Mitochondrial gene RTN4IP1 drives triple-negative breast cancer metastasis

Triple-negative breast cancer accounts for approximately 15% of all breast cancer cases. It is characterized by the absence of estrogen receptor, progesterone receptor, and HER2 expression, rendering it insensitive to conventional endocrine and targeted therapies.

Patients often face higher risks of recurrence and metastasis, with generally poor prognoses, making it a significant challenge in clinical treatment. By integrating and analyzing multiple public genomic databases, the research team found that the RTN4IP1 gene is highly expressed in triple-negative breast cancer tissues and cell lines. Its expression level is significantly associated with shorter patient survival and a higher risk of distant metastasis. To validate its function, the researchers conducted gene knockdown experiments in triple-negative breast cancer cells, demonstrating that downregulation of RTN4IP1 expression effectively inhibits the proliferation, invasion, and migration capabilities of cancer cells.

The researchers further verified the pro-metastatic function of this protein in vivo using experimental animal models. Through tail vein injection to construct a lung metastasis model, the study found that although knocking down RTN4IP1 did not affect the number of micro-metastatic lesions formed by tumor cells in the lungs, it significantly inhibited the subsequent growth and volume expansion of these metastatic lesions. This indicates that RTN4IP1 primarily regulates the post-colonization growth stage of metastasis. At the mechanistic level, the study elucidated the pathway through which RTN4IP1 promotes metastasis by interfering with cellular energy metabolism.

The protein is localized to mitochondria, and its functional loss leads to a significant decrease in the levels of the essential coenzyme NAD⁺ within cells. NAD⁺ is a key factor in maintaining the normal operation of two major energy metabolism pathways-glycolysis and mitochondrial oxidative phosphorylation. Its depletion collectively suppresses the cell's energy supply and biosynthetic capacity, thereby weakening the metastatic potential of cancer cells. Using co-immunoprecipitation techniques, the researchers also confirmed that RTN4IP1 directly interacts with the mitochondrial adenylate kinase AK4 and that AK4 is an essential downstream effector molecule for its pro-metastatic function.

The value of this study lies not only in identifying RTN4IP1 as a factor associated with poor prognosis in triple-negative breast cancer but, more importantly, in delineating a clear signaling axis from RTN4IP1-AK4 molecular interaction to NAD⁺ metabolic disruption, ultimately leading to cellular metabolic reprogramming and enhanced metastatic capacity. This provides multiple potential entry points for intervening in this pathway.

Based on these findings, the study proposes future translational research directions, such as designing small molecule inhibitors targeting the predicted NAD(P)-binding pocket in the RTN4IP1 protein structure or developing intervention strategies to disrupt the RTN4IP1-AK4 protein interaction. These ideas lay the groundwork for pioneering new targeted therapy pathways for triple-negative breast cancer.