Abstract S2-04: RANK ligand as a target for breast cancer prevention in BRCA1 mutation carriers

Background: BRCA1 mutation carriers commonly undergo prophylactic mastectomy to reduce their risk of breast cancer. The precise role of chemoprevention with tamoxifen, which reduces the incidence of ER-positive breast cancer in the general population, is uncertain for BRCA1 mutation carriers, where uptake has been modest. The identification of an effective and acceptable prevention therapy therefore remains a 'holy grail' for the field. Precancerous BRCA1mut/+ tissue harbors an aberrant population of luminal progenitor cells and deregulated progesterone signaling has been implicated in BRCA1-associated oncogenesis. Since Receptor Activator of Nuclear Factor-kappa B ligand (RANKL) is a key paracrine effector of progesterone signaling, and RANKL and its receptor RANK contribute to mammary tumorigenesis, we investigated a role for this pathway in the preneoplastic phase of BRCA1 mutation carriers.

Methods: We explored a role for the RANK/RANKL pathway during the preneoplastic phase in freshly isolated (histologically normal) patient specimens from BRCA1 mutation carriers using several approaches. RANK and RANKL expression in breast cancer was evaluated in formalin fixed paraffin embedded (FFPE) archival sections by IHC from the kConFab and the Amgen Tissue Banks. All samples were obtained with patient consent and relevant IRB approval. A role for RANKL inhibition in attenuating tumor onset was studied using the MMTV-cre/Brca1fl/fl/p53+/– mouse model that recapitulates human basal-like breast cancer.

Results: We identified two subsets of luminal progenitors (RANK+ and RANK–) in histologically normal tissue of BRCA1 mutation carriers and found that RANK+ cells are highly proliferative, exhibit grossly aberrant DNA repair and bear a molecular signature similar to that of basal-like breast cancer. Moreover, high levels of RANK expression prevailed in established BRCA1-associated tumors. These data suggest that RANK+ and not RANK– progenitors are a key target population in these women. Notably, inhibition of RANKL signaling by denosumab in 3D breast organoids derived from pre-neoplastic BRCA1mut/+ tissue attenuated progesterone-induced proliferation. Furthermore, inhibition of RANKL with either the RANKL inhibitor OPG-Fc or a RANKL monoclonal antibody in a Brca1-deficient mouse model significantly curtailed mammary tumorigenesis, when compared to controls (p<0.001).

Conclusions: Together these findings identify a targetable pathway in a putative cell of origin population in BRCA1 mutation carriers and implicate RANKL blockade as a promising breast cancer prevention strategy.

Citation Format: Lindeman GJ, Nolan E, Vaillant F, Branstetter D, Pal B, Giner G, Whitehead L, Lok SW, Mann GB, kConFab Consortium, Rohrbach K, Huang L-Y, Soriano R, Smyth GK, Dougall WC, Visvader JE. RANK ligand as a target for breast cancer prevention in BRCA1 mutation carriers [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr S2-04.

Abstract P5-03-02: Expression of RANK and RANK ligand (RANKL) in breast carcinoma and distinct breast epithelial cells from BRCA1 mutation carriers

Background: Breast tumors in BRCA1 mutation carriers likely arise from luminal progenitor (LP) cells, previously shown to exhibit aberrant growth properties. Oophorectomy, and possibly tamoxifen, reduce breast cancer risk in BRCA1 carriers, potentially via inhibition of paracrine mediated signaling to stem/progenitor cells. RANKL is a major paracrine effector of progesterone's mitogenic action in mammary epithelium via its receptor RANK, and has a role in ovarian hormone-dependent activation of stem cells. Here we assessed RANK and RANKL expression in breast tumors and normal breast epithelial subtypes from women with mutations of BRCA1 (mBRCA1) or BRCA2 (mBRCA2).

Methods: RANK and RANKL expression in breast cancer or normal breast tissue samples with mBRCA1, mBRCA2 or wildtype (WT) BRCA1/2 were analyzed in formalin fixed paraffin embedded (FFPE) sections by IHC. kConFab and The Royal Melbourne Hospital Tissue Bank provided the samples used in this analysis; these samples were obtained with relevant IRB approval. RANK expression on normal breast epithelial cells was measured by flow cytometry. Antibodies against human RANK (N-1H8, N-2B10; Amgen) and RANKL (M366; Amgen) were used in both assays. Incidence of IHC staining was scored as a positive IHC signal of any intensity. The overall expression was generated using the H scoring method which is calculated as the staining intensity of the tumor (0-3) multiplied by the percentage of positively staining cells.

