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. 2016 Jan 19;11(1):e0147218.
doi: 10.1371/journal.pone.0147218. eCollection 2016.

Sodium Butyrate Induces Endoplasmic Reticulum Stress and Autophagy in Colorectal Cells: Implications for Apoptosis

Jintao Zhang  1 , Man Yi  1 , Longying Zha  1 , Siqiang Chen  2 , Zhijia Li  1 , Cheng Li  1 , Mingxing Gong  1 , Hong Deng  1 , Xinwei Chu  1 , Jiehua Chen  1 , Zheqing Zhang  1 , Limei Mao  1 , Suxia Sun  1
Affiliations

Sodium Butyrate Induces Endoplasmic Reticulum Stress and Autophagy in Colorectal Cells: Implications for Apoptosis

Jintao Zhang et al. PLoS One. .

Abstract

Purpose: Butyrate, a short-chain fatty acid derived from dietary fiber, inhibits proliferation and induces cell death in colorectal cancer cells. However, clinical trials have shown mixed results regarding the anti-tumor activities of butyrate. We have previously shown that sodium butyrate increases endoplasmic reticulum stress by altering intracellular calcium levels, a well-known autophagy trigger. Here, we investigated whether sodium butyrate-induced endoplasmic reticulum stress mediated autophagy, and whether there was crosstalk between autophagy and the sodium butyrate-induced apoptotic response in human colorectal cancer cells.

Methods: Human colorectal cancer cell lines (HCT-116 and HT-29) were treated with sodium butyrate at concentrations ranging from 0.5-5mM. Cell proliferation was assessed using MTT tetrazolium salt formation. Autophagy induction was confirmed through a combination of Western blotting for associated proteins, acridine orange staining for acidic vesicles, detection of autolysosomes (MDC staining), and electron microscopy. Apoptosis was quantified by flow cytometry using standard annexinV/propidium iodide staining and by assessing PARP-1 cleavage by Western blot.

Results: Sodium butyrate suppressed colorectal cancer cell proliferation, induced autophagy, and resulted in apoptotic cell death. The induction of autophagy was supported by the accumulation of acidic vesicular organelles and autolysosomes, and the expression of autophagy-associated proteins, including microtubule-associated protein II light chain 3 (LC3-II), beclin-1, and autophagocytosis-associated protein (Atg)3. The autophagy inhibitors 3-methyladenine (3-MA) and chloroquine inhibited sodium butyrate induced autophagy. Furthermore, sodium butyrate treatment markedly enhanced the expression of endoplasmic reticulum stress-associated proteins, including BIP, CHOP, PDI, and IRE-1a. When endoplasmic reticulum stress was inhibited by pharmacological (cycloheximide and mithramycin) and genetic (siRNA targeting BIP and CHOP) methods, the induction of BIP, PDI, IRE1a, and LC3-II was blocked, but PARP cleavage was markedly enhanced.

