Indole phytoalexin derivatives induce mitochondrial-mediated apoptosis in human colorectal carcinoma cells

To investigate the mechanism of the antiproliferative effect of synthetic indole phytoalexin derivatives on human colorectal cancer cell lines.

Changes in cell proliferation and the cytotoxic effect of the tested compounds on human colorectal cancer cell lines and human fibroblasts were evaluated using MTS and BrdU assay, allowing us to choose the most potent substance. Cell cycle alterations were analyzed using flow cytometric analysis. The apoptosis-inducing effect of compound K-453 on the HCT116 cell line was examined with annexin V/PI double staining using flow cytometry, as well as acridine orange/propidium iodide (AO/PI) staining. The flow cytometry method also allowed us to measure changes in levels or activation states of other factors associated with apoptosis, such as poly (ADP-ribose) polymerase (PARP), caspase-3 and -9, cytochrome c, Bcl-2 family proteins, and also the integrity of the mitochondrial membrane. To evaluate activity of the transcription factors and proteins involved in signaling pathways we used Western blot analysis together with flow cytometry.

Among the ten tested compounds, compound K-453 {(±)-trans-1,2-dimethoxy-2'-(3,5-bis-trifluoromethylphenylamino)spiro{indoline-3,5'[4',5']dihydrothiazol} exhibited the most potent activity with IC₅₀ = 32.22 ± 1.14 μmol/L in human colorectal HCT116 cells and was thus selected for further studies. Flow cytometric analysis revealed a K-453-induced increase in the population of cells with sub-G₁ DNA content, which is considered as a marker of apoptotic cell death. The apoptosis-inducing effect of compound K453 was also confirmed by annexin V/PI double staining and AO/PI staining. The apoptosis was associated with the loss of mitochondrial membrane potential, PARP cleavage, caspase-3 and caspase-9 activation, release of cytochrome c, as well as changes in the levels of Bcl-2 family members. Moreover, flow cytometry showed that compound K-453 stimulates phosphorylation of p38 MAPK but decreases phosphorylation of Akt and Erk 1/2. Activation of p38 MAPK was also confirmed using Western blot analysis. This analysis also revealed down-regulation of NF-κB1 (p50) and RelA (p65) proteins and the loss of their anti-apoptotic activity.

In our study compound K-453 exhibited an antiproliferative effect by induction of intrinsic apoptosis as well as modulation of several signaling pathways.