The Human Adenovirus Type 5 E4orf4 Protein Targets Two Phosphatase Regulators of the Hippo Signaling Pathway.

UNLABELLED: When expressed alone at high levels, the human adenovirus E4orf4 protein exhibits tumor cell-specific p53-independent toxicity. A major E4orf4 target is the B55 class of PP2A regulatory subunits, and we have shown recently that binding of E4orf4 inhibits PP2A(B55) phosphatase activity in a dose-dependent fashion by preventing access of substrates (M. Z. Mui et al., PLoS Pathog 9:e1003742, 2013, http://dx.doi.org/10.1371/journal.ppat.1003742). While interaction with B55 subunits is essential for toxicity, E4orf4 mutants exist that, despite binding B55 at high levels, are defective in cell killing, suggesting that other essential targets exist. In an attempt to identify additional targets, we undertook a proteomics approach to characterize E4orf4-interacting proteins. Our findings indicated that, in addition to PP2A(B55) subunits, ASPP-PP1 complex subunits were found among the major E4orf4-binding species. Both the PP2A and ASPP-PP1 phosphatases are known to positively regulate effectors of the Hippo signaling pathway, which controls the expression of cell growth/survival genes by dephosphorylating the YAP transcriptional coactivator. We find here that expression of E4orf4 results in hyperphosphorylation of YAP, suggesting that Hippo signaling is affected by E4orf4 interactions with PP2A(B55) and/or ASPP-PP1 phosphatases. Furthermore, knockdown of YAP1 expression was seen to enhance E4orf4 killing, again consistent with a link between E4orf4 toxicity and inhibition of the Hippo pathway. This effect may in fact contribute to the cancer cell specificity of E4orf4 toxicity, as many human cancer cells rely heavily on the Hippo pathway for their enhanced proliferation. IMPORTANCE: The human adenovirus E4orf4 protein has been known for some time to induce tumor cell-specific death when expressed at high levels; thus, knowledge of its mode of action could be of importance for development of new cancer therapies. Although the B55 form of the phosphatase PP2A has long been known as an essential E4orf4 target, genetic analyses indicated that others must exist. To identify additional E4orf4 targets, we performed, for the first time, a large-scale affinity purification/mass spectrometry analysis of E4orf4 binding partners. Several additional candidates were detected, including key regulators of the Hippo signaling pathway, which enhances cell viability in many cancers, and results of preliminary studies suggested a link between inhibition of Hippo signaling and E4orf4 toxicity.

Met synergizes with p53 loss to induce mammary tumors that possess features of claudin-low breast cancer.

Triple-negative breast cancer (TNBC) accounts for ∼20% of cases and contributes to basal and claudin-low molecular subclasses of the disease. TNBCs have poor prognosis, display frequent mutations in tumor suppressor gene p53 (TP53), and lack targeted therapies. The MET receptor tyrosine kinase is elevated in TNBC and transgenic Met models (Met(mt)) develop basal-like tumors. To investigate collaborating events in the genesis of TNBC, we generated Met(mt) mice with conditional loss of murine p53 (Trp53) in mammary epithelia. Somatic Trp53 loss, in combination with Met(mt), significantly increased tumor penetrance over Met(mt) or Trp53 loss alone. Unlike Met(mt) tumors, which are histologically diverse and enriched in a basal-like molecular signature, the majority of Met(mt) tumors with Trp53 loss displayed a spindloid pathology with a distinct molecular signature that resembles the human claudin-low subtype of TNBC, including diminished claudins, an epithelial-to-mesenchymal transition signature, and decreased expression of the microRNA-200 family. Moreover, although mammary specific loss of Trp53 promotes tumors with diverse pathologies, those with spindloid pathology and claudin-low signature display genomic Met amplification. In both models, MET activity is required for maintenance of the claudin-low morphological phenotype, in which MET inhibitors restore cell-cell junctions, rescue claudin 1 expression, and abrogate growth and dissemination of cells in vivo. Among human breast cancers, elevated levels of MET and stabilized TP53, indicative of mutation, correlate with highly proliferative TNBCs of poor outcome. This work shows synergy between MET and TP53 loss for claudin-low breast cancer, identifies a restricted claudin-low gene signature, and provides a rationale for anti-MET therapies in TNBC.

