Interferon regulatory factor 1 and a variant of heterogeneous nuclear ribonucleoprotein L coordinately silence the gene for adhesion protein CEACAM1.

The adhesion protein carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is widely expressed in epithelial cells as a short cytoplasmic isoform (S-iso) and in leukocytes as a long cytoplasmic isoform (L-iso) and is frequently silenced in cancer by unknown mechanisms. Previously, we reported that interferon response factor 1 (IRF1) biases alternative splicing (AS) to include the variable exon 7 (E7) in CEACAM1, generating long cytoplasmic isoforms. We now show that IRF1 and a variant of heterogeneous nuclear ribonucleoprotein L (Lv1) coordinately silence the gene. RNAi-mediated Lv1 depletion in IRF1-treated HeLa and melanoma cells induced significant CEACAM1 protein expression, reversed by ectopic Lv1 expression. The Lv1-mediated repression resided in residues Gly-Gly and Ala-Gly in Lv1's N-terminal extension. ChIP analysis of IRF1- and FLAG-tagged Lv1-treated HeLa cells and global treatment with the global epigenetic modifiers 5-aza-2'-deoxycytidine and trichostatin A indicated that IRF1 and Lv1 together induce chromatin remodeling, restricting IRF1 access to the promoter. In interferon γ-treated HeLa cells, the transcription factor SP1 did not associate with the promoter, but binding by upstream transcription factor 1 (USF1), a known regulator, was greatly enhanced. ChIP-sequencing revealed that Lv1 overexpression in IRF1-treated cells induces transcriptional silencing across many genes, including (eleted in olorectal arcinoma), associated with CEACAM5 in colon cancer. Notably, IRF1, but not IRF3 and IRF7, affected CEACAM1 expression via translational repression. We conclude that IRF1 and Lv1 coordinately regulate transcription, alternative splicing, and translation and may significantly contribute to silencing in cancer.