Role of protein kinase C in estrogen protection against apoptotic cerebellar cell death in ethanol-withdrawn rats

Intermittent hypoxia training: Powerful, non-invasive cerebroprotection against ethanol withdrawal excitotoxicity

Ethanol intoxication and withdrawal exact a devastating toll on the central nervous system. Abrupt ethanol withdrawal provokes massive release of the excitatory neurotransmitter glutamate, which over-activates its postsynaptic receptors, causing intense Ca2+ loading, p38 mitogen activated protein kinase activation and oxidative stress, culminating in ATP depletion, mitochondrial injury, amyloid β deposition and neuronal death. Collectively, these mechanisms produce neurocognitive and sensorimotor dysfunction that discourages continued abstinence. Although the brain is heavily dependent on blood-borne O2 to sustain its aerobic ATP production, brief, cyclic episodes of moderate hypoxia and reoxygenation, when judiciously applied over the course of days or weeks, evoke adaptations that protect the brain from ethanol withdrawal-induced glutamate excitotoxicity, mitochondrial damage, oxidative stress and amyloid β accumulation. This review summarizes evidence from ongoing preclinical research that demonstrates intermittent hypoxia training to be a potentially powerful yet non-invasive intervention capable of affording robust, sustained neuroprotection during ethanol withdrawal.

Alcohol withdrawal and brain injuries: beyond classical mechanisms

Unmanaged sudden withdrawal from the excessive consumption of alcohol (ethanol) adversely alters neuronal integrity in vulnerable brain regions such as the cerebellum, hippocampus, or cortex. In addition to well known hyperexcitatory neurotransmissions, ethanol withdrawal (EW) provokes the intense generation of reactive oxygen species (ROS) and the activation of stress-responding protein kinases, which are the focus of this review article. EW also inflicts mitochondrial membranes/membrane potential, perturbs redox balance, and suppresses mitochondrial enzymes, all of which impair a fundamental function of mitochondria. Moreover, EW acts as an age-provoking stressor. The vulnerable age to EW stress is not necessarily the oldest age and varies depending upon the target molecule of EW. A major female sex steroid, 17β-estradiol (E2), interferes with the EW-induced alteration of oxidative signaling pathways and thereby protects neurons, mitochondria, and behaviors. The current review attempts to provide integrated information at the levels of oxidative signaling mechanisms by which EW provokes brain injuries and E2 protects against it.

Mitochondrial mechanisms of estrogen neuroprotection

Mitochondria have become a primary focus in our search not only for the mechanism(s) of neuronal death but also for neuroprotective drugs and therapies that can delay or prevent Alzheimer's disease and other chronic neurodegenerative conditions. This is because mitochrondria play a central role in regulating viability and death of neurons, and mitochondrial dysfunction has been shown to contribute to neuronal death seen in neurodegenerative diseases. In this article, we review the evidence for the role of mitochondria in cell death and neurodegeneration and provide evidence that estrogens have multiple effects on mitochondria that enhance or preserve mitochondrial function during pathologic circumstances such as excitotoxicity, oxidative stress, and others. As such, estrogens and novel non-hormonal analogs have come to figure prominently in our efforts to protect neurons against both acute brain injury and chronic neurodegeneration.

Rapid signaling by steroid receptors

Steroid receptors transcribe genes that lead to important biological processes, including normal organ development and function, tissue differentiation, and promotion of oncogenic transformation. These actions mainly result from nuclear steroid receptor action. However, for 50 years, it has been known that rapid effects of steroid hormones occur and could result from rapid signal transduction. Examples of these effects include stress responses to secreted glucocorticoids, rapid actions of thyroid hormones in the heart, and acute uterine/vaginal responses to injected estrogen. These types of responses have increasingly been attributed to rapid signaling by steroid hormones, upon engaging binding proteins most often at the cell surface of target organs. It is clear that rapid signal transduction serves an integrated role to modify existing proteins, altering their structure and activity, and to modulate gene transcription, often through collaboration with the nuclear pool of steroid receptors. The biological outcomes of steroid hormone actions thus reflect input from various cellular pools, cocoordinating the necessary events that are restrained in temporal and kinetic fashion. Here I describe the current understanding of rapid steroid signaling that is now appreciated to extend to virtually all members of this family of hormones and their receptors.

