Ochratoxin A-induced cell proliferation and tumor promotion in mouse skin by activating the expression of cyclin-D1 and cyclooxygenase-2 through nuclear factor-kappa B and activator protein-1

Abstract

Our prior studies have indicated that ochratoxin A (OTA), a mycotoxin, has skin tumor initiating activity. In the present investigation, skin tumor promoting activity of OTA and the mechanism/(s) involved therein was undertaken. A single topical application of OTA (100 nmol/mouse) caused significant enhancement in short-term markers of skin tumor promotion such as ornithine decarboxylase activity, DNA synthesis, hyperplasia as well as expression of cyclin-D1 and COX-2 in mouse skin. In a two-stage mouse skin tumorigenesis protocol, twice-weekly exposure of OTA (50 nmol/mouse) to 7,12-dimethylbenz[α]anthracene (120 nmol/mouse) initiated mice skin for 24 weeks leads to tumor formation. Further, exposure of primary murine keratinocytes (PMKs) with non-cytotoxic dose of OTA (5.0 µM) caused (i) significant enhancement of DNA synthesis, (ii) enhanced phosphorylation and subsequent activation of epidermal growth factor receptor (EGFR) and its downstream signaling pathways viz Akt, ERK1/2, p38 and JNK mitogen-activated protein kinases (MAPKs), (iii) overexpression of c-jun, c-fos, cyclin-D1 and COX-2 and (iv) increased binding of nuclear factor-kappaB (NF-κB) and AP-1 transcription factors to the promoter region of cyclin-D1 and COX-2 genes. It was also observed that knocking down the messenger RNA expression of NF-κB, c-jun, c-fos, cyclin-D1 and COX-2 results in significant inhibition in OTA-induced PMKs proliferation. These results suggest that OTA has cell proliferative and tumor-promoting potential in mouse skin, which involves EGFR-mediated MAPKs and Akt pathways along with NF-κB and AP-1 transcription factors and that cyclin-D1 and COX-2 are the target genes responsible for tumor-promoting activity of OTA.