Inhibition of Constitutively Activated JAK/STAT3 Pathway in Multiple Myeloma Cells Leads to Induction of Protective Autophagy.

Abstract 4914

Constitutive activation of Janus Kinase-2 (JAK-2)/signal transducer and activator of transcription-3 (STAT3) has been implicated to play a crucial role in the pathogenesis of Multiple Myeloma (MM). Therefore, targeted inhibition of STAT3 is an attractive therapy for MM patients. JSI-124 is a natural product isolated from various plant families and has been recently described as a specific inhibitor of JAK2/STAT3. It has been shown to exert strong apoptosis-inducing effects against STAT3+ tumor cell lines in vitro and in vivo. However, there is little information available about the anti-tumor effects of JSI-124 on myeloma cells. We therefore tested the role of JSI-124 as a potential novel anti-MM compound and were able to determine that JSI-124 has potent anti-myeloma properties against human MM cell lines. Our results revealed that JSI-124 could inhibit both constitutively expressed and IL-6 induced phosphorylation of Tyr705 STAT3 as well as other important cell signal pathways that regulate cell proliferation in MM cell lines. However, the anti-myeloma effects were not restricted to STAT3+ MM cells. In STAT3+ cells JSI-124 induced cell death, which was predominantly caspase dependent. In contrast, JSI-124 induced both caspase-dependent and caspase-independent cell death in STAT3-MM cells. Furthermore, JSI-124 induced DNA damage only in STAT3- MM cells. Surprisingly, STAT3- cell lines were more sensitive to JSI-124-induced growth inhibition and induction of cell death than STAT3+ MM cell lines. Further analysis revealed that JSI-124 led to induction of autophagy only in STAT3+ MM cells as determined by upregulation of LC3B. A dramatic increase of JSI-124-dependent apoptosis was observed when the autophagy pathway was blocked by Chloroquine suggesting that STAT3-inhibition in STAT3+ MM cells leads to protective autophagy, which can be overcome by Chlorquine treatment. Interestingly, inhibition of autophagy by Chloroquine or 3-mA in STAT3+ cell lines U266 or ARH 77 resulted in AIF release and well as caspase-independent cell death. The results in this study suggest that JSI-124 could be an effective anti-myeloma agent regardless of STAT3 activation status and that combination with Chloroquine may enhance its anti-MM effect.