Potential implications of protein kinase Cα in pathophysiological conditions and therapeutic interventions

PKCα is a molecule with many functions that play an important role in cell survival and death to maintain cellular homeostasis. Alteration in the normal functioning of PKCα is responsible for the complicated etiology of many pathologies, including cancer, cardiovascular diseases, kidney complications, neurodegenerative diseases, diabetics, and many others. Several studies have been carried out over the years on this kinase's function, and regulation in normal physiology and pathological conditions. A lot of data with antithetical results have therefore accumulated over time to create a complex framework of physiological implications connected to the PKCα function that needs comprehensive elucidation. In light of this information, we critically analyze the multiple roles played by PKCα in basic cellular processes and their molecular mechanism during various pathological conditions. This review further discusses the current approaches to manipulating PKCα signaling amplitude in the patient's favour and proposed PKCα as a therapeutic target to reverse pathological states.

Anti-Inflammatory Effects of Rhamnetin on Bradykinin-Induced Matrix Metalloproteinase-9 Expression and Cell Migration in Rat Brain Astrocytes

Bradykinin (BK) has been shown to induce matrix metalloproteinase (MMP)-9 expression and participate in neuroinflammation. The BK/MMP-9 axis can be a target for managing neuroinflammation. Our previous reports have indicated that reactive oxygen species (ROS)-mediated nuclear factor-kappaB (NF-κB) activity is involved in BK-induced MMP-9 expression in rat brain astrocytes (RBA-1). Rhamnetin (RNT), a flavonoid compound, possesses antioxidant and anti-inflammatory effects. Thus, we proposed RNT could attenuate BK-induced response in RBA-1. This study aims to approach mechanisms underlying RNT regulating BK-stimulated MMP-9 expression, especially ROS and NF-κB. We used pharmacological inhibitors and siRNAs to dissect molecular mechanisms. Western blotting and gelatin zymography were used to evaluate protein and MMP-9 expression. Real-time PCR was used for gene expression. Wound healing assay was applied for cell migration. 2',7'-dichlorodihydrofluorescein diacetate (H2DCF-DA) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) were used for ROS generation and NOX activity, respectively. Promoter luciferase assay and chromatin immunoprecipitation (ChIP) assay were applied to detect gene transcription. Our results showed that RNT inhibits BK-induced MMP-9 protein and mRNA expression, promoter activity, and cell migration in RBA-1 cells. Besides, the levels of phospho-PKCδ, NOX activity, ROS, phospho-ERK1/2, phospho-p65, and NF-κB p65 binding to MMP-9 promoter were attenuated by RNT. In summary, RNT attenuates BK-enhanced MMP-9 upregulation through inhibiting PKCδ/NOX/ROS/ERK1/2-dependent NF-κB activity in RBA-1.

Comprehensive Account on the Synthesis of (-)-Balanol and its Analogues

BACKGROUND

A variety of diseases have been associated with hyperactivation of protein kinase C (PKC) enzymes such as cancer, diabetes, asthma, cardiovascular and central nervous system disorders. There is a dire need to selectively inhibit these enzymes by synthesizing new potent inhibitors. Balanol, a fungal metabolite belonging to the PKC inhibitor family, is especially included in this aspect. Tremendous effort has been put towards the synthesis of balanol by different research groups.

OBJECTIVES

The aim of this review is to provide a detailed description of synthetic approaches adopted for the synthesis of key fragments of balanol (azepane and benzophenone). All the factors that resulted in excellent yield and high enantioselectivity have also been mentioned.

CONCLUSION

It has been shown throughout this review that the synthesis of hexahydroazepine and benzophenone cores of balanol was achieved by employing a variety of important key steps with commercially available starting precursors, which make this total synthesis more valuable. Moreover, this article provides ideas to the synthetic as well as pharmaceutical chemists for the synthesis of (-)-balanol and its analogues.

