HIF-1-Dependent Induction of Jumonji Domain-Containing Protein (JMJD) 3 under Hypoxic Conditions

Jumonji domain-containing proteins (JMJD) catalyze the oxidative demethylation of a methylated lysine residue of histones by using O₂, α-ketoglutarate, vitamin C, and Fe(II). Several JMJDs are induced by hypoxic stress to compensate their presumed reduction in catalytic activity under hypoxia. In this study, we showed that an H3K27me3 specific histone demethylase, JMJD3 was induced by hypoxia-inducible factor (HIF)-1α/β under hypoxia and that treatment with Clioquinol, a HIF-1α activator, increased JMJD3 expression even under normoxia. Chromatin immunoprecipitation (ChIP) analyses showed that both HIF-1α and its dimerization partner HIF-1β/Arnt occupied the first intron region of the mouse JMJD3 gene, whereas the HIF-1α/β heterodimer bound to the upstream region of the human JMJD3, indicating that human and mouse JMJD3 have hypoxia-responsive regulatory regions in different locations. This study shows that both mouse and human JMJD3 are induced by HIF-1.     

BCR/ABL Recruits p53 Tumor Suppressor Protein to Induce Drug Resistance

Tumors expressing the ABL oncoproteins (BCR/ABL, TEL/ABL, v-ABL) can avoidapoptosis triggered by DNA damaging agents. The tumor suppressor protein p53 is animportant activator of apoptosis in normal cells; conversely its functional loss may causedrug resistance. The ABL oncoprotein - p53 paradigm represents the relationship between anoncogenic tyrosine kinase and a tumor suppressor gene. Here we show that BCR/ABLoncoproteins employ p53 to induce resistance to DNA damage in myeloid leukemia cells.Cells transformed by the ABL oncoproteins displayed accumulation of p53 upon DNAdamage. In contrast, only a modest increase of p53 expression followed by activation ofcaspase-3 were detected in normal cells expressing endogenous c-ABL. Phosphatidylinositol-3 kinase-like protein kinases (ATR and also ATM) -dependent phosphorylation of p53-Ser15residue was associated with the accumulation of p53, and stimulation of p21Waf-1 andGADD45, resulting in G2/M delay in BCR/ABL cells after genotoxic treatment. Inhibition ofp53 by siRNA or by the temperature-sensitive mutation reduced G2/M accumulation anddrug resistance of BCR/ABL cells. In conclusion, accumulation of the p53 proteincontributed to prolonged G2/M checkpoint activation and drug resistance in myeloid cellsexpressing the BCR/ABL oncoproteins.     

Histone Demethylase Jumonji AT-rich Interactive Domain 1B (JARID1B) Controls Mammary Gland Development by Regulating Key Developmental and Lineage Specification Genes

The JmjC domain-containing H3K4 histone demethylase jumonji AT-rich interactive domain 1B (JARID1B) (also known as KDM5B and PLU1) is overexpressed in breast cancer and is a potential target for breast cancer treatment. To investigate the in vivo function of JARID1B, we developed Jarid1b^−/− mice and characterized their phenotypes in detail. Unlike previously reported Jarid1b^−/− strains, the majority of these Jarid1b^−/− mice were viable beyond embryonic and neonatal stages. This allowed us to further examine phenotypes associated with the loss of JARID1B in pubertal development and pregnancy. These Jarid1b^−/− mice exhibited decreased body weight, premature mortality, decreased female fertility, and delayed mammary gland development. Related to these phenotypes, JARID1B loss decreased serum estrogen level and reduced mammary epithelial cell proliferation in early puberty. In mammary epithelial cells, JARID1B loss diminished the expression of key regulators for mammary morphogenesis and luminal lineage specification, including FOXA1 and estrogen receptor α. Mechanistically, JARID1B was required for GATA3 recruitment to the Foxa1 promoter to activate Foxa1 expression. These results indicate that JARID1B positively regulates mammary ductal development through both extrinsic and cell-autonomous mechanisms.     

The Positive Regulation of p53 by the Tumor Suppressor VHL

The ubiquitin-mediated degradation of hypoxia-inducible factor-α (HIF-α) by a von Hippel-Lindau tumor suppressor protein (pVHL) is mechanistically responsible for controlling gene expression due to oxygen availability. Germline mutations in the VHL gene cause dysregulation of HIF and induce an autosomal dominant cancer syndrome referred to as VHL disease. However, it is unclear whether HIF accumulation caused by VHL mutations is sufficient for tumorigenesis. Recently, we found that pVHL directly associates and positively regulates the tumor suppressor p53 by inhibiting Mdm2-mediated ubiquitination, and by subsequently recruiting p53-modifying enzymes. Moreover, VHL-deleted RCC cells showed attenuated apoptosis or abnormal cell-cycle arrest upon DNA damage, but became normal when pVHL was restored. Thus, pVHL appears to play a pivotal role in tumor suppression by participating actively as a component of p53 transactivation complex during DNA damage response.     

Control of Tumor Suppressor p53 Function by Endoplasmic Reticulum Stress

Alterations in the homeostasis of the endoplasmic reticulum (ER) by various forms of stress can lead to the accumulation of unfolded proteins and protein aggregates that are detrimental to cell survival. Eukaryotic cells can adapt to ER stress by activating specific signalling pathways and mechanisms, whose primary purpose is to limit the accumulation of unfolded proteins in the ER. We recently reported a novel mechanism of cell adaptation to ER stress, which proceeds through the inhibition of the apoptotic function of the tumour suppressor p53 [Genes & Development 2004;18:261-277]. We found that ER stress increases the cytoplasmic localization and enhances the destabilization of the tumour suppressor. This process requires the phosphorylation of p53 at serine 315 and serine 376, which is mediated by the activation of glycogen synthase kinase-3beta (GSK-3?). ER stress also prevents p53 activation and p53-mediated apoptosis in response to DNA damage. These findings demonstrate that ER stress utilizes mechanisms that are distinct from other types of stress to modulate p53. In addition, they reveal that ER stress and nuclear DNA damage can induce inter-organellar cross-talk pathways targeting p53 with important implications for the treatment of tumours with dysfunctional ER.     

Paracrine Apoptotic Effect of p53 Mediated by Tumor Suppressor Par-4

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