Zbtb7c is a critical gluconeogenic transcription factor that induces glucose-6-phosphatase and phosphoenylpyruvate carboxykinase 1 genes expression during mice fasting.

Gluconeogenesis is essential for blood glucose homeostasis during fasting and is regulated by various enzymes, which are encoded by gluconeogenic genes. Those genes are controlled by various transcription factors. Zinc finger and BTB domain-containing 7c (Zbtb7c, also called Kr-pok) is a BTB-POZ family transcription factor with proto-oncogenic activity. Previous findings have indicated that Zbtb7c is involved in the regulation of fatty acid biosynthesis, suggesting an involvement also in primary metabolism. We found here that fasting induced Zbtb7c expression in the mouse liver and in primary liver hepatocytes. We also observed that Zbtb7c-knockout mice have decreased blood glucose levels, so we investigated whether Zbtb7c plays a role in gluconeogenesis. Indeed, differential gene expression analysis of Zbtb7c-knockout versus wild type mouse livers showed downregulated transcription of gluconeogenic genes encoding the glucose 6-phosphatase catalytic subunit (G6pc) and phosphoenolpyruvate carboxykinase 1 (Pck1), while Zbtb7c expression upregulated these two genes, under fasting conditions. Mechanistically, we found that when complexed with histone deacetylase 3 (Hdac3), Zbtb7c binds insulin response elements (IREs) within the G6pc and Pck1 promoters. Moreover, complexed Zbtb7c deacetylated forkhead box O1 (Foxo1), thereby increasing Foxo1 binding to the G6pc and Pck1 IREs, resulting in their transcriptional activation. These results demonstrate Zbtb7c to be a crucial metabolic regulator of blood glucose homeostasis, during mammalian fasting.

ZNF509S1 downregulates PUMA by inhibiting p53K382 acetylation and p53-DNA binding.

Expression of the POK family protein ZNF509L, and -its S1 isoform, is induced by p53 upon exposure to genotoxic stress. Due to alternative splicing of the ZNF509 primary transcript, ZNF509S1 lacks the 6 zinc-fingers and C-terminus of ZNF509L, resulting in only one zinc-finger. ZNF509L and -S1 inhibit cell proliferation by activating p21/CDKN1A and RB transcription, respectively. When cells are exposed to severe DNA damage, p53 activates PUMA (p53-upregulated modulator of apoptosis) transcription. Interestingly, apoptosis due to transcriptional activation of PUMA by p53 is attenuated by ZNF509S1. Thus we investigated the molecular mechanism(s) underlying the transcriptional attenuation and anti-apoptotic effects of ZNF509S1. We show that ZNF509S1 modulation of p53 activity is important in PUMA gene transcription by modulating post-translational modification of p53 by p300. ZNF509S1 directly interacts with p53 and inhibits p300-mediated acetylation of p53 lysine K382, with deacetylation of p53 K382 leading to decreased DNA binding at the p53 response element 1 of the PUMA promoter. ZNF509S1 may play a role not only in cell cycle arrest, by activating RB expression, but also in rescuing cells from apoptotic death by repressing PUMA expression in cells exposed to severe DNA damage.

Kr-POK (ZBTB7c) regulates cancer cell proliferation through glutamine metabolism.

Kr-POK (ZBTB7c) is a kidney cancer-related POK transcription factor that not only represses transcription of CDKN1A but also increases expression of FASN. However, precisely how Kr-POK affects cell metabolism by controlling gene expression in response to an energy source in rapidly proliferating cells remains unknown. In this study, we characterized the molecular and functional features of Kr-POK in the context of tumor growth and glutamine metabolism. We found that cells expressing Kr-POK shRNA exhibited more severe cell death than control cells in glucose-deprived medium, and that knockdown of Kr-POK decreased glutamine uptake. Glutamine is critical for tumor cell proliferation. Glutaminase (GLS1), which is activated by p-STAT1, catalyzes the initial reaction in the pathway of glutaminolysis. Kr-POK interacts with PIAS1 to disrupt the interaction between PIAS1 and p-STAT1, and free p-STAT1 can activate GLS1 transcription through an interaction with p300. Kr-POK can be also sumoylated by PIAS1, facilitating Kr-POK degradation by the ubiquitin-mediated proteasomal pathway. Finally, we showed that repression of Kr-POK inhibited tumor growth in vivo in a xenograft model by repressing GLS1 expression. Taken together, our data reveal that Kr-POK activates GLS1 transcription and increases glutamine uptake to support rapid cancer cell proliferation.

Zbtb7c is a molecular 'off' and 'on' switch of Mmp gene transcription.

