Phosphorylation of the rat pancreatic bile-salt-dependent lipase by caseinkinase II is essential for secretion

Bile-salt-dependent lipase (BSDL, EC 3.1.1.-) is an enzyme expressed by the pancreatic acinar cells and secreted as a component of the pancreatic juic e of all examined species. During its secretion route BSDL is associated wi th intracellular membranes. This association allows the complete glycosylat ion of the enzyme or participates in the inhibition of the enzyme activity, which can deleterious for the acinar pancreatic cell. Thereafter, the huma n BSDL is phosphorylated by a serine/threonine protein kinase and released from intracellular membranes. In the present study, we show that the rat pa ncreatic BSDL, expressed by AR4-2J cells used as a model, is phosphorylated by a protein kinase that is insensitive to inhibitors of protein kinases A , C or G and that the phosphorylation process is favoured by okadaic acid ( an inhibitor of protein phosphatases 1 and 2A). However, 5,6-dichloro-1-bet a-D-ribofuranosylbenzimidazole (DRB), which is a specific inhibitor of case in kinase II, abolishes the phosphorylation in vitro of BSDL within microso mes of AR4-2J pancreatic cells. We showed further that the alpha-subunit of casein kinase II co-locates with BSDL within the lumenal compartment of th e Golgi. Genistein, which perturbs the trans-Golgi network, also inhibits t he phosphorylation of BSDL, suggesting that this post-translational modific ation of BSDL probably occurred within this cell compartment. The inhibitio n of the phosphorylation of BSDL by DRB also decreases the rate at which th e enzyme is secreted. Under the same conditions, the rate of alpha-amylase secretion was not modified. These data strongly suggest that phosphorylatio n is a posttranslational event, which appears to be essential for the secre tion of BSDL.