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• 1. [Article] The cyclin E regulator cullin 3 prevents mouse hepatic progenitor cells from becoming tumor-initiating cells

  Cyclin E is often overexpressed in cancer tissue, leading to genetic instability and aneuploidy. Cullin 3 (Cul3) is a component of the BTB-Cul3-Rbx1 (BCR) ubiquitin ligase that is involved in the turnover ...

  Citation × Citation Citation Abstract Copy Title: The cyclin E regulator cullin 3 prevents mouse hepatic progenitor cells from becoming tumor-initiating cells Author: Kossatz, Uta, Breuhahn, Kai, Wolf, Benita, Hardtke-Wolenski, Matthias, Wilkens, Ludwig, Steinemann, Doris, Singer, Stephan, Brass, Felicitas, Kubicka, Stefan, Schlegelberger, Brigitte, Schirmacher, Peter, Manns, Michael P., Singer, Jeffrey D., Malek, Nisar P. Year: 2010 Cyclin E is often overexpressed in cancer tissue, leading to genetic instability and aneuploidy. Cullin 3 (Cul3) is a component of the BTB-Cul3-Rbx1 (BCR) ubiquitin ligase that is involved in the turnover of cyclin E. Here we show that liver-specific ablation of Cul3 in mice results in the persistence and massive expansion of hepatic progenitor cells. Upon induction of differentiation, Cul3-deficient progenitor cells underwent substantial DNA damage in vivo and in vitro, thereby triggering the activation of a cellular senescence response that selectively blocked the expansion of the differentiated offspring. Positive selection of undifferentiated progenitor cells required the expression of the tumor suppressor protein p53. Simultaneous loss of Cul3 and p53 in hepatic progenitors turned these cells into highly malignant tumor-initiating cells that formed largely undifferentiated tumors in nude mice. In addition, loss of Cul3 and p53 led to the formation of primary hepatocellular carcinomas. Importantly, loss of Cul3 expression was also detected in a large series of human liver cancers and correlated directly with tumor de-differentiation. The expression of Cul3 during hepatic differentiation therefore safeguards against the formation of progenitor cells that carry a great potential for transformation into tumor-initiating cells. Title: The cyclin E regulator cullin 3 prevents mouse hepatic progenitor cells from becoming tumor-initiating cells Author: Kossatz, Uta, Breuhahn, Kai, Wolf, Benita, Hardtke-Wolenski, Matthias, Wilkens, Ludwig, Steinemann, Doris, Singer, Stephan, Brass, Felicitas, Kubicka, Stefan, Schlegelberger, Brigitte, Schirmacher, Peter, Manns, Michael P., Singer, Jeffrey D., Malek, Nisar P. Year: 2010 Cyclin E is often overexpressed in cancer tissue, leading to genetic instability and aneuploidy. Cullin 3 (Cul3) is a component of the BTB-Cul3-Rbx1 (BCR) ubiquitin ligase that is involved in the turnover of cyclin E. Here we show that liver-specific ablation of Cul3 in mice results in the persistence and massive expansion of hepatic progenitor cells. Upon induction of differentiation, Cul3-deficient progenitor cells underwent substantial DNA damage in vivo and in vitro, thereby triggering the activation of a cellular senescence response that selectively blocked the expansion of the differentiated offspring. Positive selection of undifferentiated progenitor cells required the expression of the tumor suppressor protein p53. Simultaneous loss of Cul3 and p53 in hepatic progenitors turned these cells into highly malignant tumor-initiating cells that formed largely undifferentiated tumors in nude mice. In addition, loss of Cul3 and p53 led to the formation of primary hepatocellular carcinomas. Importantly, loss of Cul3 expression was also detected in a large series of human liver cancers and correlated directly with tumor de-differentiation. The expression of Cul3 during hepatic differentiation therefore safeguards against the formation of progenitor cells that carry a great potential for transformation into tumor-initiating cells. Close

• 2. [Article] Pcif1 modulates Pdx1 protein stability and pancreatic β cell function and survival in mice

  The homeodomain transcription factor pancreatic duodenal homeobox 1 (Pdx1) is a major mediator of insulin transcription and a key regulator of the β cell phenotype. Heterozygous mutations in PDX1 are associated ...

