Cooperative functions of ZnT1, metallothionein and ZnT4 in the cytoplasm are required for full activation of TNAP in the early secretory pathway

PLoS One. 2013 Oct 18;8(10):e77445. doi: 10.1371/journal.pone.0077445. eCollection 2013.

Abstract

The activation process of secretory or membrane-bound zinc enzymes is thought to be a highly coordinated process involving zinc transport, trafficking, transfer and coordination. We have previously shown that secretory and membrane-bound zinc enzymes are activated in the early secretory pathway (ESP) via zinc-loading by the zinc transporter 5 (ZnT5)-ZnT6 hetero-complex and ZnT7 homo-complex (zinc transport complexes). However, how other proteins conducting zinc metabolism affect the activation of these enzymes remains unknown. Here, we investigated this issue by disruption and re-expression of genes known to be involved in cytoplasmic zinc metabolism, using a zinc enzyme, tissue non-specific alkaline phosphatase (TNAP), as a reporter. We found that TNAP activity was significantly reduced in cells deficient in ZnT1, Metallothionein (MT) and ZnT4 genes (ZnT1(-/-) MT(-/-) ZnT4(-/-) cells), in spite of increased cytosolic zinc levels. The reduced TNAP activity in ZnT1(-/-) MT(-/-) ZnT4(-/-) cells was not restored when cytosolic zinc levels were normalized to levels comparable with those of wild-type cells, but was reversely restored by extreme zinc supplementation via zinc-loading by the zinc transport complexes. Moreover, the reduced TNAP activity was adequately restored by re-expression of mammalian counterparts of ZnT1, MT and ZnT4, but not by zinc transport-incompetent mutants of ZnT1 and ZnT4. In ZnT1(-/-) MT(-/-) ZnT4(-/-) cells, the secretory pathway normally operates. These findings suggest that cooperative zinc handling of ZnT1, MT and ZnT4 in the cytoplasm is required for full activation of TNAP in the ESP, and present clear evidence that the activation process of zinc enzymes is elaborately controlled.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alkaline Phosphatase / genetics*
  • Alkaline Phosphatase / metabolism
  • Animals
  • Avian Proteins / genetics*
  • Avian Proteins / metabolism
  • B-Lymphocytes / cytology
  • B-Lymphocytes / metabolism
  • Cation Transport Proteins / deficiency
  • Cation Transport Proteins / genetics*
  • Cell Line, Transformed
  • Chickens / genetics
  • Chickens / metabolism
  • Cytoplasm / metabolism
  • Enzyme Activation
  • Gene Expression Regulation
  • Metallothionein / deficiency
  • Metallothionein / genetics*
  • Protein Isoforms / deficiency
  • Protein Isoforms / genetics
  • Protein Transport
  • Secretory Pathway / genetics*
  • Signal Transduction / genetics*
  • Zinc / metabolism*

Substances

  • Avian Proteins
  • Cation Transport Proteins
  • Protein Isoforms
  • Metallothionein
  • Alkaline Phosphatase
  • Zinc

Grants and funding

This work was supported by grants-in-aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan and by the Mochida Memorial Foundation for Medical and Pharmaceutical Research, the Suzuken Memorial Foundation, the Takeda Science Foundation and the Japan Foundation for Applied Enzymology (to T. Kambe). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.