Chiral 2-phenyl-3-hydroxypropyl esters as PKC-alpha modulators: HPLC enantioseparation, NMR absolute configuration assignment, and molecular docking studies

Protein kinase C (PKC) isoforms play a pivotal role in the regulation of numerous cellular functions, making them extensively studied and highly attractive drug targets. In our previous work, we identified in racemate 1-2, based on the 2-benzyl-3-hydroxypropyl ester scaffold, two new potent and promising PKCα and PKCδ ligands, targeting the C1 domain of these two kinases. Herein, we report the resolution of the racemates by enantioselective semi-preparative HPLC. The attribution of the absolute configuration (AC) of homochirals 1 was performed by NMR, via methoxy-α-trifluoromethyl-α-phenylacetic acid derivatization (MTPA or Mosher's acid). Moreover, the match between the experimental and predicted electronic circular dichroism (ECD) spectra confirmed the assigned AC. These results proved that Mosher's esters can be properly exploited for the determination of the AC also for chiral primary alcohols. Lastly, homochiral 1 and 2 were assessed for binding affinity and functional activity against PKCα. No significative differences in the K_i of the enantiopure compounds was observed, thus suggesting that chirality does not seem to play a significant role in targeting PKC C1 domain. These results are in accordance with the molecular docking studies performed using a new homology model for the human PKCαC1B domain.

Synthesis of hydrophilic caged DAG-lactones for chemical biology applications

The 6-bromo-7-hydroxy-coumarin-4-ylmethyl (Bhc) group has been used widely in cage chemistry because of its high molar absorptivity and photolytic efficiency. One of the drawbacks of coumarins however is their low aqueous solubility. Aqueous solubility is important in the behavior of caged compounds because hydrophobic caged compounds might be aggregated in physiological conditions and consequently the photocleavage would be impaired. The 8-azacoumarin-4-ylmethyl derivatives with bromine (8-aza-Bhc) or iodine (8-aza-Ihc), which were previously developed in this laboratory, have aqueous solubilities that are higher than those of related coumarins. Here, to improve the hydrophilicity and management of caged diacylglycerol lactones (DAG-lactones), 8-aza-Bhc and 8-aza-Ihc were introduced into the DAG-lactone structure. The synthesized caged compounds showed high hydrophilicity compared with the parent Bhc-caged DAG-lactone, and the 8-aza-Ihc-caged DAG-lactone (2) showed excellent photolytic efficiency, which allows rapid release of the DAG-lactone (1) by brief photoirradiation. The 8-aza-7-hydroxy-6-iodo-coumarin-4-ylmethyl group might be useful for caging of bioactive compounds, especially hydrophobic compounds.

Kaempferol modulates TCDD- and t-BHQ-induced drug-metabolizing enzymes and luteolin enhances this effect

The expression of drug-metabolizing enzymes is deeply involved in chemical-induced cancer progression and prevention. The aryl hydrocarbon receptor (AhR) induces phase I, and certain phase II drug-metabolizing enzymes after the binding of ligands, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We have previously demonstrated that luteolin inhibited TCDD-induced AhR transformation, and modulated the expression of drug-metabolizing enzymes through not only the AhR, but also the nuclear factor-erythroid-2-related factor 2 (Nrf2). We have examined the effect of kaempferol on the expression of drug-metabolizing enzymes through modulation of the AhR- and Nrf2-pathways, and the effect of co-treatment with kaempferol and luteolin. Kaempferol dose-dependently inhibited not only the TCDD-induced expression of phase I and phase II drug-metabolizing enzymes, but also the tertiary butylhydroquinone (t-BHQ)-induced expression of phase II drug-metabolizing enzymes, by modulating the AhR- and Nrf2-pathways. Co-treatment with kaempferol and luteolin enhanced the inhibitory effect on the expression of drug-metabolizing enzymes, compared with either kaempferol or luteolin alone. Moreover, co-treatment with kaempferol and luteolin increased the cellular levels of kaempferol without affecting the levels of luteolin. An in vivo study was also performed and the results demonstrated that co-treatment with kaempferol and luteolin enhanced the inhibition of benzo[a]pyrene-induced drug-metabolizing enzymes compared with either kaempferol or luteolin alone, in the liver of ICR mice. These results suggest that luteolin promoted the incorporation of kaempferol into hepatocytes and enhanced the inhibitory effect of kaempferol on chemical-induced drug-metabolizing enzymes. Thus, luteolin enhances the kaempferol-inhibited expression of drug-metabolizing enzymes.

