Membrane Proximal Ectodomain Cleavage of MUC16 Occurs in the Acidifyinggolgi/post-Golgi Compartments

MUC16, precursor of the most widely used ovarian cancer biomarker CA125, is upregulated in multiple malignancies and is associated with poor prognosis. While thepro-tumorigenic and metastatic roles of MUC16 are ascribed to the cell-associatedcarboxyl-terminal MUC16 (MUC16-Cter), the exact biochemical nature of MUC16 cleavagegenerating MUC16-Cter has remained unknown. Using different lengths of dual-epitope(N-terminal FLAG- and C-terminal HA-Tag) tagged C-terminal MUC16 fragments, wedemonstrate that MUC16 cleavage takes place in the juxta-membrane ectodomain stretchof twelve amino acids that generates a ~17 kDa cleaved product and isdistinct from the predicted sites. This was further corroborated by domain swappingexperiment. Further, the cleavage of MUC16 was found to take place in theGolgi/post-Golgi compartments and is dependent on the acidic pH in the secretorypathway. A similar pattern of ~17 kDa cleaved MUC16 was observed inmultiple cell types eliminating the possibility of cell type specific phenomenon.MUC16-Cter translocates to the nucleus in a cleavage dependent manner and binds tothe chromatin suggesting its involvement in regulation of gene expression. Takentogether, we demonstrate for the first time the oft-predicted cleavage of MUC16 thatis critical in designing successful therapeutic interventions based on MUC16.