Results: Breast tumors from women with mBRCA1 had a higher incidence of RANK expression (68/162; 42%) compared with mBRCA2 (17/113; 15%) or WT (34/314; 11%) and higher overall H score (21.3) compared with mBRCA2 (8.0) or WT (3.4); RANKL expression did not vary greatly between groups: mBRCA1 (13/135; 10%), mBRCA2 (5/114; 4%), WT (23/212; 11%). In normal breast tissue, LP (Lin−EpCAM+CD49f+) and basal/stem cells (Lin−EpCAM−CD49fhi) expressed RANK on their surface. Similar expression patterns were seen in these epithelial subtypes from each BRCA1/2 genotype. Stromal cells (Lin−EpCAM−CD49f−) had minimal RANK expression.

Conclusions: RANK expression intensity and incidence scores are both enriched approximately 4-fold in breast tumors from BRCA1 carriers compared with other genotypes. Also, RANK is normally expressed in breast LP cells as well as the basal/stem cell containing population. Ongoing studies will assess functional regulation of LP or mammary stem cell activity by RANKL and determine if the RANKL/RANK signaling pathway affects the aberrant growth characteristics of these cells from BRCA1 mutation carriers.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-03-02.

P3-04-05: Expression Profiles of RANK and RANKL mRNA and Protein in the Mammary Gland of Female Cynomolgus Monkeys after Long-Term Treatment with Different Menopausal Hormone Therapies

Background: RANK and its ligand (RANKL), key factors for bone remodeling and metastasis, are crucial for the development of the mouse mammary gland during pregnancy. OPG also binds RANKL and inhibits RANKL action. Treatment of mice with progesterone increases RANKL expression in mammary epithelium. RANKL functions as a major paracrine effector of both progesterone-dependent mitogenic action in mammary tissue, and also in ovarian hormone-dependent expansion and regenerative potential of mammary stem cells. RANKL inhibition in vivo reduces progestin-induced mammary preneoplasias and adenocarcinomas. In postmenopausal women, the addition of a progestin to estrogen therapy has been associated with significant increases in mammary proliferation, breast density, and breast cancer risk and recurrence. The goal of this study was to characterize the expression of RANK, RANKL, and OPG in the mammary gland following estrogen+progestin therapy in comparison with estrogen-alone therapy or placebo in a postmenopausal primate model.

Methods: Ovariectomized female cynomolgus monkeys (Macaca fascicularis) were randomized into 3 treatment groups: placebo, oral conjugated equine estrogens at 0.625 mg/d (CEE) and CEE with medroxyprogesterone acetate (MPA) at 2.5 mg/d (CEE+MPA). Treatments were administered in the diet for 2 years. RANK, RANKL, and OPG mRNA were measured in mammary gland samples (n = 28 to 31/group) using quantitative real-time reverse transcriptase polymerase chain reaction (qPCR). Antibodies against human RANK (N-2B10 and N-1H8; Amgen) and human RANKL (M366; Amgen) were used for immunohistochemistry (IHC) of formalin-fixed tissues (n = 20 to 23/group) along with an isotype control. The intensity of IHC staining was scored on a semiquantitative scale (0 = absent, 1 = weak, 2 = moderate, 3 = intense). Incidence was scored as a positive IHC signal (any intensity).

Results: Analysis of mRNA revealed that treatment with CEE+MPA resulted in a significantly greater ratio of RANKL to OPG compared to treatment with placebo (P < 0.01) or CEE (P < 0.05). IHC demonstrated RANK protein was predominately localized to basal cells of ducts and acinar cells, while RANKL protein was localized to luminal cells. RANK protein was detected in mammary glands of animals in all groups: placebo (74%), CEE (55%), and CEE +MPA (53%). RANKL protein was expressed in the animals treated with CEE+MPA (74%) and CEE (15%), but not in placebo-treated animals. Quantitative expression of specific mRNA markers for epithelial proliferation (MKI67) and density (KRT19) was highest for CEE+MPA, intermediate for CEE, and lowest for placebo.

