Phosphorylation dysregulation has been implicated in various diseases including
cancer. The phosphorylation change of
proteins in secretome may be a novel source for the discovery of
biomarkers and
drug targets. In this study, the
phosphoproteins in
cancer secretome (phosphosecretome) were globally analyzed for the first time by phosphoproteomics. One hundred forty-two phosphorylation sites on 62 unique
phosphopeptides representing 49 nonredundant
proteins were identified, several of which are known as secreted
proteins involved in
carcinogenesis, invasion, and
metastasis. Most of them were first found as secreted
proteins with no previously known function.
Protein sublocation analysis showed that 33
proteins were found to be secreted as
phosphoproteins, in which 27 (81.81%) were secreted by a nonclassic, ER/Golgi-independent pathway, suggesting that the phosphorylation modification of these
proteins might play an important role in their nonconventional secretion processes. Their
protein kinases and regulatory phosphosites involved in the secretion regulation of these
phosphoproteins, such as
stanniocalcin 2,
annexin A2, and HSP90 alphạ, were first identified. The phosphosecretome data enriched the secretome database and phosphoproteome database, and will help us to discover
cancer biomarkers and
drug targets, illustrating the mystery of the nonclassic
protein secretion pathway.