Careen K. Tang

Epidermal growth factor and betacellulin mediate signal transduction through co‐expressed ErbB2 and ErbB3 receptors

Interleukin‐3 (IL‐3)‐dependent murine 32D cells do not detectably express epidermal growth factor receptors (EGFRs) and do not proliferate in response to EGF, heregulin (HRG) or other known EGF‐like ligands. Here, we report that EGF specifically binds to and can be crosslinked to 32D transfectants co‐expressing ErbB2 and ErbB3 (32D.E2/E3), but not to transfectants expressing either ErbB2 or ErbB3 individually. [125I]EGF‐crosslinked species detected in 32D.E2/E3 cells were displaced by HRG and betacellulin (BTC) but not by other EGF‐like ligands that were analyzed. EGF, BTC and HRG also induced receptor tyrosine phosphorylation, activation of downstream signaling molecules and proliferation of 32D.E2/E3 cells. 32D transfectants were also generated which expressed an ErbB3–EGFR chimera alone (32D.E3‐E1) or in combination with ErbB2 (32D.E2/E3‐E1). While HRG stimulation of 32D.E3‐E1 cells resulted in DNA synthesis and receptor phosphorylation, EGF and BTC were inactive. However, EGF and BTC were as effective as HRG in mediating signaling when ErbB2 was co‐expressed with the chimera in the 32D.E2/E3‐E1 transfectant. These results provide evidence that ErbB2/ErbB3 binding sites for EGF and BTC are formed by a previously undescribed mechanism that requires co‐expression of two distinct receptors. Additional data utilizing MDA MB134 human breast carcinoma cells, which naturally express ErbB2 and ErbB3 in the absence of EGFRs, supported the results obtained employing 32D cells and suggest that EGF and BTC may contribute to the progression of carcinomas that co‐express ErbB2 and ErbB3.

Evidence of high incidence of EGFRvIII expression and coexpression with EGFR in human invasive breast cancer by laser capture microdissection and immunohistochemical analysis

EGFRvIII was first reported in human glioblastomas. Subsequent reports indicated EGFRvIII protein to be frequently detected in several other human cancers, but not in normal tissues. Our previous studies suggested that EGFRvIII could induce a transformation from ligand‐dependent non‐tumorigenic cell line to ligand‐independent malignant phenotype cells in vitro and in vivo. Transfection of EGFRvIII in MCF‐7 cell line resulted in a 3‐fold increase in colony formation and significantly enhanced tumorigenicity in nude mice (p < 0.001). EGFRvIII could also induce ErbB‐2 phosphorylation. The existence and significance of EGFRvIII transcript in human breast cancer, however, was not reported. In our study, we detected the presence of EGFRvIII mRNA and revealed a high incidence (67.8%) of EGFRvIII transcript in human primary invasive breast cancer by utilizing laser capture microdissection (LCM)/RT‐PCR to capture pure breast cancer cells. In addition, 57.1% of the infiltrating breast carcinomas expressed both EGFRwt and EGFRvIII mRNA in the same tumor. There is no detectable EGFRvIII mRNA in normal breast tissue. Evaluation of the EGFRwt and EGFRvIII protein levels in the same sample sets by immunohistochemical analysis further confirmed the LCM/RT‐PCR finding. Our study provides first direct evidence of high incidence of coexpression of EGFRvIII and EGFRwt in human invasive breast cancer tissue. The unique characteristics and high prevalence of EGFRvIII in invasive human breast cancer as well as negative expression in normal breast may suggest its important role in breast carcinogenesis and make it an ideally potential target for treatment of breast cancer without interrupting normal EGFR signaling.

The significance of heregulin in breast cancer tumor progression and drug resistance

