Amino acids can mimic or block protein functions and produce physiological changes in cellular function. In some cases, the VE-822 synthetic peptides used to produce cellular changes corresponded to peptides found in peptidomics analyses of the tissue. For example, the addition of specific peptides was shown to modulate the signal transduction elicited by agonists of G-protein coupled receptors in HEK293 and CHO cells. Intracellular peptides derived from rat adipose tissue proteins facilitate insulin-induced glucose uptake in 3T3-L1 adipocytes. In C.elegans, peptides produced from mitochondrial proteins were shown to signal nuclear-encoded mitochondrial chaperone genes and indicate the stress of mitochondrial protein misfolding. In Drosophila, peptides encoded by small open reading frame genes were found to control epidermal differentiation by modifying the activity of transcription factors. Thus, an emerging concept is that peptides produced from cytosolic, mitochondrial, and/or nuclear proteins have functional roles in cellular processes, and are not merely intermediates in the protein degradation pathway. There are four major peptide-generating systems within cells: proteasomes, calpains, caspases, and lysosomes. The LY2157299 TGF-beta inhibitor proteasome complex plays a major role in protein turnover, degrading proteins into peptides of 4�C25 amino acids with an average size around 10 amino acids. Calpains are a family of calcium-regulated proteases that perform limited proteolysis. Caspases are also a family of intracellular proteases, but with a strict substrate specificity for cleavage at sites containing an Asp residue. Lysosomes are organelles that degrade proteins by a series of endo- and exopeptidase activities. In addition to these proteases, a number of cytosolic oligopeptidases exist, including thimet oligopeptidase, neurolysin, post-prolyl oligopeptidase, nardilysin, and insulin degrading enzyme. These oligopeptidases are not capable of cleaving proteins; they selectively cleave a subset of cellular peptides into smaller fragments. Degradation of intracellular peptides into amino acids occurs through the action of aminopeptidases and other enzymes. Previous studies aimed at determining the proteolytic system involved in producing the intracellular peptides of human embryonic kidney 293Tcells implicated the proteasome complex and not calpains based on the observation that epoxomicinbut not A23187affected intracellular peptide levels. Epoxomicin is an irreversible inhibitor of the proteasome, potently inhibiting the beta-5 subunitand less potently inhibiting the beta-2 subunit. Consistent with this activity of epoxomicin, most of the intracellular peptides that resulted from protein cleavage at hydrophobic sites were greatly reduced by 0.2 mM epoxomicin while those peptides that resulted from protein cleavage at basic amino acids were reduced by 2 mM epoxomicin but not by 0.2 mM epoxomicin. Furthermore, many of the intracellular peptides that resulted from cleavage at beta-1 siteswere elevated by epoxomicin treatment; this is consistent with the idea that proteins transported into the epoxomicin-inhibited proteasome cannot be cleaved at their normal sitesand as a result there is increased activity at alternate sites. Bortezomib has 
been reported to be a highly selective proteasome inhibitor with greatest potency for the beta-5 subunit and lower potency for the beta-1 subunit. Bortezomib has been successful for the treatment of several types of cancer, including multiple myeloma. A major side effect of bortezomib is neuropathy, presumably due to the action of the drug on nerve cells. In the present study, we tested the effect of bortezomib on levels of peptides in two different.