Extranuclear steroid receptors: nature and actions

Rapid effects of steroid hormones result from the actions of specific receptors localized most often to the plasma membrane. Fast-acting membrane-initiated steroid signaling (MISS) leads to the modification of existing proteins and cell behaviors. Rapid steroid-triggered signaling through calcium, amine release, and kinase activation also impacts the regulation of gene expression by steroids, sometimes requiring integration with nuclear steroid receptor function. In this and other ways, the integration of all steroid actions in the cell coordinates outcomes such as cell fate, proliferation, differentiation, and migration. The nature of the receptors is of intense interest, and significant data suggest that extranuclear and nuclear steroid receptor pools are the same proteins. Insights regarding the structural determinants for membrane localization and function, as well as the nature of interactions with G proteins and other signaling molecules in confined areas of the membrane, have led to a fuller understanding of how steroid receptors effect rapid actions. Increasingly, the relevance of rapid signaling for the in vivo functions of steroid hormones has been established. Examples include steroid effects on reproductive organ development and function, cardiovascular responsiveness, and cancer biology. However, although great strides have been made, much remains to be understood concerning the integration of extranuclear and nuclear receptor functions to organ biology. In this review, we highlight the significant progress that has been made in these areas.

Conventional protein kinase C isoforms mediate neuroprotection induced by phorbol ester and estrogen

Rapid signal transduction pathways play a prominent role in mediating neuroprotective actions of estrogen in the CNS. We have previously shown that estrogen-induced neuroprotection of primary cerebrocortical neurons from beta-amyloid peptide (Abeta) toxicity depends on activation of protein kinase C (PKC). PKC activation with phorbol-12-myristate-13-acetate (PMA) also provides neuroprotection in this paradigm. Because the PKC family includes several isoforms that have opposing roles in regulating cell survival, we sought to identify which PKC isoforms contribute to neuroprotection induced by PMA and estrogen. We detected protein expression of multiple PKC isoforms in primary neuron cultures, including conventional (alpha, betaI, betaII), novel (delta, epsilon, theta) and atypical (zeta, iota/lambda) PKC. Using a panel of isoform-specific peptide inhibitors and activators, we find that novel and atypical PKC isoforms do not participate in the mechanism of either PMA or estrogen neuroprotection. In contrast, a selective peptide activator of conventional PKC isoforms provides dose-dependent neuroprotection against Abeta toxicity. In addition, peptide inhibitors of conventional, betaI, or betaII PKC isoforms significantly reduce protection afforded by PMA or 17beta-estradiol. Taken together, these data provide evidence that conventional PKC isoforms mediate phorbol ester and estrogen neuroprotection of cultured neurons challenged by Abeta toxicity.

Reversal of ethanol toxicity in embryonic neurons with growth factors and estrogen

Prenatal exposure to ethanol is the cause of fetal alcohol syndrome, which is characterized by brain abnormalities and decreased mental capacity. In the current study, cultured neurons from embryonic rat cortices were used to study the reversal of ethanol toxicity on neuronal survival and neurite outgrowth. Ethanol treatment followed by treatment with estrogen and certain growth factors were used to assess the potential of these growth factors and estrogen to reverse the effects of ethanol damage. Cortical neurons from embryonic day (E) 16 rats were grown in defined medium with a glial plane at a distance of 1mm from the neurons. Ethanol (45 mM) was administered on day in vitro 1 (DIV 1) and DIV 4. Insulin-like growth factor-I (IGF-I, 10 ng/ml), insulin-like growth factor-II (IGF-II, 10 ng/ml), basic fibroblast growth factor (bFGF, 5 ng/ml), nerve growth factor (NGF, 100 ng/ml), and estrogen (Es, 10 ng/ml) were administered on DIV 4 and DIV 5. Cell viability was determined on DIV 6 using the intravital dyes fluorescein diacetate and propidium iodide. IGF-I and bFGF reduced ethanol's toxic effect on neuronal survival. Estrogen, bFGF, and NGF increased total neurite length after ethanol treatment. Although none of the treatments had a statistically significant effect on the mean number of primary neurites, all caused a statistically significant increase in the mean number of secondary neurites per cell (a measure of neuritic branching) relative to the ethanol treatment alone.

