Subcellular localization of pro-gastrin releasing peptide in human Ewing sarcoma cell lines

Progastrin-releasing peptide (ProGRP), a neuropeptide, is a stable tumor marker for small cell lung carcinoma (SCLC) and other neuroendocrine tumors, such as medullary thyroid carcinoma. It has recently been reported as highly expressed in Ewing sarcoma (ES) and as a useful marker for this type of tumor. However, the mechanisms underlying ProGRP production remain unclear. This study aimed to elucidate its subcellular localization in human ES cell lines. Three ES cell lines (A-673, SK-N-MC, and SK-NEP-1) and one SCLC line (DMS 53) were transplanted into athymic mice. After 3-4 weeks, the tumors were excised, and the expression and localization of ProGRP were examined using immunohistochemistry and immunoelectron microscopy. Immunohistochemistry revealed that all three ES cell lines expressed ProGRP in a dot-like pattern in their cytoplasm, whereas SCLC cells showed a diffuse granular pattern. Immunoelectron microscopy revealed that ProGRP localized in neurosecretory granules (NSGs) in SCLC cells and in autolysosome-like structures in ES cells, with no NSGs observed in the ES cell lines. ES cells secrete high levels of ProGRP, similar to SCLC cells. However, in ES cells, ProGRP is localized in autolysosome-like structures, unlike the NSGs in SCLC, suggesting that ProGRP in ES can be transported and secreted without following the typical pathway for neuropeptides.