Discussion: Taken together, these results suggested that sodium butyrate-induced autophagy was mediated by endoplasmic reticulum stress, and that preventing autophagy by blocking the endoplasmic reticulum stress response enhanced sodium butyrate-induced apoptosis. These results provide novel insights into the anti-tumor mechanisms of butyric acid.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Sodium butyrate inhibited proliferation and induced apoptosis in colorectal cancer cells.
HCT-116 (A) and HT-29 (B) cells were treated with the indicated concentrations of sodium butyrate (NaB) for 24 (grey bars), 48 (pink bars), or 72 (yellow bars) hours and cell proliferation was assessed. HCT-116 (C, E) and HT-29 (D, F) cells were treated with the indicated concentrations of NaB for 24 h. C.D. The percentage of apoptotic cells in HCT-116 (C) and HT-29 (D) cells was quantified in three independent experiments using an annexinV/PI assay (annexinV+/PI-) by flow cytometry and expressed as mean ± SD. One-way ANOVA was used to compare between the control cells and NaB treatments. *p
Fig 2
Fig 2. Sodium butyrate induced autophagy in colorectal cancer cells.
HCT-116 (A, C) and HT-29 (B, D) cells were treated with 2mM sodium butyrate (NaB) for 24 h and then stained with acridine orange (AO; A-B) or monodansylcadaverine (MDC; C-D). Flow cytometry measured the percentage of AO+ cells and the results of three independent AO fluorescence experiments for HTC-116 (E) and HT-29 (F) cells were expressed as mean ± SD. One-way ANOVA was used to compare control cells and NaB treatments. *p
Fig 3
Fig 3. Sodium butyrate increased expression of LC3-II.
HCT-116 (A) or HT-29 (B) cells were treated with the indicated concentrations of sodium butyrate (NaB) for 24 h. Representative blots showed expression of Beclin1, ATG3, and LC3 as well as GAPDH, the loading control. C. Electron microscopy images of control and NaB (2mM) treated HCT-116 cells. ER, endoplasmic reticulum; Mt, mitochondrion; AL.
Fig 4
Fig 4. 3-MA and chloroquine blocked sodium butyrate induced autophagy in colorectal cancer cells.
HCT-116 or HT-29 cells were treated with 5mM 3-MA or 5μM chloroquine (CQ) for 30 min, and then with 2mM sodium butyrate (NaB) for 24 h. Representative Western blots of LC3-I and LC3-II expression were quantified by densitometry and normalized to GAPDH (ratio of LC3:GAPDH). The fold change from control is indicated as the mean ± SD of three independent experiments. One-way ANOVA was used for statistical analysis. ** p
Fig 5
Fig 5. Sodium butyrate induced endoplasmic reticulum stress in colorectal cancer cells.
HCT-116 (A) and HT-29 (B) cells were treated with the indicated concentrations of sodium butyrate (NaB) for 24 h. Representative Western blots show the expression of IRE-1a, BIP, PDI, and CHOP. GAPDH was used as the loading control.
Fig 6
Fig 6. Cycloheximide blocked sodium butyrate induced endoplasmic reticulum stress in colorectal cancer cells.
HCT-116 or HT-29 cells were treated with 10 μg/mL cycloheximide for 30 min followed by sodium butyrate (NaB) for 24 h. Representative Western blots showed the expression of IRE-1a, BIP, PDI, and GADPH (loading control) in HCT-116 (A) or HT-29 (C) cells. Protein expression was quantified by densitometry and normalized to GAPDH (ratio of protein:GAPDH). The fold change from control for each protein is expressed as mean ± SD of three independent experiments for HCT-116 (B) and HT-29 (D) cells. are shown. One-way ANOVA was used for statistical analysis. * P
Fig 7
Fig 7. Mithramycin blocked sodium butyrate induced endoplasmic reticulum stress in colorectal cancer cells.
HCT-116 (A, B) or HT-29 (C,D) cells were treated with 0.1μM mithramycin for 30 min followed by sodium butyrate (NaB) for 24 h. Representative Western blots showing the expression of IRE-1a, BIP, and PDI in HCT-116 (A) or HT-29 (C) cells are shown. GAPDH was used as loading control. Protein expression (IRE-1a, BIP, and PDI) was quantified by densitometry and normalized to GAPDH (ratio of protein:GAPDH). The fold change from control for each protein is expressed as mean ± SD of three independent experiments for HCT-116 (B) and HT-29 (D) cells. One-way ANOVA was used for statistical analysis. * P
Fig 8
Fig 8. Cyclohexamide and mithramycin blocked sodium butyrate induced autophagy in colorectal cancer cells.
HCT-116 or HT-29 cells were treated with 10 μg/mL cycloheximide or 0.1μM mithramycin for 30 min and then with 2mM sodium butyrate (NaB) for 24 h. Representative Western blots of the expression of LC3-II are shown. The level of LC3-II expression was quantified by densitometry and normalized to GAPDH (ratio of LC3-II:GAPDH). The fold change from control cells is shown. Means and standard deviation (SD) of three independent experiments are shown. One-way ANOVA was used for statistical analysis. * P
Fig 9
Fig 9. RNA interference targeting BIP or CHOP blocked sodium butyrate induced autophagy in colorectal cancer cells.
HCT-116 cells were transfected with BIP or CHOP specific siRNAs for 48 h and then treated with or without 2mM sodium butyrate (NaB) for 24 h. Negative control (NC) scramble siRNA was used the negative control for the transfection. Representative Western blots are shown for BIP siRNA (A) and CHOP siRNA (C). The expression level of each protein was determined by densitometry and normalized to GAPDH (ratio of protein:GAPDH). (B) Normalized expression levels of BIP and LC3-II in HCT-116 cells treated with BIP specific siRNA. (C) Normalized expression levels of CHOP and LC3-II in HCT-116 cells treated with CHOP specific siRNA. Means and standard deviation (SD) of three independent experiments are shown. One-way ANOVA was used for statistical analysis. * P
Fig 10
Fig 10. Inhibition of autophagy potentiates sodium butyrate-induced apoptotic cell death.
HCT-116 (A) and HT-29 (B) cells were exposed to 5mM 3-MA or 5μM chloroquine (CQ) for 30 min and then treated with 2mM sodium butyrate (NaB) for 24 h. The percentage of apoptotic cells (annexinV+/PI-) was quantified by flow cytometry using an annexinV/PI assay and expressed as mean ± SD of three independent experiments. One-way ANOVA was used for statistical analysis to compare control cells and NaB treatments. *p
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