Oligomerization of the Nrdp1 E3 ubiquitin ligase is necessary for efficient autoubiquitination but not ErbB3 ubiquitination.

Overexpression of the ErbB3 receptor tyrosine kinase protein in breast and other cancers contributes to tumor malignancy and therapeutic resistance. The RBCC/TRIM family RING finger E3 ubiquitin ligase Nrdp1 mediates the ubiquitination of ErbB3 in normal mammary epithelial cells to facilitate receptor degradation and suppress steady-state receptor levels. Post-transcriptional loss of Nrdp1 in patient breast tumors allows ErbB3 overexpression and receptor contribution to tumor progression, and elevated lability through autoubiquitination contributes to the observed loss of Nrdp1 in tumors relative to normal tissue. To begin to understand the mechanisms underlying Nrdp1 protein self-regulation through lability, we investigated the structural determinants required for efficient autoubiquitination and ErbB3 ubiquitination. Using mutagenesis, chemical cross-linking, size exclusion chromatography, and native polyacrylamide gel electrophoresis, we demonstrate that Nrdp1 self-associates into a stable oligomeric complex in cells. Deletion of its coiled-coil domain abrogates oligomerization but does not affect Nrdp1-mediated ErbB3 ubiquitination or degradation. On the other hand, the presence of the coiled-coil domain is necessary for efficient Nrdp1 autoubiquitination via a trans mechanism, indicating that Nrdp1 ubiquitination of its various targets is functionally separable. Finally, a GFP fusion of the coiled-coil domain stabilizes Nrdp1 and potentiates ErbB3 ubiquitination and degradation. These observations point to a model whereby the coiled-coil domain plays a key role in regulating Nrdp1 lability by promoting its assembly into an oligomeric complex, and raise the possibility that inhibition of ligase oligomerization via its coiled-coil domain could be of therapeutic benefit to breast cancer patients by restoring Nrdp1 protein.

Post-transcriptional mechanisms contribute to the suppression of the ErbB3 negative regulator protein Nrdp1 in mammary tumors.

The ErbB2 and ErbB3 receptor tyrosine kinases act synergistically to promote cellular properties associated with tumor development. Previous studies indicate that endogenous ErbB3 protein is markedly elevated in mouse mammary tumors induced by transgenic ErbB2 overexpression. However, this occurs in the absence of elevated ErbB3 transcript, indicating that post-transcriptional regulatory mechanisms play crucial roles in suppressing ErbB3 protein in normal tissue. Our previous studies also demonstrate that protein levels of Nrdp1, an E3 ubiquitin ligase that targets ErbB3 for degradation, are markedly suppressed in tumors from ErbB2 transgenic animals relative to normal tissue. Here we demonstrate that transgenic expression of Nrdp1 cDNA in the mouse mammary gland is not sufficient to suppress elevated ErbB3 levels or tumor initiation and growth in ErbB2 transgenic mice. Unexpectedly, Nrdp1 protein is absent in tumors from Nrdp1/ErbB2 bigenic mice, and real time PCR analysis indicates that Nrdp1 protein levels are suppressed post-transcriptionally. Nrdp1 protein is more resistant to proteasome-dependent degradation when exogenously expressed in cultured MCF10A nontransformed human breast epithelial cells than in breast tumor cells. These observations indicate that mammary tumors use potent post-transcriptional mechanisms to suppress Nrdp1 protein levels and that protein destabilization may play a central role in Nrdp1 loss in tumors.

Suppression of the negative regulator LRIG1 contributes to ErbB2 overexpression in breast cancer.

The ErbB2 receptor tyrosine kinase is overexpressed in approximately 25% of breast tumors and contributes to poor patient prognosis and therapeutic resistance. Here, we examine the role of the recently discovered ErbB negative regulator LRIG1 in ErbB2(+) breast cancer. We observe that LRIG1 protein levels are significantly suppressed in ErbB2-induced mammary tumors in transgenic mice as well as in the majority of ErbB2(+) human breast tumors. These observations raise the possibility that LRIG1 loss could contribute to the initiation or growth of ErbB2(+) breast tumors. RNA interference-mediated knockdown of endogenous LRIG1 in the ErbB2-overexpressing breast tumor cell lines MDA-MB-453 and BT474 further elevates ErbB2 in these cells and augments cellular proliferation. In contrast, ectopic expression of LRIG1 reverses these trends. Interestingly, we observe that LRIG1 protein levels are suppressed in response to ErbB receptor activation in breast tumor cells but are unaffected by ErbB activation in immortalized nontransformed breast epithelial cells. Our observations indicate that the suppression of LRIG1 protein levels is a common feature of breast tumors. Moreover, our observations point to the existence of a feed-forward regulatory loop in breast tumor cells where aberrant ErbB2 signaling suppresses LRIG1 protein levels, which in turn contributes to ErbB2 overexpression.

