ErbB3 (HER3) interaction with the p85 regulatory subunit of phosphoinositide 3-kinase

Pharmacological strategies to overcome HER2 cross-talk and Trastuzumab resistance

Approximately 20-30% of breast cancers show increased expression of the HER2 receptor tyrosine kinase. Trastuzumab (Herceptin) is a clinically approved anti-HER2 monoclonal antibody. Many patients with HER2-overexpressing metastatic breast cancer respond to trastuzumab; however, a subset display primary drug resistance. In addition, many patients who initially respond to trastuzumab ultimately develop disease progression. Multiple molecular mechanisms contributing to trastuzumab resistance have been proposed in the literature. These mechanisms include cross-signaling from related HER/erbB receptors and compensatory signaling from receptors outside of the HER/erbB family, including receptors for insulin-like growth factor-I, vascular endothelial growth factor, and transforming growth factor beta. The major downstream signaling pathway activated by HER2 cross-talk is PI3K/mTOR, and a potential integrator of receptor cross-talk is Src-focal adhesion kinase (FAK) signaling. PI3K, Src, and FAK have independently been implicated in trastuzumab resistance. In this review, we will discuss pharmacological inhibition of HER2 cross-talk as a strategy to treat trastuzumab-refractory HER2-overexpresssing breast cancer.

HER3 is required for HER2-induced preneoplastic changes to the breast epithelium and tumor formation

Increasing evidence suggests that HER2-amplified breast cancer cells use HER3/ErbB3 to drive therapeutic resistance to HER2 inhibitors. However, the role of ErbB3 in the earliest events of breast epithelial transformation remains unknown. Using mouse mammary specific models of Cre-mediated ErbB3 ablation, we show that ErbB3 loss prevents the progressive transformation of HER2-overexpressing mammary epithelium. Decreased proliferation and increased apoptosis were seen in MMTV-HER2 and MMTV-Neu mammary glands lacking ErbB3, thus inhibiting premalignant HER2-induced hyperplasia. Using a transgenic model in which HER2 and Cre are expressed from a single polycistronic transcript, we showed that palpable tumor penetrance decreased from 93.3% to 6.7% upon ErbB3 ablation. Penetrance of ductal carcinomas in situ was also decreased. In addition, loss of ErbB3 impaired Akt and p44/42 phosphorylation in preneoplastic HER2-overexpressing mammary glands and in tumors, decreased growth of preexisting HER2-overexpressing tumors, and improved tumor response to the HER2 tyrosine kinase inhibitor lapatinib. These events were rescued by reexpression of ErbB3, but were only partially rescued by ErbB36F, an ErbB3 mutant harboring six tyrosine-to-phenylalanine mutations that block its interaction with phosphatidyl inositol 3-kinase. Taken together, our findings suggest that ErbB3 promotes HER2-induced changes in the breast epithelium before, during, and after tumor formation. These results may have important translational implications for the treatment and prevention of HER2-amplified breast tumors through ErbB3 inhibition.

Chemopreventive activity of vitamin E in breast cancer: a focus on γ- and δ-tocopherol

Vitamin E consists of eight different variants: α-, β-, γ-, and δ-tocopherols (saturated phytyl tail) and α-, β-, γ-, and δ-tocotrienols (unsaturated phytyl tail). Cancer prevention studies with vitamin E have primarily utilized the variant α-tocopherol. To no avail, a majority of these studies focused on variant α-tocopherol with inconsistent results. However, γ-tocopherol, and more recently δ-tocopherol, have shown greater ability to reduce inflammation, cell proliferation, and tumor burden. Recent results have shown that γ-enriched mixed tocopherols inhibit the development of mammary hyperplasia and tumorigenesis in animal models. In this review, we discuss the possible differences between the variant forms, molecular targets, and cancer-preventive effects of tocopherols. We recommend that a γ-enriched mixture, γ- and δ-tocopherol, but not α-tocopherol, are promising agents for breast cancer prevention and warrant further investigation.

Wnt/Β-catenin and sex hormone signaling in endometrial homeostasis and cancer

A delicate balance between estrogen and progestagen signaling underlies proper functioning of the female reproductive tract and, in particular, the monthly re- and degenerative phases characteristic of the menstrual cycle. Here, we propose that the canonical Wnt/β-catenin signaling pathway may underlie this finely tuned hormonal equilibrium in endometrial homeostasis and, upon its constitutive activation, lead to neoplastic transformation of the endometrium. During the menstrual cycle, estradiol will enhance Wnt/β-catenin signaling in the proliferative phase, while progesterone inhibits Wnt/β-catenin signaling, thus restraining estrogens' proliferative actions, during the secretory phase. In case of enhanced or unopposed estrogen signaling, constitutive activation of Wnt/β-catenin signaling will trigger endometrial hyperplasia, which may develop further into endometrial cancer.

ErbB3 ablation impairs PI3K/Akt-dependent mammary tumorigenesis

The ErbB receptor family member ErbB3 has been implicated in breast cancer growth, but it has yet to be determined whether its disruption is therapeutically valuable. In a mouse model of mammary carcinoma driven by the polyomavirus middle T (PyVmT) oncogene, the ErbB2 tyrosine kinase inhibitor lapatinib reduced the activation of ErbB3 and Akt as well as tumor cell growth. In this phosphatidylinositol-3 kinase (PI3K)-dependent tumor model, ErbB2 is part of a complex containing PyVmT, p85 (PI3K), and ErbB3, that is disrupted by treatment with lapatinib. Thus, full engagement of PI3K/Akt by ErbB2 in this oncogene-induced mouse tumor model may involve its ability to dimerize with and phosphorylate ErbB3, which itself directly binds PI3K. In this article, we report that ErbB3 is critical for PI3K/Akt-driven tumor formation triggered by the PyVmT oncogene. Tissue-specific, Cre-mediated deletion of ErbB3 reduced Akt phosphorylation, primary tumor growth, and pulmonary metastasis. Furthermore, EZN-3920, a chemically stabilized antisense oligonucleotide that targets the ErbB3 mRNA in vivo, produced similar effects while causing no toxicity in the mouse model. Our findings offer further preclinical evidence that ErbB3 ablation may be therapeutically effective in tumors where ErbB3 engages PI3K/Akt signaling.

Transcriptional and posttranslational up-regulation of HER3 (ErbB3) compensates for inhibition of the HER2 tyrosine kinase

Sustained and complete inhibition of HER3 and its output to PI3K/Akt are required for the optimal antitumor effect of therapeutic inhibitors of the HER2 oncogene. Here, we show that, after inhibition of the HER2 tyrosine kinase with lapatinib, there is PI3K/Akt and FoxO3a-dependent up-regulation of HER3 mRNA and protein. Up-regulated HER3 was then phosphorylated by residual HER2 activity, thus partially maintaining P-Akt and limiting the antitumor action of lapatinib. Inhibition of HER3 with siRNA or a neutralizing HER3 antibody sensitized HER2+ breast cancer cells and xenografts to lapatinib both in vitro and in vivo. Combined blockade of HER2 and HER3 inhibited pharmacodynamic biomarkers of PI3K/Akt activity more effectively than each inhibitor alone. These results suggest that because of HER3-mediated compensation, current clinical inhibitors of HER2 and PI3K/Akt will not block the PI3K pathway completely. They also suggest that therapeutic inhibitors of HER3 should be used in combination with HER2 inhibitors and PI3K pathway inhibitors in patients with HER2- and PI3K-dependent cancers.

Wnt/Β-catenin and sex hormone signaling in endometrial homeostasis and cancer

A delicate balance between estrogen and progestagen signaling underlies proper functioning of the female reproductive tract and, in particular, the monthly re- and degenerative phases characteristic of the menstrual cycle. Here, we propose that the canonical Wnt/β-catenin signaling pathway may underlie this finely tuned hormonal equilibrium in endometrial homeostasis and, upon its constitutive activation, lead to neoplastic transformation of the endometrium. During the menstrual cycle, estradiol will enhance Wnt/β-catenin signaling in the proliferative phase, while progesterone inhibits Wnt/β-catenin signaling, thus restraining estrogens' proliferative actions, during the secretory phase. In case of enhanced or unopposed estrogen signaling, constitutive activation of Wnt/β-catenin signaling will trigger endometrial hyperplasia, which may develop further into endometrial cancer.

