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

Human prostatic acid phosphatase, an authentic tyrosine phosphatase, dephosphorylates ErbB-2 and regulates prostate cancer cell growth

Cellular prostatic acid phosphatase (cPAcP), an authentic tyrosine phosphatase, is proposed to function as a negative growth regulator of prostate cancer (PCa) cells in part through its dephosphorylation of ErbB-2. Nevertheless, the direct interaction between cPAcP and ErbB-2 has not been shown nor the specific dephosphorylation site of ErbB-2 by cPAcP. In this report, our data show that the phosphorylation level of ErbB-2 primarily at Tyr(1221/2) correlates with the growth rate of both LNCaP and MDA PCa2b human PCa cells. Further, cPAcP reciprocally co-immunoprecipitated with ErbB-2 in a non-permissive growth condition. Expression of wild type cPAcP, but not inactive mutant, by cDNA in cPAcP-null LNCaP C-81 cells results in decreased tyrosine phosphorylation of ErbB-2 including Tyr(1221/2). Concurrently, Tyr(317) phosphorylation of p52(Shc), proliferating cell nuclear antigen expression, and cell growth are decreased in these cells. Conversely, decreased cPAcP expression by short hairpin RNA in LNCaP C-33 cells was associated with elevated phosphorylation of ErbB-2 initially at Tyr(1221/2). Its downstream p52(Shc), ERK1/2, Akt, Src, STAT-3, and STAT-5 were activated, and cell proliferation, proliferating cell nuclear antigen, and cyclin D1 expression were increased. Stable subclones of C-33 cells by small interfering PAcP had elevated Tyr(1221/2) phosphorylation of ErbB-2 and exhibited androgen-independent growth and increased tumorigenicity in xenograft female animals. In summary, our data together indicate that in prostate epithelia, cPAcP interacts with and dephosphorylates ErbB-2 primarily at Tyr(1221/2) and hence blocks downstream signaling, leading to reduced cell growth. In PCa cells, decreased cPAcP expression is associated with androgen-independent cell proliferation and tumorigenicity as seen in advanced hormone-refractory prostate carcinomas.

Activation (tyrosine phosphorylation) of ErbB-2 (HER-2/neu): a study of incidence and correlation with outcome in breast cancer

PURPOSE

We hypothesize that phosphorylated ErbB-2 (P-ErbB-2, identified by a novel antibody PN2A) may provide either more significant or additional prognostic marker data for breast cancer patients. This study was designed to compare the incidence and prognostic value of ErbB-2 (HER-2/neu) and P-ErbB-2 immunoexpression in archival breast cancer samples.

MATERIALS AND METHODS

Eight hundred sixteen invasive breast cancers with a median of 16.3 years of follow-up were immunostained for ErbB-2 (using antibody CB11) and P-ErbB-2 (using antibody PN2A). ErbB-2 and P-ErbB-2 data were compared with clinical, histologic, immunohistochemical, and outcome variables.

RESULTS

Of 816 primary breast cancers, 307 (38%) were positive for ErbB-2 and 37 (12% of ErbB-2 positive and 5% of the study population) expressed P-ErbB-2. P-ErbB-2 was not detected in ErbB-2-negative cases (n = 509). ErbB-2 immunohistochemical data were bimodal; patients with > or = 80% cellular expression had the shortest disease-free and disease-specific survival. P-ErbB-2 was associated with a higher percentage of ErbB-2-positive cells, a higher number of positive lymph nodes, and cellular proliferation. ErbB-2 and P-ErbB-2 were indicators of poor prognosis in node-positive patients in both univariate and multivariate analyses. We found that either P-ErbB-2 expression or high (> or = 80%) ErbB-2 expression provided the most significant prognostic value in node-positive cases by multivariate analyses. There were too few P-ErbB-2-positive cases and events in the node-negative patient group to allow statistical analysis of P-ErbB-2 in that subgroup.

