Regulation of beta-amyloid precursor protein expression by brain-derived neurotrophic factor involves activation of both the Ras and phosphatidylinositide 3-kinase signalling pathways

Brain-derived neurotrophic factor (BDNF) stimulates beta-amyloid precursor protein (APP) promoter activity by a Ras-dependent mechanism in TrkB-expressing SH-SY5Y cells. To determine the signalling pathways involved in the BDNF-induced response, we have analysed the ability of TrkB mutated forms to mediate promoter stimulation. Brain-derived neurotrophic factor causes a significant induction of promoter activity and mutation K540R in the active site of TrkB completely abolishes the neurotrophin-induced response. A substitution of the Y484 residue by phenylalanine, which blocks binding of Shc, reduces the activation of APP promoter by BDNF by approximately 50% whereas mutation Y785P, which blocks binding of phospholipase C gamma, does not affect the response. In addition, the phosphatidylinositide 3-kinase (PI3K)-specific inhibitors wortmannin and LY294002 reduced BDNF-induced activation. In agreement with a participation of both Ras/MAPK- and PI3K/Akt-mediated mechanisms, transient expression of constitutive active forms of Ras, PI3K and other components of both signalling pathways led to a significant increase of APP promoter activity. Furthermore, the stimulation of the APP promoter by BDNF was completely precluded by expression of dominant-negative forms of Ras and PI3K effectors. Taken together, our results suggest that simultaneous activation of at least two signalling pathways, Ras/MAPK and PI3K/Akt, is necessary to mediate a full activation of the APP promoter by BDNF.     

Increased processing of APLP2 and APP with concomitant formation of APP intracellular domains in BDNF and retinoic acid-differentiated human neuroblastoma cells

The amyloid precursor protein (APP) belongs to a conserved gene family, also including the amyloid precursor-like proteins, APLP1 and APLP2. We have previously shown that all members of the APP protein family are up-regulated upon retinoic acid (RA)-induced neuronal differentiation of SH-SY5Y neuroblastoma cells. Here, we demonstrate that RA also affects the processing of APLP2 and APP, as shown by increased shedding of both sAPLP2 and sAPPalpha, as well as elevated levels of the APP intracellular domains (AICDs). Brain-derived neurotrophic factor (BDNF) has been reported to induce APP promoter activity and RA induces expression of the tyrosine kinase receptor B (TrkB) in neuroblastoma cells. We show that the increase in shedding of both APLP2 and APP in response to RA is not mediated through the TrkB receptor. However, BDNF concomitant with RA increased the expression of APP even further. In addition, the secretion of sAPLP2 and sAPPalpha as well as the levels of AICDs were increased in response to BDNF. In contrast, the levels of membrane-bound APP C-terminal fragment C99 significantly decreased. Our results suggest that RA and BDNF shifts APP processing towards the alpha-secretase pathway. In addition, we show that RA and BDNF regulate N-linked glycosylation of APLP1.     

Nerve growth factor modulates the expression and secretion of beta-amyloid precursor protein through different mechanisms in PC12 cells

The beta-amyloid protein, component of the senile plaques found in Alzheimer brains is proteolytically derived from the beta-amyloid precursor protein (APP), a larger membrane-associated protein that is expressed in both neural and non-neural cells. Overexpression of APP might be one of the mechanisms that more directly contributes to the development of Alzheimer's disease. The APP gene expression is regulated by a number of cellular mediators including nerve growth factor (NGF) and other ligands of tyrosine kinase receptors. We have previously described that NGF increases APP mRNA levels in PC12 cells. However, the molecular mechanisms and the precise signalling pathways that mediate its regulation are not yet well understood. In the present study we present evidence that NGF, and to a lesser extent fibroblast growth factor and epidermal growth factor, stimulate APP promoter activity in PC12 cells. This induction is mediated by DNA sequences located between the nucleotides - 307 and - 15, and involves activation of the Ras-MAP kinase signalling pathway. In contrast, we have also found that NGF-induced secretion of soluble fragments of APP into the culture medium is mediated by a Ras independent mechanism.     

