Prostaglandins (PGs) of the J2 series form in vivo and exert effects on a variety of biological processes. While most of PGs mediate their effects through G protein-coupled receptors, the mechanism of action for the J2 series of PGs remains unclear. Here, we report the PGJ2 and its derivatives are efficacious activators of peroxisome proliferator-activated receptors alpha and gamma (PPAR alpha and PPAR gamma, respectively), orphan nuclear receptors implicated in lipid homeostasis and adipocyte differentiation. The PGJ2 metabolite 15-deoxy-delta 12,14-PGJ2 binds directly to PPAR gamma and promotes efficient differentiation of C3H10T1/2 fibroblasts to adipocytes. These data provide strong evidence that a fatty acid metabolite can function as an adipogenic agent through direct interactions with PPAR gamma and furthermore, suggest a novel mechanism of action for PGs of the J2 series.

The genetic loci agouti and extension control the relative amounts of eumelanin (brown-black) and phaeomelanin (yellow-red) pigments in mammals: extension encodes the receptor for melanocyte-stimulating hormone (MSH) and agouti encodes a novel 131-amino-acid protein containing a signal sequence. Agouti, which is produced in the hair follicle, acts on follicular melanocytes to inhibit alpha-MSH-induced eumelanin production, resulting in the subterminal band of phaeomelanin often visible in mammalian fur. Here we use partially purified agouti protein to demonstrate that agouti is a high-affinity antagonist of the MSH receptor and blocks alpha-MSH stimulation of adenylyl cyclase, the effector through which alpha-MSH induces eumelanin synthesis. Agouti was also found to be an antagonist of the melanocortin-4 receptor, a related MSH-binding receptor. Consequently, the obesity caused by ectopic expression of agouti in the lethal yellow (Ay) mouse may be due to the inhibition of melanocortin receptor(s) outside the hair follicle.

Cartilage specimens from osteoarthritis (OA)-affected patients spontaneously released PGE 2 at 48 h in ex vivo culture at levels at least 50-fold higher than in normal cartilage and 18-fold higher than in normal cartilage ϩ cytokines ϩ endotoxin. The superinduction of PGE 2 production coincides with the upregulation of cyclooxygenase-2 (COX-2) in OA-affected cartilage. Production of both nitric oxide (NO) and PGE 2 by OA cartilage explants is regulated at the level of transcription and translation. Dexamethasone inhibited only the spontaneously released PGE 2 production, and not NO, in OA-affected cartilage. The NO synthase inhibitor H N G -monomethyl-L -arginine monoacetate inhibited OA cartilage NO production by Ͼ 90%, but augmented significantly (twofold) the spontaneous production of PGE 2 in the same explants. Similarly, addition of exogenous NO donors to OA cartilage significantly inhibited PGE 2 production. Cytokine ϩ endotoxin stimulation of OA explants increased PGE 2 production above the spontaneous release. Addition of L -NMMA further augmented cytokine-induced PGE 2 production by at least fourfold. Inhibition of PGE 2 by COX-2 inhibitors (dexamethasone or indomethacin) or addition of exogenous PGE 2 did not significantly affect the spontaneous NO production. These data indicate that human OA-affected cartilage in ex vivo conditions shows ( a ) superinduction of PGE 2 due to upregulation of COX-2, and ( b ) spontaneous release of NO that acts as an autacoid to attenuate the production of the COX-2 products such as PGE 2 . These studies, together with others, also suggest that PGE 2 may be differentially regulated in normal and OA-affected chondrocytes. ( J. Clin. Invest. 1997. 99:1231-1237.)

