Abstract
Melanoma is the most serious type of skin cancer. The immunosuppressive tumor microenvironment and aberrant expression of some proto-oncogenes are the main cause of melanoma development. We have constructed a single-stranded RNA (ssRNA)–Pim-3–small hairpin RNA (shRNA) dual-function vector, which activates the toll-like receptor (TLR)7 to stimulate the antitumor immune response through ssRNA fragments and simultaneously silences the proto-oncogene Pim-3 to intensify apoptosis of the tumor cells via shRNA. Here, we found that therapy with the ssRNA-Pim-3-shRNA dual-function vector not only promotes the apoptosis and inhibits the proliferation of B16F10 melanoma cells by inhibiting the expression of Pim-3 but also enhances the activation of CD8+ T cells and natural killer (NK) cells and simultaneously reduces the proportion of intratumoral regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs). Together, these features effectively inhibit the growth of melanoma. Intriguingly, the bifunctional therapeutic effect that reverses the tumor immunosuppressive microenvironment is dependent on the activation of plasmacytoid dendritic cells (pDCs) and the secretion of type I interferon (IFN). Our study suggests that ssRNA-Pim-3-shRNA dual-function therapy is expected to become a promising therapeutic strategy for melanoma and other solid tumors with immunosuppressive microenvironment.
Highlights
Melanoma is the most serious form of skin cancer owing to the high degree of malignancy
We showed that this dual-function therapy can promote apoptosis and inhibit the proliferation of B16F10 melanoma cells by inhibiting the expression of Pim3 and can stimulate the activation of plasmacytoid dendritic cells (pDCs) by single-stranded RNA (ssRNA) to secrete a large amount of type I IFN, thereby enhancing the activity of CD8+ T cells and natural killer (NK) cells in a B16F10 tumor-bearing mouse model
Transfection with the ssRNA and dual-function vectors induced the significant activation of IRF3 and NF-κB that are the downstream signals of TLR7 (Figure 1A) and increased the secreted levels of IFNα and IFN-β in the supernatants of B16F10 cells (Figure 1B)
Summary
Melanoma is the most serious form of skin cancer owing to the high degree of malignancy. Proto-oncogenes play a key role in the growth of melanoma [1]. The abnormally high expression of the proto-oncogene causes changes in the biological characteristics of the cells, resulting in increased cell proliferation and reduced cell apoptosis [2]. The proto-oncogene Pim-3 is a highly conserved serine/threonine kinase and plays an important role in many physiology and pathology processes, including cell proliferation, survival, and apoptosis [3,4,5]. Its abnormal expression in a variety of cancers, including melanoma, leads to decreased apoptosis and increased proliferation through phosphorylation and inactivation of the proapoptotic BH3-only protein Bad, promoting the occurrence and development of tumors [6, 7]. Inhibition of Pim-3 kinase or silencing of Pim-3 inhibits tumor growth and enhances apoptosis of tumor cells [8]. Pim-3 kinase might be a candidate therapeutic target for cancer
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