Protein Exaptation by Endogenous Retroviral Elements Shapes Tumor Cell Senescence and Downstream Immune Signaling.

Cancer cell senescence in lung squamous cell carcinoma (LUSC) is associated with a poor response to chemotherapies and immunotherapies due to promotion of an immunosuppressive tumor microenvironment. This environment is shaped by the senescence-associated secretory pathway, which recruits suppressive immune cell populations. In a recent study, Attig and colleagues identified a transcription factor-activated molecular switch that circumvents cellular senescence through increased expression of the calbindin protein. A human endogenous retrovirus (HERV) sequence upstream of the calbindin gene, CALB1, promotes the transcription of an HERVH-CALB1 transcript through a splice event at the third CALB1 exon in a process known as protein exaptation. The KLF5 transcription factor mediates this transcriptional activity by binding at the HERVH sequence, subsequently initiating the chimeric HERVH-CALB1 transcription. This increased expression of calbindin reduces CXCL8 chemokine production and downstream neutrophil recruitment in LUSC tumor cells. CALB1 exaptation by HERVH is one example by which endogenous retroelements (ERE) regulate immunity in human cancers, highlighting the emerging role of EREs in tumor immunity.