FK506-binding protein 51 (FKBP51) is an immunophilin with isomerase activity, which performs important biological functions in the cell. It has recently been involved in the apoptosis resistance of malignant melanoma. The aim of this study was to investigate the possible role of FKBP51 in the control of response to ionizing radiation (Rx) in malignant melanoma. FKBP51-silenced cells showed reduced clonogenic potential after irradiation compared with non-silenced cells. After Rx, we observed apoptosis in FKBP51-silenced cells and autophagy in non-silenced cells. The FKBP51-controlled radioresistance mechanism involves NF-jB. FKBP51 was required for the activation of Rx-induced NF-jB, which in turn inhibited apoptosis by stimulating X-linked inhibitor of apoptosis protein and promoting authophagy-mediated Bax degradation. Using a tumor-xenograft mouse model, the in vivo pretreatment of tumors with FKBP51-siRNA provoked massive apoptosis after irradiation. Immunohistochemical analysis of 10 normal skin samples and 80 malignant cutaneous melanomas showed that FKBP51 is a marker of melanocyte malignancy, correlating with vertical growth phase and lesion thickness. Finally, we provide evidence that FKBP51 targeting radiosensitizes cancer stem/initiating cells. In conclusion, our study identifies a possible molecular target for radiosensitizing therapeutic strategies against malignant melanoma.

Monocyte/macrophages represent the first line of defense against protozoan parasites. Different mechanisms of monocyte suppression by Toxoplasma gondii that sustain parasite invasion and persistence have been described, including apoptosis. In the present study, we investigated the effect of microbial excretory–secretory polypeptides, namely the microneme protein MIC3 and the dense granule antigen GRA1, on apoptosis of monocytes from patients with congenital toxoplasmosis and healthy individuals. We found that GRA1 but not MIC3 could induce apoptosis of monocytes, observing the effect in samples from both Toxoplasma-infected and uninfected individuals, thus ruling out involvement of mechanisms of apoptosis linked to adaptive immunity or a cellular context related to infection. Selective inhibition of TGF-β type I receptors reversed GRA1-induced apoptosis, indicating that this apoptosis involved canonical TGF-β signaling. By using TGF-β-neutralizing antibodies, we showed that monocyte apoptosis required endogenous TGF-β and that GRA1 stimulation activated TGF-β transcription and expression in monocytes but not lymphocytes, suggesting involvement of an autocrine TGF-β-mediated mechanism in GRA1-induced apoptosis.