Nerve growth factor activates kinases that phosphorylate atypical protein kinase C

Src tyrosine kinase regulates angiotensin II-induced protein kinase Czeta activation and proliferation in vascular smooth muscle cells

Protein kinase Czeta (PKCzeta) isoform plays a critical role in angiotensin II (AngII)-elicited extracellular signal-regulated kinase 1/2 (ERK1/2) activation and proliferation in vascular smooth muscle cells (VSMCs). However, the exact signal transduction mechanism by which AngII activates PKCzeta has not been clarified. In this study, we investigated the role of Src in PKCzeta activation and VSMCs proliferation induced by AngII. AngII-induced rapid activation of PKCzeta, which was associated with its phosphorylation and nuclear translocation. AngII not only induced Src activation but also promoted the physical association between Src and PKCzeta, which was abolished by Src inhibition with PP2. Src inhibition also abrogated AngII-stimulated PKCzeta activation, ERK1/2 phosphorylation and VSMCs proliferation. In conclusion, Src kinase plays an important role in AngII-elicited PKCzeta activation and the subsequent downstream signaling including ERK1/2 activation and VSMCs proliferation.

The role of PKCzeta in NMDA-induced retinal ganglion cell death: prevention by aspirin

Intravitreal NMDA injection has been shown to induce the excitotoxic loss of retinal cells. The retinal ganglion cell apoptosis induced by NMDA is thought to play an important role in retinal ischemia injury and NMDA-injected rat has been used as a model of neuronal loss in diseases such as glaucoma. In this experimental model, we studied the early effects of NMDA leading to the degeneration of retinal ganglion cells. PKCzeta regulates the NF-kappaB pathway in cellular responses to various stresses and we have shown that aspirin inhibits purified human PKCzeta. We therefore investigated the molecular mechanism by which retinal cells limit ocular injury following NMDA treatment. We found that the NMDA-induced apoptosis of ganglion cells was mediated, at least partly, by PKCzeta. This enzyme was activated early in the cellular response to NMDA. Prolonged activation was followed by PKCzeta cleavage, and nuclear translocation of the C-terminal region of this protein-a critical event for the survival of retinal cells. We also found that pretreatment with aspirin or the coinjection of NMDA with a specific PKCzeta inhibitor counteracted the effects of NMDA. These findings provide new insight into the role played by PKCzeta in neuronal loss in glaucoma.

Activation loop phosphorylation-independent kinase activity of human protein kinase C zeta

Atypical protein kinase C zeta (PKCzeta) plays an important role in cell proliferation and survival. PKCzeta and its truncated form containing only the kinase domain, CATzeta, have been reported to be activated by the phosphorylation of threonine 410 in the activation loop. We expressed both the full length PKCzeta and CATzeta in a baculovirus/insect cell over-expression system and purified the proteins for biochemical characterization. Ion exchange chromatography of CATzeta revealed three species with different levels of phosphorylation at Thr-410 and allowed the isolation of the CATzeta protein devoid of phosphorylation at Thr-410. All three species of CATzeta were active and their activity was not correlated with phosphorylation at Thr-410, indicating that the kinase activity of CATzeta did not depend solely on activation loop phosphorylation. Tyrosine phosphorylation was detected in all three species of CATzeta and the full length PKCzeta. Homology structural modeling of PKCzeta revealed a conserved, predicted-to-be phosphorylated tyrosine residue, Tyr-428, in the close proximity of the RD motif of the catalytic loop and of Thr-410 in the activation loop. The structural analysis indicated that phospho-Tyr-428 would interact with two key, positively-charged residues to form a triad conformation similar to that formed by phospho-Thr-410. Based on these observations, it is possible that the Thr-410 phosphorylation-independent kinase activity of CATzeta is regulated by the phosphorylation of Tyr-428. This alternative mode of PKCzeta activation is supported by the observed stimulation of PKCzeta kinase activity upon phosphorylation at the equivalent site by Abl, and may be involved in resistance to drug-induced apoptosis.

Neurotrophin-regulated signalling pathways

Neurotrophins are a family of closely related proteins that were identified initially as survival factors for sensory and sympathetic neurons, and have since been shown to control many aspects of survival, development and function of neurons in both the peripheral and the central nervous systems. Each of the four mammalian neurotrophins has been shown to activate one or more of the three members of the tropomyosin-related kinase (Trk) family of receptor tyrosine kinases (TrkA, TrkB and TrkC). In addition, each neurotrophin activates p75 neurotrophin receptor (p75NTR), a member of the tumour necrosis factor receptor superfamily. Through Trk receptors, neurotrophins activate Ras, phosphatidyl inositol-3 (PI3)-kinase, phospholipase C-gamma1 and signalling pathways controlled through these proteins, such as the MAP kinases. Activation of p75NTR results in activation of the nuclear factor-kappaB (NF-kappaB) and Jun kinase as well as other signalling pathways. Limiting quantities of neurotrophins during development control the number of surviving neurons to ensure a match between neurons and the requirement for a suitable density of target innervation. The neurotrophins also regulate cell fate decisions, axon growth, dendrite growth and pruning and the expression of proteins, such as ion channels, transmitter biosynthetic enzymes and neuropeptide transmitters that are essential for normal neuronal function. Continued presence of the neurotrophins is required in the adult nervous system, where they control synaptic function and plasticity, and sustain neuronal survival, morphology and differentiation. They also have additional, subtler roles outside the nervous system. In recent years, three rare human genetic disorders, which result in deleterious effects on sensory perception, cognition and a variety of behaviours, have been shown to be attributable to mutations in brain-derived neurotrophic factor and two of the Trk receptors.