Discussion: Key components of the RANKL/RANK pathway are expressed in the normal postmenopausal primate mammary gland, modulated by long-term estrogen+progestin exposure at clinically relevant doses, and associated with changes in estrogen+progestin-driven proliferation of mammary tissue.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P3-04-05.

P2-01-02: Pharmacological Inhibition of RANKL Attenuates Mammary Tumor Development and Lung Metastases in the MMTV-neu Transgenic Spontaneous Tumor Model

Background: RANK and its ligand (RANKL), key factors for bone remodeling and metastasis, are crucial for the development of mouse mammary glands during pregnancy. In a mouse model of mammary carcinogenesis induced by the hormone medroxyprogesterone (MPA) and a carcinogen (DMBA), treatment with a RANKL inhibitor RANK-Fc reduces hormone-induced mammary epithelial proliferation, incidence of preneoplasias, and mammary adenocarcinomas. This study assessed the expression of mouse RANK and RANKL in FVB wild-type (WT) and MMTV-neu mice and whether RANKL inhibition could inhibit mammary tumor formation and lung metastases in this model.

Materials and Methods: MMTV-neu mice were treated with the RANKL inhibitor RANK-Fc (10 mg/kg, 3x/week) or vehicle (muFc) beginning at 20 weeks. Mammary tumor formation was determined by palpation and confirmed by histologic examination. The presence of preneoplastic lesions and histotype of tumors was determined on H&E-stained tissues. Entire lungs from tumor-bearing MMTV-neu mice were step-sectioned at 75 μm and assessed histologically for the presence of metastases. Epithelial proliferation was measured by BrdU labeling of the mammary epithelium. The expression of RANK, RANKL, cyclin D1, progesterone receptor (PR), F4/80, FOXP3, and CD3 was determined by immunohistochemistry (IHC).

Results: RANK-Fc treatment of MMTV-neu mice resulted in a significant reduction in total number of mammary tumors and lung metastases quantified at necropsy but did not impact median time to spontaneous mammary tumor onset. RANKL inhibition resulted in reduced pre-neoplasias and corresponding reductions in proliferation and cyclin D1 within normal mammary epithelia but not in adenocarcinomas. Treatment of MMTV-neu mice with RANK-Fc also reduced the incidence and number of lung metastases per mouse. RANK protein was detected in normal mammary epithelium, preneoplastic mammary intraepithelial neoplasia (MIN), the epithelial component of adenocarcinomas and lung metastases, and monocytic element adjacent to primary MMTV-neu tumors. RANKL protein was detected in normal mammary epithelium but not in the epithelia, stromal component or infiltrating cells of adenocarcinomas or lung metastases. Progesterone receptor expression was observed in normal mammary epithelium of MMTV-neu and FVB WT mice but decreased over time in MMTV-neu mice and was not detected in preneoplasias, adenocarcinomas, or lung metastases. CD3 was detected in mammary tumors and lung metastases but neither RANKL nor FOXP3 were present. Staining of lymph nodes was used as a positive control for RANKL and FOXP3. Expression of FOXP3 and RANKL were not measurably colocalized in dual staining of lymph nodes.

Discussion: RANKL is expressed in normal mammary epithelia of MMTV-neu mice but is not detected in any stage of atypia, including MIN, mammary adenocarcinoma, or lung metastases. Selective pharmacological inhibition of RANKL by RANK-Fc treatment attenuated mammary tumor development and lung metastases in the MMTV-neu transgenic spontaneous tumor model.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-01-02.

RANK is essential for osteoclast and lymph node development

The physiological role of the TNF receptor (TNFR) family member, RANK, was investigated by generating RANK-deficient mice. RANK(-/-) mice were characterized by profound osteopetrosis resulting from an apparent block in osteoclast differentiation. RANK expression was not required for the commitment, differentiation, and functional maturation of macrophages and dendritic cells from their myeloid precursors but provided a necessary and specific signal for the differentiation of myeloid-derived osteoclasts. RANK(-/-) mice also exhibited a marked deficiency of B cells in the spleen. RANK(-/-) mice retained mucosal-associated lymphoid tissues including Peyer's patches but completely lacked all other peripheral lymph nodes, highlighting an additional major role for RANK in lymph node formation. These experiments reveal that RANK provides critical signals necessary for lymph node organogenesis and osteoclast differentiation.

