We have been probing the molecular mechanisms of tumor promoters that stimulate distinct initial signals to define critical downstream biochemical events in carcinogenesis. The action of the novel skin tumor promoter palytoxin on signaling and gene expression in keratinocytes, the primary target cells of tumor promoters, was therefore investigated. Palytoxin stimulated an increase in mRNA for matrix metalloproteinase-13 (MMP-13), an enzyme implicated in carcinogenesis, in a keratinocyte cell line derived from initiated mouse skin (308). Palytoxin stimulated an increase in c-Fos binding to the activator protein-1 (AP-1) site present in the promoter of the mouse MMP-13 gene. This effect was specific because palytoxin had little effect on c-Jun, JunB, JunD, FosB, Fra-1, or Fra-2 binding or on overall levels of transcription factor binding. The increase in c-Fos binding corresponded to a palytoxin-stimulated increase in c-Fos protein levels. Palytoxin stimulated the activation of the mitogen-activated protein kinases (MAPKs) extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase, and p38. The MAPK kinase inhibitor PD 98059 blocked palytoxin-stimulated ERK activation. PD 98059 also blocked the palytoxin-stimulated increases in c-Fos protein levels, c-Fos binding to the AP-1 site, and MMP-13 mRNA. These studies identify important differences between palytoxin-stimulated signaling in keratinocytes derived from initiated mouse skin, the biologically relevant cell type, and other cell lines. Specifically, our data suggest that, in keratinocytes derived from initiated mouse skin, ERK plays an important role in transmitting palytoxin-stimulated signals to three downstream targets that are likely to affect carcinogenesis: c-Fos, AP-1, and MMP-13.
- Matrix metalloproteinase-13
- Mitogen-activated protein kinases
- Signal transduction
- Tumor promoters