Inverse correlation between TP53 gene status and PD-L1 protein levels in a melanoma cell model depends on an IRF1/SOX10 regulatory axis.

BACKGROUND: PD-L1 expression on cancer cells is an important mechanism of tumor immune escape, and immunotherapy targeting the PD-L1/PD1 interaction is a common treatment option for patients with melanoma. However, many patients do not respond to treatment and novel predictors of response are emerging. One suggested modifier of PD-L1 is the p53 pathway, although the relationship of p53 pathway function and activation is poorly understood. METHODS: The study was performed on human melanoma cell lines with various p53 status. We investigated PD-L1 and proteins involved in IFNγ signaling by immunoblotting and mRNA expression, as well as membrane expression of PD-L1 by flow cytometry. We evaluated differences in the ability of NK cells to recognize and kill target tumor cells on the basis of p53 status. We also investigated the influence of proteasomal degradation and protein half-life, IFNγ signaling and p53 activation on biological outcomes, and performed bioinformatic analysis using available data for melanoma cell lines and melanoma patients. RESULTS: We demonstrate that p53 status changes the level of membrane and total PD-L1 protein through IRF1 regulation and show that p53 loss influences the recently discovered SOX10/IRF1 regulatory axis. Bioinformatic analysis identified a dependency of SOX10 on p53 status in melanoma, and a co-regulation of immune signaling by both transcription factors. However, IRF1/PD-L1 regulation by p53 activation revealed complicated regulatory mechanisms that alter IRF1 mRNA but not protein levels. IFNγ activation revealed no dramatic differences based on TP53 status, although dual p53 activation and IFNγ treatment confirmed a complex regulatory loop between p53 and the IRF1/PD-L1 axis. CONCLUSIONS: We show that p53 loss influences the level of PD-L1 through IRF1 and SOX10 in an isogenic melanoma cell model, and that p53 loss affects NK-cell cytotoxicity toward tumor cells. Moreover, activation of p53 by MDM2 inhibition has a complex effect on IRF1/PD-L1 activation. These findings indicate that evaluation of p53 status in patients with melanoma will be important for predicting the response to PD-L1 monotherapy and/or dual treatments where p53 pathways participate in the overall response.

Biochemical evidence for conformational variants in the anti-viral and pro-metastatic protein IFITM1.

Interferon induced transmembrane proteins (IFITMs) play a dual role in the restriction of RNA viruses and in cancer progression, yet the mechanism of their action remains unknown. Currently, there is no data about the basic biochemical features or biophysical properties of the IFITM1 protein. In this work, we report on description and biochemical characterization of three conformational variants/oligomeric species of recombinant IFITM1 protein derived from an Escherichia coli expression system. The protein was extracted from the membrane fraction, affinity purified, and separated by size exclusion chromatography where two distinct oligomeric species were observed in addition to the expected monomer. These species remained stable upon re-chromatography and were designated as "dimer" and "oligomer" according to their estimated molecular weight. The dimer was found to be less stable compared to the oligomer using circular dichroism thermal denaturation and incubation with a reducing agent. A two-site ELISA and HDX mass spectrometry suggested the existence of structural motif within the N-terminal part of IFITM1 which might be significant in oligomer formation. Together, these data show the unusual propensity of recombinant IFITM1 to naturally assemble into very stable oligomeric species whose study might shed light on IFITM1 anti-viral and pro-oncogenic functions in cells.

IFITM protein regulation and functions: Far beyond the fight against viruses.

Interferons (IFNs) are important cytokines that regulate immune responses through the activation of hundreds of genes, including interferon-induced transmembrane proteins (IFITMs). This evolutionarily conserved protein family includes five functionally active homologs in humans. Despite the high sequence homology, IFITMs vary in expression, subcellular localization and function. The initially described adhesive and antiproliferative or pro-oncogenic functions of IFITM proteins were diluted by the discovery of their antiviral properties. The large set of viruses that is inhibited by these proteins is constantly expanding, as are the possible mechanisms of action. In addition to their beneficial antiviral effects, IFITM proteins are often upregulated in a broad spectrum of cancers. IFITM proteins have been linked to most hallmarks of cancer, including tumor cell proliferation, therapeutic resistance, angiogenesis, invasion, and metastasis. Recent studies have described the involvement of IFITM proteins in antitumor immunity. This review summarizes various levels of IFITM protein regulation and the physiological and pathological functions of these proteins, with an emphasis on tumorigenesis and antitumor immunity.