Background & aims: Growing evidence shows that some non-coding RNAs (ncRNAs) contain small open reading frames (smORFs) that are translated into short peptides. Herein, we aimed to determine where and how these short peptides might promote hepatocellular carcinoma (HCC) development.

Methods: We performed an RNA-immunoprecipitation followed by high-throughput sequencing (RIP-seq) assay with an antibody against ribosomal protein S6 (RPS6) on 4 cancer cell lines. Focusing on 1 long non-coding RNA (lncRNA), LINC00998, we used qPCR and public databases to evaluate its expression level in patients with HCC. Special vectors were constructed to confirm its coding potential. We also explored the function and mechanism of LINC00998-encoded peptide in tumor growth and metastasis.

Results: We discovered that many lncRNAs bind to RPS6 in cancer cells. One of these lncRNAs, LINC00998, encoded a small endogenous peptide, termed SMIM30. SMIM30, rather than the RNA itself, promoted HCC tumorigenesis by modulating cell proliferation and migration, and its level was correlated with poor survival in patients with HCC. Furthermore, SMIM30 was transcribed by c-Myc and then drove the membrane anchoring of the non-receptor tyrosine kinases SRC/YES1. Moreover, the downstream MAPK signaling pathway was activated by SRC/YES1.

Conclusions: Our results not only unravel a new mechanism of HCC tumorigenesis promoted by ncRNA-encoded peptides, but also suggest that these peptides can serve as a new target for HCC cancer therapy and a new biomarker for HCC diagnosis and prognosis.

Lay summary: Very little is known about how peptides activate signaling pathways that play a crucial role in diseases such as cancer. Specifically, we reported on a conserved peptide encoded by LINC00998, SMIM30. This peptide promoted the tumorigenesis of hepatocellular carcinoma (HCC) by modulating cell proliferation and migration. Of note, it bound the non-receptor tyrosine kinases, SRC/YES1, to drive their membrane anchoring and phosphorylation, activating the downstream MAPK signaling pathway. Our work not only unravels a new mechanism of HCC tumorigenesis promoted by peptides, but also demonstrates how the peptide works to activate a signaling pathway.