Matrix metalloproteinases (MMPs) are zinc-containing endopeptidases that play roles in cell proliferation, migration, differentiation, angiogenesis, and apoptosis. The expression of MMP gene is tightly regulated and shows cell- and tissue-specific expression patterns. Despite their differential expression, MMP genes have AP-1 (activator protein-1) binding elements within their promoters. Interestingly, c-JUN phosphorylation by cytokine signaling decreased its interaction with NCoR, but increased its interaction with p300, resulting in activation of MMP gene transcription. Here, we found that Zbtb7c (Kr-pok) is a critical component of a transcriptional repressor complex containing c-Jun and NCoR. c-Jun, bound at AP-1, interacts with Zbtb7c, which in turn recruits an NCoR/Hdac3 complex to repress several Mmp (-8, -10, -13, and -16) genes. The molecular interaction between c-Jun and Zbtb7c also prevents phosphorylation of c-Jun by p-Jnk, However, Zbtb7c phosphorylation by p-Jnk (induced by TNFα), and its (Zbtb7c) subsequent degradation by the ubiquitin-mediated proteasomal pathway, leads to c-Jun phosphorylation by p-Jnk. Promoter-bound p-c-Jun then recruits the coactivator p300 to upregulate Mmp gene. Overall, these findings show that Zbtb7c is a key molecule that recruits an NCoR/Hdac3 complex to inhibit phosphorylation of c-Jun, and thereby repress Mmp gene expression.

Two ZNF509 (ZBTB49) isoforms induce cell-cycle arrest by activating transcription of p21/CDKN1A and RB upon exposure to genotoxic stress.

ZNF509 is unique among POK family proteins in that four isoforms are generated by alternative splicing. Short ZNF509 (ZNF509S1, -S2 and -S3) isoforms contain one or two out of the seven zinc-fingers contained in long ZNF509 (ZNF509L). Here, we investigated the functions of ZNF509 isoforms in response to DNA damage, showing isoforms to be induced by p53. Intriguingly, to inhibit proliferation of HCT116 and HEK293 cells, we found that ZNF509L activates p21/CDKN1A transcription, while ZNF509S1 induces RB. ZNF509L binds to the p21/CDKN1A promoter either alone or by interacting with MIZ-1 to recruit the co-activator p300 to activate p21/CDKN1A transcription. In contrast, ZNF509S1 binds to the distal RB promoter to interact and interfere with the MIZF repressor, resulting in derepression and transcription of RB. Immunohistochemical analysis revealed that ZNF509 is highly expressed in normal epithelial cells, but was completely repressed in tumor tissues of the colon, lung and skin, indicating a possible role as a tumor suppressor.

Promyelocytic leukemia zinc finger-retinoic acid receptor α (PLZF-RARα), an oncogenic transcriptional repressor of cyclin-dependent kinase inhibitor 1A (p21WAF/CDKN1A) and tumor protein p53 (TP53) genes.

Promyelocytic leukemia zinc finger-retinoic acid receptor α (PLZF-RARα) is an oncogene transcriptional repressor that is generated by a chromosomal translocation between the PLZF and RARα genes in acute promyelocytic leukemia (APL-type) patients. The molecular interaction between PLZF-RARα and the histone deacetylase corepressor was proposed to be important in leukemogenesis. We found that PLZF-RARα can repress transcription of the p21WAF/CDKN1A gene, which encodes the negative cell cycle regulator p21 by binding to its proximal promoter Sp1-binding GC-boxes 3, 4, 5/6, a retinoic acid response element (RARE), and distal p53-responsive elements (p53REs). PLZF-RARα also acts as a competitive transcriptional repressor of p53, RARα, and Sp1. PLZF-RARα interacts with co-repressors such as mSin3A, NCoR, and SMRT, thereby deacetylating histones Ac-H3 and Ac-H4 at the CDKN1A promoter. PLZF-RARα also interacts with the MBD3-NuRD complex, leading to epigenetic silencing of CDKN1A through DNA methylation. Furthermore, PLZF-RARα represses TP53 and increases p53 protein degradation by ubiquitination, further repressing p21 expression. Resultantly, PLZF-RARα promotes cell proliferation and significantly increases the number of cells in S-phase.

Human Kruppel-related 3 (HKR3) is a novel transcription activator of alternate reading frame (ARF) gene.

HKR3 (Human Krüppel-related 3) is a novel POK (POZ-domain Krüppel-like zinc-finger) family transcription factor. Recently, some of the POK (POZ-domain Krüppel-like zinc finger) family proteins have been shown to play roles in cell cycle arrest, apoptosis, cell proliferation, and oncogenesis. We investigated whether HKR3, an inhibitor of cell proliferation and an uncharacterized POK family protein, could regulate the cell cycle by controlling expression of genes within the p53 pathway (ARF-MDM2-TP53-p21WAF/CDKN1A). HKR3 potently activated the transcription of the tumor suppressor gene ARF by acting on the proximal promoter region (bp, -149∼+53), which contains Sp1 and FBI-1 binding elements (FREs). HKR3 interacted with the co-activator p300 to activate ARF transcription, which increased the acetylation of histones H3 and H4 within the proximal promoter. Oligonucleotide pull-down assays and ChIP assays revealed that HKR3 interferes with the binding of the proto-oncogenic transcription repressor FBI-1 to proximal FREs, thus derepressing ARF transcription.

