Protein kinase D2 regulates chromogranin A secretion in human BON neuroendocrine tumour cells

Abstract

22165 Background: Chromogranin A is a member of the granin family of acidic secretory glycoproteins that is found in secretory granules of many endocrine cells including neuroendocrine tumour cells. This hormone serves as a model system for autonomous hormone secretion by the so called functional neuroendocrine tumours of the gastrointestinal tract. The protein kinase D (PKD) family of serine threonine kinases has so far been implicated in the regulation of constitutive secretion in epithelial cells. Here we examined whether PKD2 expression and activity could also play a role in the release of secretory granules from the trans Golgi network (TGN) in neuroendocrine tumour cells and hence be a target to block autonomous secretion by these tumours. Methods: BON neuroendocrine tumour cells are the only established model for studying the biology of these rare type of tumours. We used invivo fluorescence microscopy in order to study the distribution and transport of hormone containing vesicles in these cells in response to stimulation with phorbol esters or in response to changes in the expression and/or activity of PKD2. In addition, secretion and activity/expression of PKD2 was quantitatively assessed by western blot analysis and immunoprecipitation- experiments. Results: Our data show that expression and catalytic activity of PKD2, and not of PKD1 and PKD3, are required for the release of chromogranin A containing secretory vesicles. Inhibition of PKD2 activity or siRNA knockdown of PKD2 resulted in a marked perinuclear retention of chromogranin A immunofluorescence in the trans Golgi network and led to a marked reduction in basal as well as phorbol ester stimulated secretion of chromogranin A into the supernatant of cells. Interestingly, PKD2 expression and activity were not required for the control of microtubulus-associated transport (speed and directionality) of hormon containing vesicels. Conclusion: PKD2 controls the release of secretory granules in neuroendocrine tumour cells at the level of the Golgi apparatus and could hence serve as a novel target to block hormone secretion in functional neuroendocrine tumours. No significant financial relationships to disclose.