Activation of Ras, Raf-1 and protein kinase C in differentiating human neuroblastoma cells after treatment with phorbolester and NGF

The human neuroblastoma cell line SH-SY5Y/TrkA differentiates in vitro and acquires a sympathetic phenotype in response to phorbolester (activator of protein kinase C, PKC) in the presence of serum or growth factors, or nerve growth factor (NGF). We have now investigated to what extent phorbolester and NGF cause activation of Ras and Raf-1 and the involvement of PKC in this response in differentiating SH-SY5Y/TrkA cells. NGF stimulated increased accumulation of Ras-GTP and a threefold activation of Raf-1. In contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA) had no effect on the amount of Ras-GTP but led to a smaller activation of Raf-1. NGF caused a limited increase in phosphorylation of Raf-1 compared with TPA, and NGF-induced Raf activity was independent of PKC. Analysis of phosphorylation of the endogenous PKC substrate myristoylated alanine-rich C-kinase substrate (MARCKS), and of subcellular distribution of PKC-alpha, -delta, and -epsilon revealed that NGF only caused a very small activation of PKC in SH-SY5Y/TrkA cells. The results identify Raf-1 as a target for both TPA- and NGF-induced signals in differentiating SH-SY5Y/TrkA cells and demonstrate that signalling to Raf-1 was mediated via distinct mechanisms.     

Regulation of protein kinase C by the anti-Parkinson drug, MAO-B inhibitor, rasagiline and its derivatives, in vivo

We have recently shown that the anti-Parkinson-propargyl-containing monoamine oxidase B (MAO-B) inhibitor drug, rasagiline [N-propargyl-(1R)-aminoindan], and its cholinesterase inhibitor derivatives TV3326 and TV3279, regulate amyloid precursor protein (APP) processing by a protein kinase C (PKC)-dependent mechanism in SH-SY5Y neuroblastoma and PC12 cells. In the present study, we investigated the effect of rasagiline and its derivatives on the regulation of the PKC-dependent mechanism and APP processing under in vivo conditions. Administration of rasagiline (0.1 mg/kg) to male C57/BL mice for 14 days significantly decreased membrane-bound holoprotein APP levels in the hippocampus. Additionally, we observed that rasagiline up-regulated p-PKC levels and the expression of alpha and epsilon PKC isozymes in the hippocampus, indicating that the mechanism by which rasagiline affects APP processing may be related to PKC-associated signalling. The results also demonstrate that rasagiline treatment significantly elevated the levels of phosphorylated myristoylated alanine-rich C kinase substrate (p-MARCKS), a major substrate for PKC, as well as the levels of receptors for activated C kinase 1 (RACK1). Similar effects on APP and PKC levels were also demonstrated for the two cholinesterase inhibitor derivatives of rasagiline, TV3326 and TV3279. These results indicate that rasagiline and its derivatives regulate PKC-dependent mechanisms and APP processing. The activation and induction of PKC and MARCKS by these drugs may have a crucial role not only in their neuroprotective activity, but also in their ability to affect neuronal plasticity and spatial learning processes.     

Regulation of Secretion of Alzheimer Amyloid Precursor Protein by the Mitogen-Activated Protein Kinase Cascade

Abstract: Activation of protein kinase C (PKC) regulates the processing of Alzheimer amyloid precursor protein (APP) into its soluble form (sAPP) and amyloid β-peptide (Aβ). However, little is known about the intermediate steps between PKC activation and modulation of APP metabolism. Using a specific inhibitor of mitogen-activated protein (MAP) kinase kinase activation (PD 98059), as well as a dominant negative mutant of MAP kinase kinase, we show in various cell lines that stimulation of PKC by phorbol ester rapidly induces sAPP secretion through a mechanism involving activation of the MAP kinase cascade. In PC12-M1 cells, activation of MAP kinase by nerve growth factor was associated with stimulation of sAPP release. Conversely, M1 muscarinic receptor stimulation, which is known to act in part through a PKC-independent pathway, increased sAPP secretion mainly through a MAP kinase-independent pathway. Aβ secretion and its regulation by PKC were not affected by PD 98059, supporting the concept of distinct secretory pathways for Aβ and sAPP formation.     

PKC phosphorylates MARCKS Ser159 not only directly but also through RhoA/ROCK

It is well recognized that phorbol 12,13-dibutyrate (PDBu)-activated PKC directly phosphorylates myristoylated alanine-rich C kinase substrate (MARCKS), whose phosphorylation is used as a marker of PKC activation. However, in SH-SY5Y neuroblastoma cells, Western blotting analyses revealed that Rho-associated coiled-coil kinase (ROCK)-specific inhibitor H-1152 inhibited PDBu-induced phosphorylation, and that a small G-protein inhibitor, toxin B, also inhibited MARCKS phosphorylation. Furthermore, in GST pull-down assays, PDBu induced RhoA activation in SH-SY5Y cells, and this activation was inhibited by PKC inhibitor Ro-31-8220. Finally, we showed that the transfection of a dominant negative form of RhoA inhibited PDBu-induced MARCKS phosphorylation in immunocytochemistries. These findings suggest that some PDBu-induced MARCKS phosphorylation includes the RhoA/ROCK pathway in SH-SY5Y cells.     

