Laura J. Taylor

Nucleolar Arf sequesters Mdm2 and activates p53

The Ink4/Arf locus encodes two tumour-suppressor proteins, p16Ink4a and p19Arf, that govern the antiproliferative functions of the retinoblastoma and p53 proteins, respectively. Here we show that Arf binds to the product of the Mdm2 gene and sequesters it into the nucleolus, thereby preventing negative-feedback regulation of p53 by Mdm2 and leading to the activation of p53 in the nucleoplasm. Arf and Mdm2 co-localize in the nucleolus in response to activation of the oncoprotein Myc and as mouse fibroblasts undergo replicative senescence. These topological interactions of Arf and Mdm2 point towards a new mechanism for p53 activation.

Redox-dependent downregulation of Rho by Rac

Rac and Rho GTPases function as critical regulators of actin cytoskeleton remodelling during cell spreading and migration. Here we demonstrate that Rac-mediated reactive oxygen species (ROS) production results in the downregulation of Rho activity. The redox-dependent decrease in Rho activity is required for Rac-induced formation of membrane ruffles and integrin-mediated cell spreading. The pathway linking generation of ROS to downregulation of Rho involves inhibition of the low-molecular-weight protein tyrosine phosphatase (LMW-PTP) and then an increase in the tyrosine phosphorylation and activation of its target, p190Rho-GAP. Our findings define a novel mechanism for the coupling of changes in cellular redox state to the control of actin cytoskeleton rearrangements by Rho GTPases.

Wild-type H- and N-Ras promote mutant K-Ras driven tumorigenesis by modulating the DNA damage response

Mutations in KRAS are prevalent in human cancers and universally predictive of resistance to anticancer therapeutics. Although it is widely accepted that acquisition of an activating mutation endows RAS genes with functional autonomy, recent studies suggest that the wild-type forms of Ras may contribute to mutant Ras-driven tumorigenesis. Here, we show that downregulation of wild-type H-Ras or N-Ras in mutant K-Ras cancer cells leads to hyperactivation of the Erk/p90RSK and PI3K/Akt pathways and, consequently, the phosphorylation of Chk1 at an inhibitory site, Ser 280. The resulting inhibition of ATR/Chk1 signaling abrogates the activation of the G2 DNA damage checkpoint and confers specific sensitization of mutant K-Ras cancer cells to DNA damage chemotherapeutic agents in vitro and in vivo.

Feedback Regulation of Ras Signaling by Rabex-5-Mediated Ubiquitination

Ras proteins play a central role in transducing signals that control cell proliferation, differentiation, motility, and survival. The location-specific signaling activity of Ras has been previously shown to be regulated by ubiquitination [1]. However, the molecular machinery that controls Ras ubiquitination has not been defined. Here we demonstrate through biochemical and functional analyses that Rabex-5 (also known as RabGEF1) [2, 3] functions as an E3 ligase for Ras. Rabex-5-mediated Ras ubiquitination promotes Ras endosomal localization and leads to the suppression of ERK activation. Moreover, the Ras effector RIN1 [4, 5] is required for Rabex-5-dependent Ras ubiquitination, suggesting a feedback mechanism by which Ras activation can be coupled to ubiquitination. These findings define new elements in the regulatory circuitry that link Ras compartmentalization to signaling output.

EZH2 couples pancreatic regeneration to neoplastic progression

Although the polycomb group protein Enhancer of Zeste Homolog 2 (EZH2) is well recognized for its role as a key regulator of cell differentiation, its involvement in tissue regeneration is largely unknown. Here we show that EZH2 is up-regulated following cerulein-induced pancreatic injury and is required for tissue repair by promoting the regenerative proliferation of progenitor cells. Loss of EZH2 results in impaired pancreatic regeneration and accelerates KRas(G12D)-driven neoplasia. Our findings implicate EZH2 in constraining neoplastic progression through homeostatic mechanisms that control pancreatic regeneration and provide insights into the documented link between chronic pancreatic injury and an increased risk for pancreatic cancer.

Sos-mediated cross-activation of wild-type Ras by oncogenic Ras is essential for tumorigenesis

Mammalian cells contain three closely related ras genes, H-ras, K-ras and N-ras. Although, in a given tumor type, oncogenic mutations are selectively observed in only one of the ras genes, the acquisition of the transformed phenotype has been shown to require the contribution of the normal products of the other ras genes. Here we demonstrate that oncogenic K-Ras promotes the activation of wild type H- and N-Ras. This activation is mediated by oncogenic K-Ras-dependent allosteric stimulation of Sos and confers a growth advantage to oncogenic K-Ras harboring cancer cells. These findings underscore the complementary functions of oncogenic and wild type Ras in tumor cells and identify a potential new targeting strategy for Ras-driven tumors.

Spatial Regulation of EGFR Signaling by Sprouty2

Ligand-induced activation of the epidermal growth factor receptor (EGFR) initiates multiple signal-transduction pathways as well as trafficking events that relocalize the receptors from the cell surface to intracellular endocytic compartments. Although there is growing awareness that endocytic transport can play a direct role in signal specification, relatively little is known about the molecular mechanisms underlying this link. Here we show that human Sprouty 2 (hSpry2), a protein that has been implicated in the negative regulation of receptor tyrosine kinase (RTK) signaling [1], interferes with the trafficking of activated EGFR specifically at the step of progression from early to late endosomes. This effect is mediated by the binding of hSpry2 to the endocytic regulatory protein, hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs), and leads to a block in intracellular signal propagation. These observations suggest that EGFR signaling is controlled by a novel mechanism involving trafficking-dependent alterations in receptor compartmentalization.

