Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related mortality worldwide. Its progression is shaped not only by malignant cells themselves, but also by complex interactions within the tumor microenvironment. Among the major components of this microenvironment are tumor-associated macrophages (TAMs), which can adopt different functional states and influence tumor growth, immune suppression and disease progression. Understanding how TAM function is controlled may therefore provide new insights into the biology of HCC.
Long non-coding RNAs (lncRNAs) have traditionally been studied as RNA molecules that regulate gene expression without producing proteins. However, accumulating evidence has shown that some lncRNAs contain small open reading frames capable of producing functional micropeptides.
A study published in Science Bulletin identifies the micropeptide CTXND1 as a key functional product of lncCtxnd1 and reveals a molecular pathway connecting lncRNA-encoded micropeptides, macrophage polarization, and tumor progression.
How CTXND1 regulates macrophage function
The researchers found that the antitumor activity of lncCtxnd1 is closely associated with macrophage function. CTXND1 suppresses mTORC1 signaling in macrophages , thereby limiting alternative macrophage activation and the M2-like polarization program. Because macrophage polarization can substantially influence the tumor microenvironment, this activity ultimately restricts HCC growth. Interestingly, increasing lncCtxnd1 expression in HCC tumor cells had a relatively limited effect on tumor-cell growth. In contrast, the effects of lncCtxnd1 were much more pronounced in macrophages. “These findings suggest that lncCtxnd1 does not primarily suppress HCC by directly inhibiting the proliferation of tumor cells,” the researchers explain. “Instead, its major function is mediated through macrophages within the tumor microenvironment.”
An important question was whether the tumor-suppressive activity of lncCtxnd1 was mediated by the RNA transcript itself or by its encoded micropeptide. To address this question, the researchers used a genetic model lacking lncCtxnd1. Re-expression of CTXND1 alone in these mice significantly reduced tumor growth. This result provides genetic evidence that CTXND1, rather than the lncCtxnd1 transcript alone, is a key functional mediator of the tumor-suppressive activity associated with lncCtxnd1. The findings establish a regulatory pathway in which reduced lncCtxnd1 expression is associated with loss of CTXND1, increased mTORC1 signaling in macrophages, enhanced M2-like polarization, and a tumor microenvironment that supports HCC progression.
New insights into the tumor immune microenvironment
The study expands the emerging view that lncRNAs can function as sources of biologically active micropeptides. Although many lncRNAs were initially classified according to their apparent lack of protein-coding capacity, increasing evidence suggests that some contain previously overlooked coding regions. The resulting micropeptides can participate in diverse cellular processes despite their small size. The current study extends this concept to tumor immunology. Rather than acting only within the cells in which they are produced, lncRNA-encoded micropeptides may influence the behavior of immune cells and thereby reshape the tumor microenvironment.
“Our findings reveal a previously unrecognized connection between an lncRNA-encoded micropeptide and macrophage function in HCC,” said Prof. Runsheng Chen, one of the paper coordinators, a researcher at the Institute of Biophysics, Chinese Academy of Sciences. “The CTXND1–mTORC1 pathway provides a new framework for understanding how small peptides encoded by non-coding RNA loci can influence the tumor immune microenvironment.”
Together, the findings define a lncCtxnd1–CTXND1–mTORC1–TAM regulatory axis in HCC. The work provides new insight into the functional complexity of the non-coding genome and identifies CTXND1 as a potential molecular entry point for further investigation of macrophage-mediated regulation of HCC. Further studies will be needed to determine how CTXND1 expression is regulated in human HCC, how the micropeptide interacts with components of the mTORC1 pathway at the molecular level, and whether the CTXND1 pathway has potential relevance to therapeutic strategies targeting the tumor microenvironment.
Science Bulletin
Experimental study