The roles of macromolecular synthesis and phosphorylation in the regulation of a protein kinase activity transiently stimulated by nerve growth factor

Role of protein phosphatase 2A in kidney disease (Review)

Kidney disease affects millions of people worldwide and is a financial burden on the healthcare system. Protein phosphatase 2A (PP2A), which is involved in renal development and the function of ion-transport proteins, aquaporin-2 and podocytes, is likely to serve an important role in renal processes. PP2A is associated with the pathogenesis of a variety of different kidney diseases including podocyte injury, inflammation, tumors and chronic kidney disease. The current review aimed to discuss the structure and function of PP2A subunits in the context of kidney diseases. How dysregulation of PP2A in the kidneys causes podocyte death and the inactivation of PP2A in renal carcinoma tissues is discussed. Inhibition of PP2A activity prevents epithelial-mesenchymal transition and attenuates renal fibrosis, creating a favorable inflammatory microenvironment and promoting the initiation and progression of tumor pathogenesis. The current review also indicates that PP2A serves an important role in protection against renal inflammation. Understanding the detailed mechanisms of PP2A provides information that can be utilized in the design and application of novel therapeutics for the treatment and prevention of renal diseases.

Analysis of the phosphorylation of human heat shock transcription factor-1 by MAP kinase family members

The activation of heat shock transcription factor-1 (HSF-1) after treatment of mammalian cells with stresses such as heat shock, heavy metals, or ethanol induces the synthesis of heat shock proteins. HSF-1 is phosphorylated at normal growth temperature and is hyperphosphorylated upon stress. We recently presented evidence that HSF-1 can be phosphorylated by the mitogen activated protein kinase, ERK1, and that such phosphorylation appears to negatively regulate the activity of HSF-1. In this report, we have tested the ability of ERK1 to phosphorylate various HSF-1 deletion mutants. Our results show that ERK1 phosphorylation is dependent on a region of HSF-1 extending from amino acids 280 to 308. This region contains three serine residues that are potential ERK1 phosphorylation sites. The region falls within a previously defined regulatory domain of HSF-1. The possibility of protein kinases other than ERK1 phosphorylating HSF-1 was also examined using in-gel kinase assays. The results show that HSF-1 can be phosphorylated in a ras-dependent manner by other members of the MAP kinase family such as JNK and p38 protein kinases and possibly others.

Increased levels of mitogen activated protein kinase (MAP-K) detected in the injured adult mouse sciatic nerve

Adult mouse sciatic nerves (SNs) with attached dorsal root ganglia (DRG) were analysed for the presence of mitogen activated protein kinase (MAP-K) during normal and regenerative conditions. By immunohistochemistry, MAP-K was found to be present in the normal nerve at low levels in both Schwann cells and DRG nerve cell bodies, with a profoundly increased expression during regeneration. In axonal outgrowth assays, treatment with 2 mM 2-aminopurine (2-AP), a MAP-K antagonist, inhibited the regeneration of axons from the SN as well as from the cultured superior cervical ganglia. The reduced outgrowth was probably not due to toxic effects of the drug since the ganglionic protein synthesis was not inhibited. It is possible that 2-AP interferes with regeneration-related events by inhibition of MAP-K.

Evidence for a role of protein phosphatases 1 and 2A during early nephrogenesis

Although most transcriptional events appear to be modulated by reversible protein phosphorylation, little is known about the role of this regulatory system during the development of mammalian organs. Here we have studied the serine/threonine protein phosphatases (PP) 1 and 2A in the early embryonic rat kidney with regard to expression and effects on growth and differentiation. All isoforms of PP-1 and PP-2A were ubiquitously expressed in 15-day embryonic (E15) kidneys (in situ hybridization studies). In contrast, mRNA for inhibitor-1 (I-1), an endogenous inhibitor of PP-1, was detected only in undifferentiated stem cells in the outer cortical area. I-1 is a novel marker for these cells. The abundance of the PP-1 protein, confirmed with immunoblotting, was high in the embryonic kidney. In organ culture of E13 kidneys, okadaic acid (OA), an exogenous inhibitor of PP-1 and PP-2A, dose-dependently inhibited growth and nephron formation (apparent half-maximal effect at 6 nM). OA 10 nM had little effect on the growth of cultured E15 kidneys, whereas nephron formation was disturbed and morphological evidence of apoptosis was seen. In summary, this study points towards important roles for protein phosphatases 1 and/or 2A in regulation of mitogenic activity in the early embryonic kidney.