Neuregulin-induced ErbB3 downregulation is mediated by a protein stability cascade involving the E3 ubiquitin ligase Nrdp1.

The molecular mechanisms underlying epidermal growth factor (EGF) receptor tyrosine kinase down-regulation in response to growth factor binding are coming into focus and involve cbl-mediated receptor ubiquitination followed by lysosomal degradation. However, mechanisms underlying the ligand-stimulated degradation of the related receptor tyrosine kinases of the ErbB family do not involve cbl and remain unexplored. Previous studies have demonstrated that the E3 ubiquitin ligase Nrdp1 contributes to the maintenance of steady-state ErbB3 levels by mediating its growth factor-independent degradation. Here we demonstrate that treatment of cells with the ErbB3 ligand neuregulin-1 (NRG1) stabilizes the deubiquitinating enzyme USP8, which in turn stabilizes Nrdp1. The catalytic activity of USP8 is required for NRG1-induced Nrdp1 stabilization. We provide evidence that Akt-mediated phosphorylation of USP8 threonine residue T907 contributes to USP8 stability. Finally, we demonstrate that Nrdp1 or USP8 knockdown suppresses NRG1-induced ErbB3 ubiquitination and degradation in MCF7 breast cancer cells. We conclude that an NRG1-induced protein stability cascade involving USP8 and Nrdp1 mediates the down-regulation of ErbB3. Our observations raise the possibility that the ligand-induced augmentation of pathways involved in the maintenance of basal levels of receptor tyrosine kinases can contribute to ligand-stimulated down-regulation.

Loss of Nrdp1 enhances ErbB2/ErbB3-dependent breast tumor cell growth.

Dysregulation of ErbB receptor tyrosine kinases is thought to promote mammary tumor progression by stimulating tumor cell growth and invasion. Overexpression and aberrant activation of ErbB2/HER2 confer aggressive and malignant characteristics to breast cancer cells, and patients displaying ErbB2-amplified breast cancer face a worsened prognosis. Recent studies have established that ErbB2 and ErbB3 are commonly co-overexpressed in breast tumor cell lines and in patient samples. ErbB2 heterodimerizes with and activates the ErbB3 receptor, and the two receptors synergize in promoting growth factor-induced cell proliferation, transformation, and invasiveness. Our previous studies have shown that the neuregulin receptor degradation protein-1 (Nrdp1) E3 ubiquitin ligase specifically suppresses cellular ErbB3 levels by marking the receptor for proteolytic degradation. Here, we show that overexpression of Nrdp1 in human breast cancer cells results in the suppression of ErbB3 levels, accompanied by the inhibition of cell growth and motility and the attenuation of signal transduction pathways. In contrast, either Nrdp1 knockdown or the overexpression of a dominant-negative form enhances ErbB3 levels and cellular proliferation. Additionally, Nrdp1 expression levels inversely correlate with ErbB3 levels in primary human breast cancer tissue and in a mouse model of ErbB2 mammary tumorigenesis. Our observations suggest that Nrdp1-mediated ErbB3 degradation suppresses cellular growth and motility, and that Nrdp1 loss in breast tumors may promote tumor progression by augmenting ErbB2/ErbB3 signaling.

The leucine-rich repeat protein LRIG1 is a negative regulator of ErbB family receptor tyrosine kinases.

