Zinc Ionophore Activity of Quercetin and Epigallocatechin-gallate: From Hepa 1-6 Cells to a Liposome Model

Abstract

Labile zinc, a tiny fraction of total intracellular zinc that is loosely bound to proteins and easily interchangeable, modulates the activity of numerous signaling and metabolic pathways. Dietary plant polyphenols such as the flavonoids quercetin (QCT) and epigallocatechin-gallate act as antioxidants and as signaling molecules. Remarkably, the activities of numerous enzymes that are targeted by polyphenols are dependent on zinc. We have previously shown that these polyphenols chelate zinc cations and hypothesized that these flavonoids might be also acting as zinc ionophores, transporting zinc cations through the plasma membrane. To prove this hypothesis, herein, we have demonstrated the capacity of QCT and epigallocatechin-gallate to rapidly increase labile zinc in mouse hepatocarcinoma Hepa 1-6 cells as well as, for the first time, in liposomes. In order to confirm that the polyphenols transport zinc cations across the plasma membrane independently of plasma membrane zinc transporters, QCT, epigallocatechin-gallate, or clioquinol (CQ), alone and combined with zinc, were added to unilamellar dipalmitoylphosphocholine/cholesterol liposomes loaded with membrane-impermeant FluoZin-3. Only the combinations of the chelators with zinc triggered a rapid increase of FluoZin-3 fluorescence within the liposomes, thus demonstrating the ionophore action of QCT, epigallocatechin-gallate, and CQ on lipid membrane systems. The ionophore activity of dietary polyphenols may underlay the raising of labile zinc levels triggered in cells by polyphenols and thus many of their biological actions.

Integration of Quercetin-Iron Complexes into Phosphatidylcholine or Phosphatidylethanolamine Liposomes

“…It is possible that quercetin can increase the permeability of lipid bilayers to iron cations. Similar ionophore activity of quercetin to zinc cations was recently demonstrated on phosphatidylcholine bilayers [46]. The revealed ability of quercetin-iron complexes to incorporate into the phospholipid bilayers of liposomes extends the potential of medical and cosmetic applications of liposomes for flavonoid storage and delivery.…”

Antitumor Effects of Quercetin in Hepatocarcinoma In Vitro and In Vivo Models: A Systematic Review

“…Quercetin antitumor effects have been described in different cancer types, including HCC [1]. In 25 of the articles included in the present review, quercetin efficacy as single treatment was evaluated employing different HCC study models [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]46,48,49,[51][52][53][54]57]. Antiproliferative effect of this flavonoid alone has been demonstrated in several researches with in vitro models [22][23][24][25][26][27][30][31][32][33]35,38,46,48,49,[51][52][53][54], highlighting the HepG2 cell line as the most used in 21 of the 25 articles [24,25,<...>…”

“…Among included articles, several of them determined effects of the flavonoid in different specific mechanisms [27][28][29]34,38]. These include inhibition of chymotrypsin-like activity of the proteasome, involved in proteasomal regulation of cancer signaling pathways [27,38]; rise of intracellular labile zinc, which has second messenger molecule activities in tumor cells [34]; and modulation of microRNAs expression, highly involved in cancer development and chemoresistance [63], leading to inhibition of the oncogenic RNA-binding proteins insulin-like growth factor-2 binding protein 1 and 3 (IGF2BP1 and IGF2BP3) through the upregulation of miR-1275 [29]. Another study published in 2018 employed quercetin to analyze adequacy of the cellular antioxidant (CAA) assay in HepG2 cells in order to determine the antioxidant activity of extracts from tree nuts [28].…”

Structural basis, chemical driving forces and biological implications of flavones as Cu(II) ionophores

“…gallate has also been reported [25], to our knowledge, their ionophoric role for redox-active copper and the related biological implications have remained unknown.…”