Keigo Nishida

Essential Role for Zinc Transporter 2 (ZnT2)-mediated Zinc Transport in Mammary Gland Development and Function during Lactation

Background: ZnT2 is expressed in non-secreting and secreting mammary epithelium; however, the physiological role is not understood. Results: ZnT2-null mice have impaired mammary expansion and compromised mammary differentiation and milk secretion during lactation. Conclusion: ZnT2-mediated zinc transport is critical for mammary development and function during lactation. Significance: This study identifies novel consequences of ZnT2 function in the mammary gland. The zinc transporter ZnT2 (SLC30A2) imports zinc into vesicles in secreting mammary epithelial cells (MECs) and is critical for zinc efflux into milk during lactation. Recent studies show that ZnT2 also imports zinc into mitochondria and is expressed in the non-lactating mammary gland and non-secreting MECs, highlighting the importance of ZnT2 in general mammary gland biology. In this study we used nulliparous and lactating ZnT2-null mice and characterized the consequences on mammary gland development, function during lactation, and milk composition. We found that ZnT2 was primarily expressed in MECs and to a limited extent in macrophages in the nulliparous mammary gland and loss of ZnT2 impaired mammary expansion during development. Secondly, we found that lactating ZnT2-null mice had substantial defects in mammary gland architecture and MEC function during secretion, including fewer, condensed and disorganized alveoli, impaired Stat5 activation, and unpolarized MECs. Loss of ZnT2 led to reduced milk volume and milk containing less protein, fat, and lactose compared with wild-type littermates, implicating ZnT2 in the regulation of mammary differentiation and optimal milk production during lactation. Together, these results demonstrate that ZnT2-mediated zinc transport is critical for mammary gland function, suggesting that defects in ZnT2 not only reduce milk zinc concentration but may compromise breast health and increase the risk for lactation insufficiency in lactating women.

Zinc-binding metallothioneins are key modulators of IL-4 production by basophils.

Zinc (Zn) is an essential nutrient, and Zn deficiency causes immunodeficiency and skin disorders. Basophils express FcɛRI on their surface and release multiple mediators after receptor cross-linking, including large amounts of IL-4. However, the mechanisms involved in the FcɛRI-mediated regulation of basophil IL-4 production are currently unclear. Here, we show that the Zn-binding metallothionein (MT) proteins are essential for the FcɛRI-induced basophil production of IL-4. Basophils from MT-I/II(-/-) mice produced significantly less FcɛRI-induced IL-4 than did wild-type basophils. The MTs were involved in maintaining intracellular Zn levels, thereby regulated the calcineurin activity and nuclear factor of activated T-cell (NFAT)-mediated IL-4 production. These results suggest that the MT-dependent control of Zn homeostasis is a novel mechanism for regulating basophil IL-4 production.

Gab1 adaptor protein acts as a gatekeeper to balance hepatocyte death and proliferation during acetaminophen-induced liver injury in mice.

Acetaminophen (APAP) overdose is the leading cause of drug‐induced acute liver failure. In APAP‐induced acute liver failure, hepatocyte death and subsequent liver regeneration determines the prognosis of patients, making it necessary to identify suitable therapeutic targets based on detailed molecular mechanisms. Grb2‐associated binder 1 (Gab1) adaptor protein plays a crucial role in transmitting signals from growth factor and cytokine receptors to downstream effectors. In this study, we hypothesized that Gab1 is involved in APAP‐induced acute liver failure. Hepatocyte‐specific Gab1 conditional knockout (Gab1CKO) and control mice were treated with 250 mg/kg of APAP. After APAP treatment, Gab1CKO mice had significantly higher mortality and elevated serum alanine aminotransferase levels compared to control mice. Gab1CKO mice had increased hepatocyte death and increased serum levels of high mobility group box 1, a marker of hepatocyte necrosis. In addition, Gab1CKO mice had reduced hepatocyte proliferation. The enhanced hepatotoxicity in Gab1CKO mice was associated with increased activation of stress‐related c‐Jun N‐terminal kinase (JNK) and reduced activation of extracellular signal‐regulated kinase and AKT. Furthermore, Gab1CKO mice showed enhanced mitochondrial translocation of JNK accompanied by an increase in the release of mitochondrial enzymes into the cytosol, which is indicative of increased mitochondrial dysfunction and subsequent nuclear DNA fragmentation. Finally, in vitro experiments showed that Gab1‐deficient hepatocytes were more susceptible to APAP‐induced mitochondrial dysfunction and cell death, suggesting that hepatocyte Gab1 is a direct target of APAP‐induced hepatotoxicity. Conclusion: Our current data demonstrate that hepatocyte Gab1 plays a critical role in controlling the balance between hepatocyte death and compensatory hepatocyte proliferation during APAP‐induced liver injury. (Hepatology 2016;63:1340–1355)

Loss of Gab1 adaptor protein in hepatocytes aggravates experimental liver fibrosis in mice

Grb2-associated binder 1 (Gab1) adaptor protein amplifies signals downstream of a broad range of growth factors/receptor tyrosine kinases. Although these signals are implicated in liver fibrogenesis, the role of Gab1 remains unclear. To elucidate the role of Gab1, liver fibrosis was examined in hepatocyte-specific Gab1-conditional knockout (Gab1CKO) mice upon bile duct ligation (BDL). Gab1CKO mice developed exacerbated liver fibrosis with activation of hepatic myofibroblasts after BDL compared with control mice. The antifibrotic role of hepatocyte Gab1 was further confirmed by another well-established mouse model of liver fibrosis using chronic injections of carbon tetrachloride. After BDL, Gab1CKO mice also displayed exacerbated liver injury, decreased hepatocyte proliferation, and enhanced liver inflammation. Furthermore, cDNA microarray analysis was used to investigate the potential molecular mechanisms of the Gab1-mediated signal in liver fibrosis, and the fibrosis-promoting factor chemokine (C-C motif) ligand 5 (Ccl5) was identified as upregulated in the livers of Gab1CKO mice following BDL. Interestingly, in vitro studies using primary hepatocytes isolated from control and Gab1CKO mice revealed that the loss of Gab1 resulted in increased hepatocyte CCL5 synthesis upon lipopolysaccharide stimulation. Finally, pharmacological antagonism of CCL5 reduced BDL-induced liver fibrosis in Gab1CKO mice. In conclusion, our results demonstrate that hepatocyte Gab1 is required for liver fibrosis and that hepatocyte CCL5 could be an important contributor to this process. Thus, we present a novel antifibrotic function of hepatocyte Gab1 in liver fibrogenesis.

Zinc Signaling by “Zinc Wave”

Zinc (Zn) is an essential heavy metal for all organisms. Zn homeostasis is maintained in mammalian cells through the activity of Zn transporters and Zn-permeable channels and through metallothionein expression levels. Zn is important in nucleic acid metabolism, cell replication, and tissue growth and repair. Zn deficiency is associated with a wide range of pathological conditions, such as impaired immunity, growth retardation, disorders in brain development, and delayed wound healing. Zn binds and affects the activity of several signaling molecules and of transcription factors that have a Zn-binding motif. However, whether Zn itself, as does calcium, acts as an intracellular signaling molecule has been a point of speculation. Recently, we and other groups have demonstrated that Zn does indeed act as an intracellular signaling molecule, converting extracellular stimuli to intracellular signals and controlling various cell functions. This chapter summarizes our current understanding of Zn signaling, especially with regard to the Zn wave and the role of Zn signaling in immune cells, and discusses how these processes contribute to allergic responses.