Phosphorylation Is Involved in the Activation of Metal-regulatory Transcription Factor 1 in Response to Metal Ions

Abstract

We have studied the role of phosphorylation in the activation of metal-regulatory transcription factor-1 (MTF-1) and metallothionein (MT) gene expression. We showed that MTF-1 is phosphorylated in vivo and that zinc stimulates MTF-1 phosphorylation 2-4-fold. Several kinase inhibitors were used to examine the possible involvement of kinase cascades in the activation of MTF-1. Metal-induced MT gene expression was abrogated by protein kinase C (PKC), c-Jun N-terminal kinase (JNK), phosphoinositide 3-kinase, and tyrosine-specific protein kinases inhibitors, as assayed by Northern analysis and by cotransfection experiments using a metal regulatory element-luciferase reporter plasmid. The extracellular signal-activated protein kinase and the p38 kinase cascades did not appear to be essential for the activation of MT gene transcription by metals. By using dominantnegative mutants of PKC, JNK, mitogen-activated kinase kinase 4 (MKK4), and MKK7, we provide further evidence supporting a role for PKC and JNK in the activation of MTF-1 in response to metals. Notably, increased MTF-1 DNA binding in response to zinc and MTF-1 nuclear localization was not inhibited in cells preincubated with the different kinase inhibitors despite strong inhibition of MTF-1-mediated gene expression. This suggests that phosphorylation is essential for MTF-1 transactivation function. We hypothesize that metal-induced phosphorylation of MTF-1 is one of the primary events leading to increased MTF-1 activity. Thus, metal ions such as cadmium could activate MTF-1 and induce MT gene expression by stimulating one or several kinases in the MTF-1 signal transduction pathway.

Degradation of the Alzheimer Disease Amyloid β-Peptide by Metal-dependent Up-regulation of Metalloprotease Activity

“…9). This is consistent with reports that Zn 2ϩ can activate gene expression by a PI3K-and JNK-dependent process (30). Alternatively, elevated metal levels could promote release of growth factors or other ligands that, in turn, activate PI3K, MAPK, and additional pathways.…”

Heavy Metal-induced Metallothionein Expression Is Regulated by Specific Protein Phosphatase 2A Complexes

“…Nuclear MTF-1 binds to specific DNA sequences termed as metal response elements (MREs) and in turn activates MT transcription (22,24). It has been reported that MTF-1 activation requires stress-induced posttranslational modifications including phosphorylation (25). Although previous study demonstrates that the dephosphorylation of MTF-1 leads to its activation (24), the dynamic regulatory pattern remains unknown.…”

“…Previous reports have indicated that MTF-1 activity is mainly regulated by phosphorylation (24,25). Although protein kinases such as protein kinase C (PKC), c-Jun N-terminal kinase (JNK), or phosphoinositide 3-kinase (PI3K) have been reported to be involved in modifying the MTF-1 signaling pathway (24,25), the dynamic changes of phosphorylated MTF-1 in transactivation of MT remains to be defined. Because specific dephosphorylation of this transcription factor contributes to its activation (24), it is crucial to identify the specific protein phosphatases involved in transcriptional activation of MTF-1 under heavy metal stress.…”

“…Metal-responsive transcription factor 1 (MTF-1) is considered to be a major activator for MT gene expression (22,23). Previous reports have indicated that MTF-1 activity is mainly regulated by phosphorylation (24,25). Although protein kinases such as protein kinase C (PKC), c-Jun N-terminal kinase (JNK), or phosphoinositide 3-kinase (PI3K) have been reported to be involved in modifying the MTF-1 signaling pathway (24,25), the dynamic changes of phosphorylated MTF-1 in transactivation of MT remains to be defined.…”

Induction of Metallothionein I by Arsenic via Metal-activated Transcription Factor 1

“…Other inducers, such as glucocorticoids and alkylating agents, may also interact with MTF1-associated proteins to activate MTF1, because these inducers are unlikely to interact with the zinc fingers directly. Phosphorylation of MTF1 was observed and may contribute to nuclear translocation of activated MTF1 (33)(34)(35). Arsenic (As 3ϩ ) is a human carcinogen and a pleiotropic toxic metalloid that causes cancer (particularly of the skin, lung, bladder, and liver), neuropathy, cardiovascular lesions, ovarian dysfunction, aberrant embryonic development, and postnatal growth retardation in humans (36 -42).…”