The cholesterol metabolism of neuroendocrine cells and crosstalk of the hormone secretion

在分泌之前，受调节的肽类激素在内分泌细胞中的反式高尔基体网络（trans-Golgi network，TGN）处被选择性地分选为分泌颗粒（secretory granule，SG）。分泌颗粒蛋白III（SgIII）似乎通过将含有嗜铬颗粒蛋白A（CgA）和肽激素的蛋白质聚集体拴系到富含胆固醇的SG膜（SGM）来促进SG分选过程。在这里，我们评估的作用，SgIII在SG分选在AtT-20细胞转染小干扰RNA靶向SgIII。在SgIII敲低细胞中，CgA的细胞内滞留大大受损，并且仅痕量的CgA定位在TGN中形成的空泡内，证实了SgIII在含CgA的聚集体的束缚和适当SG形态的建立中的意义。虽然阿黑皮素原（POMC）的细胞内滞留在SgIII敲低细胞大大受损，残留的ACTH/POMC仍然定位于一些少数剩余的SGs与另一种颗粒蛋白，分泌颗粒蛋白II（SgII），并以调节的方式分泌。生化分析表明，SgII直接绑定到SGM的胆固醇依赖性的方式，并能够保留POMC的聚集形式，揭示了潜在的冗余SG分选和保留机制，确保调节分泌的生物活性肽。

Prior to secretion, regulated peptide hormones are selectively sorted to secretory granules (SGs) at the trans-Golgi network (TGN) in endocrine cells. Secretogranin III (SgIII) appears to facilitate SG sorting process by tethering of protein aggregates containing chromogranin A (CgA) and peptide hormones to the cholesterol-rich SG membrane (SGM). Here, we evaluated the role of SgIII in SG sorting in AtT-20 cells transfected with small interfering RNA targeting SgIII. In the SgIII-knockdown cells, the intracellular retention of CgA was greatly impaired, and only a trace amount of CgA was localized within the vacuoles formed in the TGN, confirming the significance of SgIII in both the tethering of CgA-containing aggregates and the establishment of the proper SG morphology. Although the intracellular retention of proopiomelanocortin (POMC) was considerably impaired in SgIII-knockdown cells, residual ACTH/POMC was still localized to some few remaining SGs together with another granin protein, secretogranin II (SgII), and was secreted in a regulated manner. Biochemical analyses indicated that SgII bound directly to the SGM in a cholesterol-dependent manner and was able to retain the aggregated form of POMC, revealing a latent redundancy in the SG sorting and retention mechanisms, that ensures the regulated secretion of bioactive peptides.