Endoplasmic reticulum stress induces secretion of high-mobility group proteins and is associated with tumor-infiltrating lymphocytes in triple-negative breast cancer

BACKGROUND Although the prognostic and predictive significance of tumor-infiltrating lymphocytes (TILs) in triple-negative breast cancer (TNBC) have been shown, the cause of the TIL influx is unclear. Here, we investigated whether extracellular secretion of HMGN1 is associated with TIL influx, as well as increased endoplasmic reticulum stress (ERS), in human TNBC. METHODS We reviewed the slides of 767 patients with TNBC and evaluated the TIL levels. We also assessed the expression of HMGs and several ERS-associated molecules using immunohistochemical staining. Western blot analysis of human TNBC cell lines and pharmacological ERS inducers was used to determine if HMGN1 migrates from the nucleus to the extracellular space in response to ERS. RESULTS On immunohistochemical staining, either higher nuclear or cytoplasmic expression of both HMGB1 and HMGN1 was significantly associated with ERS. TILs showed a positive correlation with the cytoplasmic expression of the HMGs. Western blot analysis of TNBC cell lines showed that ERS induction resulted in the secretion of HMG proteins. CONCLUSIONS This is the first study to elucidate the associations among ERS, secretion of HMGs, and degree of TILs in TNBCs. Understanding the mechanisms of TIL influx will help in the development of effective immunotherapeutic agents for TNBC.