Tetracycline analogues (doxycycline and COL‐3) induce caspase‐dependent and ‐independent apoptosis in human colon cancer cells

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Abstract
Tetracycline analogues (TCNAs) possess cytotoxic activities as well as matrix metalloproteinase (MMP) inhibitory properties. Previously, we demonstrated that doxycycline (DOXY) could induce apoptosis in human HT29 colon cancer cells. In present study, the molecular apoptotic mechanisms induced by two kinds of TCNAs, designated as DOXY and COL‐3 (chemically modified tetracycline‐3; 6‐demethyl, 6‐deoxy, 4‐dedimethylamino tetracycline), were evaluated in cultured HT29 cells. Both TCNAs inhibited the proliferation of 6 different colorectal cancer cell lines in a dose‐dependent manner. Especially, COL‐3 had a stronger effect on cancer cells than DOXY. Apoptotic changes were actually observed by 10 μg/ml COL‐3 and 20 μg/ml DOXY in a time‐dependent manner. COL‐3 produced the increase in cytosolic cytochrome c and the loss of mitochondrial membrane potential after 3 hr treatment, and thereafter activated caspases. In case of DOXY, these changes were observed after 24 hr. Bax translocation was not a prerequisite for cytochrome c releasing in COL‐3 treatment. Pretreated pancaspase inhibitor (Z‐VAD‐FMK) reduced COL‐3 and DOXY mediated apoptosis up to 81.3 and 35.3%, as compared with nontreated cells, respectively. These data indicated that TCNAs could induce mitochondria‐mediated apoptosis through both caspase‐dependent and ‐independent pathway. In fact, endonuclease G and apoptosis‐inducing factor were released into cytosol after the treatment of TCNAs, which indicated that caspase‐independent apoptotic pathway is also one of the key mechanisms for the treatment of TCNAs. Taken together, we believe that TCNAs could have strong potentials for clinical application in treating colorectal cancers and improve cancer chemotherapy.