Neuronal apoptosis linked to EglN3 prolyl hydroxylase and familial pheochromocytoma genes: developmental culling and cancer

Germline NF1, c-RET, SDH, and VHL mutations cause familial pheochromocytoma. Pheochromocytomas derive from sympathetic neuronal precursor cells. Many of these cells undergo c-Jun-dependent apoptosis during normal development as NGF becomes limiting. NF1 encodes a GAP for the NGF receptor TrkA, and NF1 mutations promote survival after NGF withdrawal. We found that pheochromocytoma-associated c-RET and VHL mutations lead to increased JunB, which blunts neuronal apoptosis after NGF withdrawal. We also found that the prolyl hydroxylase EglN3 acts downstream of c-Jun and is specifically required among the three EglN family members for apoptosis in this setting. Moreover, EglN3 proapoptotic activity requires SDH activity because EglN3 is feedback inhibited by succinate. These studies suggest that failure of developmental apoptosis plays a role in pheochromocytoma pathogenesis.

Aspirin prevention of NMDA-induced neuronal death by direct protein kinase Cζ inhibition

Abstract Aspirin has been shown to protect against glutamate neurotoxicity via the nuclear factor kappaB pathway. Some studies have implicated the atypical protein kinase C (PKC) zeta (zeta) isoform in cell protection, but the mechanism involved remains unclear. We show here that aspirin exerts at least some of its effects through PKCzeta, decreasing the NMDA-induced activation, cleavage and nuclear translocation of this molecule. Aspirin (acetylsalicylic acid) directly inhibited the protein kinase activity of PKCzeta, whereas salicylic acid did not. This direct effect of aspirin on purified human PKCzeta is consistent with PKCzeta inhibition preventing the NMDA-induced death of cortical neurones. Caspase-3 inhibition blocked the cleavage and nuclear translocation of PKCzeta, whereas caspase-1-inhibition did not. Thus, PKCzeta (protein kinase Mzeta) regulates nuclear events essential for the initiation of the apoptotic pathway. Aspirin protects cells against NMDA-induced apoptosis by means of a novel mechanism targeting PKCzeta, a key molecule in inflammatory responses and neurodegeneration.

Trk receptors: roles in neuronal signal transduction

Trk receptors are a family of three receptor tyrosine kinases, each of which can be activated by one or more of four neurotrophins-nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophins 3 and 4 (NT3 and NT4). Neurotrophin signaling through these receptors regulates cell survival, proliferation, the fate of neural precursors, axon and dendrite growth and patterning, and the expression and activity of functionally important proteins, such as ion channels and neurotransmitter receptors. In the adult nervous system, the Trk receptors regulate synaptic strength and plasticity. The cytoplasmic domains of Trk receptors contain several sites of tyrosine phosphorylation that recruit intermediates in intracellular signaling cascades. As a result, Trk receptor signaling activates several small G proteins, including Ras, Rap-1, and the Cdc-42-Rac-Rho family, as well as pathways regulated by MAP kinase, PI 3-kinase and phospholipase-C-gamma (PLC-gamma). Trk receptor activation has different consequences in different cells, and the specificity of downstream Trk receptor-mediated signaling is controlled through expression of intermediates in these signaling pathways and membrane trafficking that regulates localization of different signaling constituents. Perhaps the most fascinating aspect of Trk receptor-mediated signaling is its interplay with signaling promoted by the pan-neurotrophin receptor p75NTR. p75NTR activates a distinct set of signaling pathways within cells that are in some instances synergistic and in other instances antagonistic to those activated by Trk receptors. Several of these are proapoptotic but are suppressed by Trk receptor-initiated signaling. p75NTR also influences the conformations of Trk receptors; this modifies ligand-binding specificity and affinity with important developmental consequences.

In-gel kinase assay as a method to identify kinase substrates

Phosphorylation of proteins by kinases is central to many cellular processes, including signal transduction. Thus, assays to identify or characterize kinases are a key tool for research in this area. Kinase substrates can be incorporated into polyacrylamide gels and used to characterize kinase activity in mixed samples. This methodology can be adapted for the identification of novel kinase-substrates or kinase-kinases that participate in the regulation of cell signaling. Here, I review the rationale and principles of an in-gel kinase assay. This strategy relies on co-polymerization of a substrate within the gel matrix, followed by detection of enzymatic activity in situ. The following Protocol provides a detailed method for performing the in-gel kinase assay and discusses the uses of the assay to evaluate kinase activity in the context of proliferation, differentiation, and survival pathways.