RESUMO
The activating protein-1 (AP-1) transcription factor transduces growth signals through signal transduction pathways to the nucleus, leading to the expression of genes involved in growth and malignant transformation in many cell types. We have previously shown that overexpression of a dominant negative form of the cJun proto-oncogene, a cJun dominant negative mutant (Tam67), blocks AP-1 transcriptional activity, induces a G(1) cell cycle block and inhibits breast cancer cell growth in vitro and in vivo. We found that AP-1 blockade by Tam67 in MCF-7 breast cancer cells downregulates cyclin D1 transcriptional activity by at least two mechanisms: by suppressing transcription at the known AP-1 binding site (-934/-928) and by suppressing growth factor-induced expression through suppressing E2F activation at the E2F-responsive site (-726/-719). AP-1 blockade also led to reduced expression of E2F1 and E2F2, but not E2F4, at the mRNA and protein levels. Chromatin immunoprecipitation and supershift assays demonstrated that AP-1 blockade caused decreased binding of E2F1 protein to the E2F site in the cyclin D1 promoter. We also found that Tam67 suppressed the expression of the E2F1 dimerizing partner, DP1 and E2F-upregulated cell cycle genes (cyclins E, A, B and D3) and enhanced the expression of E2F-downregulated cell cycle genes (cyclins G(2) and I). Reduced expression of other E2F-regulated genes was also seen with AP-1 blockade and E2F suppression. Thus, the AP-1 factor regulates the expression of cyclin D and E2F (the latter in turn regulates E2F-downstream genes), leading to cell cycle progression and breast cancer cell proliferation.
Assuntos
Neoplasias da Mama/patologia , Ciclina D1/antagonistas & inibidores , Fatores de Transcrição E2F/metabolismo , Fator de Transcrição AP-1/metabolismo , Neoplasias da Mama/genética , Linhagem Celular Tumoral , Proliferação de Células , Imunoprecipitação da Cromatina , Ciclina D1/genética , Ciclina D1/metabolismo , Ciclinas/genética , Ciclinas/metabolismo , Regulação para Baixo , Fatores de Transcrição E2F/genética , Fator de Transcrição E2F1/genética , Fator de Transcrição E2F1/metabolismo , Fator de Transcrição E2F2/genética , Fator de Transcrição E2F2/metabolismo , Fator de Transcrição E2F4/genética , Fator de Transcrição E2F4/metabolismo , Feminino , Regulação Neoplásica da Expressão Gênica , Humanos , Fragmentos de Peptídeos/metabolismo , Regiões Promotoras Genéticas , Proto-Oncogene Mas , Proteínas Proto-Oncogênicas c-jun/metabolismo , RNA Mensageiro/antagonistas & inibidores , RNA Mensageiro/metabolismo , Fator de Transcrição AP-1/antagonistas & inibidores , Fator de Transcrição DP1/metabolismoRESUMO
Retinoid X receptor (RXR) belongs to a family of ligand-activated transcription factors that regulate many aspects of metazoan life. A class of nuclear receptors requires RXR as heterodimerization partner for their function. This places RXR in the crossroad of multiple distinct biological pathways. This and the fact that the debate on the endogenous ligand requirement for RXR is not yet settled make RXR still an enigmatic transcription factor. Here, we review some of the biology of RXR. We place RXR into the evolution of nuclear receptors, review structural details and ligands of the receptor. Then processes regulated by RXR are discussed focusing on the developmental roles deduced from studies on knockout animals and metabolic roles in diseases such as diabetes and atherosclerosis deduced from pharmacological studies. Finally, aspects of RXR's involvement in myeloid differentiation and apoptosis are summarized along with issues on RXR's suitability as a therapeutic target.
Assuntos
Receptores X de Retinoides/fisiologia , Animais , Apoptose/fisiologia , Diferenciação Celular/fisiologia , Humanos , Resistência à Insulina/fisiologia , Ligantes , Filogenia , Estrutura Secundária de Proteína , Receptores Citoplasmáticos e Nucleares/fisiologia , Receptores X de Retinoides/química , Receptores X de Retinoides/genéticaRESUMO
Chemotherapeutic drugs are known to eliminate cancer cells by inducing apoptosis. Tissue transglutaminase (tTG), a frequent player in apoptotic processes, is markedly induced in drug-resistant cancer cells. To better understand the action of apoptosis-inducing drugs, our study elucidates changes in the expression of tTG in the early phase of cell death, before the downstream events of apoptosis. We demonstrate that HepG2 cells uniformly induce both tTG mRNA and enzyme activity upon treatment with cisplatin, doxorubicin, and bleomycin, chemotherapeutic agents with different modes of action. The expression of fas ligand, caspase3 and baxalpha changes differentially or remain unaffected. tTG expression did not change significantly after administration of either the peroxisome proliferator activated receptor-alpha agonist WY14643 or the retinoid X receptor-specific analog LG 100268. However, both compounds blocked drug-induced tTG induction without affecting the extent of cell death. The pleiotropic cytokine interleukin-6 effectively rescued hepatoma cells from apoptosis while tTG induction still took place, along with the induction of antiapoptotic transcripts bcl-x(L), gp130, and her2/neu. These results suggest that the induction of tTG, although present in drug-induced apoptosis, is pharmacologically dissociable from the early, initiating events of apoptosis. Blocking the induction of tTG during drug-induced cell death may alleviate limiting side effects of anticancer agents, including fibrosis and neuropathies.