Epidermal squamous cell carcinoma is among the most common cancers in humans. These tumors are comprised of phenotypically diverse populations of cells that display varying potential for proliferation and differentiation. An important goal is identifying cells from this population that drive tumor formation. To enrich for tumor-forming cells, cancer cells were grown as spheroids in non-attached conditions. We show that spheroid-selected cells form faster growing and larger tumors in immune-compromised mice as compared to non-selected cells. Moreover, spheroid-selected cells gave rise to tumors following injection of as few as one hundred cells, suggesting these cells have enhanced tumor-forming potential. Cells isolated from spheroid-selected tumors retain an enhanced ability to grow as spheroids when grown in non-attached culture conditions. Thus, these tumor-forming cells retain their phenotype following in vivo passage as tumors. Detailed analysis reveals that spheroid-selected cultures are highly enriched for expression of epidermal stem cell and embryonic stem cell markers, including aldehyde dehydrogenase 1, keratin 15, CD200, keratin 19, Oct4, Bmi-1, Ezh2 and trimethylated histone H3. These studies indicate that a subpopulation of cells that possess stem cell-like properties and express stem cell markers can be derived from human epidermal cancer cells and that these cells display enhanced ability to drive tumor formation.

Epidermal growth factor receptor (EGFR) activation is absolutely required for cervical cell proliferation. This suggests that EGFR-inhibitory agents may be of therapeutic value. In the present study, we investigated the effects of epigallocatechin-3-gallate (EGCG), a bioactive green tea polyphenol, on EGFR signaling in cervical cells. EGCG inhibits epidermal growth factor-dependent activation of EGFR, and EGFR-dependent activation of the mitogen-activated protein kinases ERK1/2. EGCG also inhibits EGFR-dependent AKT activity. The EGCGdependent reduction in ERK and AKT activity is associated with reduced phosphorylation of downstream substrates, including p90RSK, FKHR, and BAD. These changes are associated with increased p53, p21 WAF-1 , and p27 KIP-1 levels, reduced cyclin E level, and reduced CDK2 kinase activity. Consistent with these findings, flow cytometry and TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling) staining revealed EGCG-dependent G 1 arrest. Moreover, sustained EGCG treatment caused apoptotic cell death. In addition to inhibiting EGFR, cell-free studies demonstrated that EGCG directly inhibits ERK1/2 and AKT, suggesting that EGCG acts simultaneously at multiple levels to inhibit EGF-dependent signaling. Importantly, the EGCG inhibition is selective, as EGCG does not effect the EGFR-dependent activation of JNK. These results suggest that EGCG acts to selectively inhibit multiple EGF-dependent kinases to inhibit cell proliferation.

(؊)-Epigallocatechin-3-gallate (EGCG) is an important bioactive constituent of green tea that efficiently reduces epidermal cancer cell proliferation. This inhibition is associated with a reduction in activator protein 1 (AP1) transcription factor level and activity. However, its effects on AP1 function in normal epidermal cells have not been extensively explored. Our present studies show that EGCG regulates normal keratinocyte function. To understand the mechanism of action, we examined the effects of EGCG on AP1 factor activity, MAPK signal transduction, and expression of the AP1 factorregulated human involucrin (hINV) gene. EGCG increases hINV promoter activity in a concentration-dependent manner that requires the presence of an intact hINV promoter AP1 factor binding site. This response appears to be physiologic, as endogenous hINV gene expression is also increased. Fra-1, Fra-2, FosB, JunB, JunD, c-Jun, and c-Fos levels are increased by EGCG treatment, as is AP1 factor binding to hINV promoter AP1 site. Gel mobility shift studies show that this complex contains Fra-1 and JunD. Signal transduction analysis indicates that the EGCG response requires Ras, MEKK1, MEK3, and p38 kinases. Kinase assays and inhibitor studies suggest that p38␦ is the p38 isoform responsible for the regulation. These changes are also associated with a cessation of cell proliferation and enhanced cornified envelope formation. These studies show that in normal human keratinocytes EGCG markedly increases, via a MAPK signaling mechanism, AP1 factor-associated responses.The polyphenol constituents of green tea inhibit carcinogenesis in a variety of tissues (1-4); however, their mechanism of action is not well understood. (Ϫ)-Epigallocatechin-3-gallate (EGCG) 1 is the major polyphenol isolated from green tea. AP1 transcription factors and AP1 factor-associated signal transduction are important targets of EGCG action (5-7). AP1 proteins consist of homodimers of Jun proteins (c-Jun, JunB, and JunD) and heterodimers of Jun and Fos (c-Fos, FosB, Fra-1, and Fra-2) factors (8). These proteins regulate transcription, differentiation, and cell proliferation by binding to specific recognition motifs in target genes (9 -13). EGCG produces specific changes in AP1 factor function in immortalized and transformed keratinocytes. These changes include an EGCGdependent reduction in phorbol ester-dependent mitogen-activated protein kinase (MAPK) activity (6), and reduced AP1 factor level and activity (7,14). EGCG also inhibits ultraviolet light-associated activation of c-Fos gene expression and the accumulation of c-Fos protein (7). Based on these studies, it has been suggested that the cancer-preventive role of EGCG may be due, in part, to its ability to reduce AP1 factor-related responses (6,7,14). However, very little information is available regarding how EGCG effects AP1 factor-regulated responses in normal keratinocytes. Our present studies show that EGCG increases AP1 factor levels in normal keratinocytes, and increases human involucrin gene expressio...

The epidermis is a dynamic renewing structure that provides life-sustaining protection from the environment. The major cell type of the epidermis, the epidermal keratinocyte, undergoes a carefully choreographed program of differentiation. Alteration of these events results in a variety of debilitating and life-threatening diseases. Understanding how this process is regulated is an important current goal in biology. In this review, we summarize the literature regarding regulation of involucrin, an important marker gene that serves as a model for understanding the mechanisms that regulate the differentiation process. Current knowledge describing the role of transcription factors and signaling cascades in regulating involucrin gene expression are presented. These studies describe a signaling cascade that includes the novel protein kinase C isoforms, Ras, MEKK1, MEK3, and a p38delta-extracellular signal regulated kinase 1/2 complex. This cascade regulates activator protein one, Sp1, and CCATT/enhancer-binding protein transcription factor DNA binding to two discrete involucrin promoter regions, the distal- and proximal-regulatory regions, to regulate involucrin gene expression.