The molecular mechanisms by which mammalian receptor tyrosine kinases are negatively regulated remain largely unexplored. Previous genetic and biochemical studies indicate that Kekkon-1, a transmembrane protein containing leucine-rich repeats and an immunoglobulin-like domain in its extracellular region, acts as a feedback negative regulator of epidermal growth factor (EGF) receptor signaling in Drosophila melanogaster development. Here we tested whether the related human LRIG1 (also called Lig-1) protein can act as a negative regulator of EGF receptor and its relatives, ErbB2, ErbB3, and ErbB4. We observed that in co-transfected 293T cells, LRIG1 forms a complex with each of the ErbB receptors independent of growth factor binding. We further observed that co-expression of LRIG1 with EGF receptor suppresses cellular receptor levels, shortens receptor half-life, and enhances ligand-stimulated receptor ubiquitination. Finally, we observed that co-expression of LRIG1 suppresses EGF-stimulated transformation of NIH3T3 fibroblasts and that the inducible expression of LRIG1 in PC3 prostate tumor cells suppresses EGF- and neuregulin-1-stimulated cell cycle progression. Our observations indicate that LRIG1 is a negative regulator of the ErbB family of receptor tyrosine kinases and suggest that LRIG1-mediated receptor ubiquitination and degradation may contribute to the suppression of ErbB receptor function.

Stabilization of the E3 ubiquitin ligase Nrdp1 by the deubiquitinating enzyme USP8.

Nrdp1 is a RING finger-containing E3 ubiquitin ligase that physically interacts with and regulates steady-state cellular levels of the ErbB3 and ErbB4 receptor tyrosine kinases and has been implicated in the degradation of the inhibitor-of-apoptosis protein BRUCE. Here we demonstrate that the Nrdp1 protein undergoes efficient proteasome-dependent degradation and that mutations in its RING finger domain that disrupt ubiquitin ligase activity enhance stability. These observations suggest that Nrdp1 self-ubiquitination and stability could play an important role in regulating the activity of this protein. Using affinity chromatography, we identified the deubiquitinating enzyme USP8 (also called Ubpy) as a protein that physically interacts with Nrdp1. Nrdp1 and USP8 could be coimmunoprecipitated, and in transfected cells USP8 specifically bound to Nrdp1 but not cbl, a RING finger E3 ligase involved in ligand-stimulated epidermal growth factor receptor down-regulation. The USP8 rhodanese and catalytic domains mediated Nrdp1 binding. USP8 markedly enhanced the stability of Nrdp1, and a point mutant that disrupts USP8 catalytic activity destabilized endogenous Nrdp1. Our results indicate that Nrdp1 is a specific target for the USP8 deubiquitinating enzyme and are consistent with a model where USP8 augments Nrdp1 activity by mediating its stabilization.

E6/E7 proteins of HPV type 16 and ErbB-2 cooperate to induce neoplastic transformation of primary normal oral epithelial cells.

Head and neck squamous cell carcinomas (HNSCC) are characterized by a marked propensity for local invasion and spread to cervical lymph nodes, with distant metastases developing in 30-40% of cases. HPV-16 is an important risk factor for HNSCC. How HPV enhances susceptibility to HNSCC is not fully understood, but seems to involve cofactors. In this study, we examined the effect of the cooperation between HPV-16 and the tyrosine kinase receptor ErbB-2 on E-cadherin/catenin complex patterns and neoplastic transformation of human normal oral epithelial (NOE) cells. We report that overexpression of ErbB-2 or E6/E7 alone does not affect E-cadherin/catenin complex patterns nor does it induce cell transformation of NOE cells. In contrast, coexpression of E6/E7 and ErbB-2 downregulates E-cadherin and catenin expression. This is accompanied by cytoplasmic localization of E-cadherin, as well as nuclear translocation of alpha, beta, and gamma-catenins. Furthermore, we demonstrate that E6/E7 cooperate with overexpressed ErbB-2 to induce tumor formation in nude mice and to upregulate cyclin D1 and c-myc expression. Our data suggest that E6/E7 cooperate with ErbB-2 in head and neck carcinogenesis, at least in part, via the conversion of beta-catenin from a cell adhesion to a nuclear function, that is, to act as a potential transcriptional regulator. This conversion leads to the upregulation of cyclin D1, c-myc and other oncoproteins necessary for alteration of the E-cadherin/catenin complex and cell transformation of NOE cells.

Regulation of multiple tumor microenvironment markers by overexpression of single or paired combinations of ErbB receptors.