Kaiso is a key regulator of spleen germinal center formation by repressing Bcl6 expression in splenocytes.

Kaiso was previously described as a methylated DNA-binding protein and a transcription repressor interacting with the corepressor protein complex NCoR. In the current study, we show that generation-3 Kaiso knockout mice show a phenotype of splenomegaly and large diffused germinal centers (GC). In the spleens of Kaiso knockout mice, Bcl6 (a transcriptional repressor that plays a critical role in GC development in spleen) and c-Myc were highly expressed, while the cell cycle arrest genes p27 (CDKN1B), p21 (CDKN1A) and Gadd45a were downregulated. Chromatin immunoprecipitation (ChIP) and transcription assays suggested that Kaiso represses Bcl6 expression, and in Kaiso knockout mice, derepressed Bcl6 increased cell proliferation by suppressing p27 (CDKN1B), p21 (CDKN1A) and Gadd45a, while upregulating the oncogene c-Myc. Further evidence for Kaiso regulation of splenomegaly was provided by B lymphocyte Ramos cells, in which ectopic KAISO repressed BCL6 and c-MYC expression, while concomitantly increasing the expression of the cell cycle arrestors p21, p27 and Gadd45a. In summary, derepressed Bcl6 expression may be responsible for increases in GC cell proliferation and splenomegaly of Kaiso knockout mice.

The proto-oncoprotein FBI-1 interacts with MBD3 to recruit the Mi-2/NuRD-HDAC complex and BCoR and to silence p21WAF/CDKN1A by DNA methylation.

The tumour-suppressor gene CDKN1A (encoding p21Waf/Cip1) is thought to be epigenetically repressed in cancer cells. FBI-1 (ZBTB7A) is a proto-oncogenic transcription factor repressing the alternative reading frame and p21WAF/CDKN1A genes of the p53 pathway. FBI-1 interacts directly with MBD3 (methyl-CpG-binding domain protein 3) in the nucleus. We demonstrated that FBI-1 binds both non-methylated and methylated DNA and that MBD3 is recruited to the CDKN1A promoter through its interaction with FBI-1, where it enhances transcriptional repression by FBI-1. FBI-1 also interacts with the co-repressors nuclear receptor corepressor (NCoR), silencing mediator for retinoid and thyroid receptors (SMRT) and BCL-6 corepressor (BCoR) to repress transcription. MBD3 regulates a molecular interaction between the co-repressor and FBI-1. MBD3 decreases the interaction between FBI-1 and NCoR/SMRT but increases the interaction between FBI-1 and BCoR. Because MBD3 is a subunit of the Mi-2 autoantigen (Mi-2)/nucleosome remodelling and histone deacetylase (NuRD)-HDAC complex, FBI-1 recruits the Mi-2/NuRD-HDAC complex via MBD3. BCoR interacts with the Mi-2/NuRD-HDAC complex, DNMTs and HP1. MBD3 and BCoR play a significant role in the recruitment of the Mi-2/NuRD-HDAC complex- and the NuRD complex-associated proteins, DNMTs and HP. By recruiting DNMTs and HP1, Mi-2/NuRD-HDAC complex appears to play key roles in epigenetic repression of CDKN1A by DNA methylation.

Kr-pok increases FASN expression by modulating the DNA binding of SREBP-1c and Sp1 at the proximal promoter.

Kr-pok (kidney cancer-related POZ domain and Krüppel-like protein) is a new proto-oncogenic POZ-domain transcription factor. Fatty acid synthase gene (FASN) encodes one of the key enzymes in fatty acids synthesis and is the only enzyme that synthesizes fatty acids in cancer cells. Sp1 and SREBP-1c are the two major transcription activators of FASN. We investigated whether Kr-pok modulates transcription of the FASN. FASN expression is significantly decreased in Kr-pok knockout murine embryonic fibroblasts. Coimmunoprecipitation, GST fusion protein pull-down, and immunocytochemistry assays show that the zinc-finger domain of Kr-pok interacts directly with the bZIP DNA binding domain of SREBP-1. Electrophoretic mobility shift assay, oligonucleotide pull-down, and chromatin immunoprecipitation assays showed that Kr-pok changes the transcription factor binding dynamics of Sp1 and SREBP-1c to the SRE/E-box elements of the proximal promoter. We found that Kr-pok expression increased during 3T3-L1 preadipocyte differentiation and that FASN expression is decreased by the knockdown of Kr-pok. Kr-pok facilitates the SREBP-1c-mediated preadipocyte differentiation and/or fatty acid synthesis. Kr-pok may act as an important regulator of fatty acid synthesis and may induce rapid cancer cell proliferation by increasing palmitate synthesis.