The involvement of multiple tumor necrosis factor receptor (TNFR)-associated factors in the signaling mechanisms of receptor activator of NF-kappaB, a member of the TNFR superfamily

Receptor activator of NF-kappaB (RANK) is a recently identified member of the tumor necrosis factor receptor superfamily and is expressed on activated T cells and dendritic cells. Its cognate ligand (RANKL) plays significant roles in the activation of dendritic cell function and osteoclast differentiation. We demonstrate here the interaction of RANK with tumor necrosis factor receptor-associated factors (TRAFs) 1, 2, 3, 5, and 6 both in vitro and in cells. Mapping of the structural requirements for TRAF/RANK interaction revealed multiple TRAF binding sites clustered in two distinct domains in the RANK cytoplasmic tail. These TRAF binding domains were shown to be functionally important for the RANK-dependent induction of NF-kappaB and c-Jun NH2-terminal kinase activities. Site-directed mutagenesis demonstrated that these TRAF binding sites exhibited selective binding for different TRAF proteins. In particular, TRAF6 interacted with membrane-proximal determinants distinct from those binding TRAFs 1, 2, 3, and 5. When this membrane-proximal TRAF6 interaction domain was deleted, RANK-mediated NF-kappaB signaling was completely inhibited while c-Jun NH2-terminal kinase activation was partially inhibited. An NH2-terminal truncation mutant of TRAF6 inhibited RANKL-mediated NF-kappaB activation, but failed to affect constitutive signaling induced by receptor overexpression, revealing a selective role for TRAF6 in ligand-induced activation events.

The novel receptor TRAIL-R4 induces NF-kappaB and protects against TRAIL-mediated apoptosis, yet retains an incomplete death domain

A fourth member of the emerging TRAIL receptor family, TRAIL-R4, has been cloned and characterized. TRAIL-R4 encodes a 386-amino acid protein with an extracellular domain showing 58%-70% identity to those of TRAIL-R1, TRAIL-R2, and TRAIL-R3. The signaling capacity of TRAIL-R4 is similar to that of TRAIL-R1 and TRAIL-R2 with respect to NF-kappaB activation, but differs in its inability to induce apoptosis. Yet TRAIL-R4 retains a C-terminal element containing one third of a consensus death domain motif. Transient overexpression of TRAIL-R4 in cells normally sensitive to TRAIL-mediated killing confers complete protection, suggesting that one function of TRAIL-R4 may be inhibition of TRAIL cytotoxicity. Like TRAIL-R1 and TRAIL-R2, this receptor shows widespread tissue expression. The human TRAIL-R4 gene has been mapped to chromosome 8p22-21, clustered with three other TRAIL receptors.

A homologue of the TNF receptor and its ligand enhance T-cell growth and dendritic-cell function

Dendritic cells are rare haematopoietic cells that reside in a number of organs and tissues. By capturing, processing and presenting antigens to T cells, dendritic cells are essential for immune surveillance and the regulation of specific immunity. Several members of the tumour necrosis factor receptor (TNFR) superfamily are integral to the regulation of the immune response. These structurally related proteins modulate cellular functions ranging from proliferation and differentiation to inflammation and cell survival or deaths. The functional activity of dendritic cells is greatly increased by signalling through the TNFR family member CD40. Here we report the characterization of RANK (for receptor activator of NF-kappaB), a new member of the TNFR family derived from dendritic cells, and the isolation of a RANK ligand (RANKL) by direct expression screening. RANKL augments the ability of dendritic cells to stimulate naive T-cell proliferation in a mixed lymphocyte reaction, and increases the survival of RANK+ T cells generated with interleukin-4 and transforming growth factor (TGF)-beta. Thus RANK and RANKL seem to be important regulators of interactions between T cells and dendritic cells.

