Uncoupling of betaIIPKC from its targeting protein RACK1 in response to ethanol in cultured cells and mouse brain

RACK1 promotes lung cancer cell growth via an MCM7/RACK1/ Akt signaling complex

MCM7, a member of the miniature chromosome maintenance (MCM) protein family, is crucial for the initiation of DNA replication and proliferation in eukaryotic cells. In this report, we demonstrate that RACK1 regulates cell growth and cell cycle progression in human non-small-cell lung cancer by mediating MCM7 phosphorylation through an MCM7/RACK1/Akt signaling complex. RACK1 functions as a central scaffold that brings Akt into physical proximity with MCM7. Overexpression of RACK1 increases interactions between Akt and MCM7 and promotes Akt-dependent MCM7 phosphorylation, which in turn increases MCM7 binding to chromatin and MCM complex formation. Together, these changes promote DNA replication and cell proliferation. Our findings reveal a novel signaling pathway that regulates growth in non-small cell lung cancer.

Ethanol inhibition of a T-type Ca²+ channel through activity of protein kinase C

BACKGROUND

T-type calcium channels (T-channels) are widely distributed in the central and peripheral nervous system, where they mediate calcium entry and regulate the intrinsic excitability of neurons. T-channels are dysregulated in response to alcohol administration and withdrawal. We therefore investigated acute ethanol (EtOH) effects and the underlying mechanism of action in human embryonic kidney (HEK) 293 cell lines, as well as effects on native currents recorded from dorsal root ganglion (DRG) neurons cultured from Long-Evans rats.

METHODS

Whole-cell voltage-clamp recordings were performed at 32 to 34°C in both HEK cell lines and DRG neurons. The recordings were taken after a 10-minute application of EtOH or protein kinase C (PKC) activator (phorbol 12-myristate 13-acetate [PMA]).

RESULTS

We recorded T-type Ca²⁺ currents (T-currents) from 3 channel isoforms (CaV3.1, CaV3.2, and CaV3.3) before and during administration of EtOH. We found that only 1 isoform, CaV3.2, was significantly affected by EtOH. EtOH reduced current density as well as producing a hyperpolarizing shift in steady-state inactivation of both CaV3.2 currents from HEK 293 cell lines and in native T-currents from DRG neurons that are known to be enriched in CaV3.2. A myristoylated PKC peptide inhibitor (MPI) blocked the major EtOH effects, in both the cell lines and the DRG neurons. However, PMA effects were more complex. Lower concentration PMA (100 nM) replicated the major effects of EtOH, while higher concentration PMA (1 μM) did not, suggesting that the EtOH effects operate through activation of PKC and were mimicked by lower concentration of PMA.

CONCLUSIONS

EtOH primarily affects the CaV3.2 isoform of T-type Ca²⁺ channels acting through PKC, highlighting a novel target and mechanism for EtOH effects on excitable membranes.

Signaling pathways mediating alcohol effects

Ethanol's effects on intracellular signaling pathways contribute to acute effects of ethanol as well as to neuroadaptive responses to repeated ethanol exposure. In this chapter we review recent discoveries that demonstrate how ethanol alters signaling pathways involving several receptor tyrosine kinases and intracellular tyrosine and serine-threonine kinases, with consequences for regulation of cell surface receptor function, gene expression, protein translation, neuronal excitability and animal behavior. We also describe recent work that demonstrates a key role for ethanol in regulating the function of scaffolding proteins that organize signaling complexes into functional units. Finally, we review recent exciting studies demonstrating ethanol modulation of DNA and histone modification and the expression of microRNAs, indicating epigenetic mechanisms by which ethanol regulates neuronal gene expression and addictive behaviors.

Ethanol disrupts axon outgrowth stimulated by netrin-1, GDNF, and L1 by blocking their convergent activation of Src family kinase signaling

Pre-natal alcohol exposure causes fetal alcohol spectrum disorders (FASD), the most common, preventable cause of developmental disability. The developing cerebellum is particularly vulnerable to the effects of ethanol. We reported that ethanol inhibits the stimulation of axon outgrowth in cerebellar granule neurons (CGN) by NAP, an active motif of activity-dependent neuroprotective protein (ADNP), by blocking NAP activation of Fyn kinase and its downstream signaling molecule, the scaffolding protein Cas. Here, we asked whether ethanol inhibits the stimulation of axon outgrowth by diverse axon guidance molecules through a common action on the Src family kinases (SFK). We first demonstrated that netrin-1, glial cell line-derived neurotrophic factor (GDNF), and neural cell adhesion molecule L1 stimulate axon outgrowth in CGNs by activating SFK, Cas, and extracellular signal-regulated kinase 1 and 2 (ERK1/2). The specific SFK inhibitor, PP2, blocked the stimulation of axon outgrowth and the activation of the SFK-Cas-ERK1/2 signaling pathway by each of these axon-guidance molecules. In contrast, brain-derived neurotrophic factor (BDNF) stimulated axon outgrowth and activated ERK1/2 without first activating SFK or Cas. Clinically relevant concentrations of ethanol inhibited axon outgrowth and the activation of the SFK-Cas-ERK1/2 pathway by netrin-1, GDNF, and L1, but did not disrupt BDNF-induced axon outgrowth or ERK1/2 activation. These results indicate that SFK, but not ERK1/2, is a primary target for ethanol inhibition of axon outgrowth. The ability of ethanol to block the convergent activation of the SFK-Cas-ERK1/2 pathway by netrin-1, GDNF, L1, and ADNP could contribute significantly to the pathogenesis of FASD.

Structure of human Rack1 protein at a resolution of 2.45 Å

The crystal structure of human receptor for activated C-kinase 1 (hRack1) protein is reported at 2.45 Å resolution. The crystals belongs to space group P4(1)2(1)2, with three molecules per asymmetric unit. The hRack1 structure features a sevenfold β-propeller, with each blade housing a sequence motif that contains a strictly conserved Trp, the indole group of which is embedded between adjacent blades. In blades 1-5 the imidazole group of a His residue is wedged between the side chains of a Ser residue and an Asp residue through two hydrogen bonds. The hRack1 crystal structure forms a starting basis for understanding the remarkable scaffolding properties of this protein.

Direct interaction between scaffolding proteins RACK1 and 14-3-3ζ regulates brain-derived neurotrophic factor (BDNF) transcription

RACK1 is a scaffolding protein that spatially and temporally regulates numerous signaling cascades. We previously found that activation of the cAMP signaling pathway induces the translocation of RACK1 to the nucleus. We further showed that nuclear RACK1 is required to promote the transcription of the brain-derived neurotrophic factor (BDNF). Here, we set out to elucidate the mechanism underlying cAMP-dependent RACK1 nuclear translocation and BDNF transcription. We identified the scaffolding protein 14-3-3ζ as a direct binding partner of RACK1. Moreover, we found that 14-3-3ζ was necessary for the cAMP-dependent translocation of RACK1 to the nucleus. We further observed that the disruption of RACK1/14-3-3ζ interaction with a peptide derived from the RACK1/14-3-3ζ binding site or shRNA-mediated 14-3-3ζ knockdown inhibited cAMP induction of BDNF transcription. Together, these data reveal that the function of nuclear RACK1 is mediated through its interaction with 14-3-3ζ. As RACK1 and 14-3-3ζ are two multifunctional scaffolding proteins that coordinate a wide variety of signaling events, their interaction is likely to regulate other essential cellular functions.

