Myeloma cell lines
U266 and MM1s myeloma cell lines were obtained from the Chinese Academy of Medical Sciences, Tianjin Institute of Hematology. The cells were cultured in RPMI1640 medium (Gibco, NY, USA) with 10% fetal bovine serum (Hao Yang, China), incubated at 37°C under saturated humidity, and passaged once every 2 days. Cells that were at the logarithmic growth phase were used for the subsequent experiments.
Preparation of the E35 solution
Sterile emodin (C15H10O5) was purchased from a pharmaceutical company (Qingze, China). E35, a novel emodin derivative (C34H50BrNO5H2O, Fig. 1A), was designed and synthesized. The purity was 98%, determined using a high-performance liquid chromatography. The agent was dissolved in the dimethyl sulfoxide (DMSO) (Sigma, USA) in 50,000 μM as a primary solution and stored at −20°C before use.
Cytotoxicity assays
The MTT assay was employed to measure the viability and proliferation of the U266 and MM1s cell lines after E35 administration. The U266 and MM1s cells were seeded into 96-well plates at a density of 2.0 × 105 per well. The E35 dilution was subsequently added. The final concentrations of E35 in the wells were 0, 0.5, 1, 2, 4, and 8 μmol/L, respectively. A control group was added with the same amount of DMSO as the highest concentration group. Then, 10 μL of the combined MTS/PMS solution (5 mg/mL MTT) were added into each well, and the plates were incubated for 4 hours. Then, the cells were incubated in 5% CO2 incubator at 37°C for various durations (24, 48, 72, and 96 hours). The optical density was measured with a STAT FAX-2100 spectrophotometer (Awareness Tech, USA) in 490 nm at test. The reference wavelength was 630 nm. The proliferation inhibitory rate (%) and IC50 values were calculated by the CalcuSyn Statistical software.
Annexin V/propidium iodide apoptosis assay
The U266 and MM1s cells were seeded into 96-well plates. Then, the E35 dilution was added. The final concentration of E35 in the wells was 1.2 μmol. After incubation with E35 for 12 hours, the cells were harvested and washed with PBS twice and then stained with Annexin V-FITC/propidium iodide (PI) (Becton-Dickinson, NJ, USA) as per the manufacturer's instructions. The early apoptotic cells were quantified by a BD FACSCanto II cytometer (BD Biosciences, USA).
Real-time polymerase chain reaction analysis
The mRNA expression of the apoptosis-related genes, namely, C-Myc, Bcl-2, Mcl-1, and Pim2, was evaluated by real-time polymerase chain reaction (RT-PCR). The U266 and MM1s cells were pretreated with different concentrations of E35 or RPMI1640 for 48 hours. The total mRNA was extracted and reverse-transcribed. The transcription levels of C-Myc, Bcl-2, Mcl-1, and Pim2 were evaluated by quantitative RT-PCR using the iCycler real-time detection system (Bio-Rad, USA) in a two-step method. The hot-start enzyme was activated at 95°C for 5 minutes. Subsequently, cDNA was amplified for 40 cycles, which consisted of denaturation at 95°C for 15 seconds and annealing/extension at 58°C for 30 seconds. A melt curve analysis was then performed (55°C for 1 minute and then increased by 0.5°C every 10 seconds) to detect the formation of primer-derived trimmers and dimmers. The primer sequences were listed as follows: C-Myc: forward, 5'-TCCTGGCAAAAGGTCAGAGT-3', and reverse, 5'-TTGTGTGTTCGCCTCTTGA-3'; Bcl-2: forward, 5'-CGACGACTTCTCCCGCCGCTACCGC-3', and reverse, 5'- CCGCATGCTGGGGCCGTACAGTTCC-3'; Mcl-1: forward, 5'-ATCTCTCGGTACCTTCGGGAGC-3', and reverse, 5'-CCTG ATGCCACCTTCTAGGTCC-3'; Pim2: forward, 5'-CAGCCATCCAGCA CTGCCATTC-3', and reverse, 5'-AGTCTGGGGAGACATGGGCTGG -3'; β-actin: forward, 5'-GGCATGGGTCAGAAGGATTCC-3', and reverse, 5'-ATGTCACGCACGATTTCCCGC-3'. β-Actin was used as the internal control. All experiments were performed in triplicates, and the data were analyzed using the 2−ΔΔCt method.
Western blotting analysis
Western blotting analysis was employed to measure changes of the protein expression associated with the treatments. The cells were exposed to E35 at varying concentrations for 48 hours and then harvested and lysed. The protein concentrations were measured using the DCTM protein assay kit (Bio-Rad, USA). Electrophoresis was performed in 8%–12% sodium dodecyl sulfate polyacrylamide gel, and the samples were subsequently transferred onto a polyvinylidene fluoride membrane. Western blotting analysis was conducted as per the kit instructions. The apoptosis-related and the Akt/mTOR/4EBP1 signaling pathway-related proteins were detected in U266 and MM1s cells, and the investigation with bortezomib was carried out in U266 cells. The following primary antibodies were used for the analysis: C-Myc, Bcl-2, Mcl-1, Pim2, poly(ADP-ribose) polymerase (PARP), caspase-3, Akt, p-Akt, mTOR, p-mTOR, 4-EBP1, p-4EBP1, EIF4E, p-EIF4E, NF-κB (Danvers, USA), and β-actin (Fremont, USA) (internal reference). The primary antibodies were incubated overnight at 4°C. The membranes were probed with secondary antibodies (goat anti-mouse or goat anti-rabbit IgG) for 1 hour at room temperature. Finally, an enhanced chemiluminescence detection system (Pierce, USA) was used for the exposure. Quantitative analyses of protein expression were performed using an Image-Pro Plus system (Media Cybernetics, USA).
Statistical analysis
Data were expressed as the mean ± standard deviation (SD) calculated by the data from at least three independent experiments. The SPSS software (v19.0.0, IBM, IL, USA) was used for the statistical analysis. One-way analysis of variance followed by a Dunnett's post hoc test was used for multiple comparisons. We selected p < 0.05 as a level of significance.