Association of signaling proteins with a nonmitogenic heterodimeric complex composed of epidermal growth factor receptor and kinase-inactive p185c-neu

The functional consequences of heterodimer formation between the epidermal growth factor receptor (EGFr) and the p185c-neu receptor tyrosine kinase include increased mitogenic and transformation potencies. To determine the possible alteration of signal transduction pathways resulting from this heteromeric complex, the capacity of several signaling proteins to associate with the heterodimeric receptors has been assayed. The in vivo interaction with the EGFr/p185c-neu heterodimer of several signal transduction proteins, including phospholipase C-gamma 1 (PLC-gamma 1), the p85 subunit of phosphotidylinositol 3-kinase, the ras GTPase activating protein, SHC, NCK, p72RAF, and the tyrosine phosphatase SHPTP2, was measured by coimmunoprecipitation. The binding of these signaling proteins to a complex composed of EGFr and a kinase-inactive form of p185 (p185K757M) was not impaired, even though the mitogenic and transformation activity of this complex had been abrogated. In addition, the EGF-induced phosphorylation of GAP, p85, and PLC-gamma 1 did not correlate with the dominant-negative action of p185K757M on EGFr function. Thus, substrate association and phosphorylation do not correlate stringently with the mitogenic and transforming activity of this receptor complex, suggesting additional pathways or mechanisms vital to EGFr/p185c-neu heterodimeric signaling.

Down-regulation of c-neu receptors by nerve growth factor in PC12 cells

A small number of p185c-neu receptors have been found on PC12 cells. These receptors show some basal phosphorylation in quiescent cells. When the cells are treated with nerve growth factor (NGF) for a short time, some increase in phosphorylation is seen, mainly on serine and threonine residues, and this is accompanied by a slight shift in the apparent molecular weight. Epidermal growth factor (EGF) also increases the phosphorylation of p185c-neu, in this case on tyrosine residues. Neither heregulin-beta 1 nor gp30 stimulates the tyrosine phosphorylation of p185c-neu, and neither has a proliferative effect on the cells. Treatment of the cells with NGF for 5 days produces a 70-80% reduction in the number of p185c-neu receptors. This down-regulation does not occur when PC12nnr5 cells, which lack the high-affinity NGF receptor, p140trk, are treated with NGF. The level of p185c-neu mRNA is not altered by NGF treatment, suggesting that the down-regulation is due to either a translational or a posttranslational alteration.

Intermolecular association and trans-phosphorylation of different neu-kinase forms permit SH2-dependent signaling and oncogenic transformation

The neu oncogene encodes a 185 kDa receptor tyrosine kinase. A single point mutation (Val664-->Glu) within the p185neu transmembrane region results in higher efficiency of receptor dimerization, constitutive activity of tyrosine kinase and cellular transformation. The oncogenic potential of this mutated form of p185neu (termed Tneu) can be inactivated by site-directed alteration of a lysine residue in the conserved catalytic domain. In this report, we have utilized the physical and functional interaction of a full-length kinase-deficient neu protein (T757) and truncated kinase-active Tneu forms to determine critical protein domains for Tneu oligomerization and the resultant biological consequences. Analysis of various truncated Tneu mutants confirmed that the transmembrane region was crucial for p185 dimerization. Receptor association facilitates intermolecular phosphorylation of kinase-deficient mutant T757 by truncated kinase-active p185 proteins, and the trans-phosphorylated kinase-deficient T757 was able to associate in vitro with proteins containing SH2 domains. Receptor-receptor interactions resulted in enhanced signal transduction potential and transformation of cell-lines co-expressing different neu-kinase forms. These studies emphasize a novel feature of protein-protein interaction and the functional significance of p185 dimerization, intermolecular phosphorylation and signaling which may result in cellular transformation.

Antibody-structure-based design of pharmacological agents

The well-studied antigen-combining sites of antibody molecules hold considerable promise as a model system for the design of bioactive peptides. These small, immunoglobulin-derived peptides can be used in the development of alternative treatments for disease and in diagnostic strategies. The general principles derived from the design of small pharmacological agents based on the structural features of antibodies may also be extended to the design of other bioactive peptides.

