miR-122 Regulates p53/Akt Signalling and the Chemotherapy-Induced Apoptosis in Cutaneous T-Cell Lymphoma

Advanced cutaneous T-cell lymphoma (CTCL) is resistant to chemotherapy and presents a major area of medical need. In view of the known role of microRNAs (miRNAs) in the regulation of cellular signalling, we aimed to identify the functionally important miRNA species, which regulate apoptosis in CTCL. Using a recently established model in which apoptosis of CTCL cell lines is induced by Notch-1 inhibition by γ-secretase inhibitors (GSIs), we found that miR-122 was significantly increased in the apoptotic cells. miR-122 up-regulation was not specific for GSI-1 but was also seen during apoptosis induced by chemotherapies including doxorubicin and proteasome blockers (bortezomib, MG132). miR-122 was not expressed in quiescent T-cells, but was detectable in CTCL: in lesional skin in mycosis fungoides and in Sézary cells purified from peripheral blood. In situ hybridization results showed that miR-122 was expressed in the malignant T-cell infiltrate and increased in the advanced stage mycosis fungoides. Surprisingly, miR-122 overexpression decreased the sensitivity to the chemotherapy-induced apoptosis via a signaling circuit involving the activation of Akt and inhibition of p53. We have also shown that induction of miR-122 occurred via p53 and that p53 post-transcriptionally up-regulated miR-122. miR-122 is thus an amplifier of the antiapoptotic Akt/p53 circuit and it is conceivable that a pharmacological intervention in this pathway may provide basis for novel therapies for CTCL.