RACK1, A multifaceted scaffolding protein: Structure and function

The Receptor for Activated C Kinase 1 (RACK1) is a member of the tryptophan-aspartate repeat (WD-repeat) family of proteins and shares significant homology to the β subunit of G-proteins (Gβ). RACK1 adopts a seven-bladed β-propeller structure which facilitates protein binding. RACK1 has a significant role to play in shuttling proteins around the cell, anchoring proteins at particular locations and in stabilising protein activity. It interacts with the ribosomal machinery, with several cell surface receptors and with proteins in the nucleus. As a result, RACK1 is a key mediator of various pathways and contributes to numerous aspects of cellular function. Here, we discuss RACK1 gene and structure and its role in specific signaling pathways, and address how posttranslational modifications facilitate subcellular location and translocation of RACK1. This review condenses several recent studies suggesting a role for RACK1 in physiological processes such as development, cell migration, central nervous system (CN) function and circadian rhythm as well as reviewing the role of RACK1 in disease.

PKCγ is required for ethanol-induced increases in GABA(A) receptor α4 subunit expression in cultured cerebral cortical neurons

Ethanol exposure produces alterations in GABA(A) receptor function and expression associated with CNS hyperexcitability, but the mechanisms of these effects are unknown. Ethanol is known to increase both GABA(A) receptor α4 subunits and protein kinase C (PKC) isozymes in vivo and in vitro. Here, we investigated ethanol regulation of GABA(A) receptor α4 subunit expression in cultured cortical neurons to delineate the role of PKC. Cultured neurons were prepared from rat pups on postnatal day 0-1 and tested after 18 days. GABA(A) receptor α4 subunit surface expression was assessed using P2 fractionation and surface biotinylation following ethanol exposure for 4 h. Miniature inhibitory post-synaptic currents were measured using whole cell patch clamp recordings. Ethanol increased GABA(A) receptor α4 subunit expression in both the P2 and biotinylated fractions, while reducing the decay time constant in miniature inhibitory post-synaptic currents, with no effect on γ2 or δ subunits. PKC activation mimicked ethanol effects, while the PKC inhibitor calphostin C prevented ethanol-induced increases in GABA(A) receptor α4 subunit expression. PKCγ siRNA knockdown prevented ethanol-induced increases in GABA(A) receptor α4 subunit expression, but inhibition of the PKCβ isoform with PKCβ pseudosubstrate had no effect. We conclude that PKCγ regulates ethanol-induced alterations in α4-containing GABA(A) receptors.

Defective translocation of PKCepsilon in EtOH-induced inhibition of Mg2+ accumulation in rat hepatocytes

BACKGROUND

Rats chronically fed ethanol for 3 weeks presented a marked decreased in total hepatic Mg(2+) content and required approximately 12 days to restore Mg(2+) homeostasis upon ethanol withdrawal. This study was aimed at investigating the mechanisms responsible for the EtOH-induced delay.

METHODS

Hepatocytes from rats fed ethanol for 3 weeks (Lieber-De Carli diet-chronic model), rats re-fed a control diet for varying periods of time following ethanol withdrawal, and age-matched control rats fed a liquid or a pellet diet were used. As acute models, hepatocytes from control animals or HepG2 cells were exposed to varying doses of ethanol in vitro for 8 minutes.

RESULTS

Hepatocytes from ethanol-fed rats presented a marked inhibition of Mg(2+) accumulation and a defective translocation of PKCepsilon to the cell membrane. Upon ethanol withdrawal, 12 days were necessary for PKCepsilon translocation and Mg(2+) accumulation to return to normal levels. Exposure of control hepatocytes or HepG2 cells to a dose of ethanol as low as 0.01% for 8 minutes was already sufficient to inhibit Mg(2+) accumulation and PKCepsilon translocation for more than 60 minutes. Also in this model, recovery of Mg(2+) accumulation was associated with restoration of PKCepsilon translocation. The use of specific antisense in HepG2 cells confirmed the involvement of PKCepsilon in modulating Mg(2+) accumulation.

CONCLUSIONS

Translocation of PKCepsilon isoform to the hepatocyte membrane is essential for Mg(2+) accumulation to occur. Both acute and chronic ethanol administrations inhibit Mg(2+) accumulation by specifically altering PKCepsilon translocation to the cell membrane.

Escalating ethanol intake is associated with altered corticostriatal BDNF expression

Alcoholism is a chronically relapsing condition, indicative of long-term neuronal adaptations maintaining the disease even after prolonged abstinence. Previously, we identified brain-derived neurotrophic factor (BDNF) in the dorsal striatum as the central mediator of a homeostatic mechanism which is activated by acute alcohol (ethanol) exposure and functions to decrease the sensitivity of rodents to ethanol-related behaviors. We hypothesized that extensive exposure to ethanol would result in dysregulation of this BDNF-mediated protective mechanism, accompanied by heightened ethanol intake. In this study, we demonstrate that while a single bout of ethanol intake increases BDNF mRNA expression in the dorsal striatum, this effect is no longer observed after 6 weeks of daily ethanol access. Additionally, 6 weeks of ethanol consumption decreases BDNF in the cortex, a main source of BDNF for the striatum. Importantly, these ethanol-induced changes in BDNF levels are not ameliorated by 2 weeks' abstinence. Together, these data suggest that the BDNF pathway, which is activated following a single bout of ethanol drinking, breaks down by the end of 6 weeks of access and does not recover its protective function after a 2-week deprivation period. These results suggest that the persistence of altered BDNF signaling may contribute to the inflexibility of addictive behaviors.

Ethanol regulation of D(1) dopamine receptor signaling is mediated by protein kinase C in an isozyme-specific manner

Ethanol consumption potentiates dopaminergic signaling that is partially mediated by the D(1) dopamine receptor; however, the mechanism(s) underlying ethanol-dependent modulation of D(1) signaling is unclear. We now show that ethanol treatment of D(1) receptor-expressing cells decreases D(1) receptor phosphorylation and concurrently potentiates dopamine-stimulated cAMP accumulation. Protein kinase C (PKC) inhibitors mimic the effects of ethanol on D(1) receptor phosphorylation and dopamine-stimulated cAMP levels in a manner that is non-additive with ethanol treatment. Ethanol was also found to modulate specific PKC activities as demonstrated using in vitro kinase assays where ethanol treatment attenuated the activities of lipid-stimulated PKCgamma and PKCdelta in membrane fractions, but did not affect the activities of PKCalpha, PKCbeta(1), or PKCvarepsilon. Importantly, ethanol treatment potentiated D(1) receptor-mediated DARPP-32 phosphorylation in rat striatal slices, supporting the notion that ethanol enhances D(1) receptor signaling in vivo. These findings suggest that ethanol inhibits the activities of specific PKC isozymes, resulting in decreased D(1) receptor phosphorylation and enhanced dopaminergic signaling.

Transcription analysis in the MeLiM swine model identifies RACK1 as a potential marker of malignancy for human melanocytic proliferation

BACKGROUND

Metastatic melanoma is a severe disease. Few experimental animal models of metastatic melanoma exist. MeLiM minipigs exhibit spontaneous melanoma. Cutaneous and metastatic lesions are histologically similar to human's. However, most of them eventually spontaneously regress. Our purpose was to investigate whether the MeLiM model could reveal markers of malignancy in human melanocytic proliferations.