ErbB3 expression promotes tumorigenesis in pancreatic adenocarcinoma

Historically, ErbB3 has been overlooked within the ErbB receptor family due to its perceived lack of tyrosine kinase activity. We have previously demonstrated that in pancreatic cancer ErbB3 is the preferred dimerization partner of EGFR, ErbB3 protein expression level directly correlates with the anti-proliferative effect of erlotinib (an EGFR-specific tyrosine kinase inhibitor), and transient knockdown of ErbB3 expression results in acquired resistance to EGFR-targeted therapy. In this study, we develop a stable isogenic model of ErbB3 expression in an attempt to decipher ErbB3's true contribution to pancreatic cancer tumorigenesis and to examine how this receptor affects cellular sensitivity to EGFR-targeted therapy. Analysis of the EGFR-ErbB3 heterodimer demonstrates that ligand-induced PI3K-AKT signaling is limited to ErbB3-expressing cells and that this signaling cascade can be partially abrogated by inhibiting EGFR function with erlotinib. Using our model of exogenous ErbB3 expression we showed a direct relationship between ErbB3 protein levels and increased pancreatic cancer cell proliferation in vitro. In vivo, ErbB3(+)PANC-1 xenografts had a significantly larger tumor volume than PANC-1 control xenografts (ErbB3-PANC-1) and displayed increased sensitivity to EGFR-targeted therapy. In pancreatic cancer, ErbB3 appears to be critically involved in EGFR signaling as evidenced by its profound effect on cellular proliferation and its ability to influence response to EGFR-targeted therapy.

Human epidermal growth factor receptor dimerization analysis in breast cancer diagnosis: potential for improving testing accuracy and treatment selection

Our understanding of the human epidermal growth factor receptor (HER) family of proteins has increased over the last few decades. It is clear from the vast assortment of research that has been and is currently being carried out that there is still a lot to be learned. HER dimerization is known to activate various downstream pathways that have an effect on treatment and therapy for breast cancer. HER dimerization acts as a mechanism not only for amplifying the signal pathway but also for signal diversification. There is clear evidence that molecular subtypes of cancer respond differently to different therapeutic options, which challenges the 'one size fits all' approach to chemotherapy in cancer. Here, we review the methods by which HER dimerization can be explored and the potential that this has in the treatment of breast cancer.

The role of HER2 in cancer therapy and targeted drug delivery

HER2 is highly expressed in a significant proportion of breast cancer, ovarian cancer, and gastric cancer. Since the discovery of its role in tumorigenesis, HER2 has received great attention in cancer research during the past two decades. Successful development of the humanized monoclonal anti-HER2 antibody (Trastuzumab) for the treatment of breast cancer further spurred scientists to develop various HER2 specific antibodies, dimerization inhibitors and kinase inhibitors for cancer therapy. On the other hand, the high expression of HER2 and the accessibility of its extracellular domain make HER2 an ideal target for the targeted delivery of anti-tumor drugs as well as imaging agents. Although there is no natural ligand for HER2, artificial ligands targeting HER2 have been developed and applied in various targeted drug delivery systems. The emphasis of this review is to elucidate the roles of HER2 in cancer therapy and targeted drug delivery. The structure and signal pathway of HER2 will be briefly described. The role of HER2 in tumorigenesis and its relationship with other tumor markers will be discussed. For the HER2 targeted cancer therapy, numerous strategies including the blockage of receptor dimerization, inhibition of the tyrosine kinase activity, and interruption of the downstream signal pathway will be summarized. For the targeted drug delivery to HER2 positive tumor cells, various targeting ligands and their delivery systems will be described in details.

Oscillatory flow-induced proliferation of osteoblast-like cells is mediated by alphavbeta3 and beta1 integrins through synergistic interactions of focal adhesion kinase and Shc with phosphatidylinositol 3-kinase and the Akt/mTOR/p70S6K pathway

Interstitial flow in and around bone tissue is oscillatory in nature and affects the mechanical microenvironment for bone cell growth and formation. We investigated the role of oscillatory shear stress (OSS) in modulating the proliferation of human osteoblast-like MG63 cells and its underlying mechanisms. Application of OSS (0.5 +/- 4 dynes/cm(2)) to MG63 cells induced sustained activation of phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR/p70S6K (p70S6 kinase) signaling cascades and hence cell proliferation, which was accompanied by increased expression of cyclins A and D1, cyclin-dependent protein kinases-2, -4, and -6, and bone formation-related genes (c-fos, Egr-1, and Cox-2) and decreased expression of p21(CIP1) and p27(KIP1). OSS-induced activation of PI3K/Akt/mTOR/p70S6K and cell proliferation were inhibited by specific antibodies or small interference RNAs of alpha(v)beta(3) and beta(1) integrins and by dominant-negative mutants of Shc (Shc-SH2) and focal adhesion kinase (FAK) (FAK(F397Y)). Co-immunoprecipitation assay showed that OSS induces sustained increases in association of Shc and FAK with alpha(v)beta(3) and beta(1) integrins and PI3K subunit p85, which were abolished by transfecting the cells with FAK(F397Y) or Shc-SH2. OSS also induced sustained activation of ERK, which was inhibited by the specific PI3K inhibitor LY294002 and was required for OSS-induced activation of mTOR/p70S6K and proliferation in MG63 cells. Our findings provide insights into the mechanisms by which OSS induces osteoblast-like cell proliferation through activation of alpha(v)beta(3) and beta(1) integrins and synergistic interactions of FAK and Shc with PI3K, leading to the modulation of downstream ERK and Akt/mTOR/p70S6K pathways.

Understanding the mechanisms behind trastuzumab therapy for human epidermal growth factor receptor 2-positive breast cancer

PURPOSE

Targeted therapy with the humanized monoclonal antibody trastuzumab has become a mainstay for human epidermal growth factor receptor 2 (HER2) -positive breast cancer (BC). The mechanisms of action of trastuzumab have not been fully elucidated, and data available to date are reviewed here. The impact of the mechanisms of action on clinical benefit also is discussed.

METHODS

An extensive literature review of trastuzumab and proposed mechanisms of action was performed.

RESULTS

At least five potential extracellular and intracellular antitumor mechanisms of trastuzumab have been identified in the preclinical setting. These include activation of antibody-dependent cellular cytotoxicity, inhibition of extracellular domain cleavage, abrogation of intracellular signaling, reduction of angiogenesis, and decreased DNA repair. These effects lead to tumor cell stasis and/or death. Clinical benefit from trastuzumab-based therapy in both early and advanced BC has been demonstrated. The benefit of trastuzumab use beyond progression has also been shown, which indicates the need for continuous suppression of the HER2 pathway. Targeting both HER2, with various approaches, and other pathways may enhance the clinical benefit observed with trastuzumab and overcome potential resistance. Novel combinations include pertuzumab (a HER2 dimerization inhibitor), lapatinib (a HER1/HER2 tyrosine kinase inhibitor), bevacizumab (an antiangiogenic agent), tanespimycin (a heat shock protein inhibitor), antiestrogen therapies, and an antibody-drug conjugate (trastuzumab-DM1).

CONCLUSION

Trastuzumab is the foundation of care for patients with HER2-positive BC. Emerging data from studies of other targeted agents may provide alternative treatment combinations to maximize the clinical benefit from trastuzumab and prevent or delay resistance. The continued development of trastuzumab highlights promising treatment approaches for the future.

