Integrin adhesion in regulation of lacrimal gland acinar cell secretion

The extracellular microenvironment regulates lacrimal gland acinar cell secretion. Culturing isolated rabbit lacrimal gland acinar cells on different extracellular matrix proteins revealed that laminin enhances carbachol-stimulated secretion to a greater extent than other extracellular matrix proteins investigated. Furthermore, immunofluorescence indicated that integrin subunits, potentially functioning as laminin receptors are present in acinar cells. Among these, the integrin α6 and β1 subunit mRNA expression was also confirmed by RT–PCR and sequence analysis. Secretion assays, which measured β-hexosaminidase activity released in the culture media, demonstrated that function-blocking integrin α6 and β1 monoclonal antibodies (mAbs) induce a rapid, transient and dose-dependent secretory response in cultured cells. To determine the intracellular pathways by which integrin α6 and β1 mAbs could induce secretion, selected second messenger molecules were inhibited. Although inhibitors of protein kinase C and IP3-induced Ca2+ mobilization attenuated carbachol-stimulated secretion, no effect on integrin mAb-induced release was observed. In addition, protein tyrosine kinases do not appear to have a role in transducing signals arising from mAb interactions. Our data clearly demonstrate, though, that cell adhesion through integrins regulates secretion from lacrimal gland acinar cells. The fact that the integrin mAbs affect the cholinergic response differently and that the integrin β1 mAb secretion, but not the α6, was attenuated by the phosphatase inhibitor, sodium orthovanadate, suggests that each subunit utilizes separate intracellular signaling pathways to induce exocytosis. The results also indicate that the secretory response triggered by the β1 integrin mAb is generated through dephosphorylation events.