Jun Xing

A signaling pathway has been elucidated whereby growth factors activate the transcription factor cyclic adenosine monophosphate response element-binding protein (CREB), a critical regulator of immediate early gene transcription. Growth factor-stimulated CREB phosphorylation at serine-133 is mediated by the RAS-mitogen-activated protein kinase (MAPK) pathway. MAPK activates CREB kinase, which in turn phosphorylates and activates CREB. Purification, sequencing, and biochemical characterization of CREB kinase revealed that it is identical to a member of the pp90(RSK) family, RSK2. RSK2 was shown to mediate growth factor induction of CREB serine-133 phosphorylation both in vitro and in vivo. These findings identify a cellular function for RSK2 and define a mechanism whereby growth factor signals mediated by RAS and MAPK are transmitted to the nucleus to activate gene expression.

The mechanisms by which growth factor-induced signals are propagated to the nucleus, leading to the activation of the transcription factor CREB, have been characterized. Nerve growth factor (NGF) was found to activate multiple signaling pathways that mediate the phosphorylation of CREB at the critical regulatory site, serine 133 (Ser-133). NGF activates the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases (MAPKs), which in turn activate the pp90 ribosomal S6 kinase (RSK) family of Ser/Thr kinases, all three members of which were found to catalyze CREB Ser-133 phosphorylation in vitro and in vivo. In addition to the ERK/RSK pathway, we found that NGF activated the p38 MAPK and its downstream effector, MAPK-activated protein kinase 2 (MAPKAP kinase 2), resulting in phosphorylation of CREB at Ser-133. Inhibition of either the ERK/RSK or the p38/MAPKAP kinase 2 pathway only partially blocked NGF-induced CREB Ser-133 phosphorylation, suggesting that either pathway alone is sufficient for coupling the NGF signal to CREB activation. However, inhibition of both the ERK/RSK and the p38/MAPKAP kinase 2 pathways completely abolished NGF-induced CREB Ser-133 phosphorylation. These findings indicate that NGF activates two distinct MAPK pathways, both of which contribute to the phosphorylation of the transcription factor CREB and the activation of immediate-early genes.

A surface loop (25/50-kDa loop) near the nucleotide pocket of myosin has been proposed to be an important element in determining the rate of ADP release from myosin, and as a consequence, the rate of actin-myosin filament sliding (Spudich, J. A. (1991) Nature 372, 515-518). To test this hypothesis, loops derived from different myosin II isoforms that display a range of actin filament sliding velocities were inserted into a smooth muscle myosin backbone. Chimeric myosins were produced by baculovirus/Sf9 cell expression. Although the nature of this loop affected the rate of ADP release (up to 9-fold), in vitro motility (2.7-fold), and the Vmax of actin-activated ATPase activity (up to 2-fold), the properties of each chimera did not correlate with the relative speed of the myosin from which the loop was derived. Rather, the rate of ADP release was a function of loop size/flexibility with the larger loops giving faster rates of ADP release. The rate of actin filament translocation was altered by the rate of ADP release, but was not solely determined by it. Through a combination of solute quenching and transient fluorescence measurements, it is concluded that, as the loop gets smaller, access to the nucleotide pocket is more restricted, ATP binding becomes less favored, and ADP binding becomes more favored. In addition, the rate of ATP hydrolysis is slowed.

OBJECTIVE: To evaluate the efficacy of clazakizumab, a monoclonal antibody with high affinity and specificity for the interleukin-6 (IL-6) cytokine, in psoriatic arthritis (PsA). METHODS: In this randomized, double-blind, placebo-controlled, dose-ranging study (ClinicalTrials. gov identifier: NCT01490450), patients with active PsA and an inadequate response to nonsteroidal antiinflammatory drugs were randomized (1:1:1:1) to receive subcutaneous placebo or clazakizumab 25 mg, 100 mg, or 200 mg every 4 weeks, with or without methotrexate. The primary end point was the response rate according to the American College of Rheumatology 20% criteria for improvement (ACR20) at week 16, with secondary efficacy end points at weeks 16 and 24. RESULTS: A total of 165 patients were randomized. At week 16, the ACR20 response rate was significantly higher with clazakizumab 100 mg versus placebo (52.4% versus 29.3%; P = 0.039). ACR20 response rates at week 16 were 46.3% with clazakizumab 25 mg (P = 0.101 versus placebo) and 39.0% with clazakizumab 200 mg (P = 0.178 versus placebo). ACR50/ACR70 response rates were numerically higher with clazakizumab versus placebo at weeks 16 and 24. Compared with placebo, clazakizumab treatment significantly improved musculoskeletal manifestations (joint signs and symptoms, enthesitis, and dactylitis), with minimal improvements in skin disease, without clear evidence of a dose response. Clazakizumab was well tolerated. CONCLUSION: This is the first clinical trial of an IL-6-targeted therapy in PsA. Clazakizumab may be an effective treatment option for musculoskeletal aspects of PsA, but because of the lack of a dose response in this study, further studies are required to confirm the appropriate dose. The safety profile is consistent with the pharmacology of IL-6 blockade and prior clinical experience with this antibody in rheumatoid arthritis.