Suppression of protein kinase Cepsilon mediates 17beta-estradiol-induced neuroprotection in an immortalized hippocampal cell line

Although estrogens are neuroprotective in a variety of neuroprotection models, the precise underlying mechanisms are currently not well understood. Here, we examined the role of protein kinase C (PKC) in mediating estrogen-induced neuroprotection in the HT-22 immortalized hippocampal cell line. The neuroprotection model utilized calcein fluorescence to quantitate cell viability following glutamate insults. 17beta-Estradiol (betaE2) protected HT-22 cells when treatment was initiated before or after the glutamate insult. The inhibition of PKC by bis-indolylmaleimide mimicked and enhanced betaE2-induced neuroprotection. In contrast, the inhibition of specific PKC isozymes (alpha and beta) by Go6976, inhibition of 1-phosphatidylinositol 3 kinase by wortmannin, or inhibition of protein kinase A by H-89, did not alter cell viability, suggesting a specific involvement of PKC in an isozyme-dependent manner. We further examined whether estrogen interacts with PKC in a PKC isozyme-specific manner. Protein levels and activity of PKC isozymes (alpha, delta, epsilon, and zeta) were assessed by western blot analysis and radiolabeled phosphorylation assays respectively. Among the isozymes tested, betaE2 altered only PKCepsilon; it reduced the activity and membrane translocation of PKCepsilon in a manner that correlated with its protection against glutamate toxicity. Furthermore, betaE2 reversed the increased activity of membrane PKCepsilon induced by glutamate. These data suggest that the neuroprotective effects of estrogens are mediated in part by inhibition of PKCepsilon activity and membrane translocation.

Estradiol protects against alteration of protein kinase Cɛ in a binge model of ethanol dependence and withdrawal

This study tested the hypothesis that a binge type of ethanol intake and ethanol withdrawal disturbs protein kinase C (PKC) homeostasis in a manner protected by 17beta-estradiol. Ovariectomized rats implanted with 17beta-estradiol or oil pellets received ethanol (7.5% weight/volume, 7 days) or control solution by a gavage method. The cerebelli were collected during ethanol exposure or ethanol withdrawal to assess the activity, protein levels, and cellular distribution of PKC(epsilon) and total PKC, using an ATP phosphorylation and immunoblot assays. While both ethanol exposure and ethanol withdrawal increased membrane protein levels and membrane translocation, only ethanol withdrawal enhanced activity of PKC(epsilon). Ethanol withdrawal not ethanol exposure increased the three parameters of total PKC. 17beta-Estradiol treatment prevented these changes in PKC profiles. These data suggest that an excessive episodic intake of ethanol followed by ethanol withdrawal disturbs PKC homeostasis and cellular distribution of PKC, in particular PKC(epsilon), in a manner that is protected by estrogen. PKC(epsilon) appears more vulnerable during ethanol withdrawal than during ethanol exposure.

