No abstract

Patients with OLP in China usually present with distinctive clinical morphology and characteristic distribution and few may display lesions with a confusing array of forms mimicking other diseases. A long time follow up is of utmost importance to detect its malignant transformation.

Krü ppel-like factor 4 (KLF4) is an epithelial cell-enriched, zinc finger-containing transcription factor, the expression of which is associated with growth arrest. Previous studies show that constitutive expression of KLF4 inhibits DNA synthesis but the manner by which KLF4 exerts this effect is unclear. In the present study, we developed a system in which expression of KLF4 is controlled by a promoter that is induced upon treatment of cells containing the receptors for the insect hormone, ecdysone, with ponasterone A, an ecdysone analogue. The rate of proliferation of a stably transfected colon cancer cell line, RKO, was significantly decreased following addition of ponasterone A when compared with untreated cells. Flow cytometric analyses indicated that the inducible expression of KLF4 caused a block in the G 1 /S phase of the cell cycle. A similar block was observed when ecdysone receptor-containing RKO cells were infected with a replication-defective recombinant adenovirus containing an inducible KLF4 and treated with ponasterone A. Results of these studies provide evidence that the inhibitory effect of KLF4 on cell proliferation is mainly exerted at the G 1 /S boundary of the cell cycle.

The tumor suppressor p53 is required for the maintenance of genomic integrity following DNA damage. One mechanism by which p53 functions is to induce a block in the transition between the G 1 and S phase of the cell cycle. Previous studies indicate that the Krüppel-like factor 4 (KLF4) gene is activated following DNA damage and that such activation depends on p53. In addition, enforced expression of KLF4 causes G 1 /S arrest. The present study examines the requirement of KLF4 in mediating the p53-dependent cell cycle arrest process in response to DNA damage. We show that the G 1 population of a colon cancer cell line, HCT116, that is null for the p53 alleles (−/−) was abolished following γ irradiation compared with cells with wild-type p53 (+/+). Conditional expression of KLF4 in irradiated HCT116 p53−/− cells restored the G 1 cell population to a level similar to that seen in irradiated HCT116 p53+/+ cells. Conversely, treatment of HCT116 p53+/+ cells with small interfering RNA (siRNA) specific for KLF4 significantly reduced the number of cells in the G 1 phase following γ irradiation compared with the untreated control or those treated with a nonspecific siRNA. In each case the increase or decrease in KLF4 level because of conditional induction or siRNA inhibition, respectively, was accompanied by an increase or decrease in the level of p21 WAF1/CIP1 . Results of our study indicate that KLF4 is an essential mediator of p53 in controlling G 1 /S progression of the cell cycle following DNA damage.The mammalian cell cycle is operationally divided into five distinct phases: gap 1 (G 1 ), DNA synthesis (S), gap 2 (G 2 ), mitosis (M), and growth arrest phase (G 0 ), also called quiescence (1). Complex networks of control mechanisms called "checkpoints" are responsible for the orderly progression of these events within the cell cycle. Defects in checkpoint control increase genetic instability, thereby contributing to uncontrolled proliferation (2). For example, damage to the DNA elicits a series of signal transduction pathways that result in an arrest of the cell cycle at various checkpoints (3). Much of the DNA damage-induced signals are funneled through p53, which directs further downstream actions that lead to inhibition of G 1 to S and G 2 to M transitions, among other events such as apoptosis (4). Therefore, it is not surprising that p53 is the most frequently mutated tumor suppressor gene in human cancers (5).The arrest in the transition between the G 1 and S phase of the cell cycle elicited by p53 requires in part the transcriptional activation of the gene encoding the cyclin-dependent kinase (Cdk) 1 inhibitor p21 WAF1/CIP1 (6,7). p21 WAF1/CIP1 binds to several G 1 cyclin-Cdk complexes and * This work was supported in part by Grants DK52230 and CA84197 from the National Institutes of Health. ¶ Recipient of a Georgia Cancer Coalition Distinguished Cancer Clinician Scientist award. To whom correspondence should be addressed: 201 Whitehead Biomedical Research Bldg., Emory University School of Medicine, 61...

Krüppel-like factor 4 (KLF4) is an epithelially enriched, zinc finger-containing transcription factor, the expression of which is associated with growth arrest. Constitutive expression of KLF4 inhibits G1/S transition of the cell cycle but the manner by which it accomplishes this effect is unclear. To better understand the biochemical function of KLF4, we identified its target genes using cDNA microarray analysis in an established human cell line containing inducible KLF4. RNA extracted from induced and control cells were hybridized differentially to microarray chips containing 9600 human cDNAs. In all, 84 genes with significantly increased expression and 107 genes with significantly reduced expression due to KLF4 induction were identified. The affected genes are sorted to several clusters on the basis of functional relatedness. A major cluster belongs to genes involved in cell-cycle control. Within this cluster, many up-regulated genes are inhibitors of the cell cycle and down-regulated genes are promoters of the cell cycle. Another up-regulated gene cluster includes nine keratin genes, of which seven are located in a specific region on chromosome 12. The results indicate that KLF4 is involved in the control of cell proliferation and does so by eliciting changes in expression of numerous cell-cycle regulatory genes in a concerted manner. Furthermore, KLF4 regulates expression of a group of epithelial-specific keratin genes in a manner consistent with a potential locus control region function.

Intestinal-enriched Krü ppel-like factor (IKLF or KLF5) belongs to the family of mammalian Krü ppel-like transcription factors. Previous studies indicate that expression of IKLF is enriched in the proliferating crypt epithelial cells of the intestinal tract. However, the biological function of IKLF is unknown. In the current study, we have shown that the level of IKLF mRNA was nearly undetectable in serum-deprived NIH3T3 fibroblasts but became acutely and significantly increased upon the addition of fetal bovine serum or the phorbol ester, PMA. This induction required protein synthesis because it was prevented by cycloheximide. Transfection of IKLF into NIH3T3 cells resulted in the formation of foci in a manner similar to that caused by the activated Ha-ras oncogene. Constitutive expression of IKLF in transfected NIH3T3 cells significantly increased the rate of proliferation when compared with cells transfected with an empty vector. The growth of IKLF-transfected cells was no longer inhibited by cell-cell contact or by low serum content. Moreover, these cells proliferated in an anchorage-independent fashion. We conclude that IKLF encodes a delayed early response gene product that positively regulates cellular proliferation and may give rise to a transformed phenotype when overexpressed.

Kru¨ppel-like factor 4 (KLF4) is a zinc-finger-containing transcription factor, the expression of which is enriched in the postmitotic cells of the intestinal epithelium. KLF4 is a target gene of the tumor suppressor adenomatous polyposis coli (APC). We sought to determine the role of KLF4 in suppressing the tumorigenecity of RKO colon cancer cells, which do not express KLF4. We utilized an established system in RKO cells, in which an inducible promoter controls expression of KLF4. Four independent assays were used to assess the effects of KLF4 induction on tumor cells. We find that KLF4 overexpression reduces colony formation, cell migration and invasion, and in vivo tumorigenecity. The mechanism of action of KLF4 does not involve apoptosis. These findings, along with our previous findings that KLF4 induces G1/S arrest, suggest that KLF4 is a cell cycle checkpoint protein that can reduce tumorigenecity of colon cancer cells.

These findings suggest that OOC is clinicopathologically separate from other types of odontogenic cysts and may thus constitute a distinct clinical entity.