Impaired proteasome function in Alzheimer's disease.

Inhibition of proteasome activity is sufficient to induce neuron degeneration and death; however, altered proteasome activity in a neurodegenerative disorder has not been demonstrated. In the present study, we analyzed proteasome activity in short-postmortem-interval autopsied brains from 16 Alzheimer's disease (AD) and nine age- and sex-matched controls. A significant decrease in proteasome activity was observed in the hippocampus and parahippocampal gyrus (48%), superior and middle temporal gyri (38%), and inferior parietal lobule (28%) of AD patients compared with controls. In contrast, no significant decrease in proteasome activity was observed in either the occipital lobe or the cerebellum. The loss of proteasome activity was not associated with a decrease in proteasome expression, suggesting that the proteasome may become inhibited in AD by a posttranslational modification. Together, these data indicate a possible role for proteasome inhibition in the neurodegeneration associated with AD.

Possible involvement of proteasome inhibition in aging: implications for oxidative stress.

Oxidative stress may contribute to the cellular alterations, which occur as the result of aging, and the nervous system is particularly vulnerable to aging associated oxidative injury. The multicatalytic proteasome (MCP) is responsible for the majority of protein degradation and is sensitive to oxidative stress. To determine if MCP activity is altered during aging, studies were conducted in multiple tissues from aged Fisher 344 rats. Analysis of heart, lung, kidney, and liver revealed decreased MCP activity in 12, 24, and 28 month old rats, compared with 3 week or 3 month old animals. The spinal cord, hippocampus, and cerebral cortex demonstrated age dependent decreases in MCP activity, but at no timepoint was MCP activity decreased in either the brain stem or cerebellum. Oxidative injury and the lipid oxidation product 4-hydroxynonenal caused decreased MCP activity in neural PC6 cells, while application of MCP inhibitors was sufficient to induce cell death in neural PC6 cells. Together, these data indicate a role for MCP inhibition in cellular dysfunction associated with aging and oxidative injury.

Oxidized high-density lipoprotein induces neuron death.

High-density lipoprotein (HDL) exists within the brain and is highly vulnerable to oxidative modifications. The focus of the present study was to determine the effect of HDL and oxidized HDL (oxHDL) upon neurons, astrocytes, and microglia. Administration of highly oxidized HDL, but not native, minimally, or moderately modified HDL resulted in a dose- and time-dependent increase in oxidative stress and death of cultured rat embryonic neurons. Astrocyte and microglia cultures treated with highly oxidized HDL displayed increased reactive oxygen species formation but no toxicity. Application of oxHDL exacerbated oxidative stress and neuron death induced by beta-amyloid peptide. Studies using pharmacological inhibitors implicate the involvement of calcium and reactive oxygen species in oxHDL-induced neuronal loss. Neural cells expressing increased levels of BCL-2 had decreased levels of oxidative stress and neuron death following exposure to oxHDL. Together, these data demonstrate that oxHDL increases oxidative stress in neurons, astrocytes, and microglia which ultimately culminate in neuron death.

4-hydroxynonenal increases neuronal susceptibility to oxidative stress.

Increased levels of reactive oxygen species occur in neurodegenerative disorders and may promote neuron death. The lipid peroxidation product 4-hydroxynonenal (HNE) is increased in neurons following oxidative stress and promotes neuron death in vitro and in vivo. The present study examined the possibility that HNE can increase neuron vulnerability to oxidative stress. Application of low concentrations of HNE (50-500 nM) increased neuron death induced by beta-amyloid or glutamate when added within 3 hr of injury. In addition, treatment with HNE exacerbated mitochondrial reactive oxygen species formation and loss of mitochondrial membrane potential in response to beta-amyloid and glutamate. The ability to exacerbate oxidative stress, mitochondrial dysfunction, and neuron death appears to be specific to HNE, because application of other lipid peroxidation products had no effect. These data indicate a role for low levels of HNE in promoting reactive oxygen species accumulation and neuron degeneration by altering mitochondrial homeostasis. In addition, the present study indicates a possible mechanism for reactive oxygen species and lipid peroxidation toxicity in neurodegenerative conditions.