CONCLUSION

PN2A immunostaining identified a subset (approximately 12% of ErbB-2-positive breast cancers) with activation (phosphorylation) of the receptor ErbB-2. P-ErbB-2 expression was strongly associated with higher levels of ErbB-2 expression (> or = 80%), although it was not a surrogate. Identification of cases with a high percentage of invasive breast cancer cells expressing ErbB-2 or determination of receptor activation via P-ErbB-2 may provide additional prognostic value in node-positive breast cancers.

Signal transduction in mammary tumorigenesis: a transgenic perspective

A number of genes have been implicated in breast cancer development, yet few have been demonstrated to play causative roles in mammary tumor formation. The advent of transgenic mouse and embryonic stem cell technologies now permits manipulation of the mouse genome in such a way as to temporally and spatially control a gene product's expression. Thus, the basic researcher now can directly assess the involvement of particular genes in tumorigenesis and disease progression and, in the process, to develop mouse models of human genetic disease. The utility of such technologies is emphasized in transgenic mice expressing genes thought to play important roles in the initiation and progression of mammary carcinomas. As these transgenic strains have been the subject of several reviews, here we focus on two mouse mammary tumor models, Polyomavirus middle T antigen and the Neu/ErbB-2 receptor tyrosine kinase, which are most amenable to study specific signaling pathways in process of mammary tumorigenesis.

Domain-specific interactions between the p185(neu) and epidermal growth factor receptor kinases determine differential signaling outcomes

We expressed the epidermal growth factor receptor (EGFR) along with mutant p185(neu) proteins containing the rat transmembrane point mutation. The work concerned the study of the contributions made by various p185(neu) subdomains to signaling induced by a heterodimeric ErbB complex. Co-expression of full-length EGFR and oncogenic p185(neu) receptors resulted in an increased EGF-induced phosphotyrosine content of p185(neu), increased cell proliferation to limiting concentrations of EGF, and increases in both EGF-induced MAPK and phosphatidylinositol 3-kinase (PI 3-kinase) activation. Intracellular domain-deleted p185(neu) receptors (T691stop neu) were able to associate with full-length EGFR, but induced antagonistic effects on EGF-dependent EGF receptor down-regulation, cell proliferation, and activation of MAPK and PI 3-kinase pathways. Ectodomain-deleted p185(neu) proteins (TDelta5) were unable to physically associate with EGFR, and extracellular domain-deleted p185(neu) forms failed to augment activation of MAPK and PI 3-kinase in response to EGF. Association of EGFR with a carboxyl-terminally truncated p185(neu) mutant (TAPstop) form did not increase transforming efficiency and phosphotyrosine content of the TAPstop species, and proliferation of EGFR.TAPstop-co-expressing cells in response to EGF was similar to cells containing EGFR only. Thus, neither cooperative nor inhibitory effects were observed in cell lines co-expressing either TDelta5 or TAPstop mutant proteins. Unlike the formation of potent homodimer assemblies composed of oncogenic p185(neu), the induction of signaling from p185(neu).EGFR heteroreceptor assemblies requires the ectodomain for ligand-dependent physical association and intracellular domain contacts for efficient intermolecular kinase activation.

Identification of p185neu sequences required for monoclonal antibody- or ligand-mediated receptor signal attenuation

Anti-p185neu antibodies downmodulate constitutively active p185neu receptors from the cell surface, which is associated with a reduction in the transformed phenotype. We have analyzed a group of mutant p185neu forms with carboxyl (C)-terminal truncations and/or an internal deletion of amino acids 1008-1057. Receptor endocytosis and degradation were examined by flow cytometric analysis and pulse-chase assays following anti-p185neu monoclonal antibody (MAb) treatment. Deletion of a sequence within the distal carboxyl terminus, including three known autophosphorylation sites, did not affect MAb-mediated receptor surface downmodulation and degradation of surface receptor. However, kinase-active deletion mutants with elimination of the putative internalization sequence (Tint delta), or Tint delta mutants also containing a large C-terminal truncation, displayed markedly impaired receptor endocytosis in response to MAb treatment. Cells expressing endocytosis-defective mutant proteins became insensitive to anti-p185neu MAb-mediated inhibition of anchorage-independent growth and were more oncogenic in vivo. Cells expressing endocytosis-defective mutant EGFR/neu chimeric proteins were more transforming upon EGF addition when compared to cells expressing wild-type EGFR/neu receptors. Taken together, these data suggest that, in addition to kinase activity, p185neu receptor endocytosis requires a functional modular structure, i.e., an internalization sequence, possibly to serve as target for endocytotic adapter proteins. Unattenuated signaling from oncogenic p185neu forms resulting from prolonged surface localization may result in enhanced cellular transformation and desensitization to MAb-mediated downregulation and phenotypic reversion.