  Citation × Citation Citation Abstract Copy Title: Pcif1 modulates Pdx1 protein stability and pancreatic β cell function and survival in mice Author: Claiborn, Kathryn C., Sachdeva, Mira M., Cannon, Corey E., Groff, David N., Singer, Jeffrey D., Stoffers, Doris A. Year: 2010 The homeodomain transcription factor pancreatic duodenal homeobox 1 (Pdx1) is a major mediator of insulin transcription and a key regulator of the β cell phenotype. Heterozygous mutations in PDX1 are associated with the development of diabetes in humans. Understanding how Pdx1 expression levels are controlled is therefore of intense interest in the study and treatment of diabetes. Pdx1 C terminus–interacting factor-1 (Pcif1, also known as SPOP) is a nuclear protein that inhibits Pdx1 transactivation. Here, we show that Pcif1 targets Pdx1 for ubiquitination and proteasomal degradation. Silencing of Pcif1 increased Pdx1 protein levels in cultured mouse β cells, and Pcif1 heterozygosity normalized Pdx1 protein levels in Pdx1+/– mouse islets, thereby increasing expression of key Pdx1 transcriptional targets. Remarkably, Pcif1 heterozygosity improved glucose homeostasis and β cell function and normalized β cell mass in Pdx1+/– mice by modulating β cell survival. These findings indicate that in adult mouse β cells, Pcif1 limits Pdx1 protein accumulation and thus the expression of insulin and other gene targets important in the maintenance of β cell mass and function. They also provide evidence that targeting the turnover of a pancreatic transcription factor in vivo can improve glucose homeostasis. Title: Pcif1 modulates Pdx1 protein stability and pancreatic β cell function and survival in mice Author: Claiborn, Kathryn C., Sachdeva, Mira M., Cannon, Corey E., Groff, David N., Singer, Jeffrey D., Stoffers, Doris A. Year: 2010 The homeodomain transcription factor pancreatic duodenal homeobox 1 (Pdx1) is a major mediator of insulin transcription and a key regulator of the β cell phenotype. Heterozygous mutations in PDX1 are associated with the development of diabetes in humans. Understanding how Pdx1 expression levels are controlled is therefore of intense interest in the study and treatment of diabetes. Pdx1 C terminus–interacting factor-1 (Pcif1, also known as SPOP) is a nuclear protein that inhibits Pdx1 transactivation. Here, we show that Pcif1 targets Pdx1 for ubiquitination and proteasomal degradation. Silencing of Pcif1 increased Pdx1 protein levels in cultured mouse β cells, and Pcif1 heterozygosity normalized Pdx1 protein levels in Pdx1+/– mouse islets, thereby increasing expression of key Pdx1 transcriptional targets. Remarkably, Pcif1 heterozygosity improved glucose homeostasis and β cell function and normalized β cell mass in Pdx1+/– mice by modulating β cell survival. These findings indicate that in adult mouse β cells, Pcif1 limits Pdx1 protein accumulation and thus the expression of insulin and other gene targets important in the maintenance of β cell mass and function. They also provide evidence that targeting the turnover of a pancreatic transcription factor in vivo can improve glucose homeostasis. Close

• 3. [Article] Cullin-3 Targets Cyclin E for Ubiquitination and Controls S Phase in Mammalian Cells

  Cyclin E is an unstable protein that is degraded in a ubiquitin- and proteasome- dependent pathway. Two factors stimulate cyclin E ubiquitination in vivo: when it is free of its CDK partner, and when it ...

  Citation × Citation Citation Abstract Copy Title: Cullin-3 Targets Cyclin E for Ubiquitination and Controls S Phase in Mammalian Cells Author: Singer, Jeffrey D., Gurian-West, Mark, Clurman, Bruce, Roberts, James M. Year: 1999 Cyclin E is an unstable protein that is degraded in a ubiquitin- and proteasome- dependent pathway. Two factors stimulate cyclin E ubiquitination in vivo: when it is free of its CDK partner, and when it is phosphorylated on threonine 380. We pursued the first of these pathways by using a two-hybrid screen to identify proteins that could bind only to free cyclin E. This resulted in the isolation of human Cul-3, a member of the cullin family of E3 ubiquitin–protein ligases. We found that Cul-3 was bound to cyclin E but not to cyclin E-Cdk2 complexes in mammalian cells, and that overexpression of Cul-3 increased ubiquitination of cyclin E but not other cyclins. Conversely, deletion of the Cul-3 gene in mice caused increased accumulation of cyclin E protein, and had cell-type-specific effects on S-phase regulation. In the extraembryonic ectoderm, in which cells undergo a standard mitotic cycle, there was a greatly increased number of cells in S phase. In the trophectoderm, in which cells go through endocycles, there was a block to entry into S phase. The SCF pathway, which targets cyclins for ubiquitination on the basis of their phosphorylation state, and the Cul-3 pathway, which selects cyclin E for ubiquitination on the basis of its assembly into CDK complexes, may be complementary ways to control cyclin abundance. Title: Cullin-3 Targets Cyclin E for Ubiquitination and Controls S Phase in Mammalian Cells Author: Singer, Jeffrey D., Gurian-West, Mark, Clurman, Bruce, Roberts, James M. Year: 1999 Cyclin E is an unstable protein that is degraded in a ubiquitin- and proteasome- dependent pathway. Two factors stimulate cyclin E ubiquitination in vivo: when it is free of its CDK partner, and when it is phosphorylated on threonine 380. We pursued the first of these pathways by using a two-hybrid screen to identify proteins that could bind only to free cyclin E. This resulted in the isolation of human Cul-3, a member of the cullin family of E3 ubiquitin–protein ligases. We found that Cul-3 was bound to cyclin E but not to cyclin E-Cdk2 complexes in mammalian cells, and that overexpression of Cul-3 increased ubiquitination of cyclin E but not other cyclins. Conversely, deletion of the Cul-3 gene in mice caused increased accumulation of cyclin E protein, and had cell-type-specific effects on S-phase regulation. In the extraembryonic ectoderm, in which cells undergo a standard mitotic cycle, there was a greatly increased number of cells in S phase. In the trophectoderm, in which cells go through endocycles, there was a block to entry into S phase. The SCF pathway, which targets cyclins for ubiquitination on the basis of their phosphorylation state, and the Cul-3 pathway, which selects cyclin E for ubiquitination on the basis of its assembly into CDK complexes, may be complementary ways to control cyclin abundance. Close