The correlation of keratin expression with in-vitro epithelial cell line differentiation
The mechanism of transcriptional regulation which allows oral mucosal-specific expression of the human keratin 13 (K13) gene remains unknown. The pattern of expression of keratin genes in the oral mucosa shows sequential synthesis of specific basal and suprabasal keratins. K15 and K13 are respectively basal and suprabasal markers of buccal epithelial tissue. K15 and K13 expression are known to alter in oral squamous cell carcinoma. When differentiation occurs basal cells migrate to the suprabasal layer and K15 synthesis stops and the synthesis of suprabasal markers like K13 begins. The aims of the study are to determine whether the K13 promoter can confer 1) oral mucosal cell-specific differentiation in in-vitro culture, 2) understand the mechanism by which different positive or negative regulators impose basal cellspecific promoter expression and how this differs from suprabasal promoter expression and 3) whether K15 and K13 expression can be correlated with in-vitro cell differentiation stage. To provide insight into the basal and suprabasal expression of K15 and K13, their 5′-upstream sequence regions were linked to EGFP. The results show that the expression of K15 and K13 can be correlated with in-vitro cell differentiation stages in malignant epithelial cell lines. The full length K13 promoter construct drove negligible GFP expression in the human foreskin fibroblast cell line and had a greater expression in the oral SCC25 cell line than the epidermal-derived RTS3b cell line. Therefore, the K13 promoter sequence appears to contain suppressor elements conferring oral cell-specificity. The expression patterns of truncated K13 promoter fragments after transient transfection into permissive, non permissive and non-epithelial cell lines, suggest binding of a negative regulatory element to the full length K13-3089bp promoter. K13 expression is induced by pharmacological alteration of chromatin structure in non-permissive skin-derived RTS3b cells. The MEK 1/ 2 pathway seems not necessary for the expression of K13, but PI3K/Akt pathway is important for expression of K13 in oral mucosa. In conclusion, the results suggest that keratin gene promoter expression can be correlated with in-vitro cell differentiation stages and these restricted differences in keratin expression might be co-ordinated by the synergistic action of transcription factors.
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