Dissecting the Process of Activation of Cancer-promoting Zinc-requiring Ectoenzymes by Zinc Metalation Mediated by ZNT Transporters*

Abstract

Zinc-requiring ectoenzymes, including both secreted and membrane-bound enzymes, are considered to capture zinc in their active site for their activation in the early secretory pathway. This idea has been confirmed by our studies conducted using tissue-nonspecific alkaline phosphatase (TNAP), which is elaborately activated by means of a two-step mechanism by zinc transporter 5 (ZNT5)-ZNT6 heterodimers and ZNT7 homodimers, through protein stabilization followed by enzyme activation with zinc in the early secretory pathway. However, the molecular basis of the activation process in other zinc-requiring ectoenzymes remains largely unknown. In this study, we investigated this activation process by using three cancer-promoting zinc-requiring ectoenzymes, autotaxin (ATX), matrix metalloproteinase 9 (MMP9), and carbonic anhydrase IX (CAIX), and the chicken DT40 cell mutants that we generated; we specifically focused on clarifying whether the same or a similar activation mechanism operates in these ectoenzymes. ATX activation required ZNT5-ZNT6 heterodimers and ZNT7 homodimers in a manner similar to TNAP activation, although the protein stability of ATX was differently regulated from that of TNAP. MMP9 required ZNT5-ZNT6 heterodimers and ZNT7 homodimers for its activation as well as secretion; MMP9 was not secreted into the spent medium unless both zinc-transport complexes were present. Finally, CAIX activation by zinc was mediated not only by ZNT5-ZNT6 heterodimers and ZNT7 homodimers but also by ZNT4 homodimers; thus, these three zinc-transport complexes redundantly contribute to CAIX activation. Our results provide pivotal insights into the activation processes of zinc-requiring ectoenzymes, and furthermore, they offer novel insights for potential cancer therapy applications given the cancer-promoting potencies of ATX, MMP9, and CAIX.