In turn, ATF4 induces growth arrest and DNA damage-inducible protein (GADD153/CHOP), resulting in cell cycle arrest and thereby preventing further damage to the cell.29,30Second, IRE1- mediates splicing of x-boxbinding protein (Xbp1), which increases transcription of ER-resident chaperones, folding enzymes, and components of the protein degradation machinery. inhibition. Furthermore, accumulated -catenin contributes to enzastaurin-induced cell death. Sequential knockdown of -catenin, c-Jun, and p73, as well as overexpression of -catenin or p73 confirmed that accumulated -catenin triggers c-Jundependent induction of p73, thereby conferring MM cell apoptosis. Our data reveal a novel role of -catenin in endoplasmic reticulum (ER) stress-mediated growth inhibition and a new proapoptotic mechanism triggered by -catenin on inhibition TLQP 21 of PKC isoforms. Moreover, we identify p73 as a potential novel therapeutic target in MM. Based on these and previous data, enzastaurin is currently under clinical investigation in a variety of hematologic malignancies, including MM. == Introduction == Multiple myeloma (MM) is the second most common hematologic neoplasm in the United States and is characterized by the malignant transformation of a terminally differentiated plasma cell within the bone marrow. Although clear incremental improvements in progression-free and overall survival have followed the recent introduction of the novel targeted therapies thalidomide, lenalidomide, and bortezomib, MM remains TLQP 21 an incurable malignancy. Further delineation of mechanisms underlying the therapeutic specificity of these and other new agents will provide insight into disease pathogenesis and identify novel therapeutic targets to improve patient outcome. The macrocyclic bisindolylmaleimide enzastaurin (LY317615.HCL) is a novel orally available inhibitor of protein kinase C (PKC) isoforms.1Preclinical activity has been demonstrated in a wide spectrum of tumor types.2Importantly, our own and other previous data strongly support a therapeutic role for enzastaurin in MM.36Specifically, enzastaurin blocks activation of classic and novel PKC TLQP 21 isoforms triggered by growth factors and cytokines and thereby inhibits MM cell proliferation, survival, migration, and drug resistance. Moreover, enzastaurin blocks angiogenesis both in TLQP 21 vitro as well as in vivo in a murine xenograft model of human MM.4Interestingly, growth inhibition of MM cells was detectable within a few hours of treatment, whereas actual cell death did not occur until 48 hours. Functionally, enzastaurin’s Mouse monoclonal to CD276 antitumor activity has been primarily attributed to the inhibition of Akt and its downstream targets.1,4,5,7,8However, very limited data are available on its precise mechanism of action. -Catenin is a key component of the WNT pathway, as well as a signal integrator for the Ras and phosphatidylinositol 3-kinase (PI3K) pathways.9,10As a mediator of the WNT signaling in MM, -catenin has been implicated in cell proliferation, migration, and bone disease, and was investigated as a promising therapeutic target.1114Cytoplasmic levels of -catenin are tightly regulated by phosphorylation at serine residues (S33, S37, and S45) and by formation of a complex with glycogen synthase kinase (GSK)3, adenomatous polyposis of the colon (APC), axin, and -TrCP (transducin repeat-containing protein).9,15,16Phosphorylation occurs first at S45, which is required for subsequent phosphorylation at S33 and S37. Fully phosphorylated -catenin is rapidly ubiquitinated and degraded by the proteasome.15,17Importantly, PKC isoforms have been reported to contribute to the phosphorylation and degradation of -catenin. Consequently, a marked accumulation of -catenin has been seen after treatment with bisindoylmaleimide inhibitors of PKC (eg, BIM I).18,19Whereas -catenin may be best known for its oncogenic role in the pathogenesis of colon cancer,20,21several reports have described an additional role in the induction of apoptosis. Proposed mechanisms include activation of p53 signaling, p14 ARF, and cyclin D1.2225However, other studies found -cateninmediated apoptosis to be independent of these factors.23Thus, the proapoptotic mechanism of -catenin still remains elusive. Another promising therapeutic target in MM is the endoplasmic reticulum (ER)associated stress pathway, also known as the unfolded protein response (UPR).26,27Under physiologic conditions, the UPR is an adaptive response, activated by the accumulation of misfolded proteins in the ER, to maintain cell survival.28Specifically, ER stress leads to the activation of 3 major UPR sensors: pancreatic eIF2- kinase (PERK), high inositol-requiring 1 TLQP 21 (IRE1-), and ATF6. First, PERK phosphorylates the eukaryotic translation initiation factor-2a (eIF2a), which results in both the initial decrease in general translation initiation and the selective translation of the transcription factor ATF4. In turn, ATF4 induces growth arrest and DNA damage-inducible protein (GADD153/CHOP), resulting in cell cycle arrest and thereby preventing further damage to the cell.29,30Second, IRE1- mediates splicing of x-boxbinding protein (Xbp1), which increases transcription of ER-resident chaperones, folding enzymes, and components of the protein degradation machinery. Third, ATF6, after activating cleavage, contributes to both induction of CHOP and up-regulation of protein folding and degradation.30In addition to the 3 UPR branches, the transcription.