Activation of ErbB2 during wallerian degeneration of sciatic nerve

We used anti-phosphopeptide-immunodetecting antibodies as immunohistochemical reagents to define the location and activity state of p185(erbB2) during Wallerian degeneration. Nerve damage induces a phosphorylation event at Y1248, a site that couples p185(erbB2) to the Ras-Raf-MAP kinase signal transduction pathway. Phosphorylation of p185(erbB2) occurs within Schwann cells and coincides in time and space with Schwann cell mitotic activity, as measured by bromodeoxyuridine uptake. These visual images of receptor autophosphorylation link activation of p185(erbB2) to the Schwann cell proliferation that accompanies nerve regeneration.

Activation of ErbB2 during wallerian degeneration of sciatic nerve

We used anti-phosphopeptide-immunodetecting antibodies as immunohistochemical reagents to define the location and activity state of p185(erbB2) during Wallerian degeneration. Nerve damage induces a phosphorylation event at Y1248, a site that couples p185(erbB2) to the Ras-Raf-MAP kinase signal transduction pathway. Phosphorylation of p185(erbB2) occurs within Schwann cells and coincides in time and space with Schwann cell mitotic activity, as measured by bromodeoxyuridine uptake. These visual images of receptor autophosphorylation link activation of p185(erbB2) to the Schwann cell proliferation that accompanies nerve regeneration.

Distinct tyrosine autophosphorylation sites negatively and positively modulate neu-mediated transformation

A number of cytoplasmic signaling molecules are thought to mediate mitogenic signaling from the activated Neu receptor tyrosine kinase through binding specific phosphotyrosine residues located within the intracellular portion of Neu/c-ErbB-2. An activated neu oncogene containing tyrosine-to-phenylalanine substitutions at each of the known autophosphorylation sites was generated and assessed for its specific transforming potential in Rat1 and NIH 3T3 fibroblasts. Mutation of these sites resulted in a dramatic impairment of the transforming potential of neu. To assess the role of these tyrosine phosphorylation sites in cellular transformation, the transforming potential of a series of mutants in which individual tyrosine residues were restored to this transformation-debilitated neu mutant was evaluated. Reversion of any one of four mutated sites to tyrosine residues restored wild-type transforming activity. While each of these transforming mutants displayed Ras-dependent signaling, the transforming activity of two of these mutants was correlated with their ability to bind either the GRB2 or SHC adapter molecules that couple receptor tyrosine kinases to the Ras signaling pathway. By contrast, restoration of a tyrosine residue located at position 1028 completely suppressed the basal transforming activity of this mutated neu molecule or other transforming neu molecules which possessed single tyrosine residues. These data argue that the transforming potential of activated neu is mediated both by positive and negative regulatory tyrosine phosphorylation sites.

HER-2/neu signal transduction in human breast and ovarian cancer

The HER-2/neu proto-oncogene encodes a 185 kDa transmembrane receptor tyrosine kinase with significant sequence homology to other members of the class I receptor tyrosine kinase family. The HER-2/neu gene is amplified and/or overexpressed in 25%-30% of human breast and ovarian cancers, and overexpression of the receptor is associated with poor prognosis. Tyrosine phosphorylation and activation of the HER-2 receptor lead to activation of specific signal transduction pathways in breast and ovarian cancer cells, including the ras/MAP kinase cascade, phosphatidylinositol 3-kinase, and phospholipase C-gamma. HER-2/neu signal transduction pathways ultimately converge on the cell nucleus, where the expression of diverse genes is induced after activation of the receptor. A more complete understanding of HER-2/neu signal transduction pathways may allow the development of specific therapeutics for the treatment of those human breast and ovarian cancers containing this alteration.

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.