SummaryTheerbB-2 receptor plays an important role in the prognosis of breast cancer and is expressed at high levels in nearly 30% of tumors in breast cancer patients. While evidence accumulates to support the relationship betweenerbB-2 overexpression and poor overall survival in human breast cancer, understanding of the biological consequence(s) oferbB-2 overexpression remains elusive. The discovery ofheregulin has allowed us to identify a number of related but distinct biological endpoints which appear responsive to signal transduction through theerbB-2/4 receptor. These endpoints of growth, invasiveness, and differentiation have clear implications for the emergence, maintenance, and/or control of malignancy, and represent established endpoints in the assessment of malignant progression in human breast cancer. Preliminary studiesin vitro have shown thatheregulin induces a biphasic growth effect on cells witherbB-2 overexpression. Interestingly, we observed that expression ofheregulin correlates with a more aggressive/invasive, vimentin-positive phenotype in breast cancer cells lines. Therefore, we have postulated thatheregulin is involved in breast cancer tumor progression. We have shown thatheregulin inducesin vitro chemoinvasion and chemotaxis of breast cancer cells as well as growth in an anchorage dependent and independent manner. Interestingly, aheregulin neutralizing antibody inhibits chemotaxis and results in cell growth inhibition and blockade of the invasive phenotype. Strikingly, genetically engineered cells which constitutively expressheregulin demonstrate critical phenotypic changes that are associated with a more aggressive phenotype. Specifically, these cells are no longer dependent on estrogen for growth and are resistant to tamoxifenin vitro andin vivo, and moreover these cells metastasize to lymph nodes in athymic nude mice. These tumors appear to have lostbcl-2 expression as compared with the control tumors. In addition, presumably by activation/regulation of topoisomerase II, theheregulin-transfected cells become exquisitely sensitive to doxorubicin and VP-16. Clearly, mechanistic aspects of theerbB-2/4 andheregulin interaction need to be understood from a therapeutic standpoint which could provide additional insights into synergistic treatments for certain patients, or improve treatment regimens for a large number of women. The study ofheregulin and its co-expression witherbB-2/4 receptor and the assessment of its involvement in the progression from the in situ stage of breast tumors to the invasive one will additionally increase the relevance ofheregulin as a prognostic/diagnostic factor. We believe that our studies provide new insights into breast cancer diagnosis, prognosis, and treatment.

Cripto-1 indirectly stimulates the tyrosine phosphorylation of erb B-4 through a novel receptor.

Cripto-1 (CR-1) is a recently discovered protein of the epidermal growth factor family that fails to directly bind to any of the four known erb B type 1 receptor tyrosine kinases. The present study demonstrates that CR-1 indirectly induces tyrosine phosphorylation of erb B-4 but not of the epidermal growth factor-related receptors erb B-2 anderb B-3 in different mouse and human mammary epithelial cell lines. In addition, down-regulation of erb B-4 in NMuMG mouse mammary epithelial cells and in T47D human breast cancer cells, using an anti-erb B-4 blocking antibody or a hammerhead ribozyme vector targeted to erb B-4 mRNA, impairs the ability of CR-1 to fully activate mitogen-activated protein kinase. Finally, chemical cross-linking of 125I-CR-1 to mouse and human mammary epithelial cell membranes results in the labeling of two specific bands with a molecular weight of 130 and 60 kDa, suggesting that the CR-1 receptor represents a novel receptor structurally unrelated to any of the known type I receptor tyrosine kinases. In conclusion, these data demonstrate that CR-1, upon binding to an unknown receptor, can enhance the tyrosine kinase activity oferb B-4 and that a functional erb B-4 receptor is required for CR-1-induced MAPK activation.

Hypophosphorylation of residue Y1045 leads to defective downregulation of EGFRvIII.

Down-regulation of the EGF receptor is the net result of receptor degradation and recycling. Cbl functions by specifically targeting activated ErbB receptors for ubiquitination, facilitating ligand-induced desensitization of EGFR. The interaction between EGFR and c-Cbl has been shown to depend upon receptor phosphorylation at tyrosine residue 1045, the major docking site for c-Cbl. To better understand the biological consequences of EGFR mutants in human cancers, we compared wild-type EGFR and EGFRvIII internalization, as well as gefitinib sensitive and resistant EGFR kinase mutations found in non-small cell lung carcinoma. We observed that Cbl failed to associate with EGFRvIII as well as an inability of the receptor to undergo ubiquitination and degradation. The most intriguing observation is that EGFRvIII tyrosine 1045 residue is either un-phosphorylated or hypophosphorylated. This is in contrast to other tyrosine residues in EGFRvIII, such as Y1173, which exhibit levels of phosphorylation comparable to those of wild-type EGFR. These results suggest that hypophosphorylation of tyrosine residue 1045 is likely to be the cause for EGFRvIII escape from c-Cbl-induced ubiquitination and degradation, enhancing EGFRvIII’s ability to increase proliferation in breast cancer cells. Interestingly, inefficient degradation was only observed in the gefitinib resistant EGFR kinase mutant, despite the fact that this mutant receptor is capable of recruiting c-Cbl and undergoes ubiquitination. The gefitinib sensitive EGFR kinase mutant exhibits similar ubiquitination and degradation patterns as the wild-type EGFR. Collectively, different EGFR mutations exert various negative mechanisms that have the potential to modify receptor internalization and degradation, and may play a critical role in resistance to tyrosine kinase inhibitory treatments.

