Plasma membrane rupture (PMR), the terminal lytic event in cells undergoing pyroptotic, necrotic or apoptotic cell death (i.e. different forms of programmed cell death) requires 16 kDa cell-surface protein NINJ1 (Ninjurin 1), which has two transmembrane regions with both N and C termini on the outside of the cell (Kayagaki et al., 2021; Kayagaki et al., 2023; Degen et al., 2023; Wang and Shao, 2021; Newton et al., 2021; Whisstock and Law, 2023; Li et al., 2023; Dias et al., 2022). Highly conserved putative N-terminal α-helices of NINJ1 insert into plasma membrane to mediate NINJ1 oligomerization which induce PMR, by forming large and irregular holes (Kayagaki et al., 2021; Kayagaki et al., 2023; Degen et al., 2023). Pro-inflammatory cytoplasmic molecules, i.e. damage-associated molecular patterns (DAMPs), that activate immune cells can be released non-selectively into the extracellular space following PMR, resulting in the propagation of inflammation (Kayagaki et al., 2021; Kayagaki et al., 2023). Inhibition of plasma membrane rupture with anti-NINJ1 monoclonal antibodies was shown to limit tissue injury that is associated with excessive cell death, by blocking NINJ1 oligomerization (Kayagaki et al., 2023). Similarly, glycine, a known cytoprotector, was found to inhibit NINJ1 membrane clustering to suppress PMR following the initiation of cell death (Borges et al., 2022). In short, PMR is an actively regulated event by NINJ1, in contrast to previous idea that it is a passive processes that follows osmotic swelling in cell death.
Heterogeneity in breast tumors is understood in terms of differential expression of hormone receptors (HR) (estrogen receptor (ER) and progesterone receptor (PR)), and human epidermal growth factor receptor 2 (HER2; ERBB2). These molecular markers have come to clinically define breast cancer, informing both patient prognosis and treatment paradigms (Turner et al., 2021). HR-positive (ER-positive and PR-positive) breast cancers usually have a favorable prognosis, but HR-negative breast cancers have a poor prognosis. PR is an estrogen-regulated gene; therefore, ER-positive tumors are usually also PR-positive, whereas ER-negative tumors are usually PR-negative (Bae et al., 2015). Also, around 20% of breast cancer patients have tumors that overexpress HER2 (National Cancer Institute, 2021). Triple-negative breast cancers (TNBC) constitute around 15–20% of all cases of breast cancer and do not express ER, PR and HER2. Patients with TNBC have worse overall survival than patients with non-TNBC (Amirkhani Namagerdi et al., 2020).
We have previously showed that the percentage of NINJ1 copy number loss events is the highest in ovarian cancer among other cancers, and that high expression of NINJ1 is associated with better overall survival in patients with ovarian cancer (Berkel and Cacan, 2023). However, NINJ1 has not been studied in the context of breast cancer, another hormone-regulated cancer type (Berkel and Cacan, 2021). Here, I showed that estrogen receptor (ER)-positive or progesterone receptor (PR)-positive breast cancer cells have higher NINJ1 transcript levels than ER-negative or PR-negative breast cancer cells, respectively. I also found that this increased mRNA levels of NINJ1 in ER-positive or PR-positive breast cancer cells is independent of menopausal status, and that tumor stage and race might influence NINJ1 expression levels in breast cancer. Finally, I showed that breast cancer patients with high NINJ1 expression have a more favorable prognosis in terms of multiple parameters compared to those with low NINJ1 expression. Combined, this study points that increased expression of NINJ1 in ER-positive or PR-positive breast cancer patients might contribute to more favorable prognosis observed in these patients, compared to, for instance, patients with triple-negative breast cancer (TNBC) which is the most malignant subtype with a poor prognosis.