Oxidized low-density lipoprotein induces neuronal death: implications for calcium, reactive oxygen species, and caspases.

Low-density lipoprotein (LDL) exists within the brain and is highly vulnerable to oxidative modifications. Once formed, oxidized LDL (oxLDL) is capable of eliciting cytotoxicity, differentiation, and inflammation in nonneuronal cells. Although oxLDL has been studied primarily for its role in the development of atherosclerosis, recent studies have identified a possible role for it in neurological disorders associated with oxidative stress. In the present study application of oxLDL, but not LDL, resulted in a dose- and time-dependent death of cultured rat embryonic neurons. Studies using pharmacological inhibitors implicate the involvement of calcium, reactive oxygen species, and caspases in oxLDL-induced neuronal death. Coapplication of oxLDL with either amyloid beta-peptide or glutamate, agents that enhance oxidative stress, resulted in increased neuronal death. Taken together, these data demonstrate that oxLDL induces neuronal death and implicate a possible role for oxLDL in conditions associated with increased levels of reactive oxygen species, including Alzheimer's disease.

Oxidized lipoproteins increase reactive oxygen species formation in microglia and astrocyte cell lines.

Lipoproteins exist in the central nervous system and surrounding vasculature possibly mediating effects upon cells in the brain during times of oxidative stress or compromised blood-brain barrier. The focus of the present study was to determine the effect of unmodified and oxidatively modified lipoproteins on astrocytes and microglia. Application of oxidized low-density lipoprotein resulted in an increase in DCF fluorescence, which was inhibited by pretreatment with antioxidants, consistent with the formation of reactive oxygen species (ROS). Low-density at concentrations below 20 microg/ml likewise increased ROS formation. Because ROS are associated with numerous astrocyte and microglia activities including proliferation, activation, and cytokine production it is possible that lipoproteins may mediate such effects on glial cells in the central nervous system.

Opposing actions of native and oxidized lipoprotein on motor neuron-like cells.

Lipoproteins are present in the central nervous system and surrounding vasculature and possibly mediate effects relevant to neuronal physiology and pathology. To determine the effects of lipoproteins on motor neurons, native low density lipoproteins (LDL) and oxidized LDL (oxLDL) were applied to a motor neuron cell line. Oxidized LDL, but not native LDL, resulted in a dose- and time-dependent increase in reactive oxygen species and neuron death. Oxidized LDL-induced toxicity was attenuated by a calcium chelator, antioxidants, caspase inhibitors, and inhibitors of macromolecular synthesis. In addition to being nontoxic, application of native LDL attenuated reactive oxygen species formation and neuron loss following glucose deprivation injury. Together, these data demonstrate a possible neuroprotective role for unmodified lipoproteins and suggest oxidized lipoproteins may amplify oxidative stress and neuron loss.

Cyclic nucleotides attenuate lipid peroxidation-mediated neuron toxicity.

Recent studies suggest that increased lipid peroxidation and lipid peroxidation products, such as 4-hydroxynonenal (HNE), contribute to neuronal loss in conditions associated with oxidative stress. The focus of the present study was to determine possible neuroprotective effects of elevated cyclic nucleotide levels against lipid peroxidation and HNE-mediated neural toxicity. Application of 8-bromo derivative analogs of cAMP or cGMP resulted in attenuation of HNE-induced increases in mitochondrial calcium, reactive oxygen species, and neuron loss. Similar results were obtained when neural cells were pretreated with the phosphodiesterase inhibitors zaprinast or isobutylmethylxanthanine (IBMX). These data are consistent with a possible neuroprotective role for elevated cyclic nucleotide levels in disorders associated with increases in lipid peroxidation and HNE.