As an important component in the innate immune system, natural killer (NK) cells have been demonstrated to be clinically associated with prostate cancer (PCa) progression and castration resistance. Therefore, the development of novel agents that may enhance the cytotoxicity of NK cells possesses promising therapeutic applications. In the present study, lienal polypeptide (IT) solution was supplemented into a co-culture system of NK and PCa cells, as it was previously demonstrated that LP are able to activate NK cells, which kill PCa cells based on an MTT cell viability assay. Mechanistic dissection demonstrated that LP enhanced androgen receptor degradation, which resulted in an upregulation of MHC class I polypeptide-related sequence A (MICA) and MICB. In turn, the induced expression of MICA and MICB was able to further trigger NK cell activation, forming a positive loop between NK cells and PCa cells in the presence of LP solution.