Bourboulia, Dimitra; Sánchez Pozo, Javier (2018)
      Matrix metalloproteinases (MMPs) are secreted zinc-dependent endopeptidases that are involved in many extracellular biological processes due to their matrix-degrading function. The majority of theseenzymes are released intothe extracellular space in their inactive form and require activation. The tissue inhibitors of metalloproteinases (TIMPs) are also secreted proteins and mainly function to inhibit all members of the MMP family. Interestingly, TIMP-2 also participates in the activationprocessof proMMP-2. Although the interaction between TIMP-2 and proMMP-2 has been known for decades, the molecular signal that triggersthis association has only recently beendetermined. Studies in our lab haveshown that TIMP-2 is tyrosine phosphorylatedby the c-Srctyrosinekinase. Also, phosphorylation of TIMP-2 Tyr90is essential for its interaction with proMMP-2in vivo. Our hypothesis is that c-Src-mediated TIMP-2 phosphorylation happens outside the cell. Here, wedemonstratethat TIMP-2 and c-Src are secreted through different secretory pathways and that TIMP-2 phosphorylation takes place in the extracellular space. Our workalso showsthatextracellularc-Srcisactive, reinforcing the fact that phosphorylation can happen extracellularly. We also hypothesize that extracellular c-Src plays a critical role in facilitating TIMP-2:proMMP-2 interaction. We first confirmed thatTIMP-2 and proMMP-2 endogenously interact only in cells containing endogenous c-Src. This interaction,as well as TIMP-2 phosphorylation,was blocked by treating cells with acustom-made anti-c-Src polyclonal antibody (pAb)that targets amino acids 84-110. We also showthat ananti-c-Src antibody that targets the first 79 amino acids does not inhibit TIMP-2 phosphorylation and interaction with proMMP-2. Therefore, since TIMP-2:proMMP-2 complex formation promotes proMMP-2 activation, we hypothesize that c-Src is an essential player in this process. Our data showsthatthe non-phosphorylatable TIMP-2Tyr90mutant does not promote proMMP-2 activation. Furthermore, pretreatment with the anti-c-Src pAbblockedTIMP-2-mediated proMMP-2 activation, whereasthe anti-c-Src mAb6 did not affect proMMP-2 activation. Overall, these findings provide further evidence that secreted c-Src-mediated TIMP-2 phosphorylation occurs in the extracellular space, where thesecretedkinase is also active. Moreover, c-Src is essential for TIMP-2:proMMP-2 complex formation as well as proMMP-2 activation.