The neu-oncogene: signal transduction pathways, transformation mechanisms and evolving therapies

A variety of pathways exist to transmit biological signals. One mechanism used for the regulated control of cell growth and differentiation is through the transduction of signals resulting from the binding of soluble polypeptide growth factors to their cognate receptors. The specificity of growth factor action is mediated by the interaction of ligand with cognate receptors which can lead to exquisite control in a tissue- and developmental-specific manner. In addition, individual receptors on the cell surface can form complex assemblies with other receptor/signal transduction molecules that potentially lead to additional levels of signal transmissions. Biological signaling by peptide ligands can be mediated through the enzymatic activation of the receptor resulting in the triggering of a defined biochemical pathway. Ultimately, a mitogenic or differentiation signal is delivered to the nucleus, completing the biological action of the growth factor. The biochemical mechanisms of signal transduction by the p185 neu/c-erbB-2 growth factor receptor and the subsequent physiological responses are the topics of this review. Study of the p185 growth factor receptor has helped to illustrate the functional role of receptor homo- (and hetero-) dimerization in enzyme activation and, in malignant cells, the detrimental results of structural mutations or aberrant gene expression which may effect this dimerization. The ability of one type of growth factor receptor to affect the activity of another (as illustrated by the p185/epidermal growth factor receptor heterodimeric complex) is likely to be a common regulatory feature of growth factor receptor action. The nomenclature to be used in this review will refer to the oncogenic mutated form of the rat protein as 'p185neu', the proto-oncogenic rat protein as 'p185c-neu' and the human form as 'p185c-erbB-2'. The term 'p185' will be used to refer to any type of protein, regardless of the source.

Kinase-deficient neu proteins suppress epidermal growth factor receptor function and abolish cell transformation

p185c-neu and epidermal growth factor receptor (EGFR) associate into an active heterodimer, and overexpression of these two receptors leads to a transformed phenotype. However, the association of EGFR and kinase-deficient Neu proteins (point mutant N757 or cytoplasmic domain deletion mutant N691stop) results in a defective or inactive heterodimeric complex. In this report we explore the biological consequences of heterodimerization between EGFR and wild-type (WT) or kinase-deficient mutant Neu proteins in living cells. We show that co-expression of EGFR and kinase-deficient Neu proteins abolished the synergistic transformation and tumorigenicity. Moreover, the normal responses of EGFR to ligand were significantly suppressed, e.g., loss of EGF-dependent transformation, reduced rate of receptor endocytosis and turnover, diminished DNA synthesis, and decreased EGF binding affinity. These results provide the first evidence that kinase-deficient Neu proteins suppress normal EGFR function and display a dominant negative mutant phenotype. Together with the stimulatory effects observed in cells forming active heterodimers, these studies provide a role for heterodimerization of EGFR and Neu/c-erbB2 in interreceptor activation and synergistic signaling which may be responsible for the transition from normal receptor function into oncogenesis.

Ligand and p185c-neu density govern receptor interactions and tyrosine kinase activation

The neu protooncogene (also known as c-erbB2, NGL, and HER2) encodes a 185-kDa transmembrane glycoprotein with intrinsic tyrosine kinase activity that resembles the receptor for epidermal growth factor. The p185 gene and protein were originally identified in the brain and are thought to play a critical role in neurogenesis. Aberrant c-erbB2 protein overexpression also occurs in several human adenocarcinomas. A ligand for p185, neu-activating factor (NAF), specifically binds to neu receptor and increases the p185c-neu tyrosine phosphorylation in vitro and in vivo in a dose-dependent manner. We now show that NAF specifically binds to purified p185 expressed in baculovirus. Direct binding analysis showed that NAF binds with high affinity (Kd = 1.3 nM). We have investigated changes in the structure and association state of baculovirus-produced neu holoreceptor that are induced by ligand binding. In this study, we used sucrose gradients to show that purified p185c-neu exists mainly in the monomeric form at low concentrations, whereas at higher concentrations p185c-neu exists as dimers or multimers. At low concentrations, but in the presence of ligand, p185c-neu sediments as a dimeric or multimeric form. Monomer-oligomer interconversion is absolutely ligand dependent at low receptor concentrations. The high molecular weight form of the receptor is enzymatically more active, as a consequence of ligand-driven activation of the receptor kinase. Oncogenic p185neu receptors sediment predominantly as high molecular weight forms and have constitutively active kinases.

