDE inhibitors mimic this effect in preclinical studies. Since all osteoanabolic drugs are injectable and that oral drugs are considered to improve the treatment adherence and persistence, osteogenic PDE inhibitors could be a promising alternative to the currently available osteogenic therapies and directly assessed clinically in drug repurposing mode. Similar to teriparatide/abaloparatide, PDE inhibitors while stimulating osteoblast function also promote osteoclast function through stimulation of receptor activator of nuclear factor kappa-B ligand production from osteoblasts. In this review, we critically discussed the effects of PDE inhibitors in bone cells from cellular signalling to a variety of preclinical models that evaluated the bone formation mechanisms. We identified pentoxifylline (a non-selective PDE inhibitor) and rolipram (a PDE4 selective inhibitor) being the most studied inhibitors with osteogenic effect in preclinical models of bone loss at ≤ human equivalent doses, which suggest their potential for post-menopausal osteoporosis treatment through therapeutic repurposing. Subsequently, we treated pentoxifylline and rolipram as prototypical osteogenic PDEs to predict new chemotypes via the computer-aided design strategies for new drugs, based on the structural biology of PDEs.

Analysis of short-term treatment with the phosphodiesterase type 5 inhibitor tadalafil on long bone development in young rats

Cyclic GMP (cGMP) is an important intracellular regulator of endochondral bone growth and skeletal remodeling. Tadalafil, an inhibitor of the phosphodiesterase (PDE) type 5 (PDE5) that specifically hydrolyzes cGMP, is increasingly used to treat children with pulmonary arterial hypertension (PAH), but the effect of tadalafil on bone growth and strength has not been previously investigated. In this study, we first analyzed the expression of transcripts encoding PDEs in primary cultures of chondrocytes from newborn rat epiphyses. We detected robust expression of PDE5 as the major phosphodiesterase hydrolyzing cGMP. Time-course experiments showed that C-type natriuretic peptide increased intracellular levels of cGMP in primary chondrocytes with a peak at 2 min, and in the presence of tadalafil the peak level of intracellular cGMP was 37% greater ( P < 0.01) and the decline was significantly attenuated. Next, we treated 1-mo-old Sprague Dawley rats with vehicle or tadalafil for 3 wk. Although 10 mg·kg^-1·day^-1 tadalafil led to a significant 52% ( P < 0.01) increase in tissue levels of cGMP and a 9% reduction ( P < 0.01) in bodyweight gain, it did not alter long bone length, cortical or trabecular bone properties, and histological features. In conclusion, our results indicate that PDE5 is highly expressed in growth plate chondrocytes, and short-term tadalafil treatment of growing rats at doses comparable to those used in children with PAH has neither obvious beneficial effect on long bone growth nor any observable adverse effect on growth plate structure and trabecular and cortical bone structure.

Tadalafil modulates aromatase activity and androgen receptor expression in a human osteoblastic cell in vitro model

PURPOSE

Phosphodiesterase type-5 inhibitor (PDE5i) tadalafil administration in men with erectile dysfunction is associated with increased testosterone/estradiol ratio, leading to hypothesize a potential increased effect of androgen action on target tissues. We aimed to characterize, in a cellular model system in vitro, the potential modulation of aromatase and sex steroid hormone receptors upon exposure to tadalafil (TAD).

METHODS

Human osteoblast-like cells SAOS-2 were chosen as an in vitro model system since osteoblasts are target of steroid hormones. Cells were tested for viability upon TAD exposure, which increased cell proliferation. Then, cells were treated with/without TAD for several times to evaluate potential modulation in PDE5, aromatase (ARO), androgen (AR) and estrogen (ER) receptor expression.

RESULTS

Osteoblasts express significant levels of both PDE5 mRNA and protein. Exposure of cells to increasing concentrations of TAD (10(-8)-10(-7) M) decreased PDE5 mRNA and protein expression. Also, TAD inhibited ARO mRNA and protein expression leading to an increase in testosterone levels in the supernatants. Interestingly, TAD increased total AR mRNA and protein expression and decreased ERα, with an increased ratio of AR/ER, suggesting preferential androgenic vs estrogenic pathway activation.

CONCLUSIONS

Our results demonstrate for the first time that TAD decreases ARO expression and increases AR protein expression in human SAOS-2, strongly suggesting a new control of steroid hormones pathway by PDE5i. These findings might represent the first evidence of translational actions of PDE5i on AR, which leads to hypothesize a growing relevance of this molecule in men with prostate cancer long-term treated with TAD for sexual rehabilitation.

