γ-Klotho is correlated with resistance to docetaxel in castration-resistant prostate cancer

The Klotho (KL) gene was first identified as a potent aging suppressor. The KL family currently comprises of three proteins: α-Klotho (KLA), β-Klotho (KLB), and γ-Klotho (KLG). Many studies have shown that KLA and KLB participate in tumor progression or suppression, depending on the type of cancer; however, the relationship between KLG and prostate cancer has not yet been studied. Some studies have claimed that KL is correlated to sensitivity to chemotherapy. Here, we investigated the oncogenic potential of KLG in castration-resistant prostate cancer (CRPC). Immunohistochemical analysis using prostate biopsy specimens revealed that patients with high KLG expression in primary prostate cancer tissue had a significantly poor prognosis for overall survival. In addition, the prostate-specific antigen response rate after docetaxel (DTX) therapy in patients with high KLG expression was lower than that in patients with low KLG expression. To evaluate the potential of KLG as a therapeutic target in human prostate cancer, we generated a xenograft model of human CRPC cell line (PC-3) in male athymic mice. The animals were randomly divided into four groups as follows: i) control group (vehicle only); ii) DTX group (intraperitoneal administration); iii) small interfering RNA targeting KLG (KLG siRNA) group (intratumoral administration); and iv) a combination group (DTX plus KLG siRNA). After 3 weeks of treatment, the tumor weight and tumor Ki-67 labeling index were significantly lower in the KLG siRNA group and the combination group than in the control group. Sensitivity to DTX was increased upon treatment with KLG siRNA. These findings suggest that KLG expression in primary prostate cancer lesions is associated with resistance to DTX in CRPC and has potential as a diagnostic and therapeutic target for patients with CRPC.