Mutant Kras (V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog) is observed in more than 20% of non-small-cell lung cancers; however, no effective Kras target therapy is available at present. The Kras DNA vaccine may represent as a novel immunotherapeutic agent in lung cancer. In this study, we investigated the antitumor efficacy of the Kras DNA vaccine in a genetically engineered inducible mouse lung tumor model driven by Kras(G12D). Lung tumors were induced by doxycycline, and the therapeutic effects of Kras DNA vaccine were evaluated with delivery of Kras(G12D) plasmids. Mutant Kras(G12D) DNA vaccine significantly decreased the tumor nodules. A dominant-negative mutant Kras(G12D)N17, devoid of oncogenic activity, achieved similar therapeutic effects. The T-helper 1 immune response was enhanced in mice treated with Kras DNA vaccine. Splenocytes from mice receiving Kras DNA vaccine presented an antigen-specific response by treatment with peptides of Kras but not Hras or OVA. The number of tumor-infiltrating CD8(+) T cells increased after Kras vaccination. In contrast, Kras DNA vaccine was not effective in the lung tumor in transgenic mice, which was induced by mutant L858R epidermal growth factor receptor. Overall, these results indicate that Kras DNA vaccine produces an effective antitumor response in transgenic mice, and may be useful in treating lung cancer-carrying Ras mutation.

Histone deacetylase (HDAC) inhibitors are used in treating certain human malignancies. Our laboratories demonstrated their capability in enhancing antitumor effect of DNA vaccine driven by an RNA polymerase II (RNA pol II) promoter. However, it is unknown whether HDAC inhibitors enhance the therapeutic short hairpin RNA (shRNA) expressed by an RNA polymerase III (RNA pol III) promoter. We investigated whether HDAC inhibitors augmented antitumor effect of indoleamine 2,3 dioxygenase (IDO) shRNA. HDAC inhibitor OSU-HDAC42 and suberoylanilide hydroxamic acid enhanced RNA pol III-driven U6 and H1 promoter activity in three different cell types in vitro: 293, NIH3T3 and dendritic cell line DC2.4. Subcutaneous injection of OSU-HDAC42 enhanced U6 and H1 promoter activity on abdominal skin of mice in vivo. Combination of IDO shRNA and OSU-HDAC42 increased antitumor effect of IDO shRNA in MBT-2 murine bladder tumor model. IDO shRNA induced tumor-infiltrating CD8⁺ and CD4⁺ T cells, whereas OSU-HDAC42 treatment induced tumor-infiltrating CD4⁺ T cells. Combination of OSU-HDAC42 and IDO shRNA further induced tumor-infiltrating natural killer cells and enhanced interferon-γ in lymphocytes, but suppressed interleukin (IL)-4 expression of lymphocytes. In addition, OSU-HDAC42 treatment did not alter mRNA expression of IL-12 and tumor necrosis factor-α. In conclusion, HDAC inhibitor OSU-HDAC42 may serve as adjuvant of the therapeutic shRNA expressed by an RNA pol III promoter.