Scaffold hopping from (5-hydroxymethyl) isophthalates to multisubstituted pyrimidines diminishes binding affinity to the C1 domain of protein kinase C

Protein kinase C (PKC) isoforms play a pivotal role in the regulation of numerous cellular functions, making them extensively studied and highly attractive drug targets. Utilizing the crystal structure of the PKCδ C1B domain, we have developed hydrophobic isophthalic acid derivatives that modify PKC functions by binding to the C1 domain of the enzyme. In the present study, we aimed to improve the drug-like properties of the isophthalic acid derivatives by increasing their solubility and enhancing the binding affinity. Here we describe the design and synthesis of a series of multisubstituted pyrimidines as analogs of C1 domain–targeted isophthalates and characterize their binding affinities to the PKCα isoform. In contrast to our computational predictions, the scaffold hopping from phenyl to pyrimidine core diminished the binding affinity. Although the novel pyrimidines did not establish improved binding affinity for PKCα compared to our previous isophthalic acid derivatives, the present results provide useful structure-activity relationship data for further development of ligands targeted to the C1 domain of PKC.

Polyphenol compounds and PKC signaling

BACKGROUND

Naturally occurring polyphenols found in food sources provide huge health benefits. Several polyphenolic compounds are implicated in the prevention of disease states, such as cancer. One of the mechanisms by which polyphenols exert their biological actions is by interfering in the protein kinase C (PKC) signaling pathways. PKC belongs to a superfamily of serine-threonine kinase and are primarily involved in phosphorylation of target proteins controlling activation and inhibition of many cellular processes directly or indirectly.

SCOPE OF REVIEW

Despite the availability of substantial literature data on polyphenols' regulation of PKC, no comprehensive review article is currently available on this subject. This article reviews PKC-polyphenol interactions and its relevance to various disease states. In particular, salient features of polyphenols, PKC, interactions of naturally occurring polyphenols with PKC, and future perspective of research on this subject are discussed.

MAJOR CONCLUSIONS

Some polyphenols exert their antioxidant properties by regulating the transcription of the antioxidant enzyme genes through PKC signaling. Regulation of PKC by polyphenols is isoform dependent. The activation or inhibition of PKC by polyphenols has been found to be dependent on the presence of membrane, Ca(2+) ion, cofactors, cell and tissue types etc. Two polyphenols, curcumin and resveratrol are in clinical trials for the treatment of colon cancer.

GENERAL SIGNIFICANCE

The fact that 74% of the cancer drugs are derived from natural sources, naturally occurring polyphenols or its simple analogs with improved bioavailability may have the potential to be cancer drugs in the future.

Effect of Glucuronidation on the Potential of Kaempferol to Inhibit Serine/Threonine Protein Kinases

To study the effect of metabolic conjugation of flavonoids on the potential to inhibit protein kinase activity, the inhibitory effects of the dietary flavonol kaempferol and its major plasma conjugate kaempferol-3-O-glucuronide on protein kinases were studied. To this end, the inhibition of the phosphorylation activity of recombinant protein kinase A (PKA) and of cell lysate from the hepatocellular carcinoma cell line HepG2 on 141 putative serine/threonine phosphorylation sites derived from human proteins was assessed. Glucuronidation reduced the inhibitory potency of kaempferol on the phosphorylation activity of PKA and HepG2 lysate on average about 16 and 3.5 times, respectively, but did not appear to affect the target selectivity for kinases present in the lysate. The data demonstrate that, upon glucuronidation, kaempferol retains part of its intrinsic kinase inhibition potential, which implies that K3G does not necessarily need to be deconjugated to the aglycone for a potential inhibitory effect on protein kinases.

Bioactive peptides/chitosan nanoparticles enhance cellular antioxidant activity of (-)-epigallocatechin-3-gallate

(-)-Epigallocatechin-3-gallate (EGCG), one representative of the well-studied chemopreventive phytochemicals but with low bioavailability, was encapsulated in monodispersed nanoparticles that were assembled from bioactive caseinophosphopeptide (CPP) and chitosan (CS). The encapsulation efficiency of EGCG in CS-CPP nanoparticles ranged from 70.5 to 81.7%; meanwhile, the in vitro release of EGCG from CS-CPP nanoparticles was in a controllable manner. The EGCG-loaded CS-CPP nanoparticles exerted stronger activity of scavenging free radical than the free EGCG (p < 0.01) in the cellular antioxidant activity assay. Furthermore, cellular uptake of the EGCG-loaded CS-CPP nanoparticles was confirmed by the green fluorescence inside the human hepatocellular caricinoma (HepG2) cells, which was considered to play an important role in the improvement of the antioxidant activity of the nanoencapsulated EGCG. The results suggested that encapsulation of EGCG using CS-CPP nanoparticles should be a potential approach to enhance its antioxidant activity in biological systems.