Stimulation of vgf gene expression by NGF is mediated through multiple signal transduction pathways involving protein phosphorylation

The vgf gene encodes one of the most rapidly induced neuronal mRNAs identified in NGF-treated PC12 cells. Maximal inhibition of VGF mRNA induction was achieved using K-252a, an inhibitor of the NGF-receptor Trk tyrosine kinase, and by mutating both Y490 (SHC association site) and Y785 (PLC-gamma 1 association site) of Trk. Inhibitors of the NGF-activated protein kinase N (PKN) were found to partially and in some cases transiently block VGF induction by NGF while in PKA-deficient PC12 cells, VGF induction by NGF was comparable to that observed in parental PC12 cells. The binding of NGF to Trk therefore activates redundant signal transduction pathways which converge to regulate vgf gene expression.

Progesterone but not ras requires MPF for in vivo activation of MAPK and S6 KII: MAPK is an essential conexion point of both signaling pathways

Induction of mitosis in Xenopus laevis oocytes by hormones and the oncogenic ras-p21 protein has been shown to correlate with a cascade of phosphorylations of the Ser/Thr family of kinases. However, the exact hierarchy of enzymes and their mutual interdependency has not been fully elucidated yet. We have used the Xenopus laevis system to investigate the mechanism of activation of the Ser/Thr kinases cascade and their relationship. Comparison between progesterone-induced germinal vesicle breakdown (GVBD), a hallmark of mitosis in oocytes, to that triggered by ras-p21, revealed the existence of at least two independent mechanisms to activate the MAP kinase enzyme in vivo. While progesterone function is dependent of cdc2 protein kinase activity, ras-p21 is independent of this enzyme. However, both progesterone and ras-p21 converge at the MAP kinase level, and depletion of MAP kinase activity inhibits the GVBD and S6 kinase II activation induced by both progesterone and ras-p21. These results provides further evidence that MAP kinase is a critical step for regulation of the cell cycle in oocytes and a critical point where ras and progesterone signaling converge.

ras-p21 activates phospholipase D and A2, but not phospholipase C or PKC, in Xenopus laevis oocytes

Xenopus laevis oocytes are a powerful tool for the characterization of signal transduction pathways leading to the induction of DNA synthesis. Since activation of PLA2, PLC, or PLD has been postulated as a mediator of ras function, we have used the oocyte system to study the putative functional relationship between ras-p21 and these phospholipases. A rapid generation of PA and DAG was observed after ras-p21 microinjection, suggesting the activation of both PLC and PLD enzymes. However, production of DAG was sensitive to inhibition of the PA-hydrolase by propranolol, indicating that PLD is the enzyme responsible for the generation of both PA and DAG. Microinjection of PLD or ras-p21 induced the late production of lysophosphatidylcholine on a p42MAPK-dependent manner, an indication of the activation of a PLA2. Inhibition of this enzyme by quinacrine does not inhibit PLD- or ras-induced GVBD, suggesting that PLA2 activation is not needed for ras or PLD function. Contrary to 3T3 fibroblasts, where ras-p21 is functionally dependent for its mitogenic activity on TPA- and staurosporine-sensitive PKC isoforms, in Xenopus oocytes, induction of GVBD by ras-p21 was independent of PKC, while PLC-induced GVBD was sensitive to PKC inhibition. Thus, our results demonstrate the activation of PLD and PLA2 by ras-p21 proteins, while no effect on PLC was observed.

A purine analog-sensitive protein kinase activity associates with Trk nerve growth factor receptors

Previous studies showed that purine analogs block with varying efficiency and specificity certain effects of nerve growth factor (NGF) on PC12 cells. These compounds also inhibit protein kinase activities. The analog 6-thioguanine has thus far been shown to inhibit only protein kinase N, an NGF-activated protein kinase, whereas 2-aminopurine also blocks other kinases. In the present study, immunoprecipitates of Trk NGF receptors from PC12 cells (+/- NGF treatment) were assayed for protein kinase activity by using the substrates myelin basic protein and histone HF1 under phosphorylating conditions optimal for protein kinase N and in the presence or absence of purine analogs. Activity was detected and approximately 50-80% was inhibited by these compounds. The purine analog-sensitive activity was maximally stimulated by NGF within 5 min, was partially decreased by 10 min, and still remained over basal levels after 15 h of NGF treatment. Analysis of myelin basic protein phosphorylated by anti-Trk immunoprecipitates revealed an NGF-stimulated increase in phosphothreonine and phosphotyrosine. Phosphorylation of threonine, but not of tyrosine residues, was inhibited by 6-thioguanine, which therefore inhibits a serine/threonine kinase associated with NGF receptor rather than the receptor kinase itself. Neither 2-aminopurine nor 6-thioguanine inhibited the NGF-dependent induction of Trk-associated kinase activity. Our findings thus indicate association of a purine analog-sensitive serine/threonine protein kinase activity with Trk NGF receptors.

