Carfilzomib is a proteasome inhibitor in clinical development that primarily targets the chymotrypsin-like (CT-L) subunits in both the constitutive proteasome (c20S) and the immunoproteasome (i20S). To investigate the impact of inhibiting the CT-L activity with carfilzomib, we set out to quantitate the levels of CT-L subunits ␤5 from the c20S and LMP7 from the i20S in normal and malignant hematopoietic cells. We found that the i20S is a major form of the proteasome expressed in cells of hematopoietic origin, including multiple myeloma (MM) CD138 ؉ tumor cells. Although specific inhibition of either LMP7 or ␤5 alone was insufficient to produce an antitumor response, inhibition of all proteasome subunits was cytotoxic to both hematologic tumor cells and peripheral blood mononuclear cells. However, selective inhibition of both ␤5 and LMP7 was sufficient to induce an antitumor effect in MM, non-Hodgkin lymphoma, and leukemia cells while minimizing the toxicity toward nontransformed cells. In MM tumor cells, CT-L inhibition alone was sufficient to induce proapoptotic sequelae, including proteasome substrate accumulation, Noxa and caspase 3/7 induction, and phospho-eIF2␣ suppression. These data support a hypothesis that hematologic tumor cells are uniquely sensitive to CT-L inhibition and provide a mechanistic understanding of the clinical safety profile and antitumor activity of proteasome inhibitors. (Blood.

Purpose: Bortezomib (Velcade), a dipeptide boronate 20S proteasome inhibitor and an approved treatment option for multiple myeloma, is associated with a treatment-emergent, painful peripheral neuropathy (PN) in more than 30% of patients. Carfilzomib, a tetrapeptide epoxyketone proteasome inhibitor, currently in clinical investigation in myeloma, is associated with low rates of PN. We sought to determine whether PN represents a target-mediated adverse drug reaction (ADR).Experimental Design: Neurodegenerative effects of proteasome inhibitors were assessed in an in vitro model utilizing a differentiated neuronal cell line. Secondary targets of both inhibitors were identified by a multifaceted approach involving candidate screening, profiling with an activity-based probe, and database mining. Secondary target activity was measured in rats and patients receiving both inhibitors.Results: Despite equivalent levels of proteasome inhibition, only bortezomib reduced neurite length, suggesting a nonproteasomal mechanism. In cell lysates, bortezomib, but not carfilzomib, significantly inhibited the serine proteases cathepsin G (CatG), cathepsin A, chymase, dipeptidyl peptidase II, and HtrA2/Omi at potencies near or equivalent to that for the proteasome. Inhibition of CatG was detected in splenocytes of rats receiving bortezomib and in peripheral blood mononuclear cells derived from bortezomib-treated patients. Levels of HtrA2/Omi, which is known to be involved in neuronal survival, were upregulated in neuronal cells exposed to both proteasome inhibitors but was inhibited only by bortezomib exposure.Conclusion: These data show that bortezomib-induced neurodegeneration in vitro occurs via a proteasome-independent mechanism and that bortezomib inhibits several nonproteasomal targets in vitro and in vivo, which may play a role in its clinical ADR profile. Clin Cancer Res; 17(9); 2734-43. Ó2011 AACR.

Originally known for its regulation of reproductive functions, estradiol, a lipophilic hormone that can easily cross plasma membranes as well as the blood-brain barrier, maintains brain systems subserving arousal, attention, mood, and cognition. In addition, both synthetic and natural estrogens exert neurotrophic and neuroprotective effects. There is increasing evidence that estrogen actions are mediated by nongenomic as well as direct and indirect genomic pathways. Although in vitro models have provided the most extensive evidence for neurotrophic and neuroprotective actions to date, there are also in vivo studies that support these actions.

The molecular circuitry underlying innate immunity is constructed of multiple, evolutionarily conserved signaling modules with distinct regulatory targets. The MAP kinases and the IKK-NF-B molecules play important roles in the initiation of immune effector responses. We have found that the Drosophila NF-B protein Relish plays a crucial role in limiting the duration of JNK activation and output in response to Gram-negative infections. Relish activation is linked to proteasomal degradation of TAK1, the upstream MAP kinase kinase kinase required for JNK activation. Degradation of TAK1 leads to a rapid termination of JNK signaling, resulting in a transient JNK-dependent response that precedes the sustained induction of Relish-dependent innate immune loci. Because the IKK-NF-B module also negatively regulates JNK activation in mammals, thereby controlling inflammation-induced apoptosis, the regulatory cross-talk between the JNK and NF-B pathways appears to be broadly conserved.

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