Kinetics of PKCε activating and inhibiting llama single chain antibodies and their effect on PKCε translocation in HeLa cells

Dysregulation of PKCε is involved in several serious diseases such as cancer, type II diabetes and Alzheimer's disease. Therefore, specific activators and inhibitors of PKCε hold promise as future therapeutics, in addition to being useful in research into PKCε regulated pathways. We have previously described llama single chain antibodies (VHHs) that specifically activate (A10, C1 and D1) or inhibit (E6 and G8) human recombinant PKCε. Here we report a thorough kinetic analysis of these VHHs. The inhibiting VHHs act as non-competitive inhibitors of PKCε activity, whereas the activating VHHs have several different modes of action, either increasing V_max and/or decreasing K_m values. We also show that the binding of the VHHs to PKCε is conformation-dependent, rendering the determination of affinities difficult. Apparent affinities are in the micromolar range based on surface plasmon resonance studies. Furthermore, the VHHs have no effect on the activity of rat PKCε nor can they bind the rat form of the protein in immunoprecipitation studies despite the 98% identity between the human and rat PKCε proteins. Finally, we show for the first time that the VHHs can influence PKCε function also in cells, since an activating VHH increases the rate of PKCε translocation in response to PMA in HeLa cells, whereas an inhibiting VHH slows down the translocation. These results give insight into the mechanisms of PKCε activity modulation and highlight the importance of protein conformation on VHH binding.

Cellular uptake and cytotoxicity of chitosan-caseinophosphopeptides nanocomplexes loaded with epigallocatechin gallate

Epigallocatechin gallate (EGCG) was successfully encapsulated in novel nanocomplexes assembled from bioactive peptides, caseinophosphopeptides (CPPs), and chitosan (CS), a natural cationic polymer. Their particle sizes and surface charges were determined to be in the range of 150.0±4.3 nm and 32.2±3.3 mV respectively. Crosslinking between the -NH3+ groups of CS with the -P=O- and -COO- groups of CPP, as well as the hydrogen bonding were confirmed from the FTIR results. Atomic force microscopy (AFM) images showed that EGCG loaded CS-CPP nanocomplexes were spherical in shape. Maintaining the surface charge as high as +32.2 mV, crosslinking CS with peptides reduced the cytotoxicity of CS nanoparticles. In addition, cellular internalization of EGCG-loaded CS-CPP nanoparticles was confirmed from green fluorescence inside the Caco-2 cells. The process of nanoparticle uptake was dose and time dependent in the range of time and concentration studied. Furthermore, the intestinal permeability of EGCG using Caco-2 monolayer was enhanced significantly as delivered by nanoparticles, which indicated the promising elevation of EGCG bioavailability.

The development of activating and inhibiting camelid VHH domains against human protein kinase C epsilon

The 10 isozymes of the protein kinase C (PKC) family can have different roles on the same biological process, making isozyme specific analysis of function crucial. Currently, only few pharmacological compounds with moderate isozyme specific effects exist thus hampering research into individual PKC isozymes. The antigen binding regions of camelid single chain antibodies (VHHs) could provide a solution for obtaining PKC isozyme specific modulators. In the present study, we have successfully selected and characterized PKCɛ specific VHH antibodies from two immune VHH libraries using phage display. The VHHs were shown to exclusively bind to PKCɛ in ELISA and immunoprecipitation studies. Strikingly, five of the VHHs had an effect on PKCɛ kinase activity in vitro. VHHs A10, C1 and D1 increased PKCɛ kinase activity in a concentration-dependent manner (EC(50) values: 212-310nM), whereas E6 and G8 inhibited PKCɛ activity (IC(50) values: 103-233nM). None of these VHHs had an effect on the activity of the other novel PKC isozymes PKCδ and PKCθ. To our knowledge, these antibodies are the first described VHH activators and inhibitors for a protein kinase. Furthermore, the development of PKCɛ specific modulators is an important contribution to PKC research.