RESULTS

We compared the serial analysis of gene expression (SAGE) between normal pig skin melanocytes and melanoma cells from an early pulmonary metastasis of MeLiM minipigs. Tag identification revealed 55 regulated genes, including GNB2L1 which was found upregulated in the melanoma library. In situ hybridisation confirmed GNB2L1 overexpression in MeLiM melanocytic lesions. GNB2L1 encodes the adaptor protein RACK1, recently shown to influence melanoma cell lines tumorigenicity. We studied the expression of RACK1 by immunofluorescence and confocal microscopy in tissues specimens of normal skin, in cutaneous and metastatic melanoma developped in MeLiM minipigs and in human patients. In pig and human samples, the results were similar. RACK1 protein was not detected in normal epidermal melanocytes. By contrast, RACK1 signal was highly increased in the cytoplasm of all melanocytic cells of superficial spreading melanoma, recurrent dermal lesions and metastatic melanoma. RACK1 partially colocalised with activated PKCalphabeta. In pig metastases, additional nuclear RACK1 did not associate to BDNF expression. In human nevi, the RACK1 signal was low.

CONCLUSION

RACK1 overexpression detected in situ in human melanoma specimens characterized cutaneous and metastatic melanoma raising the possibility that RACK1 can be a potential marker of malignancy in human melanoma. The MeLiM strain provides a relevant model for exploring mechanisms of melanocytic malignant transformation in humans. This study may contribute to a better understanding of melanoma pathophysiology and to progress in diagnosis.

Alcohol related changes in regulation of NMDA receptor functions

Long-term alcohol exposure may lead to development of alcohol dependence in consequence of altered neurotransmitter functions. Accumulating evidence suggests that the N-methyl-D-aspartate (NMDA) type of glutamate receptors is a particularly important site of ethanol's action. Several studies showed that ethanol potently inhibits NMDA receptors (NMDARs) and prolonged ethanol exposition leads to a compensatory "up-regulation" of NMDAR mediated functions. Therefore, alterations in NMDAR function are supposed to contribute to the development of ethanol tolerance, dependence as well as to the acute and late signs of ethanol withdrawal.A number of publications report alterations in the expression and phosphorylation states of NMDAR subunits, in their interaction with scaffolding proteins or other receptors in consequence of chronic ethanol treatment. Our knowledge on the regulatory processes, which modulate NMDAR functions including factors altering transcription, protein expression and post-translational modifications of NMDAR subunits, as well as those influencing their interactions with different regulatory proteins or other downstream signaling elements are incessantly increasing. The aim of this review is to summarize the complex chain of events supposedly playing a role in the up-regulation of NMDAR functions in consequence of chronic ethanol exposure.

Dynorphin is a downstream effector of striatal BDNF regulation of ethanol intake

We recently identified brain-derived neurotrophic factor (BDNF) in the dorsal striatum to be a major component of a homeostatic pathway controlling ethanol consumption. We hypothesized that ethanol-mediated activation of the BDNF signaling cascade is required for the ethanol-related function of the neurotrophic factor. Here, we demonstrate that exposure of striatal neurons to ethanol results in the activation of the BDNF receptor TrkB, leading to the activation of the mitogen-activated protein kinase (MAP kinase) signaling pathway and the subsequent increase in the expression of preprodynorphin (Pdyn) via BDNF. Finally, we show that activation of the dynorphin receptor, the kappa opioid receptor (KOR), is required for the BDNF-mediated decrease in ethanol intake, illustrating a function of dynorphin in BDNF's homeostatic control of ethanol consumption. Taken together, these results demonstrate that BDNF regulates ethanol intake by initiation of MAP kinase signaling and the ensuing production of downstream gene products, including Pdyn.

Ethanol-BDNF interactions: still more questions than answers

Brain-derived neurotrophic factor (BDNF) has emerged as a regulator of development, plasticity and, recently, addiction. Decreased neurotrophic activity may be involved in ethanol-induced neurodegeneration in the adult brain and in the etiology of alcohol-related neurodevelopmental disorders. This can occur through decreased expression of BDNF or through inability of the receptor to transduce signals in the presence of ethanol. In contrast, recent studies implicate region-specific up-regulation of BDNF and associated signaling pathways in anxiety, addiction and homeostasis after ethanol exposure. Anxiety and depression are precipitating factors for substance abuse and these disorders also involve region-specific changes in BDNF in both pathogenesis and response to pharmacotherapy. Polymorphisms in the genes coding for BDNF and its receptor TrkB are linked to affective, substance abuse and appetitive disorders and therefore may play a role in the development of alcoholism. This review summarizes historical and pre-clinical data on BDNF and TrkB as it relates to ethanol toxicity and addiction. Many unresolved questions about region-specific changes in BDNF expression and the precise role of BDNF in neuropsychiatric disorders and addiction remain to be elucidated. Resolution of these questions will require significant integration of the literature on addiction and comorbid psychiatric disorders that contribute to the development of alcoholism.

Acute ethanol administration rapidly increases phosphorylation of conventional protein kinase C in specific mammalian brain regions in vivo

BACKGROUND

Protein kinase C (PKC) is a family of isoenzymes that regulate a variety of functions in the central nervous system including neurotransmitter release, ion channel activity, and cell differentiation. Growing evidence suggests that specific isoforms of PKC influence a variety of behavioral, biochemical, and physiological effects of ethanol in mammals. The purpose of this study was to determine whether acute ethanol exposure alters phosphorylation of conventional PKC isoforms at a threonine 674 (p-cPKC) site in the hydrophobic domain of the kinase, which is required for its catalytic activity.

METHODS

Male rats were administered a dose range of ethanol (0, 0.5, 1, or 2 g/kg, intragastric) and brain tissue was removed 10 minutes later for evaluation of changes in p-cPKC expression using immunohistochemistry and Western blot methods.

RESULTS

Immunohistochemical data show that the highest dose of ethanol (2 g/kg) rapidly increases p-cPKC immunoreactivity specifically in the nucleus accumbens (core and shell), lateral septum, and hippocampus (CA3 and dentate gyrus). Western blot analysis further showed that ethanol (2 g/kg) increased p-cPKC expression in the P2 membrane fraction of tissue from the nucleus accumbens and hippocampus. Although p-cPKC was expressed in numerous other brain regions, including the caudate nucleus, amygdala, and cortex, no changes were observed in response to acute ethanol. Total PKCgamma immunoreactivity was surveyed throughout the brain and showed no change following acute ethanol injection.

CONCLUSIONS

These results suggest that ethanol rapidly promotes phosphorylation of cPKC in limbic brain regions, which may underlie effects of acute ethanol on the nervous system and behavior.

