Abstract
Multiple myeloma (MM) is a malignant neoplasm of bone marrow plasma B cells with high morbidity. Clofazimine (CLF) is an FDA-approved leprostatic, anti-tuberculosis, and anti-inflammatory drug that was previously shown to have growth suppression effects on various cancer types such as hepatocellular, lung, cervix, esophageal, colon, and breast cancers as well as melanoma, neuroblastoma, and leukemia cells. The objective of this study was to evaluate the anticancer effect of CLF on U266 resistant MM cell line.
The relative cell viability of a panel of hematological cell lines (Jurkat, U266, Namalwa, K562, HL60) treated with 10 µM CLF after 24 h of treatment significantly reduced the viability in all cell lines, with percentages ranging between 28% (U266) and 38% (Jurkat) (p<0.001). IC50 value of CLF was found as 9.8 ± 0.7 µM on the U266 cell line. Previous studies showed that this level of CLF does not inhibit growth of healthy cells, which supports safety of CLF. CLF had both dose (2, 5, 10 µM) and time (12, 24, and 48 h) dependent growth inhibitory effect. Combination chemotherapy is an approach to increase the effectiveness of chemotherapeutics as well as overcome drug resistance and suppresses side effects of drugs. Therefore, we evaluate the combination effect of CLF in U266 cells and showed that combination with cisplatin led to a synergistic interaction between two compounds in all tested dose regimes, resulting in a 2.5-7.1 fold marked increase in cell death. Importantly this synergism was observed in U266 cells, which have mutant p53 at A161T showing resistance to cytotoxic agents such as platinum analogs (cisplatin etc.). <>Depolarization of the mitochondrial membrane is one of the first events in apoptosis. JC-1 is a lipophilic and cationic dye that reversibly changes color from green to red as the mitochondrial membrane potential increases (depolarization). JC-1 assay used in both flow cytometry analyses and fluorescence microscopy images have shown that relative to the control, CLF treatment results in the depolarization of mitochondrial membrane 15, 20.5, 14.3 fold respectively at 12, 24, and 48 h in U266 cell line (Figure 1).
The caspase family of cysteine proteases plays an important role in apoptosis. Caspase-3 is a major protease activated during the early stages of programmed cell death. 10 µM CLF was applied for 12, 24, and 48 h and anti-active caspase-3 PE stained U266 cells were analyzed by flow cytometry. Caspase-3 activity is enhanced 5.6, 24.5 and 13.6-fold relative to untreated controls at 12h, 24h and 48 h respectively.
Phosphatidylserine (PS) translocation to the outer leaflet of the cellular membrane is one of the key steps in early stages of apoptosis. To support our previous findings on apoptotic effect of CLF, we employed Annexin-V assay. CLF treatment caused a significant increase in the percentage of early and late apoptotic cells at 12 h (2.1 and 1.8 fold respectively), 24 h (4.1 and 12.3 fold) and 48 h (10.1 and 11.5 fold). Fluorescence microscopy images also supported flow cytometry data (Figure 2). Collectively, all three apoptosis assay results show that CLF significantly induces apoptosis in U266 cells.
Our study is the first to show apoptotic and growth inhibitory effects of CLF on a p53-mutant resistant MM cell line U266. Our results also proved that combined therapy employing CLF together with chemotherapeutics seems to be a possible future therapeutic approach for MM. Further in vivo and clinical studies are warranted to evaluate its therapeutic potential for resistant MM treatment.
Figure 1 Effect of 10 µM CLF on mitochondrial membrane potential. Flow cytometry fluorescence intensity A) Dot plots B) Bar plots of cells stained with JC-1 (n=3). C) Fluorescence microscopy image of JC-1-stained untreated cells indicating healthy mitochondria (red), D) In CLF-treated cells, green color shows diffusion of JC-1 from damaged mitochondria. Figure 1. Effect of 10 µM CLF on mitochondrial membrane potential. Flow cytometry fluorescence intensity A) Dot plots B) Bar plots of cells stained with JC-1 (n=3). C) Fluorescence microscopy image of JC-1-stained untreated cells indicating healthy mitochondria (red), D) In CLF-treated cells, green color shows diffusion of JC-1 from damaged mitochondria. Figure 2 Flow cytometry analysis of Annexin V-PE/7-AAD stained U266 cells treated with 10 µM CLF. A) Representative dot plots of Annexin V-PE vs 7-AAD signals gated as live, early apoptotic and late apoptotic quadrants B) Cell population bar graphs of corresponding dot plot quadrants (n=3). C) Early apoptotic U266 cell (right) stained with Annexin V-PE (green) and a late apoptotic U266 cell (left) stained with both Annexin V-PE (green) and nuclear dye PI (red) D) Close-up micrograph (160X) of a late apoptotic U266 cell. Figure 2. Flow cytometry analysis of Annexin V-PE/7-AAD stained U266 cells treated with 10 µM CLF. A) Representative dot plots of Annexin V-PE vs 7-AAD signals gated as live, early apoptotic and late apoptotic quadrants B) Cell population bar graphs of corresponding dot plot quadrants (n=3). C) Early apoptotic U266 cell (right) stained with Annexin V-PE (green) and a late apoptotic U266 cell (left) stained with both Annexin V-PE (green) and nuclear dye PI (red) D) Close-up micrograph (160X) of a late apoptotic U266 cell.
Disclosures
No relevant conflicts of interest to declare.
Monitoring of relative mitochondrial membrane potential in living cells by fluorescence microscopy.
