Mitogen-activated protein (MAP) kinases are important players in signal transduction pathways activated by a range of stimuli and mediate a number of physiological and pathological changes in cell function. MAP kinase activation requires phosphorylation on a threonine and tyrosine residue located within the activation loop of kinase subdomain VIII. This process is reversible even in the continued presence of activating stimuli, indicating that protein phosphatases provide an important mechanism for MAP kinase control. Dual specificity phosphatases (DSPs) are an emerging subclass of the protein tyrosine phosphatase (PTP) gene superfamily, which appears to be selective for dephosphorylating the critical phosphothreonine and phosphotyrosine residues within MAP kinases. Some DSPs are localized to different subcellular compartments and moreover, certain family members appear highly selective for inactivating distinct MAP kinase isoforms. This enzymatic specificity is due in part to powerful catalytic activation of the DSP phosphatase after tight binding of its amino-terminal to the target MAP kinase. DSP gene expression is induced strongly by various growth factors and/or cellular stresses, providing a sophisticated transcriptional mechanism for targeted inactivation of selected MAP kinase activities.

Phosphoinositide 3-kinases (PI3K) have long been considered promising drug targets for the treatment of inflammatory and autoimmune disorders as well as cancer and cardiovascular diseases. But the lack of specificity, isoform selectivity and poor biopharmaceutical profile of PI3K inhibitors have so far hampered rigorous disease-relevant target validation. Here we describe the identification and development of specific, selective and orally active small-molecule inhibitors of PI3Kgamma (encoded by Pik3cg). We show that Pik3cg(-/-) mice are largely protected in mouse models of rheumatoid arthritis; this protection correlates with defective neutrophil migration, further validating PI3Kgamma as a therapeutic target. We also describe that oral treatment with a PI3Kgamma inhibitor suppresses the progression of joint inflammation and damage in two distinct mouse models of rheumatoid arthritis, reproducing the protective effects shown by Pik3cg(-/-) mice. Our results identify selective PI3Kgamma inhibitors as potential therapeutic molecules for the treatment of chronic inflammatory disorders such as rheumatoid arthritis.

Dual-specificity phosphatases (DUSPs) are a subset of protein tyrosine phosphatases, many of which dephosphorylate threonine and tyrosine residues on mitogen-activated protein kinases (MAPKs), and hence are also referred to as MAPK phosphatases (MKPs). The regulated expression and activity of DUSP family members in different cells and tissues controls MAPK intensity and duration to determine the type of physiological response. For immune cells, DUSPs regulate responses in both positive and negative ways, and DUSP-deficient mice have been used to identify individual DUSPs as key regulators of immune responses. From a drug discovery perspective, DUSP family members are promising drug targets for manipulating MAPK-dependent immune responses in a cell-type and disease-context-dependent manner, to either boost or subdue immune responses in cancers, infectious diseases or inflammatory disorders.

MAP kinase phosphatase-3 (MKP-3) dephosphorylates phosphotyrosine and phosphothreonine and inactivates selectively ERK family mitogen-activated protein (MAP) kinases. MKP-3 was activated by direct binding to purified ERK2. Activation was independent of protein kinase activity and required binding of ERK2 to the noncatalytic amino-terminus of MKP-3. Neither the gain-of-function Sevenmaker ERK2 mutant D319N nor c-Jun amino-terminal kinase-stress-activated protein kinase (JNK/SAPK) or p38 MAP kinases bound MKP-3 or caused its catalytic activation. These kinases were also resistant to enzymatic inactivation by MKP-3. Another homologous but nonselective phosphatase, MKP-4, bound and was activated by ERK2, JNK/SAPK, and p38 MAP kinases. Catalytic activation of MAP kinase phosphatases through substrate binding may regulate MAP kinase activation by a large number of receptor systems.Signal transduction pathways that lead to activation of MAP kinases control many diverse and essential functions in yeast, worms, flies, and mammals. Extracellular signal-regulated kinase-1 (ERK1) and ERK2 exemplify one class of MAP kinase that undergoes activation by a range of stimuli including growth factors, cytokines, cell adhesion, tumor-promoting phorbol esters, and oncogenes (1). Specific functions assigned to ERK activity include chemotaxis, neuronal differentiation, and synaptic changes underlying memory and learning, as well as cellular mitogenesis and oncogenic transformation (1, 2).Full activation of ERK requires phosphorylation of threonine and tyrosine residues by a class of MAP kinase/ERK kinase (MEK) exemplified by MEK-1 (1, 3). Conversely, an emerging family of dual-specificity phosphatases that act on both phosphotyrosine and phosphothreonine reverse this process and also appear to be critical regulators of MAP kinase activity. CL100/3CH134 or MAP kinase phosphatase-1 (MKP-1) is the archetypal member of this gene family and has high substrate specificity for MAP kinases (4). Up to nine other mammalian dual-specificity phosphatases have been identified, and several of these are under tight transcriptional control and display distinct tissue, cell, and subcellular expression patterns (5, 6). MKP-3 appears exceptional in that it specifically inactivates ERK as compared with c-Jun NH 2 -terminal kinases/stress-activated protein kinases (JNK/SAPK) or p38 MAP kinases (7). We now show that ERK, but not other MAP kinases, cause substratetriggered activation of MKP-3.We purified various MKP-3 deletion mutants expressed in Escherichia coli and found that the NH 2 -terminal noncatalytic domain (amino acids 1 to 221) binds tightly to its target MAP kinases p44 ERK1 and p42 ERK2 (8). Binding to purified ERK2 (9) stimulates p-nitrophenyl phosphate (p-NPP) phosphatase activity of full-length MKP-3 by up to 30-fold (Fig. 1A) (10). Both glutathione-S-transferase (GST)-ERK2 and ERK2 caused similar activation of either the fusion protein GST-MKP-3, His-tagged MKP-3, or free MKP-3 (11). Activation of MKP-3 was dose-dependent and saturable wit...

Dysregulated signal transduction in innate and adaptive immune cells is known to be associated with the development of various autoimmune and inflammatory diseases. Consequently, targeting intracellular signalling of the pro-inflammatory cytokine network heralds hope for the next generation of anti-inflammatory drugs. Phosphoinositide 3-kinases (PI3Ks) generate lipid-based second messengers that control an array of intracellular signalling pathways that are known to have important roles in leukocytes. In light of the recent progress in the development of selective PI3K inhibitors, and the beneficial effects of these inhibitors in models of acute and chronic inflammatory disorders, we discuss the therapeutic potential of blocking PI3K isoforms for the treatment of rheumatoid arthritis and other immune-mediated diseases.