Tumor-specific apoptosis caused by deletion of the ERBB3 pseudo-kinase in mouse intestinal epithelium

Pharmacologic blockade of EGFR or the closely related receptor ERBB2 has modest efficacy against colorectal cancers in the clinic. Although the upregulation of ERBB3, a pseudo-kinase member of the EGFR/ERBB family, is known to contribute to EGFR inhibitor resistance in other cancers, its functions in normal and malignant intestinal epithelium have not been defined. We have shown here that the intestinal epithelium of mice with intestine-specific genetic ablation of Erbb3 exhibits no cytological abnormalities but does exhibit loss of expression of ERBB4 and sensitivity to intestinal damage. By contrast, intestine-specific Erbb3 ablation resulted in almost complete absence of intestinal tumors in the ApcMin mouse model of colon cancer. Unlike nontransformed epithelium lacking ERBB3, intestinal tumors lacking ERBB3 had reduced PI3K/AKT signaling, which led to attenuation of tumorigenesis via a tumor-specific increase in caspase-3-mediated apoptosis. Consistent with the mouse data, which suggest that ERBB3-ERBB4 heterodimers contribute to colon cancer survival, experimentally induced loss of ERBB3 in a KRAS mutant human colon cancer cell line was associated with loss of ERBB4 expression, and siRNA knockdown of either ERBB3 or ERBB4 resulted in elevated levels of apoptosis. These results indicate that the ERBB3 pseudo-kinase has essential roles in supporting intestinal tumorigenesis and suggest that ERBB3 may be a promising target for the treatment of colorectal cancers.

Novel mechanism of lapatinib resistance in HER2-positive breast tumor cells: activation of AXL

HER2-directed therapies, such as trastuzumab and lapatinib, are important treatments for breast cancer. However, some tumors do not respond or develop resistance to these agents. We isolated and characterized multiple lapatinib-resistant, HER2-positive, estrogen receptor (ER)-positive breast cancer clones derived from lapatinib-sensitive BT474 cells by chronic exposure to lapatinib. We show overexpression of AXL as a novel mechanism of acquired resistance to HER2-targeted agents in these models. GSK1363089 (foretinib), a multikinase inhibitor of AXL, MET, and vascular endothelial growth factor receptor currently in phase II clinical trials, restores lapatinib and trastuzumab sensitivity in these resistant cells that exhibit increased AXL expression. Furthermore, small interfering RNA to AXL, estrogen deprivation, or fulvestrant, an ER antagonist, decreases AXL expression and restores sensitivity to lapatinib in these cells. Taken together, these data provide scientific evidence to assess the expression of AXL in HER2-positive, ER-positive patients who have progressed on either lapatinib or trastuzumab and to test the combination of HER2-targeted agents and GSK1363089 in the clinic.

Novel anticancer targets: revisiting ERBB2 and discovering ERBB3

Aberrant receptor expression or functioning of the epidermal growth factor receptor (Erbb) family plays a crucial part in the development and evolution of cancer. Inhibiting the signalling activity of individual receptors in this family has advanced the treatment of a range of human cancers. In this Review we re-evaluate the role of two important family members, ERBB2 (also known as HER2) and ERBB3 (also known as HER3), and explore the mechanisms of action and preclinical and clinical data for new therapies that target signalling through these pivotal receptors. These new therapies include tyrosine kinase inhibitors, antibody-chemotherapy conjugates, heat-shock protein inhibitors and antibodies that interfere with the formation of ERBB2-ERBB3 dimers.

Trophic factor expression in phrenic motor neurons

The function of a motor neuron and the muscle fibers it innervates (i.e., a motor unit) determines neuromotor output. Unlike other skeletal muscles, respiratory muscles (e.g., the diaphragm, DIAm) must function from birth onwards in sustaining ventilation. DIAm motor units are capable of both ventilatory and non-ventilatory behaviors, including expulsive behaviors important for airway clearance. There is significant diversity in motor unit properties across different types of motor units in the DIAm. The mechanisms underlying the development and maintenance of motor unit diversity in respiratory muscles (including the DIAm) are not well understood. Recent studies suggest that trophic factor influences contribute to this diversity. Remarkably little is known about the expression of trophic factors and their receptors in phrenic motor neurons. This review will focus on the contribution of trophic factors to the establishment and maintenance of motor unit diversity in the DIAm, during development and in response to injury or disease.

TLR4/MyD88/PI3K interactions regulate TLR4 signaling

TLRs activate immune responses by sensing microbial structures such as bacterial LPS, viral RNA, and endogenous "danger" molecules released by damaged host cells. MyD88 is an adapter protein that mediates signal transduction for most TLRs and leads to activation of NF-kappaB and MAPKs and production of proinflammatory cytokines. TLR4-mediated signaling also leads to rapid activation of PI3K, one of a family of kinases involved in regulation of cell growth, apoptosis, and motility. LPS stimulates phosphorylation of Akt, a downstream target of PI3K, in wild-type (WT) mouse macrophages. LPS-induced phosphorylation of Akt serine 473 was blunted in MyD88(-/-) macrophages and was completely TLR4-dependent. MyD88 and p85 were shown previously to co-immunoprecipitate, and a YXXM motif within the Toll-IL-1 resistance (TIR) domain of MyD88 was suggested to be important for this interaction. To test this hypothesis, we compared expressed MyD88 variants with mutations within the YXXM motif or lacking the TIR domain or death domain and measured their capacities to bind PI3K p85, MyD88, and TLR4 by co-immunoprecipitation analyses. The YXXM --> YXXA mutant MyD88 bound more strongly to p85, TLR4, and WT MyD88 than the other variants, yet was significantly less active than WT MyD88, suggesting that sustained interaction of MyD88/PI3K with the TLR4 intracellular "signaling platform" negatively regulates signaling. We propose a hypothetical model in which sustained PI3K activity at the membrane limits the availability of the PI3K substrate, thereby negatively regulating signaling.

Breast tumor cells with PI3K mutation or HER2 amplification are selectively addicted to Akt signaling

Background

Dysregulated PI3K/Akt signaling occurs commonly in breast cancers and is due to HER2 amplification, PI3K mutation or PTEN inactivation. The objective of this study was to determine the role of Akt activation in breast cancer as a function of mechanism of activation and whether inhibition of Akt signaling is a feasible approach to therapy.

Methodology/Principal Findings

A selective allosteric inhibitor of Akt kinase was used to interrogate a panel of breast cancer cell lines characterized for genetic lesions that activate PI3K/Akt signaling: HER2 amplification or PI3K or PTEN mutations in order to determine the biochemical and biologic consequences of inhibition of this pathway. A variety of molecular techniques and tissue culture and in vivo xenograft models revealed that tumors with mutational activation of Akt signaling were selectively dependent on the pathway. In sensitive cells, pathway inhibition resulted in D-cyclin loss, G1 arrest and induction of apoptosis, whereas cells without pathway activation were unaffected. Most importantly, the drug effectively inhibited Akt kinase and its downstream effectors in vivo and caused complete suppression of the growth of breast cancer xenografts with PI3K mutation or HER2 amplification, including models of the latter selected for resistance to Herceptin. Furthermore, chronic administration of the drug was well-tolerated, causing only transient hyperglycemia without gross toxicity to the host despite the pleiotropic normal functions of Akt.

Conclusions/Significance

These data demonstrate that breast cancers with PI3K mutation or HER2 amplification are selectively dependent on Akt signaling, and that effective inhibition of Akt in tumors is feasible and effective in vivo. These findings suggest that direct inhibition of Akt may represent a therapeutic strategy for breast and other cancers that are addicted to the pathway including tumors with resistant to Herceptin.

Involvement of secretory and endosomal compartments in presentation of an exogenous self-glycolipid to type II NKT cells

Natural Killer T (NKT) cells recognize both self and foreign lipid Ags presented by CD1 molecules. Although presentation of the marine sponge-derived lipid alphaGalCer to type I NKT cells has been well studied, little is known about self-glycolipid presentation to either type I or type II NKT cells. Here we have investigated presentation of the self-glycolipid sulfatide to a type II NKT cell that specifically recognizes a single species of sulfatide, namely lyso-sulfatide but not other sulfatides containing additional acyl chains. In comparison to other sulfatides or alphaGalCer, lyso-sulfatide binds with lower affinity to CD1d. Although plate-bound CD1d is inefficient in presenting lyso-sulfatide at neutral pH, it is efficiently presented at acidic pH and in the presence of saposin C. The lysosomal trafficking of mCD1d is required for alphaGalCer presentation to type I NKT cells, it is not important for presentation of lyso-sulfatide to type II NKT cells. Consistently, APCs deficient in a lysosomal lipid-transfer protein effectively present lyso-sulfatide. Presentation of lyso-sulfatide is inhibited in the presence of primaquine, concanamycin A, monensin, cycloheximide, and an inhibitor of microsomal triglyceride transfer protein but remains unchanged following treatment with brefeldin A. Wortmannin-mediated inhibition of lipid presentation indicates an important role for the PI-3kinase in mCD1d trafficking. Our data collectively suggest that weak CD1d-binding self-glycolipid ligands such as lyso-sulfatide can be presented via the secretory and endosomal compartments. Thus this study provides important insights into the exogenous self-glycolipid presentation to CD1d-restricted T cells.

