Cancer Prevention Research All-Trans Retinoic Acid Suppresses Stat3 Signaling during Skin Carcinogenesis

Squamous cell carcinoma (SCC) of the skin is the most clinically aggressive form of nonmelanoma skin cancer. We have determined the effects of all-trans retinoic acid (ATRA), a naturally occurring chemopreventive retinoid, on signal transducer and activator of transcription 3 (Stat3) signaling during the development of skin SCC. Stat3 is a transcription factor that plays a critical role in cell proliferation and survival, and it is constitutively active in several malignant cell types. We have previously shown that Stat3 is required for the initiation, promotion, and progression of skin SCC. ATRA is a highly efficient suppressor of tumor formation in the two-stage mouse skin carcinogenesis model and we have shown that this effect correlates with the suppression of the B-Raf/Mek/Erk signaling pathway. In this study, we have determined the pattern of Stat3 phosphorylation throughout the course of the two-stage protocol, both in the presence and absence of ATRA. We have used both SENCAR mice and K5.Stat3C transgenic mice, which express the Stat3C protein, a constitutively active form of Stat3, in the skin. Using Western blotting and immunohistochemical staining with phosphospecific antibodies, we show that coadministration of ATRA suppressed the 12-O-tetradecanoylphorbol-13-acetate–induced phosphorylation of Stat3 in both models, but was only able to suppress tumor formation in the SENCAR mice. Surprisingly, ATRA actually enhanced tumor formation in 12-O-tetradecanoylphorbol-13-acetate– treated K5.Stat3C mice. We hypothesize that ATRA blocks tumor formation, at least in part, by targeting events upstream of Stat3, such as the B-Raf/Mek/Erk pathway, and that in the K5.Stat3C mice, in which Stat3 activity is constitutive, it cannot suppress tumor formation. Nonmelanoma skin cancer is the most common cancer in the United States, with over a million new cases of the two most common forms, squamous cell carcinoma (SCC) and basal cell carcinoma, anticipated annually (1). The more clinically aggressive form is SCC (2),which has been increasing in incidence since the 1960s at annual rates from 4% to as much as 10% in recent years. Advanced disease–related and treatment-relatedmorbidity have a profound effect on the patients' quality of life. Unlike basal cell carcinoma, which bears a single genetic hallmark of disruption of the patched-sonic hedgehog signaling pathway, the genetic alterations leading to SCC seem more complex and varied, and are poorly understood (3). Better control of advanced skin SCC is clearly necessary, and will be greatly helped by improving our understanding of the molecular basis for skin carcinogenesis and of the action of chemopreventive drugs. The two-stage mouse skin chemical carcinogenesis model is one of the best studied andmost informative with regard to understandingmolecular mechanisms and the evolution of cancer cells (4). It has proven to be ideal for the study of events leading to the transition from initiation, to promotion, and then progression to carcinoma. Molecular analysis of multistage human cancers such as prostate and colon cancerhave shownahigh level of genetic and biological similarity to mouse skin in the two-stage model. (5). The SENCAR (sensitive to carcinogen) mouse strain was developed for this assay due to its high susceptibility to chemical-induced tumor formation, relative to most other strains of mice tested (4). Skin tumors can be readily induced by the sequential application of a sub-threshold dose of carcinogen such as 7,12-dimethylbenz(a)anthracene (DMBA), referred to as the initiation stage, followed by repetitive treatment with a noncarcinogenic tumor promoter such as 12-O-tetradecanoylphorbol-13-acetate (TPA), referred to as the promotion stage. Authors' Affiliations: Biochemistry and Molecular Biology, Otorhinolaryngology, Pharmacology, and Pathology, Louisiana State University Health Sciences Center-Shreveport and Feist-Weiller Cancer Center, Shreveport, Louisiana; and Department of Carcinogenesis, University of Texas M.D. Anderson Cancer Center, Science Park Research Division, Smithville,