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Keratinocyte differentiation marker suppression by arsenic: mediation by AP1 response elements and antagonism by
B A Jessen1, Q Qin, M A Phillips
1Department of Environmental Toxicology, University of California, One Shields Avenue, Davis, California, 95616-8588, USA.
Abstract:
Culture models of target cells are anticipated to help elucidate the mechanism by which inorganic arsenic acts as a carcinogen in humans. Present work characterizes the response of human keratinocytes, a target cell type, to arsenic suppression of their differentiation program. Four representative differentiation marker mRNAs (involucrin, keratinocyte transglutaminase, small proline-rich protein 1, and filaggrin) were suppressed by both arsenate and arsenite in normal, spontaneously immortalized (premalignant), and malignant keratinocytes with EC50 values in the low micromolar range. The suppression was almost completely reversed 9 days after removal of arsenate from the culture medium. In the case of the involucrin gene, suppression was mediated primarily by two functional AP1 response elements in the gene promoter. Both glucocorticoid and serum stimulation of differentiation occurred to a similar extent in the presence and absence of arsenic, indicating neither stimulation was a specific target of arsenic action and neither agent could overcome arsenic suppression. In contrast, 12-O-tetradecanoylphorbol-13-acetate prevented the suppression of keratinocyte transglutaminase, suggesting that arsenic acts upstream of protein kinase C.
Insights
Arsenic exposure suppresses human keratinocyte differentiation markers, impacting carcinogen mechanisms. This suppression is reversible and suggests arsenic acts upstream of protein kinase C.
Area of Science:
- Toxicology
- Cell Biology
- Dermatology
Background:
- Inorganic arsenic is a human carcinogen.
- Understanding arsenic's carcinogenic mechanisms requires studying target cells like keratinocytes.
- Arsenic's effect on keratinocyte differentiation is not fully understood.
Purpose of the Study:
- To characterize the response of human keratinocytes to arsenic exposure.
- To investigate arsenic's impact on keratinocyte differentiation markers.
- To elucidate the molecular mechanisms underlying arsenic-induced suppression of differentiation.
Main Methods:
- Utilized human keratinocyte culture models, including normal, premalignant, and malignant cells.
- Quantified the suppression of four key differentiation marker mRNAs (involucrin, keratinocyte transglutaminase, small proline-rich protein 1, and filaggrin) by arsenate and arsenite.
- Investigated the reversibility of arsenic-induced suppression and its interaction with differentiation stimuli (glucocorticoid, serum, 12-O-tetradecanoylphorbol-13-acetate).
- Analyzed the role of AP1 response elements in the involucrin gene promoter.
Main Results:
- Both arsenate and arsenite suppressed differentiation marker mRNAs in all keratinocyte types (EC50 in low micromolar range).
- Suppression was largely reversible upon arsenic removal.
- Arsenic did not specifically target glucocorticoid or serum-stimulated differentiation pathways.
- 12-O-tetradecanoylphorbol-13-acetate partially prevented suppression, indicating arsenic acts upstream of protein kinase C.
Conclusions:
- Arsenic effectively suppresses human keratinocyte differentiation programs.
- The observed suppression is reversible and suggests a mechanism upstream of protein kinase C.
- These findings contribute to understanding arsenic's role in human carcinogenesis via keratinocyte targets.