Fig. 6: Illustrative draw of the Metallothionein regulation and the involvement of MTF1 and NRF2 regulation.

The unique chemical properties described above determine the key roles of MTs in cellular Zn++ homeostasis by low affinity binding of ~5–15% of the cell’s Zn ion pool in combination with two classes families of Zn transporters, namely, Zrt- and Irt-like proteins (ZIP, transporting Zn++ into the cytoplasm) and Zn transporters (ZnT, transporting Zn++ away from the cytoplasm). One MT can contain up to 7 atoms of Zn2+. MT are involved in heavy metals detoxification, like Cu2+, Cd+ or Pb+. For instance, MT oligomerization can occur either in native/non-oxidative forms or in oxidative forms induced by high Cd2+ concentrations, where MT subunits are covalently linked via disulfide bridges. Under oxidative stress, MT can release Zn2+ (apo-MT) to form thiol groups and participates in GSSG/ GSH reduction. Basal TM activity is regulated by several general transcription factors. Additionally, MT can be activated by a variety of stimuli, including metal ions, cytokines, and growth factors. Several inducible expression regulators of MT genes have been identified, including metal-responsive element (MRE). MREs are classically required for induction by metals. MREs act in conjunction with the zinc-dependent and zinc-responsive transcription factor MTF1 which is involved in the inducible expression of MTs. In this sense, MREs, MTF1 and the essential Zn ions that associate with MTF1, contribute to the basal and inducible expression of MTs. MTF1 also independently facilitates the recruitment of Sp1 and p300 to the protein complex in response to zinc. Along with MREs, many MT promoters contain an ARE consensus sequence, and are induced by NRF2 mediated antioxidant response. All promoters of the MT gene also contain at least one GC box (consensus sequence GGGGCGGGG) that responds to members of the constitutive Sp/XKLF zinc finger transcription factor family, including Sp1. Finally, MTF1 has been shown to be constitutively phosphorylated at serine and tyrosine residues under basal conditions in both human and mouse cells. Induction with transition metals (both cadmium and zinc) increases the level of MTF1 phosphorylation through a complex pathway involving protein kinase C (PKC), phosphoinositol-3 kinase (PI3K), c-jun N-terminal kinase (JNK), tyrosine kinase, casein kinase II, and calcium signaling, as well as PTEN, which can interact with MTF1 via its phosphatase/ C2 ___domain.