In the present study, we studied the effect of the EGFR inhibitor erlotinib on the growth of human melanoma and found that erlotinib dose-dependently inhibited tumor cell growth in different melanoma cell lines. We also demonstrated that the novel combination of IL-24 and erlotinib increased the effectiveness of erlotinib. Treatment with Ad-IL-24 or purified IL-24 protein in combination with erlotinib significantly inhibited tumor cell growth and increased apoptosis induction in various melanoma cell lines. These results show that this novel combination treatment is a promising therapeutic approach for melanoma treatment. To our knowledge, this is the first report regarding EGFR tyrosine kinase inhibitor erlotinib and its combination with IL-24 tumor suppressor for the treatment of melanoma. These results will serve as a basis for guiding the combination treatment designs in future preclinical and clinical trials.
Mutations in apoptotic pathways and decreased susceptibility to apoptosis are associated with cellular resistance to chemotherapeutic drugs in various human cancers. The IL-24/MDA7 protein has been found to physically interact with dsRNA-dependent protein kinase (PKR), a potent growth inhibitory protein, in human lung cancer cells. This direct interaction between PKR and IL-24 is important for PKR-mediated apoptosis [18
]. Previous studies also showed that overexpression of IL-24 in melanoma cells activated the p38 mitogen-activated protein kinase (MAPK) pathway, resulting in up-regulated expression of the genes associated with cell growth-arrest and apoptotic processes [54
]. However, the precise molecular mechanisms and signaling pathways of IL-24 for tumor suppression and apoptosis induction remain largely unclear in melanoma cells. Our current study has demonstrated that the enhancement of apoptosis by the IL-24 and erlotinib combination is mediated by activation of the caspase- and Apaf-1-dependent apoptotic pathway in melanoma cells. Apaf-1 is an important signaling protein involved in the activation of caspase-9 during apoptosis. Apaf-1 forms a complex with caspase-9 in the presence of cytochrome c and dATP, ultimately leading to activation of caspase-9 and caspase-3 and subsequently inducing apoptosis [55
]. Our study provides new insights into understanding the molecular mechanisms of IL-24 for tumor suppression.
Previous studies have shown that EGFR signaling pathway is involved in the development and progression of melanoma [46
]. EGFR tyrosine kinase inhibitor erlotinib has been used to treat various solid tumors, but have not been tested in melanoma. We hypothesized that inhibition of EGFR-dependent signaling pathway by erlotinib would also suppress the growth of melanoma cells, and that a combination treatment consisting of IL-24 and erlotinib would enhance antitumor activity by simultaneously activating the tumor suppression and apoptotic pathways and inhibiting the anti-apoptotic and cell survival signaling pathways in melanoma cells. The results from our study support our hypothesis and have important implications in exploring novel therapeutic strategies for melanoma. Although sensitivity to erlotinib varied in our panel of melanoma cell lines, our study demonstrates that IL-24-mediated molecular therapy increases the response to erlotinib, and suggests a potential of this combination as a promising strategy for the treatment of melanoma.
A strong positive association between erolinib toxicity and patient survival has been reported for several epithelial malignancies such as including lung cancer, head and neck cancer, and ovarian cancer. The toxicities experienced by patients taking erlotinib are multifactorial and determined by distinct parameters in different tissues. The most common adverse effects of erlotinib are skin rash and diarrhea. [57
]. Adenovirus-mediated IL-24/MDA-7 gene therapy alone has been shown to inhibit the growth and kills a broad spectrum of cancer cells, with no toxicities or other deleterious effects in normal human epithelial or fibroblast cells [60
]. A recent study in our laboratory has also shown that IL-24 plays an important role in the resolution of skin wound healing (Poindexter, N., submitted for publication). Therefore, IL-24 may prove useful to regulate the severity of the skin rash and provides additional rationale to explore the dose and scheduling in patients experiencing toxicities of erlotinib.
The persistent activity of the Akt signaling pathway contributes to resistance of tumor cells to EGFR inhibitors. In this study, we also identified the combined effect of IL-24 and erlotinib on EGFR and Akt signaling and found that the combination of IL-24 and erlotinib leaded to an enhanced inactivation of the Akt signaling pathway. Phosphorylated Akt is a mediator of EGFR-induced cell survival and clinical response to EGFR inhibitors, and a reduction of phosphorylated Akt expression is thought to be an important event in sensitizing tumor cells to EGFR inhibitor treatment. Our current study showed that combination of IL-24 and erlotinib significantly increased the inhibition of phosphorylation of the EGFR, PI3K, and Akt proteins. Therefore, our findings revealed a novel pathway for IL-24 in melanoma cells.
In summary, we demonstrated that treatment with Ad-IL-24 in combination with erlotinib significantly inhibited tumor cell growth and induced apoptosis in melanoma cells. The enhancement of antitumor activity by IL-24 and erlotinib combination is associated with the activation of the Apaf-1-dependent apoptotic pathway and the inactivation of the Akt-dependent cell survival signaling pathway. Our findings provide new insights into the molecular mechanisms of IL-24 and suggest that a combination of IL-24 and erlotinib may be an effective treatment strategy for human melanoma.