Met induces mammary tumors with diverse histologies and is associated with poor outcome and human basal breast cancer.

Elevated MET receptor tyrosine kinase correlates with poor outcome in breast cancer, yet the reasons for this are poorly understood. We thus generated a transgenic mouse model targeting expression of an oncogenic Met receptor (Met(mt)) to the mammary epithelium. We show that Met(mt) induces mammary tumors with multiple phenotypes. These reflect tumor subtypes with gene expression and immunostaining profiles sharing similarities to human basal and luminal breast cancers. Within the basal subtype, Met(mt) induces tumors with signatures of WNT and epithelial to mesenchymal transition (EMT). Among human breast cancers, MET is primarily elevated in basal and ERBB2-positive subtypes with poor prognosis, and we show that MET, together with EMT marker, SNAIL, are highly predictive of poor prognosis in lymph node-negative patients. By generating a unique mouse model in which the Met receptor tyrosine kinase is expressed in the mammary epithelium, along with the examination of MET expression in human breast cancer, we have established a specific link between MET and basal breast cancer. This work identifies basal breast cancers and, additionally, poor-outcome breast cancers, as those that may benefit from anti-MET receptor therapies.

Defining the role of prolactin as an invasion suppressor hormone in breast cancer cells.

Prolactin hormone (PRL) is well characterized as a terminal differentiation factor for mammary epithelial cells and as an autocrine growth/survival factor in breast cancer cells. However, this function of PRL may not fully signify its role in breast tumorigenesis. Cancer is a complex multistep progressive disease resulting not only from defects in cell growth but also in cell differentiation. Indeed, dedifferentiation of tumor cells is now recognized as a crucial event in invasion and metastasis. PRL plays a critical role in inducing/maintaining differentiation of mammary epithelial cells, suggesting that PRL signaling could serve to inhibit tumor progression. We show here that in breast cancer cells, PRL and Janus-activated kinase 2, a major kinase involved in PRL signaling, play a critical role in regulating epithelial-mesenchymal transformation (EMT), an essential process associated with tumor metastasis. Activation of the PRL receptor (PRLR), achieved by restoring PRL/JAK2 signaling in mesenchymal-like breast cancer cells, MDA-MB-231, suppressed their mesenchymal properties and reduced their invasive behavior. While blocking PRL autocrine function in epithelial-like breast cancer cells, T47D, using pharmacologic and genetic approaches induced mesenchymal-like phenotypic changes and enhanced their invasive propensity. Moreover, our results indicate that blocking PRL signaling led to activation of mitogen-activated protein kinase (extracellular signal-regulated kinase 1/2) and transforming growth factor-beta/Smad signaling pathways, two major prometastatic pathways. Furthermore, our results indicate that following PRL/JAK2 inhibition, ERK1/2 activation precedes and is required for Smad2 activation and EMT induction in breast cancer cells. Together, these results highlight PRL as a critical regulator of epithelial plasticity and implicate PRL as an invasion suppressor hormone in breast cancer.

5'-Inositol phosphatase SHIP2 recruits Mena to stabilize invadopodia for cancer cell invasion.

Invadopodia are specialized membrane protrusions that support degradation of extracellular matrix (ECM) by cancer cells, allowing invasion and metastatic spread. Although early stages of invadopodia assembly have been elucidated, little is known about maturation of invadopodia into structures competent for ECM proteolysis. The localized conversion of phosphatidylinositol(3,4,5)-triphosphate and accumulation of phosphatidylinositol(3,4)-bisphosphate at invadopodia is a key determinant for invadopodia maturation. Here we investigate the role of the 5'-inositol phosphatase, SHIP2, and reveal an unexpected scaffold function of SHIP2 as a prerequisite for invadopodia-mediated ECM degradation. Through biochemical and structure-function analyses, we identify specific interactions between SHIP2 and Mena, an Ena/VASP-family actin regulatory protein. We demonstrate that SHIP2 recruits Mena, but not VASP, to invadopodia and that disruption of SHIP2-Mena interaction in cancer cells leads to attenuated capacity for ECM degradation and invasion in vitro, as well as reduced metastasis in vivo. Together, these findings identify SHIP2 as a key modulator of carcinoma invasiveness and a target for metastatic disease.

