Abstract
The protein kinase D (PKD) family of serine/threonine kinases, which can be activated by gastrointestinal hormones, consists of three distinct isoforms that modulate a variety of cellular processes including intracellular protein transport as well as constitutive and regulated secretion. Although isoform-specific functions have been identified in a variety of cell lines, the expression and function of PKD isoforms in normal, differentiated secretory tissues is unknown. Here, we demonstrate that PKD isoforms are differentially expressed in the exocrine and endocrine cells of the pancreas. Specifically, PKD3 is the predominant isoform expressed in exocrine cells of the mouse and human pancreas, whereas PKD1 and PKD2 are more abundantly expressed in the pancreatic islets. Within isolated mouse pancreatic acinar cells, PKD3 undergoes rapid membrane translocation, trans-activating phosphorylation, and kinase activation after gastrointestinal hormone or cholinergic stimulation. PKD phosphorylation in pancreatic acinar cells occurs viaaCa2+-independent, diacylglycerol- and protein kinase C-dependent mechanism. PKD phosphorylation can also be induced by physiologic concentrations of secretagogues and by in vivo stimulation of the pancreas. Furthermore, activation of PKD3 potentiates MEK/ERK/RSK (RSK, ribosomal S6 kinase) signaling and significantly enhances cholecystokinin-mediated pancreatic amylase secretion. These findings reveal a novel distinction between the exocrine and endocrine cells of the pancreas and further identify PKD3 as a signaling molecule that promotes hormone-stimulated amylase secretion.
Highlights
Recent functional studies have shown that protein kinase D (PKD) isoforms differentially regulate exocytic protein trafficking and cargo specificity [9, 12,13,14]
We demonstrated that PKD1 mediates NT peptide secretion from a pancreas-derived neuroendocrine cell line, BON, and that PKD1 activation is regulated by protein kinase C (PKC) and Rho/Rho kinase pathways [4]; PKD1 and PKD2 isoforms are highly expressed in this endocrine cell line with little to no PKD3 expression, suggesting that PKD1/2 may be the predominant isoforms for endocrine secretion
We have previously identified a critical role for PKD1 in stimulated hormone secretion from the BON endocrine cell line [4]
Summary
Recent functional studies have shown that PKD isoforms differentially regulate exocytic protein trafficking and cargo specificity [9, 12,13,14]. E, isolated human pancreatic acini from a different donor were treated with carbachol of the indicated concentration for 30 min and analyzed by Western blotting using anti-phospho- PKD and total PKD antibodies on parallel blots. To further identify potential upstream signaling kinases that contribute to PKD phosphorylation, we analyzed the effects of inhibiting three pathways (i.e. PI3K/Akt, MEK/ERK, PKC) that are activated by CCK stimulation of pancreatic acini [54].
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