The role of ErbB3 and its binding partners in breast cancer progression and resistance to hormone and tyrosine kinase directed therapies

An increasingly important role for the ErbB3 receptor in the genesis and progression of breast cancer is emerging. ErbB3 is frequently overexpressed in breast cancer and coexpression of ErbB2/3 is a poor prognostic indicator. ErbB3 has also been implicated in the development of resistance to antiestrogens such as tamoxifen and ErbB tyrosine kinase inhibitors such as gefitinib. Persistent activation of the AKT pathway has been postulated to contribute to ErbB3-mediated resistance to these therapies. This activation may be due in part to the inappropriate production of the ErbB3 ligand heregulin. ErbB3 binding proteins, which negatively regulate ErbB3 protein levels and the ability of ErbB3 to transmit proliferative signals, also contribute to breast cancer progression and treatment resistance. These proteins include the intracellular RING finger E3 ubiquitin ligase Nrdp1 and the leucine-rich protein LRIG-1 that mediate receptor degradation. Ebp1, another ErbB3 binding protein, suppresses HRG driven breast cancer cell growth and contributes to tamoxifen sensitivity. These studies point to the importance of the evaluation of protein levels and functional activity of ErbB3 and its binding proteins in breast cancer prognosis and prediction of clinical response to treatment.

The ERBB3 receptor in cancer and cancer gene therapy

ERBB3, a member of the epidermal growth factor receptor (EGFR) family, is unique in that its tyrosine kinase domain is functionally defective. It is activated by neuregulins, by other ERBB and nonERBB receptors as well as by other kinases, and by novel mechanisms. Downstream it interacts prominently with the phosphoinositol 3-kinase/AKT survival/mitogenic pathway, but also with GRB, SHC, SRC, ABL, rasGAP, SYK and the transcription regulator EBP1. There are likely important but poorly understood roles for nuclear localization and for secreted isoforms. Studies of ERBB3 expression in primary cancers and of its mechanistic contributions in cultured cells have implicated it, with varying degrees of certainty, with causation or sustenance of cancers of the breast, ovary, prostate, certain brain cells, retina, melanocytes, colon, pancreas, stomach, oral cavity and lung. Recent results link high ERBB3 activity with escape from therapy targeting other ERBBs in lung and breast cancers. Thus a wide and centrally important role for ERBB3 in cancer is becoming increasingly apparent. Several approaches for targeting ERBB3 in cancers have been tested or proposed. Small inhibitory RNA (siRNA) to ERBB3 or AKT is showing promise as a therapeutic approach to treatment of lung adenocarcinoma.

Induction of ErbB-3 expression by alpha6beta4 integrin contributes to tamoxifen resistance in ERbeta1-negative breast carcinomas

Background

Tamoxifen is still the most widely used drug in hormone therapy for the treatment of breast cancer. Its benefits in adjuvant treatment are well documented in controlled and randomized clinical studies, which have demonstrated an increase in disease-free intervals of patients with positive hormonal receptors. However, the mechanisms involved in endocrine resistance are not clear. Laboratory and clinical data now indicate that bi-directional molecular cross-talk between nuclear or membrane ER and growth factor receptor pathways may be involved in endocrine resistance. We recently found a functional interaction between α6β4 integrin and ErbB-3 receptor to maintain the PI3K/Akt survival pathway of mammary tumour cells. We sought to improve understanding of this process in order to provide the involvement of both receptors insight into mechanism of Tamoxifen resistance.

Methods and Findings

Using human breast cancer cell lines displaying different levels of α6β4 and ErbB-3 receptors and a series of 232 breast cancer biopsies from patients submitted to adjuvant Tamoxifen monotherapy for five years, we evaluated the functional interaction between both receptors in relationship to Tamoxifen responsiveness. In mammary carcinoma cells, we evidenced that the α6β4 integrin strongly influence Akt phosphorylation through ErbB-3 protein regulation. Moreover, the ErbB-3 inactivation inhibits Akt phosphorylation, induces apoptosis and inhibits in vitro invasion favouring Tamoxifen responsiveness. The analysis of human tumors revealed a significant relationship between α6β4 and ErbB-3 in P-Akt-positive and ERβ1-negative breast cancers derived from patients with lower disease free survival.

Conclusions

We provided evidence that a strong relationship occurs between α6β4 and ErbB-3 positivity in ERβ1-negative breast cancers. We also found that the association between ErbB-3 and P-Akt positivity mainly occurs in ERβ1-negative breast cancer derived from patients with lower DFS indicating that both receptors are clinically relevant in predicting the response to Tamoxifen.

HER-3 in colorectal tumourigenesis: from mRNA levels through protein status to clinicopathologic relationships

INTRODUCTION

Colorectal cancer is a major cause of cancer mortality in the Western world. Although HER-3 signalling is known to be implicated in colorectal carcinogenesis, the significance of its expression, localisation and phosphorylation remains elusive.

METHODS

Quantitative RT-PCR for HER-3 mRNA and immunohistochemistry for HER-3 and phosphorylated HER-3 (pHER-3) protein were performed in normal tissue, adenomas and carcinomas from 140 patients with colorectal cancer.

RESULTS

HER-3 was detected both in the cytoplasm and nucleus, whereas pHER-3 was observed in the nucleus and membrane of cells. A possible switch in HER-3 topography from the nucleus to the cytoplasm during colorectal tumourigenesis is suggested. The expression of pHER-3 did not differ significantly in normal tissue, adenomas and carcinomas, but was related to disease stage. HER-3 mRNA overexpression was significantly associated with decreased time to disease progression. It was also correlated with higher median age, left colon and rectal tumour sites and lymph node involvement.

CONCLUSION

We postulate that HER-3 is critically involved in colorectal tumourigenesis and its expression/phosphorylation might be of prognostic significance.

Lethal congenital contractural syndrome type 2 (LCCS2) is caused by a mutation in ERBB3 (Her3), a modulator of the phosphatidylinositol-3-kinase/Akt pathway

Lethal congenital contractural syndrome type 2 (LCCS2) is an autosomal recessive neurogenic form of arthrogryposis that is associated with atrophy of the anterior horn of the spinal cord. We previously mapped LCCS2 to 6.4 Mb on chromosome 12q13 and have now narrowed the locus to 4.6 Mb. We show that the disease is caused by aberrant splicing of ERBB3, which leads to a predicted truncated protein. ERBB3 (Her3), an activator of the phosphatidylinositol-3-kinase/Akt pathway--regulating cell survival and vesicle trafficking--is essential for the generation of precursors of Schwann cells that normally accompany peripheral axons of motor neurons. Gain-of-function mutations in members of the epidermal growth-factor tyrosine kinase-receptor family have been associated with predilection to cancer. This is the first report of a human phenotype resulting from loss of function of a member of this group.

Analysis of interactions between nutrient germinant receptors and SpoVA proteins ofBacillus subtilisspores

Yeast two-hybrid and Far Western analyses were used to detect interactions between Bacillus subtilis spores' nutrient germinant receptor proteins and proteins encoded by the spoVA operon, all of which are involved in spore germination and located in the spores' inner membrane. These analyses indicated that two subunits of the GerA nutrient germinant receptor interact, consistent with previous genetic data, and that some GerA proteins interact with SpoVAD and some with SpoVAE. SpoVA proteins appear to be involved in the release of the spore's dipicolinic acid during spore germination, an event triggered by the binding of nutrient germinants to their receptors. Consequently, these new findings suggest that nutrient germinant receptors physically contact SpoVA proteins, and presumably this is a route for signal transduction during spore germination.