Induction of the Cellular E2F-1 Promoter by the Adenovirus E4-6/7 Protein

ABSTRACT The adenovirus type 5 (Ad5) E4-6/7 protein interacts directly with different members of the E2F family and mediates the cooperative and stable binding of E2F to a unique pair of binding sites in the Ad5 E2a promoter region. This induction of E2F DNA binding activity strongly correlates with increased E2a transcription when analyzed using virus infection and transient expression assays. Here we show that while different adenovirus isolates express an E4-6/7 protein that is capable of induction of E2F dimerization and stable DNA binding to the Ad5 E2a promoter region, not all of these viruses carry the inverted E2F binding site targets in their E2a promoter regions. The Ad12 and Ad40 E2a promoter regions bind E2F via a single binding site. However, these promoters bind adenovirus-induced (dimerized) E2F very weakly. The Ad3 E2a promoter region binds E2F very poorly, even via a single binding site. A possible explanation of these results is that the Ad E4-6/7 protein evolved to induce cellular gene expression. Consistent with this notion, we show that infection with different adenovirus isolates induces the binding of E2F to an inverted configuration of binding sites present in the cellular E2F-1 promoter. Transient expression of the E4-6/7 protein alone in uninfected cells is sufficient to induce transactivation of the E2F-1 promoter linked to chloramphenicol acetyltransferase or green fluorescent protein reporter genes. Further, expression of the E4-6/7 protein in the context of adenovirus infection induces E2F-1 protein accumulation. Thus, the induction of E2F binding to the E2F-1 promoter by the E4-6/7 protein observed in vitro correlates with transactivation of E2F-1 promoter activity in vivo. These results suggest that adenovirus has evolved two distinct mechanisms to induce the expression of the E2F-1 gene. The E1A proteins displace repressors of E2F activity (the Rb family members) and thus relieve E2F-1 promoter repression; the E4-6/7 protein complements this function by stably recruiting active E2F to the E2F-1 promoter to transactivate expression.

Perturbation of cytoskeleton dynamics by the opposing effects of Rac1 and Rac1b

Rac1, a ubiquitously expressed member of the Rho GTPase family, plays a pivotal role in the regulation of multiple cellular processes including cytoskeleton reorganization, cell growth, differentiation and motility. Here we show that the tumor-specific splice variant of Rac1, Rac1b, negatively regulates Rac1 activity. The expression of Rac1b in HeLa cells interferes with Rac1 activation by PDGF, leads to a reduction in membrane-bound Rac1 and promotes an increase in Rho activity. The antagonistic relationship between Rac1 and Rac1b perturbs the regulatory circuitry that controls actin cytoskeleton dynamics thereby leading to tumor-linked alterations in cell morphology and motility.

Abstract 3001: Accurins are endocytosed by KRAS-mutant cells via macropinocytosis

Background: BIND-014 is a novel prostate-specific membrane antigen (PSMA)-targeted Accurin (polymeric nanoparticle) encapsulating docetaxel. In a Phase 2 non-small cell lung cancer (NSCLC) trial, BIND-014 demonstrated promising anti-tumor activity in patients with tumors expressing KRAS mutations (mutated Kirsten ras oncogene homolog). KRAS mutations in NSCLC are generally associated with poor response to currently available drug therapy regimens, including docetaxel. Mutant KRAS stimulates an endocytic process known as macropinocytosis, in which extracellular fluid and its constituents are internalized non-selectively into cells. We hypothesized that Accurins preferentially accumulate in KRAS-mutant cells via macropinocytosis, potentially contributing to anti-tumor activity of BIND-014 in KRAS-mutant cancer. Methods: To determine whether BIND-014 is internalized in cells with high levels of macropinocytosis, we treated KRAS-mutant, macropinocytosis-high human NSCLC A549 cells and human bladder cancer T24 cells with PSMA targeted Accurins in which docetaxel was replaced by a fluorescent label, and visualized co-localization with fluorescently labeled markers of macropinocytosis, dextran and BSA, by confocal microscopy. In addition, we quantified the levels of labeled Accurin uptake in A549 cells by fluorescence-activated cell sorting (FACS) and compared them to labeled Accurin internalization in macropinocytosis-low, KRAS-wild-type BxPC3 cells. We further tested the macropinocytosis-dependence of labeled Accurin uptake by treating A549 and BxPC3 cells with the macropinocytosis inhibitor EIPA and measuring changes in Accurin internalization by FACS. Results: Confocal microscopy analysis of A549 and T24 cells co-treated with labeled Accurin and dextran or BSA showed substantial accumulation of Accurin in dextran- and BSA-positive macropinosomes. Moreover, FACS analysis of A549 cells treated with 0.1 ug and 1.0 ug of labeled Accurin revealed 60.4% and 81.8% uptake, respectively, that could be reduced to 3.6% and 3.7% with 50 uM EIPA. By contrast, BxPC3 cells showed 1.8% and 1.5% uptake of 0.1 ug and 1.0 ug labeled Accurin that was reduced to 1.3% and 0.9%, respectively, with 50 uM EIPA by FACS analysis. Conclusions: These data demonstrate that Accurins preferentially enter human cancer cells that display high levels of macropinocytosis and co-localize with several markers of macropinosomes. Moreover, Accurin internalization can be inhibited in macropinocytosis-positive cells with an established chemical inhibitor of macropinocytosis. BIND-014 may therefore preferentially accumulate in KRAS-mutant NSCLC tumors, thereby contributing to the clinical activity seen to date in this tumor type. Citation Format: Jane E. Cullis, Laura Taylor, Susan Low, Dafna Bar-Sagi. Accurins are endocytosed by KRAS-mutant cells via macropinocytosis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3001.