Icariin from Epimedium brevicornum Maxim promotes the biosynthesis of estrogen by aromatase (CYP19)

ETHNOPHARMACOLOGICAL RELEVANCE

Epimedium brevicornum Maxim has long been used for the treatment of osteoporosis in China and other Asian countries. However, the mechanism behind the antiosteoporotic activity of this medicinal plant is not fully understood.

AIM OF THE STUDY

The present study was designed to investigate the effects of five widely used antiosteoporotic medicinal plants (Epimedium brevicornum, Cuscuta chinensis, Rhizoma drynariae, Polygonum multiflorum, and Ligustrum lucidum) on the production of estrogen, and identify the bioactive compounds responsible for the estrogen biosynthesis-promoting effect.

MATERIALS AND METHODS

Human ovarian granulosa-like KGN cells were used to evaluate estrogen biosynthesis, and the production of 17β-estradiol was quantified by a magnetic particle-based enzyme-linked immunosorbent assay (ELISA) kit. Further, the mRNA expression of aromatase was determined by a quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR), and the protein expression of aromatase was detected by western blotting. The activity of alkaline phosphatase (ALP) in rat osteoblastic UMR-106 cells was measured using p-nitrophenyl sodium phosphate assay.

RESULTS

Among the 5 antiosteoporotic medicinal plants, the extract of Epimedium brevicornum was found to significantly promote estrogen biosynthesis in KGN cells. Icariin, the major compound in Epimedium brevicornum, was identified to be the active compound for the estrogen biosynthesis-promoting effect. Icariin promoted estrogen biosynthesis in KGN cells in a concentration- and time-dependant manner and enhanced the mRNA and protein expressions of aromatase, which is the only enzyme for the conversion of androgens to estrogens in vertebrates. Further study showed that icariin also promoted estrogen biosynthesis and ALP activity in osteoblastic UMR-106 cells.

CONCLUSIONS

These results show that the promotion of estrogen biosynthesis is a novel effect of Epimedium brevicornum, and icariin could be utilized for the prevention and treatment of osteoporosis.

Effects of phosphodiesterase 7 inhibition by RNA interference on the gene expression and differentiation of human mesenchymal stem cell-derived osteoblasts

The second messenger molecule cyclic adenosine monophosphate (cAMP) plays an important role in the hormonal regulation of bone metabolism. cAMP is inactivated by the cyclic nucleotide phosphodiesterases (PDEs), a superfamily of enzymes divided into 11 known families designated PDE 1-11. The aim of this study was to investigate the effect of PDE7 and PDE8 inhibition on the gene expression and differentiation of human osteoblasts. Osteoblasts differentiated from human mesenchymal stem cells (hMSC) were cultured and treated with short interfering RNAs (siRNAs) generated from PDE7 and PDE8 PCR products. Total RNA was isolated from the cells, and gene expression was assayed with cDNA microarray and quantitative real-time PCR. bALP measurements were assayed during differentiation, and mineralization was determined by quantitative Alizarin red S staining. PDE7 and PDE8 inhibition by RNA interference decreased the gene expression of PDE7A by 60-70%, PDE7B by 40-50%, and PDE8A by 30%. PDE7 silencing increased the expression of beta-catenin, osteocalcin, caspase-8, and cAMP-responsive element-binding protein 5 (CREB-5) genes and decreased the expression of the 1, 25-dihydroxyvitamin D3 receptor gene. PDE8A silencing increased the expression of anti-apoptotic genes, but decreased the expression of osteoglycin (osteoinductive factor) and bone morphogenetic protein 1 (BMP-1). PDE7 silencing increased bALP and mineralization up to three-fold compared to controls. Treatment with the PDE7-selective PDE inhibitor BRL-50481 had similar effects on mineralization as the gene silencing. The PDE7 silencing also increased forskolin stimulated cAMP response, but had no effect on the proliferation rate. Furthermore, osteocalcin expression was increased by PDE7 silencing by a mechanism dependent on protein kinase A. Our results show that specific gene silencing with the RNAi method is a useful tool for inhibiting the gene expression of specific PDEs and that PDE7 silencing upregulates several osteogenic genes and increases mineralization. PDE7 may play an important role in the regulation of osteoblastic differentiation.