Lethal contractural syndrome type 3 (LCCS3) is caused by a mutation in PIP5K1C, which encodes PIPKI gamma of the phophatidylinsitol pathway

Lethal congenital contractural syndrome (LCCS) is a severe form of arthrogryposis. To date, two autosomal recessive forms of the disease (LCCS and LCCS2) have been described and mapped to chromosomes 9q34 and 12q13, respectively. We now describe a third LCCS phenotype (LCCS3)--similar to LCCS2 yet without neurogenic bladder. Using 10K single-nucleotide-polymorphism arrays, we mapped the disease-associated gene to 8.8 Mb on chromosome 19p13. Further analysis using microsatallite markers narrowed the locus to a 3.4-Mb region harboring 120 genes. Of these genes, 30 candidates were sequenced, which identified a single homozygous mutation in PIP5K1C. PIP5K1C encodes phosphatidylinositol-4-phosphate 5-kinase, type I, gamma (PIPKI gamma ), an enzyme that phophorylates phosphatidylinositol 4-phosphate to generate phosphatidylinositol-4,5-bisphosphate (PIP(2)). We demonstrate that the mutation causes substitution of aspartic acid with asparagine at amino acid 253 (D253N), abrogating the kinase activity of PIPKI gamma . Thus, a defect in the phosphatidylinositol pathway leading to a decrease in synthesis of PIP(2), a molecule active in endocytosis of synaptic vesicle proteins, culminates in lethal congenital arthrogryposis.

Role of HER2/HER3 co-receptor in breast carcinogenesis

ErbB receptors are essential mediators of cell proliferation and differentiation. Their aberrant activation is associated with the development and severity of many cancers. Homo- and heterodimerization of ErbB receptors result in a wide variety of cellular signal transduction. Dimerization of human epidermal growth-factor receptor (HER)2 and HER3 occurs frequently and is a preferred heterodimer. The HER2/HER3 dimer constitutes a high affinity co-receptor for heregulin, which is capable of potent mitogenic signaling. HER3 is a kinase-defective protein that is phosphorylated by HER2. Tyrosine phosphorylated HER3 is able to directly couple to phosphatidylinositide 3-kinase, a lipid kinase involved in the proliferation, survival, adhesion and motility of tumor cells. The authors' research provides mechanistic evidence that apigenin induces apoptosis by depleting the HER2 protein and, in turn, suppressing the signaling of the HER2/HER3-phosphatidylinositide 3-kinase/Akt pathway. This indicates that inhibition of HER2/HER3 heterodimer function may be an especially effective and unique strategy for blocking the HER2-mediated carcinogenesis of breast cancer cells.

The alpha6beta4 integrin can regulate ErbB-3 expression: implications for alpha6beta4 signaling and function

The integrin alpha(6)beta(4) has been shown to facilitate key functions of carcinoma cells, including their ability to migrate, invade, and evade apoptosis. The mechanism involved seems to be a profound effect of alpha(6)beta(4) on specific signaling pathways, especially the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. An intimate relationship between alpha(6)beta(4) and growth factor receptors may explain this effect of alpha(6)beta(4) on signaling. Previously, we showed that alpha(6)beta(4) and ErbB-2 can function synergistically to activate the PI3K/Akt pathway. Given that ErbB-2 can activate PI3K only when it heterodimerizes with other members of the epidermal growth factor receptor family, these data imply that other receptors cooperate in this process. Here, we report that alpha(6)beta(4) can regulate the expression of ErbB-3 using several different models and that the consequent formation of an ErbB-2/ErbB-3 heterodimer promotes the alpha(6)beta(4)-dependent activation of PI3K/Akt and the ability of this integrin to impede apoptosis of carcinoma cells. Our data also support the hypothesis that alpha(6)beta(4) can regulate ErbB-3 expression at the translational level as evidenced by the findings that alpha(6)beta(4) does not increase ErbB-3 mRNA significantly, and that this regulation is both rapamycin sensitive and dependent on eukaryotic translation initiation factor 4E. These findings provide one mechanism to account for the activation of PI3K by alpha(6)beta(4) and they also provide insight into the regulation of ErbB-3 in carcinoma cells.

Compensatory ErbB3/c-Src signaling enhances carcinoma cell survival to ionizing radiation

EGFR and ErbB2 are two members of the ErbB family of receptor Tyr Kinases identified as therapeutic targets for treating carcinomas. Breast carcinoma cells express different complements and variable proportions of ErbB receptor Tyr kinases, which activate unique and redundant signaling cascades that are essential for cell survival. Previously it was shown that a COOH-terminal truncation mutant of the EGFR (EGFR-CD533) blocks EGFR dependent signals and radiosensitizes breast carcinoma cells. In this study the effects of EGFR-CD533 and an analogous truncation mutant of ErbB2 (ErbB2-CD572) on ErbB receptor family dimerization and signaling are further investigated. Using adenoviral vectors in breast carcinoma cell lines with variable ErbB expression profiles, we demonstrate different effects for each deletion mutant. EGFR-CD533 blocks ligand stimulation of EGFR, ErbB2, and ErbB4, but is associated with a compensatory Tyr kinase activity resulting in phosphorylation of ErbB3. In contrast, ErbB2-CD572 produces a weaker, non-specific pattern of ErbB receptor family inhibition, based upon the ErbB expression pattern of the cell type. Investigation of the compensatory Tyr kinase activity associated with EGFR-CD533 expression identified an ErbB3/c-Src signaling pathway that regulates expression of anti-apoptotic Bcl family proteins. This signaling is active in the T47D cell line, which inherently over-express ErbB3, absent in MDA-MB231 cells, which have low ErbB3 expression levels, and is restored in a MDA-MB231 cell line engineered to over-express ErbB3. Furthermore we demonstrate that ErbB3/c-Src signaling is radio-protective, and that its elimination through pharmacologic inhibition of c-Src enhances radiation-induced apoptosis. In summary, these studies identify a novel ErbB3/c-Src survival signal and point to ErbB3 expression levels as an important variable in therapeutic targeting of ErbB receptors in breast carcinoma cells.

Heregulin beta1 drives gefitinib-resistant growth and invasion in tamoxifen-resistant MCF-7 breast cancer cells

INTRODUCTION

Resistance to anti-epidermal growth factor receptor (anti-EGFR) therapies is an emerging clinical problem. The efficacy of anti-EGFR therapies can be influenced by the presence of heregulins (HRGs), which can bind erbB3/4 receptors and can activate alternative signalling pathways. In the present study we have examined whether HRG signalling can circumvent EGFR blockade in an EGFR-positive tamoxifen-resistant MCF-7 (Tam-R) breast cancer cell line.

METHODS

Tam-R cells, incubated with the selective EGFR tyrosine kinase inhibitor gefitinib ('Iressa', ZD1839), were exposed to HRGbeta1 and the effects on erbB receptor dimerization profiles and on activation of associated downstream signalling components were assessed by immunoprecipitation, western blotting and immunocytochemistry. The effects of HRGbeta1 on gefitinib-treated Tam-R cell growth and invasion were also examined, and HRGbeta1 expression levels were assessed in breast cancer tissue by immunohistochemistry to address the potential clinical relevance of such a resistance mechanism.

RESULTS

In Tam-R cells, HRGbeta1 promoted erbB3/erbB2 and erbB3/EGFR heterodimerization, promoted ERK1/2 and AKT pathway activation and increased cell proliferation and invasion. Gefitinib prevented HRGbeta1-driven erbB3/EGFR heterodimerization, ERK1/2 activation and Tam-R cell proliferation, but HRGbeta1-driven erbB3/erbB2 heterodimerization, AKT activation and Tam-R cell invasion were maintained. A combination of gefitinib and the phosphatidylinositol 3-kinase inhibitor LY294002 effectively blocked HRGbeta1-mediated intracellular signalling activity, growth and invasion in Tam-R cells. Similarly, targeting erbB2 with trastuzumab in combination with gefitinib in Tam-R cells reduced HRGbeta1-induced erbB2 and ERK1/2 activity; however, HRGbeta1-driven AKT activity and cell growth were maintained while cell invasion was significantly enhanced with this combination. In clinical tissue all samples demonstrated cytoplasmic tumour epithelial HRGbeta1 protein staining, with expression correlating with EGFR positivity and activation of both AKT and ERK1/2.

