Phosphorylation of SNAP-25 on serine-187 is induced by secretagogues in insulin-secreting cells, but is not correlated with insulin secretion

The tSNARE (the target-membrane soluble NSF-attachment protein receptor, where NSF is N-ethylmaleimide-sensitive fusion protein) synaptosomal-associated protein of 25kDa (SNAP-25) is implicated in regulated insulin secretion. In pheochromocytoma PC12 cells, SNAP-25 is phosphorylated at Ser¹⁸⁷, which lies in a region that is important for its function. The aims of the present study were to determine whether SNAP-25 is phosphorylated at Ser¹⁸⁷ in insulin-secreting cells and, if so, whether this is important for regulated insulin secretion. The major findings are: (i) SNAP-25 is rapidly and reversibly phosphorylated on Ser¹⁸⁷ in both rat insulinoma INS-1 cells and rat islets in response to the phorbol ester, PMA; (ii) less than 35% of SNAP-25 in INS-1 cells is phosphorylated in response to PMA, and phosphorylation is limited to plasma-membrane-associated SNAP-25; (iii) both SNAP-25 isoforms (a and b) are phosphorylated, with 1.8-fold greater phosphorylation for SNAP-25b in response to PMA; (iv) in rat islets, Ser¹⁸⁷ phosphorylation is stimulated by glucose or carbachol, albeit to a lesser extent than by PMA, but not by cAMP; (v) insulin secretion from botulinum neurotoxin E-treated hamster insulinoma tumour (HIT) cells, transfected with toxin-resistant Ser¹⁸⁷→Ala or Ser¹⁸⁷→Asp mutant SNAP-25, was similar to that of wild-type HIT cells. Furthermore, in rat islets no correlation was found between the extent of SNAP-25 phosphorylation at Ser¹⁸⁷ in response to secretagogues and stimulation of insulin release; (vi) use of protein kinase C (PKC) inhibitors suggests that glucose stimulates SNAP-25 phosphorylation via conventional and non-conventional PKC isoforms. In summary, although SNAP-25 phosphorylation at Ser¹⁸⁷ occurs in insulin-secreting cells and is mediated by PKC, it does not appear to play a major role in regulated insulin secretion.