Detection of mRNA transcripts of truncated opiate precursor (POMC) in human cartilage

Do Neuroendocrine Peptides and Their Receptors Qualify as Novel Therapeutic Targets in Osteoarthritis?

Joint tissues like synovium, articular cartilage, meniscus and subchondral bone, are targets for neuropeptides. Resident cells of these tissues express receptors for various neuroendocrine-derived peptides including proopiomelanocortin (POMC)-derived peptides, i.e., α-melanocyte-stimulating hormone (α-MSH), adrenocorticotropin (ACTH) and β-endorphin (β-ED), and sympathetic neuropeptides like vasoactive intestinal peptide (VIP) and neuropeptide y (NPY). Melanocortins attained particular attention due to their immunomodulatory and anti-inflammatory effects in several tissues and organs. In particular, α-MSH, ACTH and specific melanocortin-receptor (MCR) agonists appear to have promising anti-inflammatory actions demonstrated in animal models of experimentally induced arthritis and osteoarthritis (OA). Sympathetic neuropeptides have obtained increasing attention as they have crucial trophic effects that are critical for joint tissue and bone homeostasis. VIP and NPY are implicated in direct and indirect activation of several anabolic signaling pathways in bone and synovial cells. Additionally, pituitary adenylate cyclase-activating polypeptide (PACAP) proved to be chondroprotective and, thus, might be a novel target in OA. Taken together, it appears more and more likely that the anabolic effects of these neuroendocrine peptides or their respective receptor agonists/antagonists may be exploited for the treatment of patients with inflammatory and degenerative joint diseases in the future.

Role of proopiomelanocortin-derived peptides and their receptors in the osteoarticular system: from basic to translational research

Proopiomelanocortin (POMC)-derived peptides such as melanocortins and β-endorphin (β-ED) exert their pleiotropic effects via binding to melanocortin receptors (MCR) and opioid receptors (OR). There is now compelling evidence for the existence of a functional POMC system within the osteoarticular system. Accordingly, distinct cell types of the synovial tissue and bone have been identified to generate POMC-derived peptides like β-ED, ACTH, or α-MSH. MCR subtypes, especially MC1R, MC2R (the ACTH receptor), MC3R, and MC4R, but also the μ-OR and δ-OR, have been detected in various cells of the synovium, cartilage, and bone. The respective ligands of these POMC-derived peptide receptors mediate an increasing number of newly recognized biological effects in the osteoarticular system. These include bone mineralization and longitudinal growth, cell proliferation and differentiation, extracellular matrix synthesis, osteoprotection, and immunomodulation. Importantly, bone formation is also regulated by the central melanocortin system via a complex hormonal interplay with other organs and tissues involved in energy metabolism. Among the POMC-derived peptides examined in cell culture systems from osteoarticular tissue and in animal models of experimentally induced arthritis, α-MSH, ACTH, and MC3R-specific agonists appear to have the most promising antiinflammatory actions. The effects of these melanocortin peptides may be exploited in future for the treatment of patients with inflammatory and degenerative joint diseases.

Modulation of basophil activity: a novel function of the neuropeptide α-melanocyte-stimulating hormone

BACKGROUND

Little is known about the effect of neuropeptides on basophils, which are important effector cells in immune and allergic responses.

OBJECTIVE

This study aimed at revealing the role of α-melanocyte-stimulating hormone (α-MSH) on basophil function.

METHODS

Expression of melanocortin receptors and proopiomelanocortin (POMC) was analyzed by means of RT-PCR, Western immunoblotting, fluorescence-activated cell sorting, and double-immunofluorescence analysis. Signal transduction studies included cyclic AMP and Ca(2+) mobilization assays. Basophil activity was assessed based on CD63 surface expression and cytokine release.

