Identification of ATF2 as a transcriptional regulator of renin gene.

The cAMP response element (enhCRE) in the distal enhancer regulatory region of renin gene is believed to play a major role in the control of renin transcription. enhCRE binds the CRE-binding protein (CREB), which is the main transcription factor target of cAMP signaling. Using the mouse renin-producing cell line As4.1 we found that activating transcription factor-2 (ATF2) also binds to enhCRE. N-terminal phosphorylation of ATF2, which controls its transactivation, is associated with downregulation of renin gene expression by the cytokine tumor necrosis factor-α (TNFα). The ubiquitin proteasome inhibitor MG132 also phosphorylates ATF2 and inhibits renin expression. Knockdown of ATF2 attenuated the suppression of renin gene expression by MG132, thus demonstrating that ATF2 mediates the inhibitory effect of MG132. In addition, MG132 increased the DNA-binding of ATF2 as well as the ratio of bound ATF2 to CREB. Using ATF2- and CREB-Gal4 fusion protein constructs coupled with luciferase reporter system we showed that ATF2 has a weaker transactivating capacity than CREB. These data suggest that ATF2 represses renin expression by drifting the transcriptional control of renin gene away from CREB. Accordingly, TNFα completely abrogated the cAMP-dependent stimulation of renin gene expression.

Melanocortin 1 receptor-signaling deficiency results in an articular cartilage phenotype and accelerates pathogenesis of surgically induced murine osteoarthritis.

Proopiomelanocortin-derived peptides exert pleiotropic effects via binding to melanocortin receptors (MCR). MCR-subtypes have been detected in cartilage and bone and mediate an increasing number of effects in diathrodial joints. This study aims to determine the role of MC1-receptors (MC1) in joint physiology and pathogenesis of osteoarthritis (OA) using MC1-signaling deficient mice (Mc1re/e). OA was surgically induced in Mc1re/e and wild-type (WT) mice by transection of the medial meniscotibial ligament. Histomorphometry of Safranin O stained articular cartilage was performed with non-operated controls (11 weeks and 6 months) and 4/8 weeks past surgery. µCT-analysis for assessing epiphyseal bone architecture was performed as a longitudinal study at 4/8 weeks after OA-induction. Collagen II, ICAM-1 and MC1 expression was analysed by immunohistochemistry. Mc1re/e mice display less Safranin O and collagen II stained articular cartilage area compared to WT prior to OA-induction without signs of spontaneous cartilage surface erosion. This MC1-signaling deficiency related cartilage phenotype persisted in 6 month animals. At 4/8 weeks after OA-induction cartilage erosions were increased in Mc1re/e knees paralleled by weaker collagen II staining. Prior to OA-induction, Mc1re/e mice do not differ from WT with respect to bone parameters. During OA, Mc1re/e mice developed more osteophytes and had higher epiphyseal bone density and mass. Trabecular thickness was increased while concomitantly trabecular separation was decreased in Mc1re/e mice. Numbers of ICAM-positive chondrocytes were equal in non-operated 11 weeks Mc1re/e and WT whereas number of positive chondrocytes decreased during OA-progression. Unchallenged Mc1re/e mice display smaller articular cartilage covered area without OA-related surface erosions indicating that MC1-signaling is critical for proper cartilage matrix integrity and formation. When challenged with OA, Mc1re/e mice develop a more severe OA-pathology. Our data suggest that MC1-signaling protects against cartilage degradation and subchondral bone sclerosis in OA indicating a beneficial role of the POMC system in joint pathophysiology.