The different facets of protein kinases C: old and new players in neuronal signal transduction pathways

Signal transduction pathways are crucial for cell-to-cell communication. Various molecular cascades allow the translation of distinct stimuli, targeting the cell, into a language that the cell itself is able to understand, thus elaborating specific responses. Within this context, a strategic role is played by protein kinases which catalyze the phosphorylation of specific substrates. The serine/threonine protein kinase C (PKC) enzymes family (at least 10 isoforms) is implicated in the transduction of signals coupled to receptor-mediated hydrolysis of membrane phospholipids. Within this molecular pathway, protein-protein interactions play a critical role in directing the distinct activated PKCs towards selective subcellular compartments, in order to guarantee spatio-temporal and localized cellular responses. A space-specific modulation of biochemical events is particularly important during learning. Among the various mechanisms, the modulation of mRNA decay appears to be an efficient post-transcriptional way of controlling gene expression during learning, allowing changes to take place in selected neuronal regions, in particular at synaptic level. To this regard, recent studies have pointed out that PKC activation is also involved in a novel signalling cascade leading to the stabilization of specific mRNAs. This review will especially focus the attention on the implication of PKC in memory trace formation and how alterations within this molecular cascade may have consequences on physiological and pathological neuronal aging (i.e. Alzheimer's disease).

BIG news in alcohol addiction: new findings on growth factor pathways BDNF, insulin, and GDNF

In recent years, it has become clear that growth factors are not only critical for the development of the central nervous system (CNS) but may also be important contributors to other neuronal functions in the adult brain. This symposium, presented at the 2005 RSA meeting, discussed evidence to support the hypothesis that alterations in growth factor pathways produce dramatic changes in the effects of alcohol on the CNS. The 4 speakers showed that the behavioral effects of alcohol in the adult are regulated by 3 growth factors, insulin, glial cell line-derived neurotrophic factor (GDNF), and brain-derived neurotrophic factor (BDNF). Dr. Wolf from the Heberlein laboratory presented findings obtained from genetic manipulations in Drosophila melanogaster, demonstrating that the insulin pathway controls sensitivity to the intoxicating effects of alcohol. Marian Logrip from the Ron and Janak laboratories presented evidence obtained in rodents that low concentrations of alcohol increase the expression of BDNF in the brain to regulate alcohol consumption. Dr. Pandey showed that amygdalar BDNF regulates alcohol's anxiolytic effects and preference. Finally, Dr. Janak presented evidence that increases in the expression of GDNF in the midbrain reduce alcohol self-administration in rats.

Localization of the scaffolding protein RACK1 in the developing and adult mouse brain

RACK1 is a multifunctional scaffolding protein known to be involved in the regulation of various signaling cascades in the central nervous system (CNS). In order to gain insight into the neurological functions of RACK1, we examined the expression of RACK1 mRNA and protein during gestation and in the adult mouse brain. Several expression patterns were observed. At embryonic day 11.5 (E11.5), RACK1 is expressed in a high-dorsal to low-ventral gradient throughout the brain. At E13.5, RACK1 is most abundant in the telencephalon. In the developing cortical primordium, RACK1 protein is expressed in a high-rostromidline to low-caudolateral gradient that appears to be regulated post-transcriptionally. At E18.5, RACK1 is expressed most abundantly in layers 1-4 of the cortex, striatum, hippocampus, dentate gyrus and specific thalamic nuclei. In the adult mouse, RACK1 is ubiquitously expressed in neuronal perikarya in most brain regions, with relatively higher levels in hippocampus, olfactory bulb, cortex and cerebellum. Subcellular staining was detected mainly in the cell bodies and extending into dendrites, whereas RACK1 was not present significantly in axonal fibers or nuclei. We also determined brain regions in which RACK1 interacts with one of its binding partners, the betaII isoform of protein kinase C (betaIIPKC). We found that betaIIPKC had a much more restricted expression pattern than RACK1 and overlapped with the scaffolding protein only in certain regions, including the CA1 area of the hippocampus, cerebellum and striatum. Our results suggest an important role for RACK1 during CNS development and support multiple functions of the protein in the adult brain.

The "ups and downs" of signaling cascades in addiction

Drug addiction is a chronic disease characterized by compulsive drug use despite the severe negative consequences associated with it. Repeated exposure to drugs of abuse results in molecular adaptations in neuronal signaling pathways, which eventually manifest in the complex behavioral alterations that characterize addiction. These include tolerance, sensitization, dependence, drug craving, and relapse. In this Review, we focus on recent studies highlighting signaling cascades initiated by cocaine, as a representative of a drug of abuse with a defined site of action, and alcohol, as a drug with an undefined primary site of action. Specifically, we describe recent studies that emphasize the role of protein-protein interactions, phosphorylation, and compartmentalization in the molecular mechanisms that result in the cellular and behavioral adaptations that underlie addiction. Signaling cascades that contribute to addiction, as well as those that protect or delay the development of addiction, are presented.

Ethanol treatment reduces bovine bronchial epithelial cell migration

BACKGROUND

Chronic ethanol abuse is associated with significant lung disease. Excessive alcohol intake increases risk for a variety of respiratory tract diseases, including pneumonia and bronchitis. Damage to airway epithelium is critical to the pathogenesis of airway disorders such as chronic bronchitis and chronic obstructive pulmonary disease. The ability of the airway epithelium to repair itself is an important step in the resolution of airway inflammation and disease. Ethanol exposure is known to modulate signaling systems in bronchial epithelial cells. We hypothesize that chronic ethanol exposure down-regulates the adenosine 3':5'-cyclic monophosphate signaling cascade in airway epithelial cells, resulting in decreased epithelial cell migration and repair.

METHODS

We evaluated the effect of ethanol on primary cultures of bovine bronchial epithelial cells in in vitro models of cell migration, wound repair, cell attachment, and cell spreading.

RESULTS

Ethanol causes a concentration-dependent effect on closure of mechanical wounds in cell monolayers. Pretreatment of cells with 100 mm ethanol for 24 hr further slows wound closure. Ethanol pretreatment also reduced the protein kinase A response to wounding and made the cells unresponsive to stimuli of protein kinase A that accelerate wound closure. The effects of ethanol on cell migration in wound closure were confirmed in another assay of migration, the Boyden chamber cell migration assay. Prolonged treatment with ethanol also reduced other cell functions, such as spreading and attachment, which are necessary for epithelial repair.

CONCLUSIONS

Ethanol modulates signaling systems that are relevant to airway injury and repair, suggesting that chronic, heavy ethanol ingestion has a detrimental impact on airway repair. Impaired response to inflammation and injury may contribute to chronic airway disease.

Trap RACK1 with Ras to mobilize Src signaling at syndecan-2/p120-GAP upon transformation with oncogenic ras

HiTrap-syndecan-2/p120-GAP and HiTrap-syndecan-2/RACK1 affinity columns were applied to reveal that Src tyrosine kinase was highly expressed in BALB/3T3 cells transfected with plasmids pcDNA3.1-[S-ras(Q(61)K)] of shrimp Penaeus japonicus. Both columns were effective to isolate Src tyrosine kinase. The selective molecular affinity for Src was found to be stronger with HiTrap-syndecan-2/RACK1, as revealed with competitive RACK1 to dislodge Src from HiTrap-syndecan-2/p120-GAP. We thus challenged the syndecan-2/p120-GAP and syndecan-2/RACK1 with GTP-K(B)-Ras(Q(61)K). The reaction between RACK1 and syndecan-2 was sustained in the presence of mutant Ras proteins, but not the reaction between p120-GAP and syndecan-2. In the presence of syndecan-2, GTP-K(B)-Ras(Q(61)K) exhibited sufficient reactivity with p120-GAP to discontinue the reaction between p120-GAP and syndecan-2. But the interference of mutant Ras disappeared when Src tyrosine kinase was introduced to stabilize the syndecan-2/p120-GAP complex. On the other hand, in the absence of syndecan-2, GTP-K(B)-Ras(Q(61)K) was found to react with RACK1. The reaction between GTP-K(B)-Ras(Q(61)K) and RACK1 could provide a mechanism to deprive RACK1 for the organization of syndecan-2/RACK1 complex and to facilitate the formation of syndecan-2/p120-GAP complex, as well as to provide docking sites for Src signaling upon transformation with oncogenic ras.

