Identification of the B-Raf/Mek/Erk MAP kinase pathway as a target for all-trans retinoic acid during skin cancer promotion.
BACKGROUND
Retinoids have been studied extensively for their potential as therapeutic and chemopreventive agents for a variety of cancers, including nonmelanoma skin cancer (NMSC). Despite their use for many years, the mechanism of action of retinoids in the prevention of NMSC is still unclear. In this study we have attempted to understand the chemopreventive mechanism of all-trans retinoic acid (ATRA), a primary biologically active retinoid, in order to more efficiently utilize retinoids in the clinic.


RESULTS
We have used the 2-stage dimethylbenzanthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) mouse skin carcinogenesis model to investigate the chemopreventive effects of ATRA. We have compared the gene expression profiles of control skin to skin subjected to the 2-stage protocol, with or without ATRA, using Affymetrix 430 2.0 DNA microarrays. Approximately 49% of the genes showing altered expression with TPA treatment are conversely affected when ATRA is co-administered. The activity of these genes, which we refer to as 'counter-regulated', may contribute to chemoprevention by ATRA. The counter-regulated genes have been clustered into functional categories and bioinformatic analysis has identified the B-Raf/Mek/Erk branch of the MAP kinase pathway as one containing several genes whose upregulation by TPA is blocked by ATRA. We also show that ATRA blocks signaling through this pathway, as revealed by immunohistochemistry and Western blotting. Finally, we found that blocking the B-Raf/Mek/Erk pathway with a pharmacological inhibitor, Sorafenib (BAY43-9006), induces squamous differentiation of existing skin SCCs formed in the 2-stage model.


CONCLUSIONS
These results indicate that ATRA targets the B-Raf/Mek/Erk signaling pathway in the 2-stage mouse skin carcinogenesis model and this activity coincides with its chemopreventive action. This demonstrates the potential for targeting the B-Raf/Mek/Erk pathway for chemoprevention and therapy of skin SCC in humans. In addition our DNA microarray results provide the first expression signature for the chemopreventive effect of ATRA in a mouse skin cancer model. This is a potential source for novel targets for ATRA and other chemopreventive and therapeutic agents that can eventually be tested in the clinic.
Cheepala
Satish B
SB
Department of Biochemistry, Louisiana State University Health Sciences Center-Shreveport and Feist-Weiller Cancer Center, Shreveport, Louisiana 17730, USA. satish.cheepala@stjude.org
Yin
Weihong
W
Syed
Zanobia
Z
Gill
Jennifer N
JN
McMillian
Alaina
A
Kleiner
Heather E
HE
Lynch
Mark
M
Loganantharaj
Rasiah
R
Trutschl
Marjan
M
Cvek
Urska
U
Clifford
John L
JL
D015127:9,10-Dimethyl-1,2-benzanthracene / Q000633:toxicity; D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D015153:Blotting, Western; D002273:Carcinogens / Q000633:toxicity; D002471:Cell Transformation, Neoplastic / Q000187:drug effects / Q000235:genetics; D048049:Extracellular Signal-Regulated MAP Kinases / Q000187:drug effects* / Q000378:metabolism; D005260:Female; D015870:Gene Expression / Q000187:drug effects; D007150:Immunohistochemistry; D020930:MAP Kinase Kinase Kinases / Q000187:drug effects* / Q000378:metabolism; D051379:Mice; D020411:Oligonucleotide Array Sequence Analysis; D048493:Proto-Oncogene Proteins B-raf / Q000187:drug effects* / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D012878:Skin Neoplasms / Q000201:enzymology / Q000235:genetics / Q000517:prevention & control*; D013755:Tetradecanoylphorbol Acetate / Q000633:toxicity; D014212:Tretinoin / Q000494:pharmacology*
D000970:Antineoplastic Agents; D002273:Carcinogens; D014212:Tretinoin; D015127:9,10-Dimethyl-1,2-benzanthracene; D048493:Proto-Oncogene Proteins B-raf; D048049:Extracellular Signal-Regulated MAP Kinases; D020930:MAP Kinase Kinase Kinases; D013755:Tetradecanoylphorbol Acetate