Heterodimerization of epidermal growth factor receptor and wild-type or kinase-deficient Neu: a mechanism of interreceptor kinase activation and transphosphorylation

We have shown that members of the erbB family undergo homodimer and heterodimer formation. The rat p185c-neu and the epidermal growth factor receptor (EGFR) can associate into an active heterodimeric tyrosine kinase. Overexpression of these two receptors also results in a transformed phenotype. We now show that mutant Neu proteins resulting from a point mutation at the ATP-binding site (N757) or cytoplasmic domain deletions (N691stop) are still able to undergo EGF-induced heterodimerization with EGFR. Analysis of heterodimer formation between EGFR and truncated Neu proteins revealed that heterodimerization is preferred over homodimerization of EGFR. N757 can be transphosphorylated by associated EGFR upon EGF stimulation. However, the heterodimer composed of EGFR and N691stop is kinase inactive. These results provided evidence that the Neu ectodomain is sufficient to associate with EGFR physically, and the cytoplasmic domain interaction is required for heterodimeric kinase activation, indicating that Neu/c-erbB2 is not just a simple substrate for EGFR but a transactivator as well.

Biological studies and potential therapeutic applications of monoclonal antibodies and small molecules reactive with the neu/c-erbB-2 protein

The overexpression of the growth factor receptor p185neu/c-erbB-2 has been observed in a number of human adenocarcinomas and is mechanistically linked to neoplastic growth. Monoclonal antibodies raised against extracellular domains of the p185neu/c-erbB-2 receptor oncoprotein have been utilized to inhibit the pathway of neu-induced tumor development. Our laboratory has demonstrated a direct effect of anti-p185neu/c-erbB-2 antibodies which requires receptor ligation. This induced aggregation causes the downmodulation of cell-surface expression and eventual degradation of p185neu/c-erbB-2 protein. In cells transformed by the neu oncogene, the result of antibody-induced p185neu/c-erbB-2 receptor modulation is the reversion of the malignant phenotype. We are exploiting the direct efficacy of this monoclonal antibody by developing small molecules (peptides and organic mimietics) based on anti-p185neu/c-erbB-2 antibody structure that can mediate similar receptor binding and biological effects.

Interaction of the neu/p185 and EGF receptor tyrosine kinases: implications for cellular transformation and tumor therapy

Growth factor receptors such as the epidermal growth factor receptor (EGFR) and the p185c-neu protein serve vital roles in the transduction of differentiation, developmental, or mitogenic signaling within normal cells. Two methods of analysis suggest that the inappropriately high expression of either protein tyrosine kinase promotes malignant transformation. First, data from in vitro experiments indicate that overexpression of either EGFR or p185c-neu (or the human homolog c-erbB-2) transforms cell-lines. Second, analysis of primary tumors and tumor cell-lines derived from many epithelial tissues (breast, stomach, ovary, and pancreas) show growth factor receptor gene amplification and elevated protein levels. The physical and functional interaction of p185c-neu and EGFR leads to the formation of a highly active, heterodimeric tyrosine kinase complex which synergistically activates cellular transformation. Anti-receptor antibodies have shown potential utility for the down modulation of these cell-surface proteins and suppression of the malignant phenotype. Design of organic antibody "mimetics" based on the structure of antireceptor antibodies may provide useful therapies and biological reagents to affect growth factor receptor function.

Carboxyl-terminal deletion and point mutations decrease the transforming potential of the activated rat neu oncogene product

The rat neu oncogene encodes a constitutively activated growth factor receptor/transmembrane tyrosine kinase, p185Tneu, that is structurally similar to yet distinct from the epidermal growth factor receptor. To explore the role of the carboxyl-terminal region and of putative autophosphorylation sites in regulating the activity of the rat p185Tneu (T, transforming) protein, we used site-directed mutagenesis to generate a p185Tneu mutant in which a putative tyrosine autophosphorylation site (residue 1253) at the extreme carboxyl terminus was replaced by a phenylalanine residue and a mutant in which the carboxyl-terminal 122 amino acids were deleted. These proteins were expressed in NIH 3T3 cells at comparable levels and exhibited similar autophosphorylation activity, exogenous substrate phosphorylation ability, oligomerization levels, and responsiveness to a partially purified neu-activating factor. However, the mutant p185Tneu proteins displayed a decreased transforming capacity both in vitro and in vivo. This analysis demonstrated that the carboxyl-terminal domain and at least one putative tyrosine autophosphorylation site of p185Tneu play a role in positively regulating the cell growth-regulating properties of the neu protein.

