Abstract. Mitochondrial dysfunction is observed in Alzheimer's disease (AD) brain, and the amyloid-β (Aβ) peptide is known to induce mitochondrial dysfunction. The relative degree of mitochondrial dysfunction in different regions of the brain in AD is not completely understood. Moreover, the relationship between levels of synaptic mitochondrial Aβ and mitochondrial dysfunction has not been clearly established. Therefore synaptic and nonsynaptic mitochondria were isolated from the hippocampus, cortex, striatum, and amygdala of 12 month AβPPsw and AβPP+PS1 mouse models of AD as well as nontransgenic mice. Mitochondrial respiratory rates, reactive oxygen species production, membrane potential, and cytochrome c oxidase activity were measured. Hippocampal and cortical mitochondria showed the highest levels of mitochondrial dysfunction, while striatal mitochondria were moderately affected, and amygdalar mitochondria were minimally affected. Mitochondria from AβPP/PS1 brain regions were more impaired than those from AβPP mice. Mitochondrial Aβ levels nearly mirrored the extent of mitochondrial dysfunction. Synaptic mitochondria were more impaired than nonsynaptic mitochondria in the AD mouse models. The AβPP/PS1 mice showed more impairment in the cognitive interference task of working memory than the AβPP mice. The association between mitochondrial Aβ levels and mitochondrial dysfunction in mouse models of AD supports a primary role for mitochondrial Aβ in AD pathology. Moreover, the degree of cognitive impairment in AD transgenic mice can be linked to the extent of synaptic mitochondrial dysfunction and mitochondrial Aβ levels, suggesting that a mitochondrial Aβ-induced signaling cascade may contribute to cognitive impairment. Therapeutics that target this cascade could be beneficial in the treatment of AD.

Mitochondrial dysfunction is a hallmark of Alzheimer's disease (AD) and is observed in mutant amyloid precursor protein (APP) transgenic mouse models of familial AD. Melatonin is a potent antioxidant, can prevent toxic aggregation of Alzheimer's beta-amyloid (Aβ) peptide and, when taken long term, can protect against cognitive deficits in APP transgenic mice. To study the effects of melatonin on brain mitochondrial function in an AD model, APP/PS1 transgenic mice were treated for 1 month with melatonin. Analysis of isolated brain mitochondria from mice indicated that melatonin treatment decreased mitochondrial Aβ levels by two- to fourfold in different brain regions. This was accompanied by a near complete restoration of mitochondrial respiratory rates, membrane potential, and ATP levels in isolated mitochondria from the hippocampus, cortex, or striatum. When isolated mitochondria from untreated young mice were given melatonin, a slight increase in respiratory rate was observed. No such effect was observed in mitochondria from aged mice. In APP-expressing neuroblastoma cells in culture, mitochondrial function was restored by melatonin or by the structurally related compounds indole-3-propionic acid or N(1)-acetyl-N(2)-formyl-5-methoxykynuramine. This restoration was partially blocked by melatonin receptor antagonists indicating melatonin receptor signaling is required for the full effect. Therefore, treatments that stimulate melatonin receptor signaling may be beneficial for restoring mitochondrial function in AD, and preservation of mitochondrial function may an important mechanism by which long term melatonin treatment delays cognitive dysfunction in AD mice.

It has been established in preclinical models of multiple myeloma and acute myeloid leukemia (AML) that the bone marrow microenvironment provides protection from chemotherapy-and death receptor^mediated apoptosis. This form of resistance, termed de novo drug resistance, occurs independent of chronic exposure to cancer-related therapies and likely promotes the development of multidrug resistance. Consequently, it is of major interest to identify compounds or drug combinations that can overcome environment-mediated resistance. In this study, we investigated the activity of tipifarnib (Zarnestra, formerly R115777) combined with bortezomib (Velcade, formerly PS-341) in microenvironment models of multiple myeloma and AML. The combination proved to be synergistic in multiple myeloma and AML cell lines treated in suspension culture. Even in tumor cells relatively resistant to tipifarnib, combined activity was maintained. Tipifarnib and bortezomib were also effective when multiple myeloma and AML cells were adhered to fibronectin, providing evidence that the combination overcomes cell adhesion^mediated drug resistance (CAM-DR). Of importance, activation of the endoplasmic reticulum stress response was enhanced and correlated with apoptosis and reversal of CAM-DR. Multiple myeloma and AML cells cocultured with bone marrow stromal cells also remained sensitive, although stromal-adhered tumor cells were partially protected (relative to cells in suspension or fibronectin adhered). Evaluation of the combination using a transwell apparatus revealed that stromal cells produce a protective soluble factor. Investigations are under way to identify the cytokines and/or growth factors involved. In summary, our study provides the preclinical rationale for trials testing the tipifarnib and bortezomib combination in patients with multiple myeloma and AML.

A major contributing factor to the high mortality rate associated with acute myeloid leukemia and multiple myeloma is the development of resistance to chemotherapy. We have shown that the combination of tipifarnib, a nonpeptidomimetic farnesyltransferase inhibitor (FTI), with bortezomib, a proteosome inhibitor, promotes synergistic death and overcomes de novo drug resistance in acute myeloid leukemia cell lines. Experiments were undertaken to identify the molecular mechanisms by which tipifarnib produces cell death in acute myeloid leukemia and multiple myeloma cell lines (U937 and 8226, respectively). ] i overload. Preventing Ca 2ϩ influx diminished tipifarnib-evoked cell death, whereas 2-APB potentiated this effect, demonstrating a link between tipifarnib-induced Ca 2ϩ influx and apoptosis. These data suggest that tipifarnib exerts its effects by acting on a membrane channel with pharmacological properties consistent with store-operated channels containing the Orai3 subunit. It is noteworthy that Orai3 transcripts were found to be expressed at lower levels in tipifarnib-resistant 8226/R5 cells. Our results indicate tipifarnib causes cell death via a novel mechanism involving activation of a plasma membrane Ca 2ϩ channel and intracellular Ca 2ϩ overload.