HIV-1 Tat-peptide inhibits protein kinase C and protein kinase A through substrate competition

HIV-1 Tat-peptide is widely used as a vector for cargo delivery into intact cells. As a cationic, arginine-rich peptide it can readily penetrate the plasma membrane and facilitate the penetration of impermeable bioactive molecules such as proteins, peptides, nucleic acids and drugs. Although at first considered as an inert vector, recent studies have however shown that it might have effects on its own on various cellular processes. In the present study we have investigated the effects of the Tat-peptide(48-60) on two basic serine/threonine kinases, protein kinase C and A, since earlier studies have shown that certain arginine-rich peptides or proteins might have a modulatory effect on their activity. In in vitro studies, Tat-peptide inhibited PKC alpha in a concentration-dependent manner with an IC(50)-value of 22nM and PKA with an IC(50)-value of 1.2 microM. The mode of inhibition was studied in the presence of increasing concentrations of a substrate peptide or ATP. Tat-peptide competed with the kinase substrates, however it did not compete with ATP. In a panel of 70 kinases Tat-peptide showed inhibitory activity at least towards other AGC-family kinases (PKB, SGK1, S6K1, MSK1), CAMK-family kinases (CAMK1 and MELK) and a STE family kinase (MKK1). In HeLa cells Tat-peptide inhibited the phorbol ester-evoked ERK1/2 phosphorylation suggesting that Tat inhibited PKC also in intact cells. In thyroid cells Tat-peptide attenuated sphingosylphosphorylcholine-evoked Ca(2+)-fluxes, which have earlier been shown to be dependent on PKC. Taken together, these results indicate that the Tat-peptide(48-60) is a potent inhibitor which binds to the substrate binding site of the basophilic kinase domain.

Design, synthesis, and biological activity of isophthalic acid derivatives targeted to the C1 domain of protein kinase C

Protein kinase C (PKC) is a widely studied molecular target for the treatment of cancer and other diseases. We have approached the issue of modifying PKC function by targeting the C1 domain in the regulatory region of the enzyme. Using the X-ray crystal structure of the PKC delta C1b domain, we have discovered conveniently synthesizable derivatives of dialkyl 5-(hydroxymethyl)isophthalate that can act as potential C1 domain ligands. Structure-activity studies confirmed that the important functional groups predicted by modeling were indispensable for binding to the C1 domain and that the modifications of these groups diminished binding. The most promising compounds were able to displace radiolabeled phorbol ester ([(3)H]PDBu) from PKC alpha and delta at K(i) values in the range of 200-900 nM. Furthermore, the active isophthalate derivatives could modify PKC activation in living cells either by inducing PKC-dependent ERK phosphorylation or by inhibiting phorbol-induced ERK phosphorylation. In conclusion, we report here, for the first time, that derivatives of isophthalic acid represent an attractive novel group of C1 domain ligands that can be used as research tools or further modified for potential drug development.

Discovering Protein Kinase C Active Plants Growing in Finland Utilizing Automated Bioassay Combined to LC/MS

Protein kinase C (PKC) modulating activity of 81 plant extracts of Finnish origin was investigated with an automated bioassay method combined to LC/MS. Twenty-one extracts from different parts of the plants inhibited PKC significantly. Fractionation of the active extract of Filipendula ulmaria showed that this method was able to identify a PKC inhibiting compound from the extract as quercetin. Our results indicate that this method is suitable for PKC screening of complex matrices and provides a quick and low volume, non-radioactive, alternative method for PKC experiments.

Time-course regulation of quercetin on cell survival/proliferation pathways in human hepatoma cells

Quercetin, a dietary flavonoid, has been shown to possess anticarcinogenic properties, but the precise molecular mechanisms of action are not thoroughly elucidated. This study was aimed at investigating the time-course regulation effect of quercetin on survival/proliferation pathways in a human hepatoma cell line (HepG2). Quercetin induced a significant time-dependent inactivation of the major survival signaling proteins, i. e., phosphatidylinositol 3-kinase (PI 3-kinase)/protein kinase B (AKT), extracellular regulated kinase (ERK), protein kinase C-alpha (PKC-alpha), in concert with a time-dependent activation of key death-related signals: c-jun amino-terminal kinase (JNK) and PKC-delta. These data suggest that quercetin exerts a tight regulation of survival/proliferation pathways that requires the integration of different signals and persists over time, being the balance of these regulatory signals what determines the fate of HepG2 cells.