RACK1 and brain-derived neurotrophic factor: a homeostatic pathway that regulates alcohol addiction

Alcoholism is a devastating disease that manifests as uncontrolled drinking. Consumption of alcohol is regulated by neurochemical systems within specific neural circuits, but endogenous systems that may counteract and thus suppress the behavioral effects of ethanol intake are unknown. Here we demonstrate that BDNF plays a role in reducing the behavioral effects of ethanol, including consumption, in rodents. We found that decreasing the levels of BDNF leads to increased behavioral responses to ethanol, whereas increases in the levels of BDNF, mediated by the scaffolding protein RACK1, attenuate these behaviors. Interestingly, we found that acute exposure of neurons to ethanol leads to increased levels of BDNF mRNA via RACK1. Importantly, acute systemic administration of ethanol and voluntary ethanol consumption lead to increased levels of BDNF expression in the dorsal striatum. Taken together, these findings suggest that RACK1 and BDNF are part of a regulatory pathway that opposes adaptations that lead to the development of alcohol addiction.

Dimerize RACK1 upon transformation with oncogenic ras

From our previous studies, we learned that syndecan-2/p120-GAP complex provided docking site for Src to prosecute tyrosine kinase activity upon transformation with oncogenic ras. And, RACK1 protein was reactive with syndecan-2 to keep Src inactivated, but not when Ras was overexpressed. In the present study, we characterized the reaction between RACK1 protein and Ras. RACK1 was isolated from BALB/3T3 cells transfected with plasmids pcDNA3.1-[S-ras(Q61K)] of shrimp Penaeus japonicus and RACK1 was revealed to react with GTP-K(B)-Ras(Q61K), not GDP-K(B)-Ras(Q61K). This selective interaction between RACK1 and GTP-K(B)-Ras(Q61K) was further confirmed with RACK1 of human placenta and mouse RACK1-encoded fusion protein. We found that RACK1 was dimerized upon reaction with GTP-K(B)-Ras(Q61K), as well as with 14-3-3beta and geranylgeranyl pyrophosphate, as revealed by phosphorylation with Src tyrosine kinase. We reported the complex of RACK1/GTP-K(B)-Ras(Q61K) reacted selectively with p120-GAP. This interaction was sufficient to dissemble RACK1 into monomers, a preferred form to compete for the binding of syndecan-2. These data indicate that the reaction of GTP-K(B)-Ras(Q61K) with RACK1 in dimers may operate a mechanism to deplete RACK1 from reaction with syndecan-2 upon transformation by oncogenic ras and the RACK1/GTP-Ras complex may provide a route to react with p120-GAP and recycle monomeric RACK1 to syndecan-2.

P120-GAP associated with syndecan-2 to function as an active switch signal for Src upon transformation with oncogenic ras

BALB/3T3 cells transfected with plasmids pcDNA3.1-[S-ras(Q(61)K)] of shrimp Penaeus japonicus were applied to reveal a complex of p120-GAP/syndecan-2 being highly expressed upon transformation. Of interest, most of the p120-GAP/syndecan-2 complex was localized at caveolae, a membrane microdomain enriched with caveolin-1. To confirm the molecular interaction between syndecan-2 and p120-GAP, we further purified p120-GAP protein from mouse brains by using an affinity column of HiTrap-RACK1 and expressed mouse RACK1-encoded fusion protein and mouse syndecan-2-encoded fusion protein in bacteria. We report molecular affinities exist between p120-GAP and RACK1, syndecan-2 and RACK1 as well as p120-GAP and syndecan-2. The selective affinity between p120-GAP and syndecan-2 was found to be sufficient to detach RACK1. The p120-GAP/syndecan-2 complex was demonstrated to keep Src tyrosine kinase in an activated form. On the other hand, the syndecan-2/RACK1 complex was found to have Src in an inactivated form. These data indicate that the p120-GAP/syndecan-2 complex at caveolae could provide a docking site for Src to transmit tyrosine signaling, implying that syndecan-2/p120-GAP functions as a tumor promoter upon transformation with oncogenic ras of shrimp P. japonicus.

Functional expression cloning reveals a central role for the receptor for activated protein kinase C 1 (RACK1) in T cell apoptosis

Mammalian cDNA expression cloning was used to identify novel genes that regulate apoptosis. Using a functional screen, we identified a partial cDNA for the receptor for activated protein kinase C 1 (RACK1) through selection for resistance to phytohemagglutinin and gamma-irradiation. Expression of this partial cDNA in T cell lines using a mammalian expression vector produced an increase in RACK1 expression and resulted in resistance to dexamethasone- and ultraviolet-induced apoptosis. Down-regulation of RACK1 using RNA interference abolished the resistance of the transfected cells to apoptosis. Overexpression of full-length RACK1 also resulted in the suppression of apoptosis mediated by several apoptotic stimuli, and this effect was quantitatively consistent with the effects of the original cDNA isolated on endogenous RACK1 levels. Together, these findings suggest that RACK1 plays an important role in the intracellular signaling pathways that lead to apoptosis in T cells.

Signaling cascades regulating NMDA receptor sensitivity to ethanol

One of the major targets for ethanol (alcohol) in the brain is the N-methyl-D-aspartate (NMDA) receptor, a glutamate-gated ion channel. Intriguingly, the effects of ethanol on the NMDA receptor are not homogeneous throughout the brain. This review focuses on recent studies revealing molecular mechanisms that mediate the actions of ethanol on the NMDA receptor in different brain regions via changes in NMDA receptor phosphorylation and compartmentalization. Specifically, the role of the scaffolding protein RACK1 and the regulatory protein DARPP-32 in mediating the distinct effects of ethanol is presented.

Cellular adaptation to chronic ethanol results in altered compartmentalization and function of the scaffolding protein RACK1

BACKGROUND

Previously, we found that acute ethanol induces the translocation of the scaffolding protein RACK1 to the nucleus. Recently, we found that nuclear RACK1 mediates acute ethanol induction of immediate early gene c-fos expression. Alterations in gene expression are thought to lead to long-term changes that ultimately contribute to the development of alcohol addiction and toxicity. Therefore, we sought to determine the effects of chronic exposure of cells to ethanol on the cellular compartmentalization of RACK1 and on c-fos messenger RNA (mRNA) and protein expression.

METHODS

Rat C6 glioma cells were used as the cell culture model. Immunohistochemistry was implemented to visualize the localization of RACK1 and to monitor the protein level of c-fos. Reverse-transcription polymerase chain reaction was used to measure c-fos mRNA levels. The Tat-protein transduction method was used to transduce recombinant Tat-RACK1 into cells as previously described.