CONCLUSION

HRGbeta1 can overcome the inhibitory effects of gefitinib on cell growth and invasion in Tam-R cells through promotion of erbB3/erbB2 heterodimerization and activation of the phosphatidylinositol 3-kinase/AKT signalling pathway. This may have implications for the effectiveness of anti-EGFR therapies in breast cancer as HRGbeta1 is enriched in many EGFR-positive breast tumours.

Epidermal growth factor receptor mutation in lung cancer are linked to bronchioloalveolar differentiation

In lung cancer, an association was made between drastic clinical response to epidermal growth factor receptor (EGFR) inhibitors and the presence of somatic mutations within the tyrosine kinase domain of the EGFR. In some cases, patients with partial response or disease stabilization do not always have EGFR-mutated tumors. To go further in the characterization of the EGF pathway, we screened EGFR, ERBB2, ERBB3, KRAS, BRAF, and PIK3CA for mutations in 2 groups of White patients with nonsmall cell lung cancer (45 cancers from women and 46 cancers from men). Associations between TP53 mutations, clinicopathologic parameters, and EGF pathway molecular alterations were analyzed. All mutations were exclusive and essentially found in EGFR and KRAS. We demonstrated that EGFR mutations were linked to female sex, absence of smoking, late age at diagnosis, and adenocarcinoma (ADC) with bronchioloalveolar (BAC) features. Moreover, in invasive ADC with BAC component, microdissection assays showed that mutations were retrieved in both tumor subtypes suggesting that EGFR mutations appear early in lung carcinogenesis. On the contrary, KRAS mutations correlated with smoking, younger age at diagnosis, and ADC subtype regardless of BAC differentiation. These results suggest the existence of distinct carcinogenesis pathways both leading to disruption of EGF regulation and targeted either by tobacco carcinogens or by unidentified toxic. The identification of BAC features in ADC helps clustering patients that are more likely to fit the EGFR-mutated group.

Neuregulin-dependent protein synthesis in C2C12 myotubes and rat diaphragm muscle

The nerve-derived trophic factor neuregulin (NRG) is a prime candidate molecule for modulating muscle fiber growth. NRG regulates signal transduction in skeletal muscle through activation of ErbB receptors present at the neuromuscular junction. In this study, we hypothesize that NRG increases protein synthesis in maturing muscle via a phosphatidylinositol 3-kinase (PI3K)-dependent mechanism. NRG signal transduction and its ability to stimulate protein synthesis (measured by incorporation of [(3)H]phenylalanine into the protein pool) were investigated in differentiated C(2)C(12) myotubes and rat diaphragm muscle (DIAm). In C(2)C(12) myotubes, NRG dose dependently increased phosphorylation of ErbB3 and recruitment of the p85 subunit of PI3K. NRG also increased phosphorylation of Akt, a downstream effector of PI3K. NRG treatment increased total protein synthesis by 35% compared with untreated control myotubes. This NRG-induced increase in Akt phosphorylation and protein synthesis was completely blocked by wortmannin, an inhibitor of PI3K but was unaffected by PD-98059, an inhibitor of MEK. In DIAm obtained from 3-day-old rat pups, Akt phosphorylation increased approximately 30-fold with NRG treatment (vs. untreated DIAm). NRG treatment also significantly increased protein synthesis in the DIAm by 29% after 3 h of incubation with [(3)H]phenylalanine (vs. untreated DIAm). Pretreatment with wortmannin abolished the NRG-induced increase in protein synthesis, suggesting a critical role for PI3K in this response. The results of the present study support the hypothesis that nerve-derived NRG contributes to the regulation of skeletal muscle mass by increasing protein synthesis via activation of PI3K.

The role of carbonic anhydrase IX overexpression in kidney cancer

Carbonic anhydrase IX (CA IX) is a membrane isoenzyme, the overexpression of which is associated with clear cell carcinoma of the kidney. Its overexpression is restricted mainly to cancer, as it is absent in corresponding normal tissues making it a potential cancer biomarker. Several recent studies have shown that CA IX, apart from its classical enzyme activity of reversibly hydrating carbon dioxide extracellularly to facilitate the net extrusion of protons from inside to outside the cell, it can also be a key player in the modulation of cell adhesion processes and participate in the regulation of cell proliferation in response to hypoxic environment to ultimately contribute to tumour progression. Here, we have shown that the sole tyrosine moiety of CA IX present in its intracellular domain can be phosphorylated in an epidermal growth factor dependent manner, suggesting that it can feed into the growth factor receptor dependent signalling pathways. Our studies suggest that the tyrosine phosphorylated CA IX can interact with the regulatory subunit of PI-3-Kinase, contributing to Akt activation. These studies have revealed a positive feed back loop that can form the basis of a vicious cycle that could contribute to the progression of clear cell renal carcinoma and poor prognosis. These studies show that CA IX signalling may be a part of both the hypoxia driven and hypoxia independent pathways that occur in the cancer cell. Finally, our studies emphasize the need for a more refined strategy using signal transduction therapeutics to inhibit the cell surface carbonic anhydrases for the management of this malignancy.

Phosphatidylinositol 3-kinase p85{alpha} subunit-dependent interaction with BCR/ABL-related fusion tyrosine kinases: molecular mechanisms and biological consequences

The p85alpha subunit of phosphatidylinositol 3-kinase (PI-3k) forms a complex with a protein network associated with oncogenic fusion tyrosine kinases (FTKs) such as BCR/ABL, TEL/ABL, TEL/JAK2, TEL/PDGFbetaR, and NPM/ALK, resulting in constitutive activation of the p110 catalytic subunit of PI-3k. Introduction of point mutations in the N-terminal and C-terminal SH2 domain and SH3 domain of p85alpha, which disrupt their ability to bind phosphotyrosine and proline-rich motifs, respectively, abrogated their interaction with the BCR/ABL protein network. The p85alpha mutant protein (p85mut) bearing these mutations was unable to interact with BCR/ABL and other FTKs, while its binding to the p110alpha catalytic subunit of PI-3k was intact. In addition, binding of Shc, c-Cbl, and Gab2, but not Crk-L, to p85mut was abrogated. p85mut diminished BCR/ABL-dependent activation of PI-3k and Akt kinase, the downstream effector of PI-3k. This effect was associated with the inhibition of BCR/ABL-dependent growth of the hematopoietic cell line and murine bone marrow cells. Interestingly, the addition of interleukin-3 (IL-3) rescued BCR/ABL-transformed cells from the inhibitory effect of p85mut. SCID mice injected with BCR/ABL-positive hematopoietic cells expressing p85mut survived longer than the animals inoculated with BCR/ABL-transformed counterparts. In conclusion, we have identified the domains of p85alpha responsible for the interaction with the FTK protein network and transduction of leukemogenic signaling.

Neuregulin-1 enhances survival of human astrocytic glioma cells

Malignant astrocytic gliomas, referred to as astrocytomas, represent the most commonly diagnosed adult primary brain tumor. These tumors are characterized by unrelenting growth that is often resistant to chemotherapy and radiation therapy. Tumor expansion into the healthy surrounding brain tissue produces severe and often fatal consequences. In this study, we examine the potential for the neuregulin-1/erbB receptor signaling cascade to contribute to this process by modulating glioma cell growth. Using antibodies specific for the erbB receptors, we demonstrate the expression patterns for the erbB2, erbB3, and erbB4 receptors in human glioma biopsy samples. We then verify receptor expression in a panel of human glioma cell lines. Next, we investigate the status of the erbB2 and erbB3 receptors in the human glioma cell lines and find that they are constitutively tyrosine-phosphorylated and heterodimerized. Subsequently, we demonstrate that theses same cell lines express membrane bound and released forms of neuregulins, the erbB receptor ligands, suggesting a possible autocrine or paracrine signaling network. Furthermore, we show that exogenous activation of erbB2 and erbB3 receptors in U251 glioma cells by recombinant Nrg-1beta results in enhanced glioma cell growth under conditions of serum-deprivation. This enhancement is due to an increase in cell survival rather than an increase in cell proliferation and is dependent on the activation of erbB2 and phosphatidylinositol-3 kinase (PI3K). Moreover, Nrg-1beta activates an inhibitor of apoptosis, Akt, implying a possible role for this kinase in mediating Nrg-1beta effects in gliomas. This data suggests that glioma cells may use autocrine or paracrine neuregulin-1/erbB receptor signaling to enhance cell survival under conditions where growth would otherwise be limited.