Tetracyclines have recently been shown to have ''chondroprotective'' effects in inf lammatory arthritides in animal models. Since nitric oxide (NO) is spontaneously released from human cartilage affected by osteoarthritis (OA) or rheumatoid arthritis in quantities sufficient to cause cartilage damage, we evaluated the effect of tetracyclines on the expression and function of human OA-affected nitric oxide synthase (OA-NOS) and rodent inducible NOS (iNOS). Among the tetracycline group of compounds, doxycycline > minocycline blocked and reversed both spontaneous and interleukin 1␤-induced OA-NOS activity in ex vivo conditions. Similarly, minocycline > doxycycline inhibited both lipopolysaccharide-and interferon-␥-stimulated iNOS in RAW 264.7 cells in vitro, as assessed by nitrite accumulation. Although both these enzyme isoforms could be inhibited by doxycycline and minocycline, their susceptibility to each of these drugs was distinct. Unlike acetylating agents or competitive inhibitors of L-arginine that directly inhibit the specific activity of NOS, doxycycline or minocycline has no significant effect on the specific activity of iNOS in cell-free extracts. The mechanism of action of these drugs on murine iNOS expression was found to be, at least in part, at the level of RNA expression and translation of the enzyme, which would account for the decreased iNOS protein and activity of the enzyme. Tetracyclines had no significant effect on the levels of mRNA for ␤-actin and glyceraldehyde-3-phosphate dehydrogenase nor on levels of protein of ␤-actin and cyclooxygenase 2 expression. These studies indicate that a novel mechanism of action of tetracyclines is to inhibit the expression of NOS. Since the overproduction of NO has been implicated in the pathogenesis of arthritis, as well as other inf lammatory diseases, these observations suggest that tetracyclines should be evaluated as potential therapeutic modulators of NO for various pathological conditions.

Nitric oxide synthesized by inducible nitric oxide synthase (iNOS) has been implicated as a mediator of inflammation in rheumatic and autoimmune diseases. We report that exposure of lipopolysaccharide-stimulated murine macrophages to therapeutic concentrations of aspirin (IC50 = 3 mM) and hydrocortisone (IC50 = 5 ,uM) inhibited the expression of iNOS and production of nitrite. In contrast, sodium salicylate (1-3 mM), indomethacin (5-20 ,uM), and acetaminophen (60-120 ,uM) had no significant effect on the production of nitrite at pharmacological concentrations. At suprapharmacological concentrations, sodium salicylate (IC50 = 20 mM) significantly inhibited nitrite production.Immunoblot analysis of iNOS expression in the presence of aspirin showed inhibition of iNOS expression (IC5s = 3 mM). Sodium salicylate variably inhibited iNOS expression (0-35%), whereas indomethacin had no effect. Furthermore, there was no significant effect of these nonsteroidal antiinflammatory drugs on iNOS mRNA expression at pharmacological concentrations. The effect of aspirin was not due to inhibition of cyclooxygenase 2 because both aspirin and indomethacin inhibited prostaglandin E2 synthesis by >75%. Aspirin and N-acetylimidazole (an effective acetylating agent), but not sodium salicylate or indomethacin, also directly interfered with the catalytic activity of iNOS in cell-free extracts. These studies indicate that the inhibition of iNOS expression and function represents another mechanism of action for aspirin, if not for all aspirin-like drugs. The effects are exerted at the level of translational/posttranslational modification and directly on the catalytic activity of iNOS.Nitric oxide (NO), first identified as an endothelium-derived relaxation factor (1), is now recognized to be an intra-and extracellular mediator of cell function (2-5). NO produced by the constitutive isoform of nitric oxide synthase (NOS) is a key regulator of homeostasis, whereas the generation of NO by inducible NOS (iNOS) plays an important role in inflammation, host-defense responses, and tissue repair (2-4). NO formation is increased during inflammation (rheumatoid arthritis, and ulcerative colitis, Crohn disease), and several classic inflammatory symptoms (erythema and vascular leakiness) are reversed by NOS inhibitors (2-4). Vane and coworkers (6) have implicated NO as an important mediator of inflammation in animal models. Furthermore, because iNOS is up-regulated by endotoxin, interleukin 1, tumor necrosis factor, and interferon y, the increased synthesis of NO has been implicated in autoimmune diseases, allograft rejection, graft-versus-host disease, and systemic response to sepsis. Recent studies by Salvemini et al. (7) have shown that NO modulates the activity of prostaglandin endoperoxide H synthase 2 [cyclooxygenase 2 (COX-2)] in a concentrationdependent manner, through a mechanism independent of cGMP.Although nonsteroidal antiinflammatory drugs (NSAIDs) clearly inhibit the synthesis and release of prostaglandins (8, 9), these actions ar...