RESULTS

Chronic exposure of cells to 200 mM ethanol for 24 and 48 hr resulted in the gradual re-distribution of RACK1 out of the nucleus. It is interesting to note that acute ethanol re-challenge immediately after chronic treatment did not result in RACK1 translocation to the nucleus, and nuclear compartmentalization of RACK1 in response to acute ethanol was detected only after 24 hr of withdrawal. Similar patterns were obtained for c-fos expression. Chronic exposure to ethanol did not result in an increase in mRNA or protein levels of c-fos. Furthermore, acute ethanol exposure did not increase c-fos protein levels in cells that were first treated chronically with ethanol. However, transduction of exogenous RACK1 expressed as a Tat-fusion protein was able to rescue c-fos mRNA expression after chronic ethanol exposure.

CONCLUSIONS

Our data suggest that RACK1 nuclear compartmentalization and ethanol-induced c-fos expression are transient and are desensitized to ethanol during prolonged exposure to high concentrations. The desensitization is temporary, and RACK1 can respond to acute ethanol treatment after a 24-hr withdrawal period. Our data further suggest that the altered compartmentalization of RACK1 leads to differences in c-fos expression upon acute or chronic exposure to ethanol. In summary, RACK1 is an important molecular mediator of the acute and chronic actions of ethanol on the expression of c-fos. These findings could have implications for the molecular signaling pathways leading to pathologic states associated with alcoholism, including toxicity.

Translocation of protein kinase C-betaII in astrocytes requires organized actin cytoskeleton and is not accompanied by synchronous RACK1 relocation

Protein kinase C (PKC)-betaII is the most abundant PKC isoform in astrocytes. Upon activation, this isoform of PKC translocates from the cytosol to the plasma membrane (PM). In this study, we investigated in astrocytes the modality of PKC-betaII translocation as far as the participation of the receptor for activated C kinase-1 (RACK1) and the requirement for intact cytoskeleton in the process. In astrocytes, Western blots and immunocytochemistry coupled to confocal microscopic quantitative analysis showed that after 5 min of phorbol-12-myristate-13-acetate (PMA) exposure, native PKC-betaII, but not PKC-betaI, is relocated efficiently from the cytosol to the PM. Translocation of PKC-betaII was not associated with synchronous RACK1 relocation. Furthermore, the quantity of PM-associated PKC-betaII that co-immunoprecipitated with PM-bound RACK1 increased following PMA exposure, indicating a post activation binding of the two proteins in the PM. Because RACK1 and PKC-betaII relocation seemed not to be synchronous, we hypothesized that an intermediate interaction with the cytoskeleton was taking place. In fact, we were able to show that pharmacological disruption of actin-based cytoskeleton greatly deranged PKC-betaII translocation to the PM. The requirement for intact actin cytoskeleton was specific, because depolymerization of tubulin had no effect on the ability of the kinase to translocate to the PM. These results indicate that in astrocytes, RACK1 and PKC-betaII synchronous relocation is not essential for relocation of PKC-betaII to the PM. In addition, we show for the first time that the integrity of the actin cytoskeleton plays a specific role in PKC-betaII movements in these cells. We hypothesize that in glial cells, rapidly occurring changes of actin cytoskeleton arrangement may be involved in the fast reprogramming of PKC targeting to specific PM location to phosphorylate substrates in different cellular locations.

Differential effects of prenyl pyrophosphates on the phosphatase activity of phosphotyrosyl protein phosphatase

Phosphotyrosyl protein phosphatase (PTPase) 1B was purified from human placenta. Immunoprecipitation analysis revealed that the isolated PTPase 1B appears as a complex with the receptor for protein kinase C (RACK1) and protein kinase C (PKC)delta. The abilities of PTPase 1B and PKCdelta to associate with RACK1 were reconfirmed by an in vitro reconstitution experiment. The E. coli expressed and biotinylated mice-RACK1-encoded fusion protein was capable of recruiting PTPase 1B and PKCdelta in the antibiotin immunoprecipitate as a complex of PTPase 1B/RACK1/PKCdelta. Thus PTPase 1B enzyme preparation was subjected to further purification by selective binding of PTPase 1B onto PEP(Taxol) affinity column in the absence of ATP. The purified PTPase 1B enzyme exihibited dose-dependent phosphatase activity towards [gamma-(32)P]-ATP labeled mice beta-tubulin-encoded fusion protein. The dephosphorylation reaction with PTPase 1B was enhanced with geranylgeranyl pyrophosphate, but not with farnesyl pyrophosphate. Interestingly, additional incubation of the purified PTPase 1B enzyme preparation with RACK1, geranylgeranyl pyrophosphate failed to modulate the dephosphorylation activity of PTPase 1B. In contrast, the enhancement effect of farnesyl pyrophosphate on the kinase activity of PKCdelta was sustained in the presence of RACK1. That is, farnesyl pyrophosphate may function as a signal to induce the kinase activity of PKCdelta in PTPase 1B/RACK1/PKCdelta complex but geranylgeranyl pyrophosphate may not for PTPase 1B. J. Exp. Zool. 301A:307-316, 2004.

Cpc2/RACK1 is a ribosome-associated protein that promotes efficient translation in Schizosaccharomyces pombe

Cpc2/RACK1 is a highly conserved WD domain protein found in all eucaryotes. Cpc2/RACK1 functions on mammalian signal transduction pathways most notably as an adaptor protein for the betaII protein kinase C isozyme. In single cell eucaryotes, Cpc2/RACK1 regulates growth, differentiation, and entry into G0 stationary phase. The exact biochemical function of Cpc2/RACK1 is unknown. Here, we provide evidence that Cpc2 is associated with the ribosome. Using immunoaffinity purification, we isolated ribosomal proteins in association with Cpc2/RACK1. Polysome and ribosomal subunit analysis using velocity gradient centrifugation of cell lysates demonstrated that Cpc2 co-sediments with the 40 S ribosomal subunit and with polysomes. Conditions known to disrupt ribosome structure alter sedimentation of the ribosome and of Cpc2/RACK1 coordinately. Loss of cpc2 does not dramatically alter the rate of cellular protein synthesis but causes a decrease in the steady state level of numerous proteins, some of which regulate methionine metabolism. Whereas real time PCR analysis demonstrated that transcriptional mechanisms are responsible for down-regulation of some of these proteins, one protein, ribosomal protein L25, is probably regulated at the level of translation.

The scaffolding protein RACK1 interacts with androgen receptor and promotes cross-talk through a protein kinase C signaling pathway

The androgen receptor (AR), a member of the nuclear hormone receptor superfamily, functions as a ligand-dependent transcription factor that regulates genes involved in cell proliferation and differentiation. Using a C-terminal region of the human AR in a yeast two-hybrid screen, we have identified RACK1 (receptor for activated C kinase-1) as an AR-interacting protein. In this report we found that RACK1, which was previously shown to be a protein kinase C (PKC)-anchoring protein that determines the localization of activated PKCbetaII isoform, facilitates ligand-independent AR nuclear translocation upon PKC activation by indolactam V. We also observed RACK1 to suppress ligand-dependent and -independent AR transactivation through PKC activation. In chromatin immunoprecipitation assays, we demonstrate a decrease in AR recruitment to the AR-responsive prostate-specific antigen (PSA) promoter following stimulation of PKC. Furthermore, prolonged exposure to indolactam V, a PKC activator, caused a reduction in PSA mRNA expression in prostate cancer LNCaP cells. Finally, we found PKC activation to have a repressive effect on AR and PSA protein expression in androgen-treated LNCaP cells. Our data suggest that RACK1 may function as a scaffold for the association and modification of AR by PKC enabling translocation of AR to the nucleus but rendering AR unable to activate transcription of its target genes.

