Autocrine inhibition of parathyroid cell secretion requires proteolytic processing of chromogranin A

Chromogranin A (CgA, Secretory Protein-I) is a protein of about 450 amino acids representing a major soluble component of the secretory granules of parathyroid and other endocrine and neuroendocrine cells. In the parathyroid, CgA is costored and cosecreted with parathormone (PTH). We earlier found that CgA and the derived peptide, pancreastatin, inhibited secretion of PTH and CgA by parathyroid cells in culture and that CgA antiserum stimulated secretion above the maximum achieved at low (0.5 mM) Ca²⁺. In the present study, porcine parathyroid cells were incubated at different cell concentrations at low Ca²⁺. The amount of secreted CgA increased over the 6-h incubation period at 1 × 10⁶ to 4 × 10⁶ cells/ml, but plateaued after 3 h at 6 × 10⁶ cells/ml. Secretion did not plateau when antiserum to CgA and pancreastatin were present at the start of incubation or resumed when the antisera were added at 3 h. Conditioned medium contained a factor or factors that blocked secretion by fresh parathyroid cells at 0.5 mM Ca²⁺. Pulse-chase studies revealed that 40% of the secreted CgA was processed after 6 h of chase, α-2-macroglobulin, an inhibitor of proteolytic processing, increased the amount of CgA in the medium by 30% at 1 h of chase and decreased the amount processed to 20% by 6 h. Other protease inhibitors similarly enhanced the amount of CgA in the medium. These data indicate that proteolytic processing of intact CgA is requisite for its autocrine inhibitory activity.