Kigamicin (KM) is a compound isolated from Actinomycetes, which reportedly induces necrosis in pancreatic cancer cells under nutrient-starving condition but not under nutrient rich condition via PI3-kinase inhibition (Lu et al., Cancer Science 95, 547–52, 2004). Although the PI3-kinase activity is thought to be critical in the growth of myeloma cells, its actual role remains to be determined. In the present study, we evaluated KM’s anti-myeloma activity in both laboratory and primary myeloma cells and found that, contrary to the original finding in pancreatic cancer cells, KM induced necrosis in human myeloma cells both under nutrient-starving and nutrient-rich conditions.

Results and discussion

Myeloma cell lines (12PE and KHM-11) and primary myeloma cells purified with CD138-coated immune-magnetic beads were incubated with KM under nutrient-rich conditions. The CC50 value of KM for myeloma cells was approximately 100 nM after 24-hour exposure while pancreatic cancer cell line, PANC-1, did not show inhibition of viability even at 10 mM under nutrient-rich conditions, suggesting high sensitivity of myeloma cells to KM. When whole mononuclear cells obtained from a myeloma patient’s bone marrow were cultured with KM at a concentration of 500 nM in vitro, normal lymphocytes were spared while all myeloma cells underwent necrosis, suggesting that preferential cytotoxicity of KM in myeloma cells. Western blot analysis revealed that AKT phosphorylation decreased with KM treatment, suggesting that KM inhibits PI-3 kinase activity as previously reported. However, another pan-PI3 kinase inhibitor, LY294002, did not induce necrosis in myeloma cells, suggesting that PI3-kinase inhibition is not critically related to the cytotoxicity of KM in myeloma cells. A pan-caspase inhibitor, ZVAD-FMK, only partially inhibited cell death, suggesting that caspase is not involved in the cytotoxic function of KM, either. To further determine the mechanism of cytotoxicity in myeloma cells, a possible involvement of cyclin D1 and p21 was also examined. Western blot analysis revealed that KM completely reduced cyclin-D1 in myeloma cells. Moreover, KM induced translocation of p21 from cytoplasm to nucleus within 5 hours treatment, suggesting that KM disrupted cell cycle regulation. Finally, melphalan-resistant myeloma cell line, 11-EMS, showed significant cell death when exposed to KM even more efficiently than did melphalan-sensitive parental cell line, KHM-11. Since a number of anti-cancer reagents induce apoptosis in myeloma cells, KM induction of necrosis may represent a unique mechanism(s) and may overcome drug resistance of myeloma cells. Taking into a consideration a recent report by Lu et al. (Cancer Science 95, 547–52, 2004) showing that KM’s safe usage in a murine pancreatic cancer xenograft model, the present data suggest that KM could be a potential therapeutic agent for treatment of myeloma and warrant that further preclinial development of KM be continued.

Disclosure: No relevant conflicts of interest to declare.

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