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A dominant negative c-jun specifically blocks okadaic acid-induced skin tumor promotion
Eric J Thompson1, Jacalyn MacGowan, Matthew R Young
1Department of Pharmacology and Toxicology, The University of Arizona, Tucson, AZ 85724, USA.
Abstract:
Okadaic acid (OA) is a prototypical non-phorbol ester skin tumor-promoting agent that works by inhibiting protein phosphatases, leading to an increase in protein phosphorylation. Increased protein phosphorylation can lead to stimulated signaling through various signal transduction pathways. One or more of the pathways affected by OA leads to increased signaling via the activator protein 1 (AP-1) transcription factor. Because AP-1 signaling has been shown to be required for skin tumor promotion by phorbol ester, studies were undertaken to determine whether AP-1 signaling is also required for 7,12-dimethylbenz(a)anthracene (DMBA)-initiated/OA-promoted skin tumorigenesis. Transgenic mice expressing a dominant negative c-jun (TAM-67) controlled by the keratin 14 promoter in ICR mice were used to determine the effects of OA on AP-1 signaling. By crossing the TAM-67 mice with mice expressing an AP-1-responsive luciferase, it was shown that TAM-67 decreases AP-1 activation in response to OA treatment by 95%. After 7,12-dimethylbenz(a)anthracene initiation, the TAM-67 mice and nontransgenic littermates were promoted with twice weekly applications of OA. These experiments showed that TAM-67 expression decreased tumor multiplicity by 90%. Additional experiments with TAM-67 mice showed that the hyperplastic response to OA is not impaired in these mice, nor were there differences in OA-induced transcription of various genes known to be AP-1 responsive under other conditions. This result suggests that only a subset of AP-1-regulated genes is targeted by TAM-67 when it prevents tumor promotion by OA. A determination of the mechanism by which TAM-67 can block tumor promotion without affecting hyperplasia will be important.
Insights
Okadaic acid (OA) promotes skin tumors by increasing protein phosphorylation and activator protein 1 (AP-1) signaling. Inhibiting AP-1 signaling with TAM-67 significantly reduced OA-promoted skin tumor development in mice.
Area of Science:
- Biochemistry
- Molecular Biology
- Dermatology
Background:
- Okadaic acid (OA) is a potent skin tumor promoter that inhibits protein phosphatases, increasing protein phosphorylation.
- This increased phosphorylation activates signaling pathways, including the activator protein 1 (AP-1) transcription factor, which is crucial for tumor promotion.
- AP-1 signaling is known to be essential for skin tumor promotion initiated by 7,12-dimethylbenz(a)anthracene (DMBA) and promoted by phorbol ester.
Purpose of the Study:
- To investigate whether AP-1 signaling is required for 7,12-dimethylbenz(a)anthracene (DMBA)-initiated and OA-promoted skin tumorigenesis.
- To determine the effect of inhibiting AP-1 signaling on OA-induced skin tumor promotion.
- To elucidate the role of AP-1 in OA-induced hyperplasia and gene expression.
Main Methods:
- Utilized transgenic mice (TAM-67) expressing a dominant-negative c-jun under the keratin 14 promoter to inhibit AP-1 signaling.
- Crossed TAM-67 mice with AP-1-responsive luciferase reporter mice to quantify OA-induced AP-1 activation.
- Administered DMBA initiation followed by OA promotion in TAM-67 and wild-type littermate mice to assess tumor development.
Main Results:
- TAM-67 expression reduced OA-induced AP-1 activation by 95%.
- TAM-67 expression decreased DMBA/OA-induced skin tumor multiplicity by 90%.
- Hyperplastic responses and transcription of some AP-1-responsive genes to OA were not impaired in TAM-67 mice, suggesting a specific subset of AP-1 targets is involved in tumor promotion.
Conclusions:
- AP-1 signaling is essential for the promotion of DMBA-initiated skin tumors by OA.
- Inhibition of AP-1 signaling effectively blocks OA-induced skin tumor promotion.
- The mechanism by which AP-1 inhibition prevents tumor promotion without affecting OA-induced hyperplasia warrants further investigation.