Integration of the extranuclear and nuclear actions of estrogen

Estrogen receptors (ERs) are localized to many sites within the cell, potentially contributing to overall estrogen action. In the nucleus, estrogen mainly modulates gene transcription, and the resulting protein products determine the cell biological actions of the sex steroid. In addition, a small pool of ERs localize to the plasma membrane and signal mainly though coupling, directly or indirectly, to G proteins. In response to steroid, signal transduction modulates both nontranscriptional and transcriptional events and impacts both the rapid and more prolonged actions of estrogen. Cross-talk from membrane-localized ERs to nuclear ERs can be mediated through growth factor receptor tyrosine kinases, such as epidermal growth factor receptor and IGF-I receptor. Growth factor receptors enact signal transduction to kinases such as ERK and phosphatidylinositol 3-kinase that phosphorylate and activate nuclear ERs, and this can also occur in the absence of sex steroid. A complex relationship between the membrane and nuclear effects of estrogen also involves membrane-initiated phosphorylation of coactivators, recruiting these proteins to the nuclear transcriptosome. Finally, large pools of cytoplasmic ERs exist, and some are localized to mitochondria. The integration of sex steroid effects at distinct cellular locations of its receptor leads to important cellular physiological outcomes and are manifest in both reproductive and nonreproductive organs.

Estrogen neuroprotection against the neurotoxic effects of ethanol withdrawal: potential mechanisms

Ethanol withdrawal (EW) produces substantial neurotoxic effects, whereas estrogen is neuroprotective. Given observations that both human and nonhuman female subjects often show less impairment following EW, it is reasonable to hypothesize that estrogens may protect females from the neurotoxic effects of ethanol. This article is based on the assumption that the behavioral deficits seen following EW are produced in part by neuronal death triggered by oxidative insults produced by EW. The EW leads to activation of protein kinase C, especially PKCepsilon, which subsequently triggers apoptotic downstream events such as phosphorylation of nuclear factor-kappaB (NFkappaB) complex. On phosphorylation, active NFkappaB translocates to the nucleus, binds to DNA, and activates caspases, which trigger DNA fragmentation and apoptosis. In contrast, estrogens are antioxidant, inhibit overexpression of PKCepsilon, and suppress expression of NFkappaB and caspases. Estrogen treatment reduces the behavioral deficits seen during EW and attenuates molecular signals of apoptosis. The effects of ethanol and estrogen on each step in the signaling cascade from ethanol exposure to apoptosis are reviewed, and potential mechanisms by which estrogen could produce neuronal protection against the neurotoxicity produced by EW are identified. These studies serve as a guide for continuing research into the mechanisms of the neuroprotective effects of estrogen during EW and for the development of potential estrogen-based treatments for male and female alcoholics.

Thiamine deficiency in the pathogenesis of chronic ethanol-associated cerebellar damage in vitro

Nutritional deficiencies associated with long-term ethanol consumption may cause neuronal damage in ethanol-dependent individuals. Thiamine deficiency, in particular, is thought to contribute to ethanol-associated cerebellar degeneration, although damage may occur in adequately nourished alcoholics. Thus, the present study examined the effects of thiamine depletion and ethanol exposure on cytotoxicity in rat cerebellum. Organotypic cerebellar slice cultures were treated starting at 25 days in vitro with 100 mM ethanol for 11 days or 10 days followed by a 24-h withdrawal period. This exposure paradigm has previously been shown in hippocampal slice cultures to result in spontaneous cytotoxicity upon ethanol withdrawal. Additional cerebellar cultures were exposed to the thiamine depleting agent pyrithiamine (10-500 microM) for 10 or 11 days, some in the presence of ethanol exposure or withdrawal. Other cultures were co-exposed to thiamine (1-100 microM), 500 microM pyrithiamine, and ethanol for 10 or 11 days. The results demonstrated that neither 11-day ethanol treatment nor withdrawal from 10-day exposure significantly increased cerebellar cytotoxicity, as measured by propidium iodide fluorescence. The 11-day treatment with 100 or 500 microM pyrithiamine significantly increased propidium iodide fluorescence approximately 21% above levels observed in control tissue. Cultures treated with both ethanol (11 days or 10 days plus withdrawal) and 500 microM pyrithiamine displayed a marked increase in cytotoxicity approximately 60-90% above levels observed in control cultures. Pyrithiamine and ethanol-induced cytotoxicity was prevented in cultures co-exposed to thiamine (10-100 microM) for the duration of pyrithiamine treatment. Findings from this report suggest that the cerebellum may be more sensitive to the toxic effects of thiamine deficiency, as compared with alcohol withdrawal, associated with alcohol dependence.

