Knockdown of c-Fos suppresses the growth of human colon carcinoma cells in athymic mice

Here we have investigated whether inhibition of c-Fos expression in RKO human colon carcinoma cells (HCCCs) would result in reduced TGFβ1 expression and suppression of tumor growth in athymic mice. We stably transfected RKO cells with c-Fos small interfering RNA (siRNA) or with the corresponding control siRNA. Using these stable cell lines, we demonstrated that siRNA-c-Fos significantly suppressed both AP-1 binding, promoter reporter activity at the proximal AP-1 site in the TGFβ1 promoter, and TGFβ1 production. Further, we established colon cancer xenografts with each of RKO-siRNA-EV, RKO-siRNA-Ctrl and RKO-siRNA-c-Fos cells. By 24 days, the tumor size of RKO-siRNA-c-Fos xenografts was 40% that of either RKO-EV or RKO-siRNA-Ctrl. Immunohistochemistry (IHC) of tumor xenografts demonstrated that siRNA-c-Fos significantly blocked c-Fos expression, and consequently expression of TGFβ1. However, expression of TGFβ2 and TGFβ3 were unaffected. Overall, our results demonstrate that blockade of TGFβ1 production by siRNA-c-Fos effectively suppressed tumor growth in vivo.     

Human wee1 kinase is directly transactivated by and increased in association with c-Fos/AP-1: rheumatoid synovial cells overexpressing these genes go into aberrant mitosis

Wee1 kinase downregulates the M-phase promoting factor, a complex of cdc2 and cyclin B kinase, that controls mitotic cell division. We isolated human wee1 kinase gene promoter and found that it contained one AP-1-binding motif in its promoter region (5'-CGAGTCA-3'; -823/-817), through which wee1 kinase gene was directly transactivated by c-Fos/AP-1. In rheumatoid synovial cells, wee1 kinase was increased in conjunction with the increase of c-Fos/AP-1 and the substrate of wee1, cdc2, was phosphorylated. The amount of wee1 and c-Fos and the phosphorylation of cdc2 were decreased after treatment of the cells with an inhibitor of AP-1, curcumin. A significant proportion of cultured synovial cells of the patients with rheumatoid arthritis, but not those of osteoarthritis, shifted to a tetraploid (4C) state upon long-term culture. Thus, human wee1 kinase gene is directly transactivated by and increased in association with c-Fos/AP-1, and rheumatoid synovial cells overexpressing these genes go into aberrant mitosis.     

Mechanism of AP-1-mediated gene expression by select organochlorines through the p38 MAPK pathway

Organochlorine compounds have been demonstrated to have detrimental health effects in both wildlife and humans, an effect largely attributed to their ability to mimic the hormone estrogen. Our laboratory has studied cell signaling by environmental chemicals associated with the estrogen receptor (ER) and more recently via ER-independent mechanisms. Here, we show that the organochlorine pesticide dichlorodiphenyltrichloroethane (DDT) and its metabolites induce a stress mitogen-activated protein kinase (MAPK) that leads to AP-1 activation. Through the use of a dominant negative c-Fos mutant, we show that DDT exposure induces the collagenase promoter in an AP-1-dependent manner. DDT stimulates an AP-1 complex shift at the DNA to one favoring c-Jun/c-Fos dimers through both increasing c-Jun levels and by post-translational activation of c-Jun and c-Fos in HEK 293 and human endometrial Ishikawa cells. DDT treatment induces phosphorylation of ERK and p38, while JNK phosphorylation levels are slightly decreased. Using pharmacological and molecular inhibitors of the various MAPKs, we implicate the p38 signaling cascade, and to a lesser extent ERK, as necessary pathways for AP-1-mediated gene expression induction by organochlorines. Taken together, these results demonstrate that organochlorines induce the collagenase promoter via sequential activation of the p38 kinase cascade and AP-1.     

A transforming growth factorbeta1 signal peptide variant increases secretion in vitro and is associated with increased incidence of invasive breast cancer

There is evidence that transforming growth factor (TGF)beta acts as a suppressor of tumor initiation but also as a promoter of tumor progression when the antiproliferative effect of the TGFbeta signaling pathway has been overridden by other oncogenic mutations. Several somatic mutations that disrupt the TGFbeta-SMAD signaling pathway have been reported in human breast tumors. We have examined the association between single nucleotide polymorphisms (SNPs) in the TGFbeta1 gene and the incidence of invasive breast cancer in three case-control series, with a maximum of 3987 patients and 3867 controls, median age approximately 50 years, and range 22-92 years. The promoter SNP, C-509T, and the T +29C signal-peptide SNP (encoding Leu10Pro) are in strong linkage disequilibrium. They are both significantly associated with increased incidence of invasive breast cancer in a recessive manner [odds ratios: (TT versus C-carrier), 1.25; 95% confidence intervals 1.06-1.48; P = 0.009 and (ProPro versus Leu-carrier), 1.21; 95% confidence intervals 1.05-1.37; P = 0.01]. The G-800A SNP was not significantly associated with incidence of breast cancer. The C-509T SNP is not contained within a known consensus sequence for a promoter regulatory element and therefore unlikely to affect TGFbeta1 expression, whereas the Leu10Pro signal peptide substitution potentially affects TGFbeta1 secretion. Transfections of HeLa cells with constructs encoding either the Pro or Leu forms of TGFbeta1 and driven by the cytomegalovirus promoter indicate that the signal peptide with Pro at residue 10 causes a 2.8-fold increase in secretion compared with the Leu form. These data indicate that the allele encoding Pro10 is associated with increased rates of TGFbeta1 secretion and with increased incidence of invasive breast cancer for the population samples described. It is estimated that 3% of all breast cancer cases may be attributable to Pro10 homozygosity.