Academic journal article Environmental Health Perspectives

AP-1, NF[kappa]B, and ERK Activation Thresholds for Promotion of Neoplastic Transformation in the Mouse Epidermal JB6 Model. (Articles)

Academic journal article Environmental Health Perspectives

AP-1, NF[kappa]B, and ERK Activation Thresholds for Promotion of Neoplastic Transformation in the Mouse Epidermal JB6 Model. (Articles)

Article excerpt

The promotion-sensitive mouse epidermal JB6 cells (clone 41) have been used to identify the tumor-promoting activity of various compounds. Because treatment by tumor promoters [12-O-tetradecanoylphorbol-13-acetate (TPA), epidermal growth factor (EGF), or tumor necrosis factor alpha (TNF[alpha])] transforms clone 41 cells to anchorage-independent and tumorigenic phenotypes, they are considered to be undergoing late-stage tumor promotion. Here we address the question of how much activation of transformation-relevant transcription factors [activator protein-1 (AP-1), ternary complex factors (TCFs), or nuclear factor [kappa]B (NF[kappa]B)] is required for transformation response and how much tumor promoter produces significant risk of transformation. Stable transfectants harboring a reporter construct with an AP-1 response element, serum-response element (SRE), or NF[kappa]B response element were established. We examined the relationship between concentration of tumor promoters, key signaling events, and activation of the transcription factors. A concentration of > 0.2 nM TPA or 0.12 ng/mL (0.02 nM) EGF produced a significant increase in transformation response as well as in extracellular signal-regulated protein kinase (ERK), SRE, or AP-1 activation. Treatment with > 0.4 U/mL (2.35 pM) TNF[alpha] increased NF[kappa]B activity and transformation response in a dose-dependent manner. However, transformation response decreased at > 33 U/mL TNF[alpha] due to a cytotoxic response. These findings suggest that the signaling pathway leading to the activation of ERK, TCF, and AP-1 proteins constitutes a major factor determining the risk of tumor promotion by TPA or EGF. Cell toxicity in addition to NF[kappa]B activation should be considered in predicting TNF[alpha]-induced transformation response. Key words: activator protein-1, epidermal growth factor, nuclear factor [kappa]B, serum-response element, 12-O-tetradecanoylphorbol-13-acetate, transformation, tumor necrosis factor-[alpha]. Environ Health Perspect 110:865-870 (2002). [Online 18 July 2002] http://ehpnet1.niehs.nih.gov/docs/2002/110p865-870suzukawa/abstract.html

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Multistage carcinogenesis consists of initiation, promotion, and progression. Tumor promotion is a stepwise process: It occurs with comparatively low frequency, requires the chronic action of tumor promoters, and does not necessarily involve genotoxic damage. The mouse skin model is an excellent example of multistage carcinogenesis. The concentration of initiator (typically dimethylbenz[a]anthracene; DMBA) can be decreased to produce genotoxic damage in the absence of tumor formation. Sequential exposure of initiated cells to various tumor promoters (e.g., 12-O-tetradecanoyl-phorbol-13-acetate; TPA) at an optimal concentration results in a robust tumor response. Because tumor initiation occurs rapidly and tumor promotion requires several months (i.e., 20-40% of a mouse lifetime), tumor promotion is considered a rate-limiting step (1). In vivo studies support a dose threshold for skin papilloma formation in response to TPA (2,3). Therefore, defining molecular thresholds in transformation-related signal transduction pathways is expected to provide a scientific basis to improve cancer risk assessment.

Although in vivo data are significant to whole organisms, in vitro systems are more readily manipulated and have advantages in terms of cost and time. The JB6 mouse epidermal cell model of genetic variants is unique in that it allows a detailed investigation of the molecular events specific to tumor promotion. Promotion-sensitive clones of the mouse epidermal cell line JB6 (clone 41) respond irreversibly to tumor promoter treatment with colony growth under anchorage-independent conditions and induced tumor formation (1,4). Anchorage-independent transformation has been observed in clone 41 cells treated with TPA, epidermal growth factor (EGF), tumor necrosis factor alpha (TNF[alpha]) (5), and other tumor promoters (6). …

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