Estrogen protects against brain lipid peroxidation in ethanol-withdrawn rats

This study examined whether 17beta-estradiol (E2) administration protects against ethanol withdrawal (EW)-associated oxidative insults by assessing oxidative markers thiobarbituric-acid-reacting-substances (TBARS). Ovariectomized rats implanted with E2 (EW/E2) or oil pellets (EW/Oil) received chronic ethanol (7.5% wt./vol., 5 weeks) or control dextrin diet (Dextrin/Oil). At 24 or 48 h of EW, rats were tested for overt EW signs and the cerebellum, hippocampus, and cortex were prepared for TBARS assessment in the presence and absence of FeCl3. For control experiments, we assessed E2 effects on blood ethanol concentrations and TBARS levels during ethanol exposure prior to EW. The EW/Oil group showed enhanced endogenous- and FeCl3-stimulated membrane TBARS levels in the cerebellum and hippocampus in a manner inhibited by E2 treatment. There was a relationship between the severity of EW and elevation of TBARS levels, particularly in the cerebellum. The enhanced TBARS levels at 24 h of EW appeared to diminish at 48 h in the hippocampus, but persisted in the cerebellum. E2 treatment did not alter blood ethanol concentrations and ethanol exposure alone did not enhance TBARS levels. These data suggest that EW rather than ethanol enhances brain lipid peroxidation that is transient and brain-region specific. Estrogens protect against the brain lipid peroxidation in a manner independent of blood ethanol concentrations.

Role of the GABAA system in behavioral, motoric, and cerebellar protection by estrogen during ethanol withdrawal

Results of studies from our laboratory have shown that administration of 17beta-estradiol (E(2)) reduces cerebellar neuronal damage during ethanol withdrawal (EW). In the current study, we investigated whether the GABAergic system is involved in the protective effects of E(2) against the EW syndrome. To test this hypothesis, we examined the effects of GABAergic drugs, with and without E(2), on EW sign scores, motoric capacity, and caspase activation. Ovariectomized rats implanted with an E(2) or an oil pellet received liquid ethanol [7.5% weight/volume (wt./vol.)] for 5 weeks or dextrin diet, followed by 2 weeks of EW. A gamma-aminobutyric acid type A (GABA(A)) agonist, muscimol (0.125 or 0.25 mg/kg), and antagonist, bicuculline (1.25 mg/kg), were administered (intraperitoneally; three times a day for 4 days) starting 1 day before the onset of EW. On termination of chronic administration of ethanol diet, rats were tested for overt withdrawal signs and latency to fall from a rotarod. The initial latency was measured separately to assess motoric capacity before learning occurred. Cerebelli were subsequently collected for immunohistochemistry to detect caspase activation. Results showed that treatment with E(2) lowered EW sign scores and improved initial as well as subsequent rotarod latencies compared with findings without treatment with E(2) (control group). These effects of E(2) were enhanced by combined treatment with muscimol and diminished by bicuculline. Results also showed that ethanol-withdrawn rats had more caspase-3-positive cells than observed for the dextrin diet-fed group in a manner reversed by E(2) and exacerbated by bicuculline. Bicuculline also caused partial antagonism of the protective effect of E(2). These findings support the suggestion that GABA(A) agonists ameliorate, and GABA(A) antagonists exacerbate, EW signs, cerebellar neuronal damage, and motoric impairment in ethanol-withdrawn rats. Also, results of the current study provide indirect evidence that the GABAergic system is involved in protective effects of E(2) against the EW syndrome.