Interleukin 1 (IL-1), produced by both synovial cells and chondrocytes, plays a pivotal role in the pathogenesis of cartilage destruction in osteoarthritis (OA). We examined the specific expression and function of IL-1 receptor family-related genes in human joint tissues. Gene array analysis of human normal and OA-affected cartilage showed mRNA expression of IL-1 receptor accessory protein (IL-1RAcp) and IL-1 type I receptor (IL-1RI), but not IL-1 antagonist (IL-1ra) and IL-1 type II decoy receptor (IL-1RII). Similarly, human synovial and epithelial cells showed an absence of IL-1RII mRNA. Functional genomic analyses showed that soluble (s) IL-1RII, at picomolar concentrations, but not soluble TNF receptor:Fc, significantly inhibited IL-1␤-induced nitric oxide (NO) and/or prostaglandin E 2 production in chondrocytes, synovial and epithelial cells. In OA-affected cartilage, the IC 50 for inhibition of NO production by sIL-1RII was 2 log orders lower than that for sIL-1RI. Human chondrocytes that overexpressed IL-1RII were resistant to IL-1-induced IL-1␤ mRNA accumulation and inhibition of proteoglycan synthesis. In osteoarthritis, deficient expression by chondrocytes of innate regulators or antagonists of IL-1 such as IL-1ra and IL-1RII (soluble or membrane form) may allow the catabolic effects of IL-1 to proceed unopposed. The sensitivity of IL-1 action to inhibition by sIL-1RII has therapeutic implications that could be directed toward correcting this unfavorable tissue(s) dependent imbalance. Osteoarthritis (OA)1 is considered a non-inflammatory arthritis, characterized by a limited infiltration of neutrophils into the synovial space and, in general, an absence of the classical signs of inflammation. Chondrocytes embedded within articular cartilage that is avascular and aneural reside in a sequestered environment, perhaps more than other cell types, whereby cellular metabolism is regulated by an autocrine mechanism responsive to biomechanical and pericellular signals. Recent observations by this and other laboratories indicate that, despite the general absence of clinical signs of inflammation, chondrocytes derived from patients with OA, show superinduction of proinflammatory genes typically associated with the products of synovial tissues in rheumatoid arthritis, including nitric-oxide synthase, cyclooxygenase-2, TNF␣, IL-6, and IL-8. The spontaneous production of the corresponding gene products and inflammatory mediators promotes a catabolic state, which leads to progressive cartilage damage in OA (1-4). This intraarticular inflammatory response in OA-affected cartilage, which may be considered as an in situ "molecular inflammation," is partially dependent on autocrine IL-1␤ production, which induces and sustains an imbalance of cartilage homeostasis and extracellular matrix synthesis (5). The autocrine production of IL-1 in OA-affected cartilage is amplified by engagement of integrins such as ␣ 5 ␤ 1 by abnormally expressed extracellular matrix proteins, including proteolytic fragments of fibronectin (5, 6)...

Several dominant mutations at the agouti locus in the mouse cause a syndrome of marked obesity, hyperinsulinemia, and insulin resistance. Although it is known that the agouti gene is expressed in an ectopic manner in these mutants, the precise mechanism by which the agouti gene product mediates these effects is unclear. Since intra-

Pages 9465 and 9466. The sequence shown as the cleavage site of erbB4/HER4 (HGLSLPVENRLYTYDH) in Figure 1 and Table 1 is actually the cleavage site of the heparinbinding epidermal growth factor (HB-EGF). Moreover, reaction products shown in Figure 2 (bottom right panel) correspond also to the cleavage of HB-EGF by TACE, not HER4. Therefore, every reference in Materials and Methods and Results to erbB4/HER4 actually corresponds to HB-EGF. While this inadvertant mislabeling of the HB-EGF substrate as HER4 is unfortunate, it does not change any of the conclusions of our paper. We sincerely apologize for any confusion that this might have caused among readers.