Secretion of transferrin from rat parotid acinar cells

Objectives: Transferrin, major iron transporter in blood plasma, is included in saliva at a lower concentration than in plasma. Salivary transferrin has been reported as a marker of contamination of blood, whereas, little has been known about its origin, role in saliva, and secretory pathway if it is secreted from salivary glands. We studied synthesis and secretion of transferrin in rat parotid acinar cells and demonstrated secretory pathways of endogenous and exogenous transferrin.

Methods: Acinar cells were prepared from rat parotid glands by enzyme digestion using trypsin and collagenase. Subcellular fractionation and immunohistochemical study were performed according to the methods described previously by us. Uptake of exogenous transferrin was performed using biotin-transferrin; acinar cells were incubated in medium with biotin-transferrin (100 µg/ml) at 37^oC for 1 h, washed out with PBS containing 25 mM acetic acid, and incubated in serum-free medium for 1 h.

Results: Transferrin was identified from incubation medium of parotid acinar cells by protein sequence analysis and Western blotting. Transferrin mRNA was detected in rat parotid acinar cells by reverse transcription/polymerase chain reaction. Transferrin was localized in secretory granules/vesicles and apical plasma membranes but not in basolateral membranes. When parotid acinar cells were incubated with biotin-transferrin at 4^oC for 1 h, biotin-transferrin was detected in basolateral membranes and not in apical membranes. After uptake of biotin-transferrin, biotin-transferrin was detected in incubation medium and in some subcellular fractions including apical membranes but not in secretory granules.

Conclusions: Endogenous transferrin was transported to secretory granules, and secreted from apical membranes in rat parotid acinar cells. Exogenous transferrin was internalized from basolateral side and secreted from apical side via the different pathway from endogenous transferrin