Fujita-Yoshigaki J, Matsuki-Fukushima M, Sugiya H. Inhibition of Src and p38 MAP kinases suppresses the change of claudin expression induced on dedifferentiation of primary cultured parotid acinar cells.

Aquaporins (AQPs) are integral membrane proteins that function as channels for the transfer of water and small solutes across membranes. In mammalian cells, 13 isoforms (AQP0-12) have been identified, and these exhibit unique patterns of expression in various cell types and tissues. Among these isoforms, AQP6 is considered to function not as water channel, but as an anion channel. We investigated the presence and localization of AQP6 in rat parotid acinar cells. AQP6 mRNA was detected in these cells by using reverse transcription/polymerase chain reaction, and Western blotting analysis identified a protein band that reacted with an anti-AQP6 antibody in the membrane fraction and secretory granule membrane. In order to localize AQP6, we used the anti-AQP6 antibody for histological immunodetection. Under confocal microscopy, we observed positive immunoreactions near the tight junctions of parotid acinar cells. Immunolabeling of ultrathin cryosections detected AQP6 near tight junctions and around secretory granule membranes. Immunoelectron microscopy confirmed the presence of AQP6 in the membranes of isolated secretory granules. These results suggest that AQP6 participates in water and anion transport in plasma membranes near tight junctions and secretory granule membranes in rat parotid acinar cells.

Myristoylated alanine-rich C kinase substrate (MARCKS) is known as a major cellular substrate for protein kinase C (PKC). MARCKS has been implicated in the regulation of brain development and postnatal survival, cellular migration and adhesion, as well as phagocytosis, endocytosis, and exocytosis. The involvement of MARCKS phosphorylation in secretory function has been reported in Ca(2+)-mediated exocytosis. In rat parotid acinar cells, the activation of beta-adrenergic receptors provokes exocytotic amylase release via accumulation of intracellular cAMP levels. Here, we studied the involvement of MARCKS phosphorylation in the cAMP-dependent amylase release in rat parotid acinar cells. MARCKS protein was detected in rat parotid acinar cells by Western blotting. The beta-adrenergic agonist isoproterenol (IPR) induced MARCKS phosphorylation in a time-dependent manner. Translocation of a part of phosphorylated MARCKS from the membrane to the cytosol and enhancement of MARCKS phosphorylation at the apical membrane site induced by IPR were observed by immunohistochemistry. H89, a cAMP-dependent protein kinase (PKA) inhibitor, inhibited the IPR-induced MARCKS phosphorylation. The PKCdelta inhibitor rottlerin inhibited the IPR-induced MARCKS phosphorylation and amylase release. IPR activated PKCdelta, and the effects of IPR were inhibited by the PKA inhibitors. A MARCKS-related peptide partially inhibited the IPR-induced amylase release. These findings suggest that MARCKS phosphorylation via the activation of PKCdelta, which is downstream of PKA activation, is involved in the cAMP-dependent amylase release in parotid acinar cells.

No abstract

In exocrine glands, secretory proteins synthesized in the rough endoplasmic reticulum (RER) exhibit vectorial transport from ER through a succession of membrane-bounded components such as Golgi complex, condensing vacuoles and secretory granules. The secretory granules migrate to particular locations within the cell close to the apical membrane prior to the release of their contents into the acinar lumen. Currently, to release intragranular contents, secretory granules have been demonstrated to transiently dock and fuse at ‘porosome’, a permanent cup-shaped structures at the cell membranes. Then swelling of secretory granules occurs to allow explusion of intragranular contents. In this process, water and ion fluxes in the granule membrane appear to contribute to maintain secretory granule integrity and morphology via osmoregulation in secretory granules. Aquaporins (AQPs) are a family of small, hydrophobic, integral membrane proteins, which function as channels to permeate water and small solutes. The AQPs reside constitutively at the plasma membrane in most cell types. However, recent studies have demonstrated that the AQPs are present in secretory granules in exocrine glands, synaptic vesicles and intracellular vesicles in liver and kidney, implying that AQPs in secretory granules and vesicles are involved in their volume regulation. This paper reviews the possible role of AQPs on secretory granules, especially in exocrine glands, in secretory function.