Salivary Histatin 1 and 2 Are Targeted to Mitochondria and Endoplasmic Reticulum in Human Cells

Human salivary histatin 1 (Hst1) and Hst2 exhibit a series of cell-activating properties (e.g., promoting adhesion, spreading, migration and metabolic activity of mammalian cells). In contrast, Hst5 shows an anti-fungal property but no cell-activating properties. Previous findings suggest that their uptake and association with subcellular targets may play a determinant role in their functions. In this study, we studied the uptake dynamics and subcellular targets of Hst1, Hst2 and Hst5 in epithelial cells (HO1N1 human buccal carcinoma epithelial cell line). Confocal laser scanning microscopy (CLSM) revealed that fluorescently labeled Hst1 (F-Hst1) was taken up into the intracellular space of epithelial cells. Then, 60 min post-incubation, the total fluorescence of cell-associated F-Hst1, as measured using flow cytometry, was significantly higher compared to those of F-Hst2 and F-Hst5. In contrast, virtually no association occurred using the negative control-scrambled F-Hst1 (F-Hst^scr). CLSM images revealed that F-Hst1, 2 and 5 co-localized with mitotracker^TM-labeled mitochondria. In addition, F-Hst1 and F-Hst2 but neither F-Hst5 nor F-Hst1^scr co-localized with the ER-tracker^TM-labeled endoplasmic reticulum. No co-localization of Hst1, 2 and 5 with lysosomes or the Golgi apparatus was observed. Furthermore, Hst1 and Hst2 but not Hst5 or Hst1^scr significantly promoted the metabolic activity of both human epithelial cell lines, HaCaT human keratinocytes and primary human gingival fibroblasts.