The cGMP-binding, cGMP-specific phosphodiesterase (PDE5): intestinal cell expression, regulation and role in fluid secretion

The expression and regulation of the cGMP-binding, cGMP-specific phosphodiesterase, PDE5, was studied in intestinal cells. Both PDE5A1 and PDE5A2 splice forms were cloned from the cDNA prepared from human colonic T84 cells, and PDE5 activity was dependent on increases in intracellular cGMP levels which correlated with increased phosphorylation of the enzyme. PDE5 expression was monitored in different regions of the gastrointestinal tract and nearly 50% of the phosphodiesterase activity in the duodenum, jejunum, ileum and colon was inhibited by sildenafil citrate. Administration of the stable toxin to intestinal loops resulted in activation of PDE5. Inhibition of PDE5 by sildenafil citrate led to fluid accumulation in loops, suggesting a possible explanation for the side effect of diarrhoea observed in individuals administered sildenafil citrate. Our results therefore represent the first study on the expression and regulation of PDE5 in intestinal tissue, and indicate that mechanisms to control its activity may have important consequences in intestinal physiology.

Dexamethasone down-regulates cAMP-phosphodiesterase in human osteosarcoma cells

Cyclic adenosine monophosphate (cAMP) is an important second messenger in the hormonal regulation of bone metabolism. cAMP is inactivated by the cyclic nucleotide phosphodiesterases (PDEs), a superfamily of enzymes divided into 11 known families, designated PDE1-11. Interference with the cAMP signaling pathway has been suggested as one mechanism causing glucocorticoid induced osteoporosis. We speculated that glucocorticoids could affect the cAMP pathway by a down-regulation of PDE-mediated cAMP hydrolysis. The main cAMP hydrolysing enzyme families of human MG-63 and SaOS-2 osteosarcoma cells were identified as PDE1 and PDE4 by assaying the PDE activity of Q-sepharose fractions and cell homogenates with selective inhibitors. Treatment with the glucocorticoid dexamethasone (Dex) decreased cAMP-PDE activity by up to 50%, without affecting cGMP-PDE activity. Dex treatment reduced the sensitivity of the total cAMP-PDE activity towards the PDE4 selective PDE inhibitor rolipram. Forskolin stimulated cAMP accumulation was increased 30-60-fold in the presence of rolipram. Treatment with Dex did not affect the basal or forskolin stimulated cAMP accumulation, but treatment resulted in a reduced effect of rolipram on cAMP accumulation. Expression of the following cAMP-PDE subtypes were detected by reverse transcriptase PCR (RT-PCR): PDE1A, PDE1C, PDE2A, PDE3A, PDE4A, PDE4B, PDE4C, PDE4D, PDE7A, PDE7B, PDE8A, PDE10A and PDE11A. Using semi-quantitative RT-PCR, we detected a 50-70% decrease in the mRNA of PDE4A and PDE4B subtypes following Dex treatment. Further analysis revealed that Dex reduced the PDE4A4 and PDE4B1 isoforms. PDE4A1 PDE4A, PDE4A7, PDE4A10, PDE4B2 were also expressed, but Dex did not affect the transcription of these isoforms. We conclude that Dex treatment could affect the cAMP signaling pathway of human osteosarcoma cells by reducing type 4 cAMP-phosphodiesterase (PDE4).

Reconstitution of ATP-dependent cGMP transport into proteoliposomes by membrane proteins from human erythrocytes

The cellular efflux of cGMP from human erythrocytes has previously been characterized in functional studies. The purpose of the present study was to find membrane proteins with the ability to restore ATP-dependent uptake of cGMP into proteoliposomes. Human erythrocyte membranes were solubilized with CHAPS (3-([3-cholamidopropyl]dimethylammonio)-1-propanesulfonate) and gel filtration gave three protein fractions with the ability to restore active transport. Only two of these fractions were retained on a lentil lectin column. By using these two purification steps, active transport was 11 times higher in the first fraction compared to the original material and SDS-PAGE showed the presence of proteins with sizes of 145 kDa and 165 kDa. The second fraction gave 20 times higher active transport after purification and comprised proteins with sizes of 145 kDa and 180 kDa. At present three members of the MRP (multi-resistance associated protein) family have been detected in human erythrocytes: MRPI, MRP4 and MRP5. The last two proteins have been shown to transport cyclic nucleotides. The present findings are compatible with MRP4 as the 145 kDa protein, MRP5 as the 165 kDa protein and MRP1 as the 180 kDa protein. However, the 145 kDa protein could also be SMRP (short multi-resistance protein), the gene splice variant of MRP5. Immunoprecipitation of MRP5 from CHAPS-solubilized extract reduced active transport and specific binding by about 45% and 40%, respectively. This shows that MRP5 is an important cGMP-transporting protein in human erythrocytes.

