Induction of apoptosis by combined treatment with differentiation-inducing agents and interferon-alpha in human lung cancer cells.

BACKGROUND

Differentiation-inducing agents for myeloid leukemic cells, such as dimethyl sulfoxide (DMSO), Na-butyrate and hexamethylene-bis-acetamide (HMBA), barely induce irreversible differentiation or growth inhibition in solid tumors when used alone. However, we previously reported that combined treatment with differentiation-inducing agents and interferon (IFN)-alpha effectively suppressed the growth of human lung cancer cell lines in vitro and in vivo. We show here that combined treatment-induced cell death is caused by apoptosis.

MATERIALS AND METHODS

Induction of apoptosis was examined by expression of several apoptotic markers.

RESULTS

Combined treatment-induced cell death is caused by apoptosis, which is characterized by the induction of apoptotic morphological changes and the activation of caspase-3-like protease. The expression of Apo2.7, an early apoptotic change, is also induced by this combined treatment, and is suppressed by the addition of Z-Asp-CH2-DCB, a pan caspase inhibitor. The signal transduction pathway of IFN-alpha is rapidly observed in lung cancer cells, although it does not induce significant growth inhibition when used alone. We analyzed the effect of IFN-alpha on the apoptotic pathway and found that IFN-alpha but not DMSO induces the expression of caspase-4. The combination of IFN-alpha and DMSO did not cause further increase in the expression of caspase-4. In many cases of the induction of apoptosis, cytochrome c is released from mitochondria, which induces the formation of large caspase-activating complexes. Caspase-3-like-activities induced by the combination of DMSO or Na-butyrate with IFN-alpha were analyzed by gel filtration and detected equally in fractions with molecular weights of 200-300 kDa, suggesting that caspase-3 is activated in large complexes in lung cancer cells. However, Na-butyrate and HMBA, when used alone, induced the release of cytochrome c and enhanced the loss of mitochondrial membrane potential, whereas DMSO had no effect.

CONCLUSION

These results suggest that the combination of differentiation-inducing agents with IFN-alpha effectively induces apoptosis in lung cancer cells whereas the mechanism of such induction varies depending on the differentiation-inducing agent used.