Phosphorylation of dedicator of cytokinesis 1 (Dock180) at tyrosine residue Y722 by Src family kinases mediates EGFRvIII-driven glioblastoma tumorigenesis

Glioblastoma, the most common primary malignant cancer of the brain, is characterized by rapid tumor growth and infiltration of tumor cells throughout the brain. These traits cause glioblastomas to be highly resistant to current therapies with a resultant poor prognosis. Although aberrant oncogenic signaling driven by signature genetic alterations, such as EGF receptor (EGFR) gene amplification and mutation, plays a major role in glioblastoma pathogenesis, the responsible downstream mechanisms remain less clear. Here, we report that EGFRvIII (also known as ΔEGFR and de2-7EGFR), a constitutively active EGFR mutant that is frequently co-overexpressed with EGFR in human glioblastoma, promotes tumorigenesis through Src family kinase (SFK)-dependent phosphorylation of Dock180, a guanine nucleotide exchange factor for Rac1. EGFRvIII induces phosphorylation of Dock180 at tyrosine residue 722 (Dock180Y722) and stimulates Rac1-signaling, glioblastoma cell survival and migration. Consistent with this being causal, siRNA knockdown of Dock180 or expression of a Dock180Y722F mutant inhibits each of these EGFRvIII-stimulated activities. The SFKs, Src, Fyn, and Lyn, induce phosphorylation of Dock180Y722 and inhibition of these SFKs by pharmacological inhibitors or shRNA depletion markedly attenuates EGFRvIII-induced phosphorylation of Dock180Y722, Rac1 activity, and glioblastoma cell migration. Finally, phosphorylated Dock180Y722 is coexpressed with EGFRvIII and phosphorylated SrcY418 in clinical specimens, and such coexpression correlates with an extremely poor survival in glioblastoma patients. These results suggest that targeting the SFK-p-Dock180Y722-Rac1 signaling pathway may offer a novel therapeutic strategy for glioblastomas with EGFRvIII overexpression.

Suppression of EGFRvIII‐mediated proliferation and tumorigenesis of breast cancer cells by ribozyme

EGFRvIII is a tumor specific, ligand‐independent, constitutively active variant of the epidermal growth factor receptor. Its expression has been detected in many human malignancies including breast cancer. No detectable level of EGFRvIII has, however, been observed in adult tissues, including normal breast tissues. These unique features of the EGFRvIII make it an excellent target for biologically based therapies. We have designed and generated a tumor specific ribozyme targeted to EGFRvIII. This specific EGFRvIII ribozyme is able to effectively cleave EGFRvIII mRNA under physiological conditions in a cell‐free system, but does not cleave wild‐type EGFR and other EGF‐family receptors. While expressing this EGFRvIII‐ribozyme in breast cancer cells, EGFRvIII‐ribozyme is capable of downregulating endogenous EGFRvIII expression at the mRNA and protein levels. Inhibition of proliferation was observed in EGFRvIII‐ribozyme transfectants. In addition, downregulation of EGFRvIII in breast cancer cells significantly inhibited tumor growth in athymic nude mice. Furthermore, this ribozyme has no effect on EGF‐family receptor expression or the proliferation of breast cancer cells, which do not express EGFRvIII but express wild‐type EGFR and other EGF‐family receptors. These results suggest that we have generated a tumor‐specific, biologically functional ribozyme and further demonstrate that EGFRvIII plays a significant role in breast cancer cell proliferation. The ultimate goal of this approach is to provide a potential treatment for breast cancer by specifically targeting this receptor.

EGFR variant-mediated invasion by enhanced CXCR4 expression through transcriptional and post-translational mechanisms.

The expression of the potent, constitutively activated EGFR variant, EGFRvIII, has been linked to breast cancer metastasis, but the mechanisms of EGFRvIII and CXCR4 crosstalk, which may facilitate breast cancer invasion, have never been explored. Here we report that CXCR4 expression is increased in breast cancer cells expressing EGFRvIII regardless of the ER/PgR status of the cells. Treatment of EGFRvIII‐expressing breast cancer cells with the tyrosine kinase inhibitor, AG1478, reverses CXCR4 expression back to levels expressed in parental cells. In addition, expressing EGFRvIII enhances CXCL12/CXCR4‐mediated invasion, which can be inhibited by CXCR4 inhibitors. Surprisingly, CXCR4 mRNA and its transcriptional regulator, HIF‐1α, are up‐regulated only in ER+/PgR+ estrogen‐dependent EGFRvIII‐expressing breast cancer cells, but not in ER‐/PgR‐ or estrogen‐independent cell lines, suggesting that HIF‐1α and hormone receptor‐mediated actions may have a role in the transcriptional regulation of CXCR4. We also demonstrate that p38 MAPK is one of the major down‐stream signaling molecules responsible for EGFRvIII/CXCR4‐mediated invasion as p38 MAPK activity was induced by CXCL12 stimulation under both normoxic and hypoxic conditions. More interestingly, inhibition of p38 MAPK activity significantly reduced CXCR4 expression and inhibited the invasive potential of EGFRvIII‐expressing breast cancer cells, suggesting an essential role for p38 MAPK in EGFRvIII/CXCR4 induced invasion. Furthermore, CXCR4 is regulated post‐translationally through decreased expression of AIP4 and β‐arrestin 1/2, molecules involved in CXCR4 internalization, cellular trafficking and degradation. These results provide a plausible mechanism for EGFRvIII‐mediated invasion and establish a functional link between EGFRvIII and CXCR4 signaling pathways.

