Characterisation of FAP-1 expression and CD95 mediated apoptosis in the A818-6 pancreatic adenocarcinoma differentiation system

The present study investigated the expression and localisation of FAP-1 (Fas associated phosphatase-1) and CD95 in a 3D differentiation model in comparison to 2D monolayers of the pancreatic adenocarcinoma cell line A818-6. Under non-adherent growth conditions, A818-6 cells differentiate into 3D highly organised polarised epithelial hollow spheres, resembling duct-like structures. A818-6 cells showed a differentiation-dependent FAP-1 localisation. Cells grown as 2D monolayers revealed FAP-1 staining in a juxtanuclear cisternal position, as well as localisation in the nucleus. After differentiation into hollow spheres, FAP-1 was relocated towards the actin cytoskeleton beneath the outer plasma membrane of polarised cells and no further nuclear localisation was observed. CD95 surface staining was found only in a subset of A818-6 monolayer cells, while differentiated hollow spheres appeared to express CD95 in all cells of a given sphere. We rarely observed co-localisation of CD95 and FAP-1 in A818-6 monolayer cells, but strong co-localisation beneath the outer plasma membrane in polarised cells. Analysis of surface expression by flow cytometry revealed that only a subset (36%) of monolayer cells showed CD95 surface expression, and after induction of hollow spheres, CD95 presentation at the outer plasma membrane was reduced to 13% of hollow spheres. Induction of apoptosis by stimulation with agonistic anti-CD95 antibodies, resulted in increased caspase activity in both, monolayer cells and hollow spheres. Knock down of FAP-1 mRNA in A818-6 monolayer cells did not alter resposiveness to CD95 agonistic antibodies. These data suggested that CD95 signal transduction was not affected by FAP-1 expression in A818-6 monolayer cells. In differentiated 3D hollow spheres, we found a polarisation-induced co-localisation of CD95 and FAP-1. A tight control of receptor surface representation and signalling induced apoptosis ensures controlled removal of individual cells instead of a "snowball effect" of apoptotic events.