Transcriptional and Translational Regulation of the Cancer-Testis Antigen PRAME Mediated by Pathogen-Associated Molecular Patterns

Abstract 2475

PRAME (preferentially expressed antigen in melanoma) is a germinal tissue-specific gene that is expressed at high levels in a number of haematological malignancies. In AML, PRAME is a positive prognostic indicator, being associated with a favourable response to chemotherapy and increased overall survival, even in the presence of unfavourable karyotypes. In contrast, over-expression of PRAME mRNA is associated with poor prognosis in CML, NHL, HD, CLL and myeloma, being associated with progressive disease and chemo-resistance. Despite proposals to implement PRAME as a tool for monitoring minimal residual disease and as an immunotherapeutic target, the physiological functions of PRAME in normal and tumour cells are unclear. A role in the regulation of retinoic acid signalling has been proposed, although interactions of PRAME with retinoic acid receptors are weak compared to other coregulator proteins. Sequence homology and structural predictions suggest that PRAME is a member of the family of leucine-rich repeat (LRR) proteins that function in cellular immunity, bearing most resemblance to the LRR domains of Toll-like receptors (in particular TLR3 and TLR4) and internalin proteins. LRR proteins can bind pathogen-associated molecular patterns (PAMPs) including proteins, nucleic acids and sugars, and consistent with this PRAME has been reported to interact with OPA proteins from Neisseriae gonorrhoeae. Here we show that expression of PRAME in HL60 leukaemic cells is strongly induced by lipopolysaccharide (LPS) or peptidoglycan in the presence of interferon (IFN)-gamma, whereas no induction was observed after exposure to other PAMPs such as zymosan A. This burst of PRAME expression peaked between 1 and 4 hrs post treatment, but was not mediated by the proximal promoter region. Moreover, we show that treatment of HL60 with LPS and IFN-gamma increases the association of PRAME transcripts with polysomes indicating PRAME is also translationally regulated, and we provide evidence that PRAME 5'UTR contains an internal ribosome entry sequence for CAP-independent translation. Finally, we show that the subcellular localisation of PRAME protein in HL60 and U937 cells is modulated by treatment with LPS and IFN-gamma. In summary, our studies indicate that the LRR protein PRAME is subject to transcriptional and translational regulation following cellular exposure to bacterial PAMPs and interferon, suggesting it may have a role in innate immunity.