EGFRvIII-Induced Estrogen-Independence, Tamoxifen-Resistance Phenotype Correlates with PgR expression and Modulation of Apoptotic Molecules in Breast Cancer

The tumor‐specific, ligand‐independent, constitutively active epidermal growth factor receptor (EGFR) variant, EGFRvIII, remains understudied in breast cancer. Here, we report that expression of EGFRvIII in the ErbB‐2‐overexpressing, estrogen‐dependent MDA‐MB‐361 breast cancer cell line resulted in significant estrogen‐independent tumor growth in ovariectomized, athymic nude mice in comparison to MDA‐MB‐361/wt cells. MDA‐MB‐361/vIII breast cancer cells maintained estrogen‐induced tumor growth, but were tamoxifen‐resistant in the presence of estrogen, while MDA‐MB‐361/wt cells had a significant reduction in tumor growth in the presence of estrogen and tamoxifen. Tamoxifen alone did not have a significant effect on EGFRvIII‐mediated estrogen‐independent tumor growth. Constitutive signaling from the EGFRvIII receptor resulted in an increased activation of both the Akt and MAPK pathways. Compared to estrogen‐dependent, tamoxifen‐sensitive MCF‐7/vIII breast cancer cells, which had unchanged levels of ERα, but an increase in progesterone receptor (PgR) in comparison to MCF‐7/wt cells, MDA‐MB‐361/vIII cells had a reduction in ERα expression as well as a more pronounced reduction in PgR compared with MDA‐MB‐361/wt cells. EGFRvIII expression was also significantly associated with an absence of PgR protein in invasive human breast cancer specimens. Alterations of proapoptotic proteins and antiapoptotic proteins were observed in EGFRvIII transfectants. In conclusion, constitutive signaling through EGFRvIII and its downstream effector proteins crosstalks with the ERα pathway, resulting in loss of PgR expression and alterations in the apoptotic pathway, which may result in the estrogen‐independent, tamoxifen‐resistant phenotype conferred to EGFRvIII‐expressing breast cancer cells.

Co-expression of EGFRvIII with ErbB-2 enhances tumorigenesis: EGFRvIII mediated constitutively activated and sustained signaling pathways, whereas EGF-induced a transient effect on EGFR-mediated signaling pathways

Elevated levels of epidermal growth factor receptor (EGFR) have been detected in a variety of human cancers. Several reports have demonstrated that the Type III EGF receptor deletion-mutant (EGFRvIII) is frequently detected in various human cancers, including breast cancer. We generated and characterized monoclonal antibody against EGFRvIII. We demonstrated that 29% of DCIS, 40% of primary invasive breast cancers and 54% of metastatic lymph nodes express EGFRvIII by immunohistochemical analysis with two monoclonal antibodies. High levels of EGFRvIII expression were detected in about 5% of primary breast cancer and 27% of metastatic lymph-nodes. Furthermore, in the positive samples, the normal mammary gland exhibited negative staining for EGFRvIII, while the tumor cells were positive. The frequency of EGFRvIII expression correlated with breast cancer progression. We also showed that, despite the absence of gene amplification of EGFR in breast carcinoma cells, EGFRvIII was phosphorylated in breast cancer. In addition, approximately 40% of ErbB-2 positive primary breast tumors were found to co-express EGFRvIII. Even more striking is that 75% (3/4) of ErbB-2 positive metastatic lymph node specimens co-expressed with EGFRvIII. Co-expression of EGFRvIII with ErbB-2 in 32D cells amplified downstream signaling cascades and significantly enhanced tumorigenesis in vivo. Furthermore, EGFRvIII mediated constitutively activated and sustained downstream signaling pathways, whereas EGF-ligand induced a transient effect on wt-EGFR-mediated downstream signaling pathways.