Comparative molecular field analysis of flavonoid inhibitors of the PIM-1 kinase

The PIM-1 protein, the product of the pim-1 oncogene, is a serine/threonine kinase. Dysregulation of the PIM-1 kinase has been implicated in the development of human malignancies including lymphomas, leukemias, and prostate cancer. Comparative molecular field analysis (CoMFA) is a 3-D QSAR technique that has been widely used, with notable success, to correlate biological activity with the steric and electrostatic properties of ligands. We have used a set of 15 flavonoid inhibitors of the PIM-1 kinase, aligned de novo by common substructure, to generate a CoMFA model for the purpose of elucidating the steric and electrostatic properties involved in flavonoid binding to the PIM-1 kinase. Partial least squares correlation between observed and predicted inhibitor potency (expressed as -logIC50), using a non-cross-validated partial least squares analysis, generated a non-cross-validated q2=0.805 for the training set (n=15) of flavonoids. The CoMFA generated steric map indicated that the PIM-1-binding site was sterically hindered, leading to more efficient binding of planar molecules over (R) or (S) compounds. The electrostatic map identified that positive charges near the flavonoid atom C8 and negative charges near C4' increased flavonoid binding. The CoMFA model accurately predicted the potency of a test set of flavonoids (n=6), generating a correlation between observed and predicted potency of q2=0.825. CoMFA models generated from additional alignment rules, which were guided by co-crystal structure ligand orientations, did not improve the correlative value of the model. Superimposing the PIM-1 kinase crystal structure onto the CoMFA contours validated the steric and electrostatic maps, elucidating the amino acid residues that potentially contribute to the CoMFA fields. Thus we have generated the first predictive model that may be used for the rational design of small-molecule inhibitors of the PIM-1 kinase.

Inhibition of protein kinase C by dequalinium analogues: Structure–activity studies on head group variations

New dequalinium analogues and related heteroaromatic systems were synthesized and evaluated for inhibition of protein kinase Calpha. In vitro assays with recombinant human PKCalpha showed that the number of the aromatic ring head groups as well as their electron-richness, are critical factors that determine potency. The inhibitory strengths of the synthesized compounds are shown to correlate well with Mulliken charges on the head group ring nitrogen atoms making it possible to design likely candidate molecules having improved protein kinase Calpha inhibitory activity.

Nicotine-induced upregulation of human neuronal nicotinic alpha7-receptors is potentiated by modulation of cAMP and PKC in SH-EP1-halpha7 cells

Chronic nicotine exposure induces upregulation of nicotinic receptors, but the mechanisms underlying this phenomenon are not well understood. The aim of this study was to examine the role of different second messenger systems in the nicotine-induced upregulation of alpha7-nicotinic receptors in SH-EP1-halpha7 human epithelial cells. We show here that chronic exposure to nicotine results in accumulation of cAMP. Furthermore, an enhanced cAMP signalling potentiates nicotine-induced upregulation of alpha7-nicotinic receptors measured by [3H]methyllycaconitine ([3H]MLA) binding suggesting that cAMP is involved in the alpha7-nicotinic receptor upregulation. Down-regulation of protein kinase C (PKC) with a phorbol ester abolishes the nicotine-induced upregulation of alpha7-nicotinic receptors. Furthermore, overexpression of PKCalpha in SH-EP1-halpha7 cells results in potentiation of nicotine-evoked upregulation indicating that PKC has a role in regulation of alpha7-nicotinic receptor number. The Ca2+-calmodulin kinase II (CaMKII) and extracellular signal regulated kinase 1/2 (ERK1/2) appear not to participate in alpha7-nicotinic receptor upregulation since the specific inhibitors of these kinases did not have an effect on the nicotine-induced upregulation. Taken together this study provides evidence that nicotine induces accumulation of cAMP and that the upregulation mechanisms of alpha7-nicotinic receptors are potentiated both by cAMP and PKC. As nicotine-evoked upregulation of heteromeric nicotinic receptors in SH-SY5Y cells was unaffected by the treatment with drugs affecting cAMP signalling or PKC activity, our results suggest that the upregulation mechanisms of homomeric alpha7-nicotinic receptors and heteromeric nicotinic receptors differ from each other.