Ambroxol Induced Myeloma Cell Death by Inhibiting Autophagy
Recently, newly developed drugs have significantly improved prognosis of patients with multiple myeloma (MM). However, most patients relapsed sooner or later, and thus, MM is still an incurable hematological malignancy. In addition, serious adverse events occasionally hamper to continue treatment. Exploitation of the new drugs that potentiate the anti-tumor activities and alleviate the adverse effects of the existing drugs is needed. Here, we found by drug repositioning that ambroxol hydrochloride (ambroxol) induces apoptosis of MM cells. Interestingly, turnover assay and reporter assay showed that ambroxol inhibits late stage of autophagy. Transmission Electron Microscope (TEM) observation also showed that MM cells treated with ambroxol revealed accumulation of autophagy vesicles in cytoplasm, supporting the result of inhibition of late stage autophagy. Existing anti-MM drugs demonstrate various effects on autophagy; panobinostat, a histone deacetylase inhibitor, induced autophagy while bortezomib and lenalidomide did not. Ambroxol revealed synergistic anti-myeloma effect with panobinostat because ambroxol was considered to inhibit activation of panobinostat-induced autophagy and also downregulated MCL1 expression. In KMS 11-xenograft model, ambroxol alone significantly delayed tumor growth, and synergistically enhanced panobinostat-induced inhibition of tumor growth. Interestingly, concomitant use of ambroxol with panobinostat alleviated panobinostat-induced diarrhea. Gene set enrichment analyses (GSEAs) and pathway analysis also showed that ambroxol increased expression of the genes related to autophagy inhibition and unfolded protein response. These results suggested that autophagy is a promising therapeutic target for multiple myeloma. Overall design: To study molecular mechanisms for ambroxol-induced autophagy inhibition in myeloma cells, KMS21 and KMS34 were treated  with ambroxol 200 mM for 24 hours, and the comparative gene expressions were analysed between ambroxol-treated vs untreated cells.