Chronic ethanol exposure increases the binding of HuR to the TNFalpha 3'-untranslated region in macrophages

Tumor necrosis factor alpha (TNFalpha) expression is a key mediator of ethanol-induced liver disease. Increased lipopolysaccharide (LPS)-stimulated TNFalpha expression in macrophages after chronic ethanol feeding is associated with a stabilization of TNFalpha mRNA (Kishore, R., McMullen, M. R., and Nagy, L. E. (2001) J. Biol. Chem. 276, 41930-41937). Here we show that the 3'-UTR of murine TNFalpha mRNA was sufficient to mediate increased LPS-stimulated expression of a luciferase reporter in RAW 264.7 macrophages after chronic ethanol exposure. Further, we show that HuR, a nuclear/cytoplasmic shuttling protein, which binds to TNFalpha mRNA, is required for increased expression of TNFalpha after chronic ethanol. In Kupffer cells, HuR was primarily localized to the nucleus and then translocated to the cytosol in response to LPS in both pair- and ethanol-fed rats. After chronic ethanol feeding, HuR quantity in the cytosol was greater, both at baseline and in response to LPS, compared with pair-fed controls. Using RNA gel shift assays, we found that LPS treatment increased HuR binding to the 65-nucleotide A + U-rich element of the TNFalpha 3'-UTR by 2-fold over baseline in Kupffer cells from pair-fed rats. After chronic ethanol feeding, HuR binding to the TNFalpha A + U-rich element was increased by more than 5-fold at baseline and in response to LPS, compared with pair-fed controls. Down-regulation of HuR expression by RNA interference prevented the chronic ethanol-induced increase in expression of luciferase reporters containing the TNFalpha 3'-UTR. Taken together, these data demonstrate that increased binding of HuR to the TNFalpha 3'-UTR contributes to increased LPS-stimulated TNFalpha expression in macrophages after chronic ethanol exposure.

Farnesyl pyrophosphate promotes and is essential for the binding of RACK1 with beta-tubulin

Receptors for activated C kinase (RACKs) are a group of protein kinase C (PKC) binding proteins that have been shown to be crucial in the translocation and subsequent functioning of PKC on activation. RACK1 isolated from BALB/3T3 cells transformed with S-ras(Q61K) exhibits receptor activity for PKCgamma as competent as that of RACK1 from BALB/3T3 cells without transformation. However, the ability of RACK1 from transformed cells to bind with beta-tubulin peptide specific for Taxol (PEPtaxol) is defective. Interestingly, when farnesyl pyrophosphate was added at the submicrogram level, the association between RACK1 and PEPtaxol was enhanced significantly in a dosage-dependent manner. A parallel finding for the enhanced effect of farnesyl pyrophosphate on tubulin binding was established with mice RACK1 expressed in vitro. On the other hand, geranylgeranyl pyrophosphate, and retinoic acid failed to modulate the binding between RACK1 and tubulin. The dissociation of RACK1 and tubulin was not effective at damaging the binding between RACK1 and membrane receptor integrin beta1 in transformed cells. These findings indicate that depletion of farnesyl pyrophosphate provides a mechanism to seal PKC signaling on the membrane with immobile RACK1 and to divert cells to aberrant growth, such as transformation.

Scaffolding of Fyn kinase to the NMDA receptor determines brain region sensitivity to ethanol

Alcohol (ethanol) abuse is a major societal problem. Although ethanol is a structurally simple, diffusible molecule, its sites of action are surprisingly selective, and the molecular mechanisms underlying specificity in ethanol actions are not understood. The NMDA receptor channel is one of the main targets for ethanol in the brain. We report here that the brain region-specific compartmentalization of Fyn kinase determines NMDA receptor sensitivity to ethanol. We demonstrate that, in the hippocampus but not in the cerebral cortex, Fyn is targeted to the NR2B subunit of the NMDA receptor by the scaffolding protein RACK1. During acute exposure to ethanol, RACK1 is dissociated from the complex, thereby facilitating Fyn-mediated phosphorylation of NR2B, which enhances channel activity, counteracting the inhibitory actions of ethanol. In this way, the selective scaffolding can account for the ethanol-induced acute tolerance of NMDA receptor activity that is detected in the hippocampus but not in the cerebral cortex. The phosphorylation-dependent, region-specific activities of ethanol on the NMDA receptor provide a compelling molecular explanation that accounts for the selective activities of ethanol and may have important implications for elucidating pathways leading to alcohol addiction.

Pituitary adenylate cyclase-activating polypeptide (PACAP(1-38)) enhances N-methyl-D-aspartate receptor function and brain-derived neurotrophic factor expression via RACK1

We recently identified a novel mechanism for modulation of the phosphorylation state and function of the N-methyl-d-aspartate (NMDA) receptor via the scaffolding protein RACK1. We found that RACK1 binds both the NR2B subunit of the NMDA receptor and the nonreceptor protein-tyrosine kinase, Fyn. RACK1 inhibits Fyn phosphorylation of NR2B and decreases NMDA receptor-mediated currents in CA1 hippocampal slices (Yaka, R., Thornton, C., Vagts, A. J., Phamluong, K., Bonci, A., and Ron, D. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 5710-5715). Here, we identified the signaling cascade by which RACK1 is released from the NMDA receptor complex and identified the consequences of the dissociation. We found that activation of the cAMP/protein kinase A pathway in hippocampal slices induced the release of RACK1 from NR2B and Fyn. This resulted in the induction of NR2B phosphorylation and the enhancement of NMDA receptor-mediated activity via Fyn. We identified the neuropeptide, pituitary adenylate cyclase activating polypeptide (PACAP(1-38)), as a ligand that induced phosphorylation of NR2B and enhanced NMDA receptor potentials. Finally, we found that activation of the cAMP/protein kinase A pathway induced the movement of RACK1 to the nuclear compartment in dissociated hippocampal neurons. Nuclear RACK1 in turn was found to regulate the expression of brain-derived neurotrophic factor induced by PACAP(1-38). Taken together our results suggest that activation of adenylate cyclase by PACAP(1-38) results in the release of RACK1 from the NMDA receptor and Fyn. This in turn leads to NMDA receptor phosphorylation, enhanced activity mediated by Fyn, and to the induction of brain-derived neurotrophic factor expression by RACK1.

Interaction of "readthrough" acetylcholinesterase with RACK1 and PKCbeta II correlates with intensified fear-induced conflict behavior

Behavioral reactions to stress are altered in numerous psychiatric and neurodegenerative syndromes, but the corresponding molecular processes and signal transduction pathways are yet unknown. Here, we report that, in mice, the stress-induced splice variant of acetylcholinesterase, AChE-R, interacts intraneuronally with the scaffold protein RACK1 and through it, with its target, protein kinase CbetaII (PKCbetaII), which is known to be involved in fear conditioning. In stress-responsive brain regions of normal FVBN mice, the mild stress of i.p. injection increased AChE and PKCbetaII levels in a manner suppressible by antisense prevention of AChE-R accumulation. Injection stress also prolonged conflict between escape and hiding in the emergence into an open field test. Moreover, transgenic FVBN mice overexpressing AChE-R displayed prolonged delay to emerge into another field (fear-induced behavioral inhibition), associated with chronically intensified neuronal colabeling of RACK1 and PKCbetaII in stress-responsive brain regions. These findings are consistent with the hypothesis that stress-associated changes in cholinergic gene expression regulate neuronal PKCbetaII functioning, promoting fear-induced conflict behavior after stress.