RESULTS

MC-1R expression was detectable in basophils isolated from human peripheral blood, as well as in basophils within nasal tissue. In isolated basophils from human blood, truncated POMC transcripts were present, but there was no POMC protein. Treatment of basophils with α-MSH increased intracellular Ca(2+) but not cyclic AMP levels. α-MSH at physiologic doses potently suppressed basophil activation induced by N-formyl-methionyl-leucyl-phenylalanine, phorbol 12-myristate 13-acetate, or grass pollen allergen in whole blood of healthy or allergic subjects, respectively. The effect of α-MSH on basophil activation was MC-1R mediated (as shown by blockade with a peptide analogue of agouti-signaling protein) and imitated by adrenocorticotropic hormone but not elicited by the tripeptides KPV and KdPT, both of which lack the central pharmacophore of α-MSH. Moreover, α-MSH at physiologic doses significantly suppressed secretion of 3 proallergic cytokines, IL-4, IL-6, and IL-13, in basophils stimulated with anti-IgE, N-formyl-methionyl-leucyl-phenylalanine, or phorbol 12-myristate 13-acetate.

CONCLUSION

Our findings highlight a novel functional activity of α-MSH, which acts as a natural antiallergic basophil-response modifier. These findings might point to novel therapeutic strategies in treating allergic diseases.

α-Melanocyte-stimulating hormone inhibits tumor necrosis factor α-stimulated MUC5AC expression in human nasal epithelial cells

Mucin hypersecretion is an important clinical feature of several respiratory diseases, including asthma, cystic fibrosis, nasal allergy, rhinitis, and sinusitis. It has been shown that α-melanocyte-stimulating hormone (α-MSH), a proopiomelanocortin (POMC)-derived peptide, has immunomodulatory activities by inhibiting NF-κB activation induced by proinflammatory cytokines such as TNF-α. Because MUC5AC expression is known to be up-regulated by TNF-α via NF-κB activation, we evaluated the inhibitory effect of α-MSH on MUC5AC gene expression induced by TNF-α in normal human nasal epithelial (NHNE) cells. Melanocortin-1-receptor (MC-1R) was detected by RT-PCR, Western blotting, and immunofluorescent labeling in NHNE cells. α-MSH suppressed NF-κB/p65 phosphorylation induced by TNF-α as well as IkB-α degradation in a dose-dependent manner, as assessed by Western blotting. In addition, α-MSH inhibited TNF-α-induced nuclear translocation of NF-κB and NF-κB luciferase activity. Real-time quantitative PCR data showed that α-MSH inhibited TNF-α-induced expression of MUC5AC, and this effect of α-MSH was neutralized by knockdown of MC-1R using MC-1R shRNA lentivirus. Analyses using RT-PCR and Western blotting showed the expression of POMC and two key enzymes in the POMC processing, proprotein convertases (PC)1 and PC2, and 7B2, which is required for enzymatic activity of PC2, in normal human nasal mucosa. We conclude that α-MSH down-regulates MUC5AC expression by inhibiting TNF-α-induced NF-κB activity through MC-1R stimulation in NHNE cells and that normal human nasal mucosa possesses the POMC processing machinery. Therefore, α-MSH may be a promising candidate to decrease mucin overproduction initiated by NF-κB activation.

The melanocortin system in articular chondrocytes: melanocortin receptors, pro-opiomelanocortin, precursor proteases, and a regulatory effect of alpha-melanocyte-stimulating hormone on proinflammatory cytokines and extracellular matrix components

OBJECTIVE

The pro-opiomelanocortin (POMC)-derived neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) mediates its effects via melanocortin (MC) receptors. This study was carried out to investigate the expression patterns of the MC system and the effects of alpha-MSH in human articular chondrocytes.