The progression of primary tumors to an invasive phenotype requires dynamic changes in multiple cellular and local tumor microenvironment markers. In this study, we report a genomic approach to assess gene transcriptional changes upon overexpression of ErbB receptors, in vitro and in vivo, focusing on markers involved in the regulation of the tumor microenvironment. ErbB receptors (ErbB-1/epidermal growth factor receptor, ErbB-2, ErbB-3, and ErbB-4) were stably overexpressed in a polyclonal cell population as single or paired combinations using murine and human breast cell models. The overall numbers of known genes that are up- or down-regulated was significantly higher in cells and tumors overexpressing paired combinations of receptors compared with cells and tumors overexpressing single ErbB receptors. Genes encoding components of cell-cell structures, extracellular matrix, coagulation factors, and angiogenesis were predominantly affected by the most active ErbB receptor combinations and were predictive of the aggressive in vivo tumorigenicity, a feature that was not always seen in vitro. Among ErbB-regulated tumor microenvironment markers detected by the genomic analysis, thrombospondin 1, an endogenous inhibitor of angiogenesis, was additionally validated in relation to tumor growth phenotype. Thrombospondin 1 mRNA and protein were down-regulated by specific ErbB receptors, in vitro and in both rodent and human ErbB-induced tumors, consistent with the extent of tumor growth and tumor vascularization associated with specific ErbB receptors. In summary, our genomic results highlight the broad diversity of ErbB-regulated cancer-associated genes and revealed several novel targets that may have potential therapeutic applications for targeting tumor progression involving aberrations of ErbB receptors.

Differential regulation of tumor angiogenesis by distinct ErbB homo- and heterodimers.

Interactions between cancer cells and their microenvironment are critical for the development and progression of solid tumors. This study is the first to examine the role of all members of the ErbB tyrosine kinase receptors (epidermal growth factor receptor [EGFR], ErbB-2, ErbB-3, or ErbB-4), expressed singly or as paired receptor combinations, in the regulation of angiogenesis both in vitro and in vivo. Comparison of all receptor combinations reveals that EGFR/ErbB-2 and ErbB-2/ErbB-3 heterodimers are the most potent inducers of vascular endothelial growth factor (VEGF) mRNA expression compared with EGFR/ErbB-3, EGFR/ErbB-4, ErbB-2/ErbB-4, and ErbB-3/ErbB-4. Immunohistochemistry of tumor xenografts overexpressing these heterodimers shows increased VEGF expression and remarkably enhanced vascularity. Enhanced VEGF expression is associated with increased VEGF transcription. Deletional analysis reveals that ErbB-mediated transcriptional up-regulation of VEGF involves a hypoxia-inducible factor 1-independent responsive region located between nucleotides -88 to -66 of the VEGF promoter. Mutational analysis reveals that the Sp-1 and AP-2 transcription factor binding elements within this region are required for up-regulation of VEGF by heregulin beta1 and that this up-regulation is dependent on the activity of extracellular signal-related protein kinases. These results emphasize the biological implications of cell signaling diversity among members of the ErbB receptor family in regulation of the tumor microenvironment.

Regulation of E-cadherin/catenin complex patterns by epidermal growth factor receptor modulation in human lung cancer cells.

We previously demonstrated that a ligand-blocking monoclonal antibody (mAb) against the epidermal growth factor-receptor (EGF-R), LA1, induced morphological conversion from epithelial-like to epithelial of the human lung cancer cell line, H322. This was accompanied by an up-regulation of epithelial cadherin (E-cadherin) expression (Clin. Cancer Res. 5 (1999) 681). In the present paper, we show that mAb LA1 induces the epithelial-like to epithelial conversion of the human lung cancer cell line, A549. In A549 and H322 cells, which express a detectable amount of EGF-R (ErbB-1), ErbB-2, ErbB-3, and ErbB-4 receptors, the LA1 mAb induces up-regulation of the E-cadherin/catenin complex (alpha-, beta-, and gamma-catenins). This is associated with re-localization of E-cadherin, alpha-catenin, (and to a lesser extent beta-catenin), but not gamma-catenin. Additionally, we report that mAb LA1 inhibits cell motility. In contrast, epidermal growth factor (EGF) or heparin-binding EGF-like growth factor (HB-EGF) induces the epithelial-like to fibroblastoid conversion of A549 and H322 cell lines, slightly reduces the expression of E-cadherin and beta-catenin, but not alpha- and gamma-catenins, and stimulates cell motility. These studies demonstrate that EGF-R modulation regulates the E-cadherin/catenin complex and cell motility in human lung epithelial carcinoma cells. Our results may have important therapeutic implications for the treatment of invasive human lung carcinomas via the restoration of the cadherin/catenin complex using inhibitors of EGF-R.