Role of CAAT/enhancer binding protein homologous protein in panobinostat-mediated potentiation of bortezomib-induced lethal endoplasmic reticulum stress in mantle cell lymphoma cells.

PURPOSE: Bortezomib induces unfolded protein response (UPR) and endoplasmic reticulum stress, as well as exhibits clinical activity in patients with relapsed and refractory mantle cell lymphoma (MCL). Here, we determined the molecular basis of the improved in vitro and in vivo activity of the combination of the pan-histone deacetylase inhibitor panobinostat and bortezomib against human, cultured, and primary MCL cells. EXPERIMENTAL DESIGN: Immunoblot analyses, reverse transcription-PCR, and immunofluorescent and electron microscopy were used to determine the effects of panobinostat on bortezomib-induced aggresome formation and endoplasmic reticulum stress in MCL cells. RESULTS: Treatment with panobinostat induced heat shock protein 90 acetylation; depleted the levels of heat shock protein 90 client proteins, cyclin-dependent kinase 4, c-RAF, and AKT; and abrogated bortezomib-induced aggresome formation in MCL cells. Panobinostat also induced lethal UPR, associated with induction of CAAT/enhancer binding protein homologous protein (CHOP). Conversely, knockdown of CHOP attenuated panobinostat-induced cell death of MCL cells. Compared with each agent alone, cotreatment with panobinostat increased bortezomib-induced expression of CHOP and NOXA, as well as increased bortezomib-induced UPR and apoptosis of cultured and primary MCL cells. Cotreatment with panobinostat also increased bortezomib-mediated in vivo tumor growth inhibition and improved survival of mice bearing human Z138C MCL cell xenograft. CONCLUSION: These findings suggest that increased UPR and induction of CHOP are involved in enhanced anti-MCL activity of the combination of panobinostat and bortezomib.

Cotreatment with panobinostat and JAK2 inhibitor TG101209 attenuates JAK2V617F levels and signaling and exerts synergistic cytotoxic effects against human myeloproliferative neoplastic cells.

The mutant JAK2V617F tyrosine kinase (TK) is present in the majority of patients with BCR-ABL-negative myeloproliferative neoplasms (MPNs). JAK2V617F activates downstream signaling through the signal transducers and activators of transcription (STAT), RAS/mitogen-activated protein kinase (MAPK), and phosphatidylinositol 3 (PI3)/AKT pathways, conferring proliferative and survival advantages in the MPN hematopoietic progenitor cells (HPCs). Treatment with the pan-histone deacetylase (HDAC) inhibitor panobinostat (PS) is known to inhibit the chaperone function of heat shock protein 90, as well as induce growth arrest and apoptosis of transformed HPCs. Here, we demonstrate that PS treatment depletes the autophosphorylation, expression, and downstream signaling of JAK2V617F. Treatment with PS also disrupted the chaperone association of JAK2V617F with hsp90, promoting proteasomal degradation of JAK2V617F. PS also induced apoptosis of the cultured JAK2V617F-expressing human erythroleukemia HEL92.1.7 and Ba/F3-JAK2V617F cells. Treatment with the JAK2 TK inhibitor TG101209 attenuated JAK2V617F autophosphorylation and induced apoptosis of HEL92.1.7 and Ba/F3-JAK2V617F cells. Cotreatment with PS and TG101209 further depleted JAK/STAT signaling and synergistically induced apoptosis of HEL92.1.7 and Ba/F3-JAK2V617F cells. Cotreatment with TG101209 and PS exerted greater cytotoxicity against primary CD34(+) MPN cells than normal CD34(+) HPCs. These in vitro findings suggest combination therapy with HDAC and JAK2V617F inhibitors is of potential value for the treatment of JAK2V617F-positive MPN.