The expression of neuregulin and erbB receptors in human skeletal muscle: effects of progressive resistance training

The neuregulin/erbB-signaling axis contributes to the development and growth of multiple mammalian tissues including skeletal muscle. In this study, we sought to characterize the native expression of this system in human skeletal muscle and test the hypothesis that a program of progressive resistance training (PRT) would regulate the expression of neuregulin (NRG) and its cognate receptors. Twelve healthy-male subjects underwent 8-weeks of lower-extremity PRT and muscle biopsies were performed at baseline and following 1- and 8-weeks of the intervention. PRT resulted in significant gains in skeletal muscle strength without appreciable changes in fiber size or myosin heavy chain (MHC) composition. At baseline, Western Blot analysis demonstrated expression of erbB2, erbB3 and erbB4 receptors and multiple NRG isoforms. Following 1- and 8-weeks of PRT, no changes erbB2, erbB4 or NRG expression were observed. ErbB3 expression, however, was significantly increased at both time points compared to baseline. Double labeling of muscle cross-sections revealed increased expression of erbB3 following PRT was not exclusive to fibers staining positive for MHC IIa. Thus, erbB2, erbB3, erbB4 and multiple NRG isoforms are natively expressed in human skeletal muscle. Following PRT, a significant increase in erbB3 was observed. The ability to detect basal expression and alterations in response to physiologic stimuli merit further studies examining the role of this system in skeletal muscle.

Novel mechanism of interaction of p85 subunit of phosphatidylinositol 3-kinase and ErbB3 receptor-derived phosphotyrosyl peptides

Ligand-activated and tyrosine-phosphorylated ErbB3 receptor binds to the SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase and initiates intracellular signaling. Here, we studied the interactions between the N- (N-SH2) and C- (C-SH2) terminal SH2 domains of the p85 subunit of the phosphatidylinositol 3-kinase and eight ErbB3 receptor-derived phosphotyrosyl peptides (P-peptides) by using molecular dynamics, free energy, and surface plasmon resonance (SPR) analyses. In SPR analysis, these P-peptides showed no binding to the C-SH2 domain, but P-peptides containing a phospho-YXXM or a non-phospho-YXXM motif did bind to the N-SH2 domain. The N-SH2 domain has two phosphotyrosine binding sites in its N- (N1) and C- (N2) terminal regions. Interestingly, we found that P-peptides of pY1180 and pY1241 favored to bind to the N2 site, although all other P-peptides showed favorable binding to the N1 site. Remarkably, two phosphotyrosines, pY1178 and pY1243, which are just 63 amino acids apart from the pY1241 and pY1180, respectively, showed favorable binding to the N1 site. These findings indicate a possibility that the pair of phosphotyrosines, pY1178-pY1241 or pY1243-pY1180, will fold into an appropriate configuration for binding to the N1 and N2 sites simultaneously. Our model structures of the cytoplasmic C-terminal domain of ErbB3 receptor also strongly supported the speculation. The calculated binding free energies between the N-SH2 domain and P-peptides showed excellent qualitative agreement with SPR data with a correlation coefficient of 0.91. The total electrostatic solvation energy between the N-SH2 domain and P-peptide was the dominant factor for its binding affinity.

ErbB4 Expression in Neural Progenitor Cells (ST14A) Is Necessary to Mediate Neuregulin-1β1-induced Migration

Activation of the receptor tyrosine kinase ErbB4 leads to various cellular responses such as proliferation, survival, differentiation, and chemotaxis. Two pairs of naturally occurring ErbB4 isoforms differing in their juxtamembrane (JMa/JMb) and C termini (cyt1/cyt2) have been described. To examine the role of ErbB4 in neuron migration, we cloned and stably transfected each of the four ErbB4 isoforms in ST14A cells (a neural progenitor cell line derived from the striatum of embryonic day 14 rats) endogenously expressing the other members of the ErbB family: ErbB1, ErbB2, and ErbB3. Using immunoprecipitation assays, we showed that the neuregulin-1beta1 (NRG1beta1) stimulus induced ErbB4 tyrosine phosphorylation and phosphatidylinositol 3-kinase (PI3K) recruitment and activation (as demonstrated by Akt phosphorylation) either directly (ErbB4 cyt1 isoform) or indirectly (ErbB4 cyt2 isoform). We examined the ability of the four ErbB4 isoforms to induce chemotaxis and cell proliferation in response to NRG1beta1 stimulation. Using migration assays, we observed that only ErbB4-expressing cells stimulated with NRG1beta1 showed a significant increase in migration, whereas the growth rate remained unchanged. Additional assays showed that inhibition of PI3K (but not of phospholipase Cgamma) dramatically reduced migratory activity. Our data show that ErbB4 signaling via PI3K activation plays a fundamental role in controlling NRG1beta1-induced migration.

Disruption of STAT3 signaling leads to tumor cell invasion through alterations of homotypic cell-cell adhesion complexes

STAT3 is frequently overexpressed and constitutively activated by tyrosine phosphorylation during malignant transformation. Despite the clear importance of STAT3 in cell proliferation and survival in diverse human cancers, its possible contribution to tumor cell adhesion, motility and invasion remains hypothetical. We therefore compared the transforming properties of STAT3wt, its constitutively activated dimeric form STAT3C, and the dominant negative mutant STAT3-Y705F in human colorectal HCT8/S11 cancer cells. Both STAT3wt and STAT3C exert a permissive action to the proinvasive activity of the scatter factor HGF in HCT8/S11 cells. In contrast, the monomeric and cytoplasmic mutant Y705F induces a constitutive invasive phenotype through Wnt/Rho-independent and EGFR/PI3-kinase-dependent pathways. Accordingly, Y705F decreases cell-cell homotypic adhesions, and increases cell motility and scattering, as well as lamellipodia-type cellular extensions. STAT3-Y705F-transfected HCT8/S11 cells display an increased tyrosine phosphorylation of the cell-cell adhesion regulator beta-catenin and its dissociation from the invasion suppressor E-cadherin at cell-cell contacts. Our data imply that both invasion promoter and repressor genes are controlled by the canonical STAT3 transcription pathways. Disruption of this cascade by Y705F reveals the proinvasive potential of altered forms of STAT3 as a persistent signaling adaptor in cytokine/transforming growth factor receptor scaffolds and oncogenic pathways.

Apigenin induces apoptosis through proteasomal degradation of HER2/neu in HER2/neu-overexpressing breast cancer cells via the phosphatidylinositol 3-kinase/Akt-dependent pathway

Apigenin is a low toxicity and non-mutagenic phytopolyphenol and protein kinase inhibitor. It exhibits anti-proliferating effects on human breast cancer cells. Here we examined several human breast cancer cell lines having different levels of HER2/neu expression and found that apigenin exhibited potent growth-inhibitory activity in HER2/neu-overexpressing breast cancer cells but was much less effective for those cells expressing basal levels of HER2/neu. Induction of apoptosis was also observed in HER2/neu-overexpressing breast cancer cells in a dose- and time-dependent manner. However, the one or more molecular mechanisms of apigenin-induced apoptosis in HER2/neu-overexpressing breast cancer cells remained to be elucidated. A cell survival pathway involving phosphatidylinositol 3-kinase (PI3K), and Akt is known to play an important role in inhibiting apoptosis in response to HER2/neu-overexpressing breast cancer cells, which prompted us to investigate whether this pathway plays a role in apigenin-induced apoptosis in HER2/neu-overexpressing breast cancer cells. Our results showed that apigenin inhibits Akt function in tumor cells in a complex manner. First, apigenin directly inhibited the PI3K activity while indirectly inhibiting the Akt kinase activity. Second, inhibition of HER2/neu autophosphorylation and transphosphorylation resulting from depleting HER2/neu protein in vivo was also observed. In addition, apigenin inhibited Akt kinase activity by preventing the docking of PI3K to HER2/HER3 heterodimers. Therefore, we proposed that apigenin-induced cellular effects result from loss of HER2/neu and HER3 expression with subsequent inactivation of PI3K and AKT in cells that are dependent on this pathway for cell proliferation and inhibition of apoptosis. This implies that the inhibition of the HER2/HER3 heterodimer function provided an especially effective strategy for blocking the HER2/neu-mediated transformation of breast cancer cells. Our results also demonstrated that apigenin dissociated the complex of HER2/neu and GRP94 that preceded the depletion of HER2/neu. Apigenin-induced degradation of mature HER2/neu involves polyubiquitination of HER2/neu and subsequent hydrolysis by the proteasome.

