Expression of herstatin, an autoinhibitor of HER-2/neu, inhibits transactivation of HER-3 by HER-2 and blocks EGF activation of the EGF receptor.

The four members of the EGF receptor family are capable of homomeric as well as heteromeric interactions. HER-2/neu (erbB-2) dominates as the preferred coreceptor that amplifies mitogenic signaling. An alternative HER-2/neu product, herstatin, consists of a segment of the ectodomain of p185HER-2 and an intron-encoded C-terminus. Recombinant herstatin was found to bind with nM affinity and inhibit p185HER-2. To further examine the impact on receptor activity, herstatin was expressed with various receptor tyrosine kinases. In CHO cells that overexpressed HER-2, herstatin caused a sevenfold inhibition of colony formation that corresponded to a reduction in the tyrosine phosphorylation of p185HER-2. Herstatin also prevented HER-2 mediated transactivation of the kinase impaired HER-3 as reflected in transphosphorylation of HER-3 and heteromers between HER-2 and HER-3. In EGF receptor-overexpressing cells, EGF induction of receptor dimerization and tyrosine phosphorylation were reduced more than 90%, and receptor down-regulation as well as colony formation were also suppressed by coexpression with herstatin. Inhibition was selective for the EGF receptor family since herstatin expression did not reduce tyrosine phosphorylation mediated by the FGF receptor-2 or by insulin-like growth factor -1. Herstatin bound to the EGF receptor as well as to p185HER-2 in pull-down assays suggesting that complex formation may be involved in receptor inhibition. Our findings indicate that herstatin has the capability to negatively regulate combinations of interactions between group I receptor tyrosine kinases that confer synergistic growth signals.

An alternative HER-2/neu transcript of 8 kb has an extended 3'UTR and displays increased stability in SKOV-3 ovarian carcinoma cells.

UNLABELLED: HER-2/neu is a potent oncogene that predicts poor outcome when overexpressed in ovarian cancer. The SKOV-3 ovarian carcinoma cell line, one of the only models for HER2-driven ovarian cancer, expresses a major uncharacterized 8-kb alternative HER-2 transcript. OBJECTIVES: The aim of this study was to characterize the structure and determine the origin of the alternative sequence and examine the possible role of the 8-kb alternative transcript in overexpression of the HER-2 gene. METHODS: The structure of the 8-kb transcript was investigated using polymerase chain reaction (PCR) and nucleotide sequencing of cDNA clones. PCR analysis of genomic DNA was used to assess the origin of the 8-kb transcript. The stability of the 8-kb mRNA was assessed by Northern blot analysis of RNA extracted from cells treated with transcriptional inhibitors. RESULTS: Similar 5'UTR and coding sequence but an extended 3'UTR were contained in the 8-kb compared to the well-characterized 4.5-kb HER-2 transcript. Genomic DNA had continuity between the novel 3'UTR sequence from the 8-kb transcript and adjacent HER-2 terminal exon sequence. The 8-kb transcript had a half-life of 13 h compared to 5.5 h for the 4.5-kb transcript (P<0.01). CONCLUSIONS: The 8-kb transcript is generated from alternative polyadenylation site usage rather than gene rearrangement. Since the 8-kb transcript contains alternative sequence found at the 3' end of the normal HER-2 gene, it could be expressed in other cells. Increased stability of the 8-kb transcript may confer a selective advantage for SKOV-3 cells by providing enhanced HER-2 expression.

The HER-2/neu receptor tyrosine kinase gene encodes a secreted autoinhibitor.

HER-2/neu (erbB-2) encodes an 185-kDa orphan receptor tyrosine kinase that is constitutively active as a dimer and displays potent oncogenic activity when overexpressed. Here we describe a secreted protein of approximately 68 kDa, designated herstatin, as the product of an alternative HER-2 transcript that retains intron 8. This alternative transcript specifies 340 residues identical to subdomains I and II from the extracellular domain of p185HER-2 followed by a unique C-terminal sequence of 79 aa encoded by intron 8. The recombinant product of the alternative transcript specifically binds to HER-2-transfected cells with a K(D) of approximately 14 nM and was chemically crosslinked to p185HER-2, whereas the intron encoded sequence alone also binds with high affinity to transfected cells and associates with p185 solubilized from cell extracts. The herstatin mRNA is expressed in normal human fetal kidney and liver, but is at reduced levels relative to p185HER-2 mRNA in carcinoma cells that contain an amplified HER-2 gene. Herstatin appears to be an inhibitor of p185HER-2, because it disrupts dimers, reduces tyrosine phosphorylation of p185, and inhibits the anchorage-independent growth of transformed cells that overexpress HER-2.

NH2-terminally truncated HER-2/neu protein: relationship with shedding of the extracellular domain and with prognostic factors in breast cancer.

We identified an NH2-terminally truncated HER-2/neu product of M(r) 95,000 with in vitro kinase activity by Western blotting and immunoprecipitations using domain-specific antibodies. p95 levels correlated with the extracellular domain (ECD) shed from different cells under varied conditions. Both ECD and p95 were at approximately 20-fold lower levels in SKOV3 ovarian carcinoma cells, as compared to BT474 breast carcinoma cells. Both were stimulated by treatment of cells with the phorbol ester tumor promoter phorbol 12-myristate 13-acetate and the lysosomotrophic agent chloroquine. The hydroxamate inhibitor of metalloproteases, TAPI, suppressed both p95 and ECD in a dose-dependent fashion, with maximal inhibition at < or = 10 microM in BT474 cells. Cancer tissues were analyzed by Western blotting and scored for p95HER-2/neu and for p185HER-2/neu expression. Breast and ovarian cancer tissues were both found to express p95HER-2/neu in addition to p185HER-2/neu. Of 161 breast cancer tissues, 22.4% expressed p95, 21.7% overexpressed p185, and 14.3% were p95 positive and overexpressed p185. A higher proportion of node-positive patients (23 of 78) than node-negative patients (9 of 63) expressed p95 in all tumors combined (P = 0.032). In the group that overexpressed p185, those that contained p95 were associated with node-positive patients (15 of 21), whereas those that were p95 negative were associated with node-negative patients (8 of 11; P = 0.017). Neither p95- nor p185-rich patients significantly correlated with tumor size or with hormone receptor status in this study. Our findings show that breast cancers, which express the HER-2/neu oncogene, are heterogeneous with respect to HER-2/neu protein products. p95HER-2/neu appears to distinguish tumors that have metastasized to the lymph nodes from those in node-negative patients.