The adaptor function of SHP-2 downstream of the prolactin receptor is required for the recruitment of p29, a substrate of SHP-2.

SHP-2, a cytosolic protein tyrosine phosphatase with two SH2 domains and multiple tyrosine phosphorylation sites, contributes to signal transduction as an enzyme and/or adaptor molecule. Here we demonstrate that prolactin (PRL) stimulation of the PRL-responsive Nb2 cells, a rat lymphoma cell line, and T47D cells, a human breast cancer cell line, lead to the complex formation of SHP-2 and growth factor receptor-bound protein-2 (grb2). Using transient co-overexpression studies of the prolactin receptor (PRLR) and several tyrosine to phenylalanine mutants of SHP-2, we show that grb2 associates with SHP-2 through the C-terminal tyrosine residues of SHP-2, Y(546) and Y(584). Furthermore, in this study, we found a highly phosphorylated, 29-kDa protein (p29), a substrate of SHP-2. The recruitment of p29 to SHP-2 requires the carboxy-terminal tyrosine residues of SHP-2 (Y(546) and Y(584)). Together, our results indicate that SHP-2 may function as an adaptor molecule downstream of the PRLR and highlight a new recruitment mechanism of SHP-2 substrates.

Prolactin induces SHP-2 association with Stat5, nuclear translocation, and binding to the beta-casein gene promoter in mammary cells.

The Src homology 2 (SH2) domain containing protein-tyrosine phosphatase SHP-2 contributes to prolactin receptor (PRLR) signal transduction to beta-casein gene promoter activation. We report for the first time that SHP-2 physically associates with the signal transducer and activator of transcription-5a (Stat5a), an important mediator of PRLR signaling to milk protein gene activation, in the mouse mammary HC11 and the human breast cancer T47D cells when stimulated with prolactin (PRL) and human growth hormone, respectively. In addition, overexpression studies indicate that the carboxyl-terminal SH2 domain of SHP-2 is required to maintain tyrosine phosphorylation of Stat5 and its interaction with SHP-2. Furthermore, we demonstrate by nuclear co-immunoprecipitation and indirect immunofluorescence studies that PRL stimulation of mammary cells leads to the nuclear translocation of SHP-2 as a complex with Stat5a. This process was found to involve the catalytic activity of the phosphatase. Finally, using the Stat5 GAS (gamma-activated sequence) element of the beta-casein gene promoter in electrophoretic mobility shift assays, we demonstrate that PRL induces the SHP-2-Stat5a complex to bind to DNA. The presence of the phosphatase in the protein-bound DNA complex was verified by using polyclonal antisera to SHP-2. Our studies indicate a tight physical and functional interaction between SHP2 and Stat5 required for regulation and perpetuation of PRL-mediated signaling in mammary cells and suggest a potential role for SHP-2 in the nucleus.

SHP-2 regulates SOCS-1-mediated Janus kinase-2 ubiquitination/degradation downstream of the prolactin receptor.

The protein tyrosine phosphatase SHP-2 is an important regulator of the Janus kinase-2 (Jak2)/signal transducer and activator of transcription (Stat) pathway downstream of the cytokine/prolactin receptor family. We report that SHP-2 dephosphorylates tyrosine (Tyr-1007) of Jak2 kinase, a critical recruitment site for the ubiquitin ligase-associated inhibitory protein suppressor of cytokine signaling-1 (SOCS-1), thereby contributing to Jak2 stability. Inactivation of SHP-2 function by blocking receptor/SHP-2 association or by using a catalytically inactive mutant of SHP-2 led to a marked increase in Jak2 ubiquitination/degradation, Jak2 phosphorylation on Tyr-1007, and Jak2/SOCS-1 association. Furthermore, functional studies indicate that modulating the interaction of Jak2/SOCS-1 by SHP-2 is essential for prolactin/Stat5-mediated signaling. Together our results provide a novel function for SHP-2 as a positive regulator of cytokine receptor signaling by regulating ubiquitination/degradation pathways.