Roles of mitogen-activated protein kinase and phosphoinositide 3'-kinase in ErbB2/ErbB3 coreceptor-mediated heregulin signaling

ErbB2/HER2 and ErbB3/HER3, two members of the ErbB/HER family, together constitute a heregulin coreceptor complex that elicits a potent mitogenic and transforming signal. Among known intracellular effectors of the ErbB2/ErbB3 heregulin coreceptor are mitogen-activated protein kinase (MAPK) and phosphoinositide (PI) 3-kinase. Activation of the distinct MAPK and PI 3-kinase signaling pathways by the ErbB2/ErbB3 coreceptor in response to heregulin and their relative contributions to the mitogenic and transformation potentials of the activated coreceptor were investigated here. To this end, cDNAs encoding the wild-type ErbB3 protein (ErbB3-WT) and ErbB3 proteins with amino acid substitutions in either the Shc-binding site (ErbB3-Y1325F), the six putative PI 3-kinase-binding sites (ErbB3-6F), or both (ErbB3-7F) were generated and expressed in NIH-3T3 cells to form functional ErbB2/ErbB3 heregulin coreceptors. While the coreceptor incorporating ErbB3-WT activated both the MAPK and the PI 3-kinase signaling pathways, those incorporating ErbB3-Y1325F or ErbB3-6F activated either PI 3-kinase or MAPK, respectively. The ErbB2/ErbB3-7F coreceptor activated neither. Elimination of either signaling pathway lowered basal and eliminated heregulin-dependent expression of cyclin D1, which was in each case accompanied by an attenuated mitogenic response. Selective elimination of the PI 3-kinase pathway severely impaired the ability of heregulin to transform cells expressing the coreceptor, whereas attenuation of the MAPK pathway had a lesser effect. Thus, while both pathways contributed in a roughly additive manner to the mitogenic response elicited by the activated ErbB2/ErbB3 coreceptor, the PI 3-kinase pathway predominated in the induction of cellular transformation.

Activation of ErbB3-PI3-kinase pathway is correlated with malignant phenotypes of adenocarcinomas

Signet-ring cell carcinomas are malignant dedifferentiated carcinomas, which are frequently found in the stomach. We previously demonstrated that a 200 kDa protein is often constitutively phosphorylated on tyrosine and bound to phosphatidylinositol 3-kinase (PI3-kinase) in signet-ring cell carcinoma cells. In this study, we purified the 200 kDa protein from an extract of NUGC-4 cells, a cell line of signet-ring cell carcinoma, and identified it as ErbB3. ErbB3 was found to be phosphorylated selectively in dedifferentiated adenocarcinoma cell lines among various gastric cancer cell lines. Expression of a constitutively active chimeric receptor consisting of ErbB2 and ErbB3 in HCC2998 cells, a highly differentiated adenocarcinoma cell line, revealed that the signaling triggered by phosphorylation of ErbB3 was important for dedifferentiated phenotypes such as loss of cell-cell interaction and high expression of MUC1/DF3 antigen, a marker of the malignant tumors. Taken together, activation of ErbB3 pathway may contribute to the development of dedifferentiated carcinomas.

Characterization of an interleukin-7-dependent thymic cell line derived from a p53(-/-) mouse

In order to study the response of T cells to IL-7, we aimed to generate an IL-7-dependent thymocyte line. CD4(-)CD8(-) thymocytes from a p53(-)/(-) mouse were continuously propagated in interleukin-7 (IL-7), and after 2 months there developed an immortal line termed "D1." The D1 line has retained a stable dependency on IL-7. Withdrawal of IL-7 from D1 cells induced arrest in G1 phase of the cell cycle, followed by apoptosis. In addition to IL-7, several other cytokines that employ gamma(c) as part of their receptor were also capable of stimulating D1 cell survival and proliferation. Gene induction by IL-7 was analyzed in D1 cells using RNase protection and array analysis and revealed a number of transcripts potentially involved in cell cycle, apoptosis and signaling.

Analysis of membrane protein interactions using yeast-based technologies

Proteins associated with membranes total approximately a third of all proteins in a typical eukaryotic cell. However, the analysis of interactions between membrane proteins is difficult because of the hydrophobic nature of these proteins, and conventional biochemical and genetic assays are often of limited use. We summarize here recent yeast-based interaction technologies that can be applied to membrane proteins.

Ionizing radiation activates Erb-B receptor dependent Akt and p70 S6 kinase signaling in carcinoma cells

In this study we have investigated the effects of low dose ionizing radiation (2 Gy) on p70 S6 kinase and Akt signaling with respect to Erb-B receptors in both the A431 squamous and the MDA-MB-231 mammary carcinoma cell lines. Ionizing radiation caused a 2-3-fold increase in p70 S6 kinase activity that was blocked pharmacologically using an EGFR inhibitor (AG1478) alone, or in combination with an Erb-B2 inhibitor (AG825). These results suggested that both EGFR and Erb-B2 receptors could initiate radiation-induced activation of p70 S6K. EGFR dependent Erb-B3 signaling also contributed to p70 S6 kinase activity through recruitment and activation of PI3K, which has been shown to regulate p70 S6 kinase activity. Furthermore, inhibition of the EGFR blocked IR stimulated increases in protein translation, a biologic consequence of p70 S6 kinase activation. We also report that ionizing radiation stimulated Akt activity that was partially independent of PI3K activity, but dependent on Erb-B2 function. Erb-B2 inhibition also correlated with enhanced apoptosis following IR exposure, suggesting an important role for Erb-B2 in cell survival. Together this work demonstrates that the Erb-B receptor tyrosine kinase network stimulates cytoprotective p70 S6 kinase and Akt activity in response to clinically relevant doses of ionizing radiation.

Ansamycin antibiotics inhibit Akt activation and cyclin D expression in breast cancer cells that overexpress HER2

Ansamycin antibiotics, such as 17-allylaminogeldanamycin (17-AAG), bind to Hsp90 and regulate its function, resulting in the proteasomal degradation of a subset of signaling proteins that require Hsp90 for conformational maturation. HER2 is a very sensitive target of these drugs. Ansamycins cause RB-dependent G1 arrest that is associated with loss of D-cyclins via a PI3 kinase, Akt dependent pathway. Downregulation of D-cyclin was due, in part, to loss of Akt expression in response to drug. Moreover, in HER2 overexpressing breast cancer cells, 17-AAG caused rapid inhibition of Akt activity prior to any change in Akt protein. Ansamycins caused rapid degradation of HER2 and a concomitant loss in HER3 associated PI3 kinase activity. This led to a loss of Akt activity, dephosphorylation of Akt substrates, and loss of D-cyclin expression. Introduction into cells of a constitutively membrane bound form of PI3 kinase prevented the effects of the drug on Akt activity and D-cyclins. Thus, in breast cancer cells with high HER2, Akt activation by HER2/HER3 heterodimers is required for D-cyclin expression. In murine xenograft models, non-toxic doses of 17-AAG markedly reduced the expression of HER2 and phosphorylation of Akt and inhibited tumor growth. Thus, pharmacological inhibition of Akt activation is achievable with ansamycins and may be useful for the treatment of HER2 driven tumors.