Chronic EtOH administration alters liver Mg2+ homeostasis

Ethanol (EtOH) administration to rats for 4 wk markedly decreased Mg(2+) content in several tissues, including liver. Total cellular Mg(2+) accounted for 26.8 +/- 2.4 vs. 36.0 +/- 1.4 nmol Mg(2+)/mg protein in hepatocytes from EtOH-fed and control rats, respectively, and paralleled a 13% decrease in cellular ATP content. Stimulation of alpha(1)- or beta-adrenergic receptor or acute EtOH administration did not elicit an extrusion of Mg(2+) from liver cells of EtOH-fed rats while releasing 5% of total tissue Mg(2+) content from hepatocytes of control rats. Despite the 25% decrease in Mg(2+) content, hepatocytes from EtOH-fed rats did not accumulate Mg(2+) following stimulation of protein kinase C signaling pathway, whereas control hepatocytes accumulated approximately 2 nmol Mg(2+). mg protein(-1). 4 min(-1). Together, these data indicate that Mg(2+) homeostasis and transport are markedly impaired in liver cells after prolonged exposure to alcohol. The inability of liver cells, and possibly other tissues, to accumulate Mg(2+) can help explain the reduction in tissue Mg(2+) content following chronic alcohol consumption.

Molecular characterization of protein kinase C-alpha binding to lamin A

Previous results from our laboratory have identified lamin A as a protein kinase C (PKC)-binding protein. Here, we have identified the regions of PKC-alpha that are crucial for this binding. By means of overlay assays and fusion proteins made of glutathione-S-transferase (GST) fused to elements of rat PKC-alpha, we have established that binding occurs through both the V5 region and a portion of the C2 region (i.e., the calcium-dependent lipid binding (CaLB) domain) of the kinase. In particular, we have found that amino acid 200-217 of the CaLB domain are essential for binding lamin A, as a synthetic peptide corresponding to this stretch of amino acids prevented the interaction between the CaLB domain and lamin A. We also show that the presence of four lysine residues of the CaLB domain (K205, K209, K211, and K213) was essential for the binding. We have determined that binding of elements of PKC-alpha to lamin A does not require the presence of cofactors such as phosphatidylserine (PS) and Ca(2+). We have also found that the binding site of lamin A for the CaLB domain of PKC-alpha is localized in the carboxyl-terminus of the lamin, downstream of amino acid 499. Our findings may prove to be important to clarify the mechanisms regulating PKC function within the nucleus and may also lead to the synthesis of isozyme-specific drugs to attenuate or reverse PKC-dependent nuclear signaling pathways important for the pathogenesis of cancer.

Protein kinase C inhibitors counteract the ethanol effects on myelin basic protein expression in differentiating CG-4 oligodendrocytes

Abnormal formation of myelin appears to be one defect contributing to the development of the neuropathology associated with the fetal alcohol syndrome. Using the CG-4 cell line we previously showed that 25-75 mM EtOH downregulates the expression of myelin basic protein (MBP) in differentiating oligodendrocytes (OLGs) without affecting morphological development (Dev. Brain Res. 128 (2001) 9). Here we showed that a relatively low concentration of 12-phorbol-13-myristate acetate (PMA) mimicked the EtOH-caused inhibition of MBP expression without affecting morphology. The inhibition of MBP expression by 100 mM EtOH or 1 nM PMA was completely counteracted by three inhibitors of protein kinase C (PKC), bisindolylmaleimide I, chelerythrine chloride, and calphostin C, indicating that EtOH downregulated MBP expression by activating PKC. We investigated whether the EtOH activation resulted, in part, from upregulation of the expression of PKC isozymes. Of 11 PKC isozymes examined, CG-4 OLGs expressed nine; PKCs alpha, beta1, beta2; delta, epsilon, eta; lambda, zeta; mu; while PKC isozymes gamma and theta were not detected. Only five PKC isozymes, alpha, beta1, beta2, eta, and mu, displayed developmental changes in expression. However, EtOH did not upregulate the early expression of any PKC isozyme during the first 2 days of differentiation, the developmental stage when it downregulates MBP expression in CG-4 cells. The similar effects of PMA and EtOH indicate that EtOH delays MBP expression by activating at least one phorbol ester-sensitive PKC isozyme in oligodendrocytes without upregulating its expression.

Binding of elements of protein kinase C-alpha regulatory domain to lamin B1

Previous results from our laboratory have demonstrated that lamin B1 is a protein kinase C (PKC)-binding protein. Here, we have identified the regions of PKC-alpha that are important for this binding. By means of overlay assays and fusion proteins made of glutathione-S-transferase (GST) fused to elements of the regulatory domain of rat PKC-alpha, we have established that binding occurs through both the V1 region and a portion of the C2 region (i.e., the calcium-dependent lipid binding [CaLB] domain) of the kinase. In particular, we have found that amino acids 200-217 of the CaLB domain are essential for binding lamin B1, as a synthetic peptide corresponding to this stretch of amino acids prevented the interaction between the CaLB domain of PKC-alpha and lamin B1. In agreement with the results of other investigators, we have determined that binding of regulatory elements of PKC-alpha to lamin B1 does not require the presence of cofactors such as PS and Ca(2+). We have also found that the binding site of lamin B1 for PKC-alpha is localized in the carboxyl-terminus of the lamin. Our findings may prove to be important in shedding more light on the mechanisms that regulate PKC functions within the nuclear compartment and may also lead to the synthesis of isozyme-specific pharmacological tools to attenuate or reverse PKC-dependent nuclear signalling pathways important for the pathogenesis of cancer.

Ethanol induces gene expression via nuclear compartmentalization of receptor for activated C kinase 1

Scaffolding proteins such as receptor for activated C kinase (RACK) 1 are involved in the targeting of signaling proteins and play an important role in the regulation of signal transduction cascades. Recently, we found that in cultured cells and in vivo, acute ethanol exposure induces the nuclear compartmentalization of RACK1. To elucidate a physiological role for nuclear RACK1, the Tat protein transduction system was used to transduce RACK1 and RACK1-derived fragments into C6 glioma cells. We found that nuclear RACK1 is mediating the induction of the immediate early gene c-fos expression induced by ethanol. First, transduction of full-length RACK1 (Tat-RACK1) resulted in the induction of c-fos expression and enhancement of ethanol activities. Second, we determined that the C terminus of RACK1 (Tat-RACK1DeltaN) is mediating transcription. Third, we identified a dominant negative fragment of RACK1 that inhibited the nuclear compartmentalization of endogenous RACK1 and inhibited ethanol-induction of c-fos mRNA and protein expression. Last, acute exposure to ethanol or transduction of full-length Tat-RACK1 resulted in an increase in mRNA levels of an activator protein 1 site-containing gene, PAC1 (pituitary adenylate cyclase-activating polypeptide receptor type I), suggesting that nuclear RACK1 is involved in the regulation of the expression of genes that are altered upon acute ethanol treatment. These results may therefore have important implications for the study of alcohol addiction.