METHODS

Articular chondrocytes established from human osteoarthritic joint cartilage were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting for the expression of MC receptors, POMC, and prohormone convertases (PCs). MC-1 receptor (MC-1R) expression in articular cartilage was further studied by immunohistochemistry. Ca(2+) and cAMP assays were used to monitor alpha-MSH signaling, while studies of alpha-MSH function were performed in cultures with chondrocyte micromass pellets stimulated with alpha-MSH. Expression of cytokines and extracellular matrix (ECM) components was determined by real-time RT-PCR, Western immunoblotting, and enzyme-linked immunosorbent assays.

RESULTS

MC-1R expression was detected in articular chondrocytes in vitro and in articular cartilage in situ. In addition, expression of transcripts for MC-2R, MC-5R, POMC, and PCs was detected in articular chondrocytes. Stimulation with alpha-MSH increased the levels of intracellular cAMP, but not Ca(2+), in chondrocytes. Both messenger RNA and protein expression of various proinflammatory cytokines, collagens, matrix metalloproteinases (MMPs), and SOX9 was modulated by alpha-MSH.

CONCLUSION

Human articular chondrocytes are target cells for alpha-MSH. The effects of alpha-MSH on expression of cytokines and MMPs suggest that this neuropeptide plays a role in inflammatory and degenerative processes in cartilage. It is conceivable that inflammatory reactions can be mitigated by the induction of endogenous MCs or administration of alpha-MSH to the affected joints. The induction pattern of regulatory and structural ECM components such as collagens as well as SOX9 and anabolic and catabolic cytokines points to a function of alpha-MSH as a trophic factor in skeletal development during endochondral ossification rather than as a factor in homeostasis of permanent cartilage.

Beta-endorphin regulation of MAPKs in cultured human articular chondrocytes: MAPK inhibitors prevent the increase of IL-1 beta protein levels during beta-endorphin stimulation

We investigated the effect of beta-endorphin on the activities of mitogen-activated protein kinases in cultured human articular chondrocytes in order to elucidate its effect on cartilage. Monolayer cultures of chondrocytes obtained from patients undergoing total knee arthroplasty were treated with 60, 600, or 6000 ng/ml beta-endorphin, or 100 ng/ml naltrexone combined with 600 ng/ml beta-endorphin. The regulation of three major mitogen-activated protein kinases phosphorylation, ERKp44/p42, p38, and JNK, was determined by Western blotting. We also examined the influence of specific mitogen-activated protein kinase inhibitors on IL-1 beta protein levels during beta-endorphin stimulation. The results demonstrate that beta-endorphin, dependent on concentration and duration of stimulation, significantly affected the activation of the three mitogen-activated protein kinases in cultured human articular chondrocytes. Naltrexone in some cases significantly regulated the mitogen-activated protein kinases in different ways when added to beta-endorphin 600 ng/ml. Furthermore, specific mitogen-activated protein kinase inhibitors hindered the increase of IL-1 beta during beta-endorphin incubation. The effect of beta-endorphin seen in this study is considered critical for the production of several mediators of cartilage damage in an arthritic joint.

Opiate regulation of IL-1β and TNF-α in cultured human articular chondrocytes

In order to investigate if beta-endorphins anti-inflammatory effect in cartilage-damaging states is mediated via tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta), we examined its influence on these two cytokines in vitro. Human articular chondrocytes were obtained from patients undergoing total knee arthroplasty and stimulated with beta-endorphin (60-6000 ng/ml). Protein levels of TNF-alpha and IL-1 beta were measured by ELISA in supernatants from articular chondrocyte cultures. beta-Endorphin significantly increased the levels of IL-1 beta for all concentrations used after 15 min incubation, and when stimulated with 600 and 6000 ng/ml after 24 h incubation. The opioid-induced increase in IL-1 beta was blocked by naltrexone in the group tested. TNF-alpha expression was also significantly stimulated by 60 and 600 ng/ml beta-endorphin after 15 min, an effect blocked by naltrexone in the group tested. These findings indicate that the mechanism of beta-endorphins anti-inflammatory influence in cartilage-damaging states is not apparently mediated via these two cytokines modulation.