Panobinostat Inhibits JAK2/STAT3 Pathway in Multiple Myeloma.

Abstract 2849

Poster Board II-825

Histone deacetylase inhibitors (HDACi) are emerging as a potential therapy for Multiple Myeloma (MM). Their antineoplastic activity depends not only on nucleosomal histone acetylation, but also on direct modulation of non-histone proteins, including p53 or HSP90. Previous studies suggest that histone deacetylases inhibitors modulate Jak2/Stat3 signaling pathway, a cascade mediating tumor cell survival. Here we examine how Panobinostat, a class I-HDAC inhibitor currently in phase I/II clinical trial, can modulate the function of the Jak2/ Stat3 pathway in MM. We first observed that Panobinostat inhibited IL6-induced Stat3 phosphorylation (Tyr705) and Jak2 phosphorylation (Tyr 1007/1008) in MM cell lines ( MM1S and INA6) in a dose- and time- depend fashion, associated with induction of Stat3 acetylation (Lys 685). Since acetylation of Stat3 alters the distribution rather than the functional status of Stat3, we next examined whether Panobinostat altered the nuclear versus cytoplasmic localization of Stat3 in MM cell lines. Although total STAT3 protein level did not change, Panobinostat treatment did trigger decreased nuclear Stat3 phosphorylation, suggesting that Panobinostat blocks Stat3 transcriptional activity. We showed by western blot analysis that the down stream pathway induced by Stat3 (Survivin, Bcl XL, c-Myc) was also down regulated after Panobinostat treatment, further confirming inhibition of STAT3 activity. Take together, our results suggest that Panobinostat inhibits the Jak2/Stat3 pathway by inhibiting STAT3 binding to DNA consensus region, rather than modulating nuclear translocation. To establish the molecular mechanism whereby Panobinostat regulates this pathway, we examined IL6/gp130 receptor, which is upstream in the Jak2 /Stat3 pathway. Panobinostat decreased both cell surface and intracellular gp130 protein expression. Interestingly, Panobinostat also inhibited IL6-induced phosphorylation of gp130, suggesting that it can directly inhibit gp130 activation. Our study therefore suggests a dual mechanism of inhibition of the JAK2/Stat3 pathway induced by Panobinostat via modulation of STAT 3 transcriptional function and gp130 -induced STAT3 activation. Finally, we observed upregulation of the MEK/ERK signaling pathway associated with HDAC inhibition, suggesting that combined blockade of these cascades may be useful. Indeed our preliminary data demonstrate enhanced cytotoxicity in MM cell lines (MM1S and INA6) induced by treatment with combined Panobinostat and MEK inhibitors, even in the presence of bone marrow stromal cells or survival cytokines ( IL6 or IGF). Our study therefore suggests a novel mechanism of action of HDAC inhibitors that provides the rationale for clinical evaluation of novel combinations based upon targeting STAT3 signaling pathway.