Compound 6d, a spiroindoline compound, exhibits antiproliferative capability against cancer cell lines. However, the exact underlying mechanism of this compound-mediated inhibitory capability remains unclear. Here, we showed that compound 6d is an inhibitor of Bcl-2, which suppresses CRC growth by inducing caspase 3-mediated intrinsic apoptosis of mitochondria. Regarding the underlying mechanism, we identified HDAC6 as a direct substrate for caspase 3, and caspase 3 activation induced by compound 6d directly cleaves HDAC6 into two fragments. Moreover, the cleavage site was located at D1088 in the DMAD-S motif HDAC6. Apoptosis stimulated by compound 6d promoted autophagy initiation by inhibiting interaction between Bcl-2 and Beclin 1, while it led to the accumulation of ubiquitinated proteins and the reduction of autophagic flux. Collectively, our findings reveal that the Bcl-2-caspase 3-HDAC6 cascade is a crucial regulatory pathway of autophagy and identify compound 6d as a novel lead compound for disrupting the balance between apoptosis and autophagy.

Colorectal cancer (CRC) is the most common intestinal malignancy, and nearly 70% of patients with this cancer develop metastatic disease. In the present study, we synthesized a novel compound, termed N-(3-(5,7-dimethylbenzo [d]oxazol-2-yl)phenyl)-5-nitrofuran-2-carboxamide (compound 275#), and found that it exhibits antiproliferative capability in suppressing the proliferation and growth of CRC cell lines. Furthermore, compound 275# triggered caspase 3-mediated intrinsic apoptosis of mitochondria and autophagy initiation. An investigation of the molecular mechanisms demonstrated that compound 275# induced intrinsic apoptosis, and autophagy initiation was largely mediated by increasing the levels of the intracellular accumulation of reactive oxygen species (ROS) in CRC cells. Taken together, these data suggest that ROS accumulation after treatment with compound 275# leads to mitochondria-mediated apoptosis and autophagy activation, highlighting the potential of compound 275# as a novel therapeutic agent for the treatment of CRC.