Background
Sulforaphane (SUL), an isothiocyanate naturally present in widely consumed vegetables, particularly broccoli, has recently
attracted attention due to its inhibitory effects on tumor cell growth by inducing apoptosis. We investigated the ability
of SUL to induce autophagy in human colon cancer cells and whether inhibition of autophagy could potentiate the proapoptotic
effect of SUL.
Methods
The proliferation of cells treated with SUL was assessed by MTS assay and colony-forming assay. Apoptosis and caspases activity
were investigated by flow cytometry. The formation of acidic vesicular organelles (AVOs) was detected in acridine-orange-stained
cells by flow cytometry. Western blotting was used for the detection of light chain 3 (LC3). Localizations of LC3 and cytochrome
c were analyzed by immunocytochemistry.
Results
The proapoptotic effect was observed by treatment of cells with relatively high concentrations of SUL for long periods of
time. After 16 h of treatment, evident formation of AVOs and recruitment of LC3 to autophagosomes, features of autophagy,
were observed. Treatment of cells with a specific autophagy inhibitor (3-methyladenine) potentiated the proapoptotic effect
of SUL, which was dependent on the activation of caspases and the release of cytochrome c to the cytosol.
Conclusion
The present results demonstrate induction of autophagy in colon cancer cells as a protective reaction against the proapoptotic
effect of SUL, and consequently, the potentiation of the proapoptotic effect by autophagy inhibition. These findings provide
a premise for use of autophagy inhibitors in combination with chemotherapeutic agents for treatment of colorectal cancer.