Tryptanthrin inhibits MDR1 and reverses doxorubicin resistance in breast cancer cells

Development of agents to overcome multidrug resistance (MDR) is important in cancer chemotherapy. Up to date, few chemicals have been reported to down-regulate MDR1 gene expression. We evaluated the effect of tryptanthrin on P-glycoprotein (P-gp)-mediated MDR in a breast cancer cell line MCF-7. Tryptanthrin could depress overexpression of MDR1 gene. We observed reduction of P-gp protein in parallel with decreases in mRNA in MCF-7/adr cells treated with tryptanthrin. Tryptanthrin suppressed the activity of MDR1 gene promoter. Tryptanthrin also enhanced interaction of the nuclear proteins with the negatively regulatory CAAT region of MDR1 gene promoter in MCF-7/adr. It might result in suppression of MDR1 gene. In addition, tryptanthrin decreased the amount of mutant p53 protein with decreasing mutant p53 protein stability. It might contribute to negative regulation of MDR1 gene. In conclusion, tryptanthrin exhibited MDR reversing effect by down-regulation of MDR1 gene and might be a new adjuvant agent for chemotherapy.     

Increasing drug resistance in human lung cancer cells by mutant-type p53 gene mediated by retrovirus

Human mutant-type (mt) p53 cDNA was synthesized and cloned from human lung cancer cell line GL containing mt-p53 gene by using polymerase chain reaction (PCR). It was confirmed that the mt-p53 cDNA con-tained the complete coding sequence of p53 gene but mutated at codon 245 (G→T) and resulted in glycine to cysteine by sequencing analysis. The retroviral vector pD53M of the mt-p53 was constructed and introduced into the drug-sen-sitive human lung cancer cells GAO in which p53 gene did not mutate. The transfected GAO cells strongly expressed mutant-type p53 protein by immunohistochemistry, showing that pD53M vector could steadily express in GAO cells. The drug resistance to several anticancer agents of GAO cells infected by pD53M increased in varying degrees, with the highest increase of 4-fold, in vitro and in vivo. By quantitative PCR and flow cytometry (FCM) analyses, the expression of MDR1 gene and the activity of P-glycoprotein (Pgp) did not increase, the expression of MRP gene and the activity of m     

Tumour suppressor p53 down-regulates the expression of the human hepatocyte nuclear factor 4alpha (HNF4alpha) gene

The liver is exposed to a wide variety of toxic agents, many of which damage DNA and result in increased levels of the tumour suppressor protein p53. We have previously shown that p53 inhibits the transactivation function of HNF (hepatocyte nuclear factor) 4alpha1, a nuclear receptor known to be critical for early development and liver differentiation. In the present study we demonstrate that p53 also down-regulates expression of the human HNF4alpha gene via the proximal P1 promoter. Overexpression of wild-type p53 down-regulated endogenous levels of both HNF4alpha protein and mRNA in Hep3B cells. This decrease was also observed when HepG2 cells were exposed to UV irradiation or doxorubicin, both of which increased endogenous p53 protein levels. Ectopically expressed p53, but not a mutant p53 defective in DNA binding (R249S), down-regulated HNF4alpha P1 promoter activity. Chromatin immunoprecipitation also showed that endogenous p53 bound the HNF4alpha P1 promoter in vivo after doxorubicin treatment. The mechanism by which p53 down-regulates the P1 promoter appears to be multifaceted. The down-regulation was partially recovered by inhibition of HDAC activity and appears to involve the positive regulator HNF6alpha. p53 bound HNF6alpha in vivo and in vitro and prevented HNF6alpha from binding DNA in vitro. p53 also repressed stimulation of the P1 promoter by HNF6alpha in vivo. However, since the R249S p53 mutant also bound HNF6alpha, binding HNF6alpha is apparently not sufficient for the repression. Implications of the p53-mediated repression of HNF4alpha expression in response to cellular stress are discussed.     

Sp1 plays a critical role in the transcriptional activation of the human cyclin-dependent kinase inhibitor p21(WAF1/Cip1) gene by the p53 tumor suppressor protein

In the present study we present evidence for the critical role of Sp1 in the mechanism of transactivation of the human cell cycle inhibitor p21(WAF1/Cip1) (p21) gene promoter by the tumor suppressor p53 protein. We found that the distal p53-binding site of the p21 promoter acts as an enhancer on the homologous or heterologous promoters in hepatoma HepG2 cells. In transfection experiments, p53 transactivated the p21 promoter in HaCaT cells that express Sp1 but have a mutated p53 form. In contrast, p53 could not transactivate the p21 promoter in the Drosophila embryo-derived Schneider's SL2 cells that lack endogenous Sp1 or related factors. Cotransfection of SL2 cells with p53 and Sp1 resulted in a synergistic transactivation of the p21 promoter. Synergistic transactivation was greatly decreased in SL2 cells and HaCaT cells by mutations in either the p53-binding site or in the -82/-77 Sp1-binding site indicating functional cooperation between Sp1 and p53 in the transactivation of the p21 promoter. Synergistic transactivation was also decreased by mutations in the transactivation domain of p53. Physical interactions between Sp1 and p53 proteins were established by glutathione S-transferase pull-down and coimmunoprecipitation assays. By using deletion mutants we found that the DNA binding domain of Sp1 is required for its physical interaction with p53. In conclusion, Sp1 must play a critical role in regulating important biological processes controlled by p53 via p21 gene activation such as DNA repair, cell growth, differentiation, and apoptosis.