Fibroblast growth factor activation of the rat PRL promoter is mediated by PKCdelta

Fibroblast growth factors play a critical role in cell growth, development, and differentiation and are also implicated in the formation and progression of tumors in a variety of tissues including pituitary. We have previously shown that fibroblast growth factor activation of the rat PRL promoter in GH4T2 pituitary tumor cells is mediated via MAP kinase in a Ras/Raf-1-independent manner. Herein we show using biochemical, molecular, and pharmacological approaches that PKCdelta is a critical component of the fibroblast growth factor signaling pathway. PKC inhibitors, or down-regulation of PKC, rendered the rat PRL promoter refractory to subsequent stimulation by fibroblast growth factors, implying a role for PKC in fibroblast growth factor signal transduction. FGFs caused specific translocation of PKCdelta from cytosolic to membrane fractions, consistent with enzyme activation. In contrast, other PKCs expressed in GH4T2 cells (alpha, betaI, betaII, and epsilon) did not translocate in response to fibroblast growth factors. The PKCdelta subtype-selective inhibitor, rottlerin, or expression of a dominant negative PKCdelta adenoviral construct also blocked fibroblast growth factor induction of rat PRL promoter activity, confirming a role for the novel PKCdelta isoform. PKC inhibitors selective for the conventional alpha and beta isoforms or dominant negative PKCalpha adenoviral expression constructs had no effect. Induction of the endogenous PRL gene was also blocked by adenoviral dominant negative PKCdelta expression but not by an analogous dominant negative PKCalpha construct. Finally, rottlerin significantly attenuated FGF-induced MAP kinase phosphorylation. Together, these results indicate that MAP kinase-dependent fibroblast growth factor stimulation of the rat PRL promoter in pituitary cells is mediated by PKCdelta.     

Evidence for a role of MEK and MAPK during signal transduction by protein kinase C zeta.

Protein kinase C zeta (zeta PKC) is critically involved in the control of a number of cell functions, including proliferation and nuclear factor kappa B (NF-kappa B) activation. Previous studies indicate that zeta PKC is an important step downstream of Ras in the mitogenic cascade. The stimulation of Ras initiates a kinase cascade that culminates in the activation of MAP kinase (MAPK), which is required for cell growth. MAPK is activated by phosphorylation by another kinase named MAPK kinase (MEK), which is the substrate of a number of Ras-activated serine/threonine kinases such as c-Raf-1 and B-Raf. We show here that MAPK and MEK are activated in vivo by an active mutant of zeta PKC, and that a kinase-defective dominant negative mutant of zeta PKC dramatically impairs the activation of both MEK and MAPK by serum and tumour necrosis factor (TNF alpha). The stimulation of other kinases, such as stress-activated protein kinase (SAPK) or p70S6K, is shown here to be independent of zeta PKC. The importance of MEK/MAPK in the signalling mechanisms activated by zeta PKC was addressed by using the activation of a kappa B-dependent promoter as a biological read-out of zeta PKC.     

老年痴呆症相关基因Nicastrin的转录调控

APP蛋白经过降解,形成老年痴呆症患者脑内老年斑的主要成分.由PS(早老素),NCT,PEN-2和APH-14种膜蛋白组成的γ分泌酶催化该降解过程.为了了解人类nicastrin(NCT)基因的转录调控机制,确定了其在人脑中的转录起始位点以及其编码区上游大小不等片段的转录起始活性.EMSA分析证实NCT启动子区的4个AP-1结合位点和2个NFAT结合位点能够与相应的转录因子结合,能够改变转录因子调控能力的定点突变和PDTC诱导使得NCT启动子在HeLa细胞和人鼠皮质神经元中的启动活性都有所改变.以上结果说明:AP-1和NFAT确实参与了人类NCT基因的转录调控.     

A PKCbeta isoform mediates phorbol ester-induced activation of Erk1/2 and expression of neuronal differentiation genes in neuroblastoma cells.

Protein kinase C (PKC) activation induces neuronal differentiation of SH-SY5Y neuroblastoma cells. This study examines the role of PKCbeta isoforms in this process. The PKCbeta-specific inhibitor LY379196 had no effect on 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced neurite outgrowth from SH-SY5Y neuroblastoma cells. On the other hand, PKCbeta inhibition suppressed the TPA-stimulated increase in neuropeptide Y mRNA, activation of neuropeptide Y gene promoter elements, and phosphorylation of Erk1/2. The TPA-induced increase in neuropeptide Y expression was also inhibited by the MEK inhibitor PD98059. These data indicate that activation of a PKCbeta isoform, through a pathway involving Erk1/2, leads to increased expression of neuronal differentiation genes in neuroblastoma cells.