Multiple mRNA species generated by alternate polyadenylation from the rat manganese superoxide dismutase gene

The mitochondrial enzyme, manganese superoxide dismutase (MnSOD) is an integral component of the cell's defense against superoxide-mediated cellular damage. We have isolated and characterized four cDNA clones and the structural gene for rat MnSOD. Northern analyses using MnSOD cDNA probes detected at least five mRNAs in all tissues and cell types examined. Southern and Northern analysis using a 3' non-coding sequence probe, common to all the cDNAs, showed hybridization only to genomic restriction fragments that correspond to our genomic clone and the five MnSOD mRNAs. These data demonstrate that all of the rat MnSOD transcripts are derived from a single functional gene. Primer extension data indicate that transcription initiation is clustered within a few bases. Northern analysis using intron probes demonstrates that all five transcripts are fully processed. Northern analysis using cDNA and genomic probes from sequences progressively 3' to the end of the coding sequence indicates that size heterogeneity in the MnSOD transcripts results from variations in the length of the 3' non-coding sequence. From this data and the location of potential polyadenylation signals near the expected sites of transcript termination, we conclude that the existence of multiple MnSOD mRNA species originate as the result of alternate polyadenylation.

Manganese superoxide dismutase: a hepatic acute phase protein regulated by interleukin-6 and glucocorticoids

The superoxide dismutases (SODs) are important metallo-enzymes which scavenge and dismutate the superoxide free radical. They are thought to be the main enzymes in the antioxidant defense system. Identification of stimuli that control transcription of the SOD genes is essential for understanding SOD gene regulation. In this study we show that manganese SOD (MnSOD) mRNA levels are elevated by lipopolysaccharide, a bacterial endotoxin, in rat liver. However, neither lipopolysaccharide nor tumor necrosis factor-alpha had an effect on MnSOD mRNA expression in cultured primary hepatocytes. On the other hand, the inflammatory cytokines, interleukin-1 (IL-1) and IL-6 did increase MnSOD mRNA levels, either 2- or 15-fold, respectively, over a 20-h period in hepatocytes. The IL-6-induced increase in MnSOD mRNA levels was attenuated by dexamethasone, a glucocorticoid, in hepatocytes cultured for less than 16 h. In contrast, in hepatocytes originally cultured for more than 16 h, IL-6 and dexamethasone produced a synergistic increase in MnSOD mRNA levels. The induction of MnSOD expression by IL-6, which is a known inflammatory cytokine, suggests that MnSOD may play a role in the inflammation process. Since inflammation is known to result in oxidative damage to cells, the role of MnSOD may be to protect cells from inflammation-mediated oxidative damage.

Regulation of manganese superoxide dismutase by lipopolysaccharide, interleukin-1, and tumor necrosis factor. Role in the acute inflammatory response

We have demonstrated a dramatic induction of manganese superoxide dismutase (Mn-SOD) mRNA levels in response to lipopolysaccharide (LPS), interleukin-1, and tumor necrosis factor in pulmonary epithelial cells. These stimuli had no effect on the corresponding mRNA levels for the copper/zinc (Cu/Zn)-SOD. Identical treatments of pulmonary fibroblast cells with LPS showed only minor changes in the Mn-SOD mRNA levels demonstrating a cell type-specific effect for this acute inflammatory mediator. Furthermore, we have shown that hyperoxia has no effect within 24 h on Mn-or Cu/Zn-SOD mRNA levels in either fibroblasts or epithelial cells. The induction of Mn-SOD mRNA levels by LPS is completely inhibited by actinomycin. Treatment of cells with cycloheximide causes an induction equal to that for LPS, whereas co-treatment with cycloheximide and LPS resulted in a "super induction." This data is strongly suggestive of an important role for the Mn-SOD in the acute inflammatory response.