K-252 compounds: modulators of neurotrophin signal transduction

K-252 compounds, which share a common polyaromatic aglycon structure, are rather general and potent inhibitors of various protein kinases, including protein kinase C and tyrosine-specific protein kinases, and possibly act by interfering at or near the ATP binding site. However, chemical modifications in their sugar moiety can result in high specificity of the inhibitory action and, furthermore, can induce other stimulatory and inhibitory effects on nerve cells. These compounds are of particular interest because, in intact cells, they inhibit the actions of NGF and other neurotrophins without diminishing comparable actions of other growth factors. This effect seems to reflect a direct inhibitory action on trk neurotrophin receptor proteins. At concentrations lower than those necessary to inhibit neurotrophin actions, K-252a and K-252b have been shown to potentiate the stimulatory effects of NT-3 on different neurons in culture and on PC12 cells. The structural requirements for this effect seem to be different from those for the inhibition of neurotrophin actions. These findings raise the possibility of development of compounds of high selectivity, able to inhibit or potentiate the transduction mechanisms of individual neurotrophins, and identify K-252a and K-252b as lead compounds for the development of such selective molecules. Specific inhibitors and stimulators of neurotrophins would be valuable tools to investigate biological functions of the neurotrophins in vitro and in vivo. Furthermore, it is possible that, in the future, highly selective drugs with agonistic or antagonistic actions on neurotrophin mechanisms could become therapeutically useful in the treatment of neurological disease and injury.

NGF and other growth factors induce an association between ERK1 and the NGF receptor, gp140prototrk

As detected by coimmunoprecipitation from PC12 cells, NGF induces rapid association between ERK1 (a growth factor-activated serine/threonine protein kinase) and gp140prototrk NGF receptors. In contrast, no such association is found with the closely related ERK2. Anti-trk immunocomplexes generated from NGF-treated cells also contain protein kinase activity that shares many properties with soluble ERK1. The association of both ERK1 protein and ERK-like kinase activity with gp140prototrk is maximal by 5 min of NGF treatment, persists for approximately 1 hr, and subsequently declines by 18 hr. Treatment with either basic fibroblast growth factor, epidermal growth factor, or orthovanadate also leads to association of ERK1 with gp140prototrk without tyrosine phosphorylation of the latter. The interaction between ERK1 and gp140prototrk may prove relevant to the NGF mechanism.

PC12 cell neuronal differentiation is associated with prolonged p21ras activity and consequent prolonged ERK activity

Expression of oncogenic ras in PC12 cells causes neuronal differentiation and sustained protein tyrosine phosphorylation and activity of extracellular signal-regulated kinases (ERKs), p42erk2 and p44erk1. Oncogenic N-ras-induced neuronal differentiation is inhibited by compounds that block ERK protein tyrosine phosphorylation or ERK activity, indicating that ERKs are not only activated by p21ras but serve as the primary downstream effectors of p21ras. Treatment of PC12 cells with nerve growth factor or fibroblast growth factor results in neuronal differentiation and in a sustained elevation of p21ras activity, of ERK activity, and of ERK tyrosine phosphorylation. Epidermal growth factor, which does not cause neuronal differentiation, stimulates only transient (< 1 hr) activation of p21ras and ERKs. These data indicate that transient activation of p21ras and, consequently, ERKs is not sufficient for induction of neuronal differentiation. Prolonged ERK activity is required: a consequence of sustained activation of p21ras by the growth factor receptor protein tyrosine kinase.

ERKs, extracellular signal-regulated MAP-2 kinases

A family of protein kinases, known alternatively as microtubule-associated protein-2/myelin basic protein kinases or extracellular signal-regulated kinases, is activated by numerous hormones, growth factors and other extracellular stimuli. At least two members of this family function as intermediate kinases in protein phosphorylation cascades. Their mechanisms of activation may involve autophosphorylation, which occurs on both threonine and tyrosine residues.

Inhibition of nerve growth factor-induced neurite outgrowth of PC12 cells by a protein kinase inhibitor which does not permeate the cell membrane

K-252a, a protein kinase inhibitor of microbial origin, has proven to be a specific inhibitor of nerve growth factor. In this study, the effects of K-252b, the 9-carboxylic acid derivative of K-252a, on nerve growth factor-induced neurite outgrowth in PC12 cells was examined. K-252b is hydrophilic and does not permeate the cell membrane of PC12 cells, whereas K-252a clearly does. K-252b is, however, as potent as K-252a itself in inhibiting the nerve growth factor-induced neurite outgrowth. These results can be interpreted to suggest that effects of K-252b may be through surface-bound/anchored K-252b-sensitive molecules on PC12 cells.