Effect of phosphodiesterase inhibitor-4, rolipram, on new bone formations by recombinant human bone morphogenetic protein-2

Collagen sponge disks (6 mm diameter, 1 mm thickness) were impregnated with recombinant human bone morphogenetic protein-2 (rhBMP-2) (5 microg/disk) and implanted onto the back muscles of mice. Ten or 20 mg/kg per day of Rolipram, a selective inhibitory agent to phosphodiesterase type 4 (PDE-4), or vehicle, was injected subcutaneously into the host mice for 3 weeks. After treatment, rhBMP-2-induced ectopic ossicles were harvested and examined by radiographic and histologic methods to determine the size, bone quality, and mineral content of the ossicles. The ossicles from a group treated with 20 mg/kg per day Rolipram were significantly larger in size and higher in bone mineral density (BMD) and bone mineral content (BMC) than the control samples. No significant differences were noted in mice treated with 10 mg/kg per day of Rolipram. Histologically, ossicles from the high-dose (20 mg/kg per day) Rolipram-treated group showed densely packed, thicker trabeculae when compared with those from the control group. These experimental results indicate that the PDE-4 inhibitor, Rolipram, may enhance the bone-inducing capacity of BMP-2 in mesenchymal cells. This in turn may result in increased responsiveness to BMP-2 and point to a potential use of PDE-4 inhibitors for the promotion of rhBMP-dependent bone repair.

Clonidine-induced nitric oxide-dependent vasorelaxation mediated by endothelial alpha(2)-adrenoceptor activation

1. To assess the involvement of endothelial alpha(2)-adrenoceptors in the clonidine-induced vasodilatation, the mesenteric artery of Sprague Dawley rats was cannulated and perfused with Tyrode solution (2 ml min(-1)). We measured perfusion pressure, nitric oxide (NO) in the perfusate using chemiluminescence, and tissue cyclic GMP by RIA. 2. In phenylephrine-precontracted mesenteries, clonidine elicited concentration-dependent vasodilatations associated to a rise in luminal NO. One hundred nM rauwolscine or 100 microM L(omega)-nitro-L-arginine antagonized the clonidine-induced vasodilatation. Guanabenz, guanfacine, and oxymetazoline mimicked the clonidine-induced vasorelaxation. 3. In non-contracted mesenteries, 100 nM clonidine elicited a maximal rise of NO (123+/-13 pmol); associated to a peak in tissue cyclic GMP. Endothelium removal, L(omega)-nitro-L-arginine, or rauwolscine ablated the rise in NO. One hundred nM aminoclonidine, guanfacine, guanabenz, UK14,304 and oxymetazoline mimicked the clonidine-induced surge of NO. Ten microM ODQ obliterated the clonidine-induced vasorelaxation and the associated tissue cyclic GMP accumulation; 10 - 100 nM sildenafil increased tissue cyclic GMP accumulation without altering the clonidine-induced NO release. 4. alpha(2)-Adrenergic blockers antagonized the clonidine-induced rise in NO. Consistent with a preferential alpha(2D)-adrenoceptor activation, the K(B)s for yohimbine, rauwolscine, phentolamine, WB-4101, and prazosin were: 6.8, 24, 19, 165, and 1489 nM, respectively. 5. Rat pretreatment with 100 mg kg(-1) 6-hydroxydopamine reduced 95% tissue noradrenaline and 60% neuropeptide Y. In these preparations, 100 nM clonidine elicited a rise of 91.9+/-15.5 pmol NO. Perfusion with 1 microM guanethidine or 1 microM guanethidine plus 1 microM atropine did not modify the NO surge evoked by 100 nM clonidine. 6. Clonidine and congeners activate endothelial alpha(2D)-adrenoceptors coupled to the L-arginine pathway, suggesting that the antihypertensive action of clonidine involves an endothelial vasorelaxation mediated by NO release, in addition to presynaptic mechanisms.