ERBB3, a member of the Epidermal Growth Factor Receptor (EGFR) family of receptor tyrosine kinases, has been implicated in activation of the phosphatidyl-inositol 3-kinase (PI3K) pathway in human lung adenocarcinomas driven by EGFR mutations. We investigated the contribution of ERBB3 to the initiation, progression and therapeutic response of EGFR-induced lung adenocarcinomas using tetracycline- and tamoxifen- inducible transgenic mouse models. Deletion of Erbb3 at the time of induction of mutant EGFR had no effect on tumorigenesis, demonstrating that ERBB3 is not required to initiate tumorigenesis. Tumors that developed in the absence of ERBB3 remained sensitive to EGFR TKIs and retained activation of the PI3K/AKT pathway. Interestingly, acute loss of Erbb3 suppressed further growth of established EGFRL858R-mediated lung tumors. Four weeks after deletion of Erbb3, the tumors exhibited phosphorylation of EGFR, of the adaptor proteins GAB1 and GAB2 and, of the downstream signaling molecules AKT and ERK suggesting that alternative signaling pathways could compensate for loss of Erbb3. Similar to our observations with mouse tumors, we found that GAB adaptor proteins play a role in ERBB3 independent activation of the PI3K pathway by mutant EGFR in EGFR mutant human cell lines. Finally, in such cell lines, increased levels of phosphorylation of ERBB2 or MET were associated with reduced sensitivity to acute loss of ERBB3, suggesting remarkable plasticity in the signaling pathways regulated by mutant EGFR with important therapeutic implications.

SND1 is an AEG-1/MTDH/LYRIC-binding protein that is upregulated in numerous human cancers, where it has been assigned multiple functional roles. In this study, we report its association with the TGFβ1 signaling pathway, which promotes epithelial–mesenchymal transition (EMT) in breast cancer. SND1 was upregulated in breast cancer tissues, in particular in primary invasive ductal carcinomas. Transcriptional activation of the SND1 gene was controlled by the TGFβ1/Smad pathway, specifically by activation of the Smad2/Smad3 complex. The SND1 promoter region contained several Smad-specific recognition domains (RD motifs), which were recognized and bound by the Smad complex that enhanced the transcriptional activation of SND1. We found that SND1 promoted expression of the E3 ubiquitin ligase Smurf1, leading to RhoA ubiquitination and degradation. RhoA degradation in breast cancer cells disrupted F-actin cytoskeletal organization, reduced cell adhesion, increased cell migration and invasion, and promoted metastasis. Overall, our results define a novel role for SND1 in regulating breast tumorigenesis and metastasis. Cancer Res; 75(7); 1–12. ©2015 AACR.

SND1 is an AEG-1/MTDH/LYRIC-binding protein that is upregulated in numerous human cancers where it has been assigned multiple functional roles. In this study, we report its association with the TGFβ1 signaling pathway which promotes epithelial-mesenchymal transition (EMT) in breast cancer. SND1 was upregulated in breast cancer tissues, in particular in primary invasive ductal carcinomas (IDC). Transcriptional activation of the SND1 gene was controlled by the TGFβ1/Smad pathway, specifically, by activation of the Smad2/Smad3 complex. The SND1 promoter region contained several Smad specific recognition domains (RD motifs), which were recognized and bound by the Smad complex that enhanced the transcriptional activation of SND1. We found that SND1 promoted expression of the E3 ubiquitin ligase Smurf1, leading to RhoA ubiquitination and degradation. RhoA degradation in breast cancer cells disrupted F-actin cytoskeletal organization, reduced cell adhesion, increased cell migration and invasion and promoted metastasis. Overall, our results define a novel role for SND1 in regulating breast tumorigenesis and metastasis.

Merkel cell polyomavirus (MCPyV) is frequently associated with Merkel cell carcinoma (MCC), a highly aggressive neuroendocrine skin cancer. Most MCC tumors contain integrated copies of the viral genome with persistent expression of the MCPyV large T (LT) and small T (ST) antigen. MCPyV isolated from MCC typically contains wild-type ST but truncated forms of LT that retain the N-terminus but delete the C-terminus and render LT incapable of supporting virus replication. To determine the oncogenic activity of MCC tumor-derived T antigens in vivo, a conditional, tissue-specific mouse model was developed. Keratin 14-mediated Cre recombinase expression induced expression of MCPyV T antigens in stratified squamous epithelial cells and Merkel cells of the skin epidermis. Mice expressing MCPyV T antigens developed hyperplasia, hyperkeratosis, and acanthosis of the skin with additional abnormalities in whisker pads, footpads, and eyes. Nearly half of the mice also developed cutaneous papillomas. Evidence for neoplastic progression within stratified epithelia included increased cellular proliferation, unscheduled DNA synthesis, increased E2F-responsive genes levels, disrupted differentiation, and presence of a DNA damage response. These results indicate that MCPyV T antigens are tumorigenic in vivo, consistent with their suspected etiologic role in human cancer. Cancer Res; 75(6); 1–12. ©2015 AACR.

Merkel cell polyomavirus (MCPyV) is frequently associated with Merkel cell carcinoma (MCC), a highly aggressive neuroendocrine skin cancer. Most MCC tumors contain integrated copies of the viral genome with persistent expression of the MCPyV large T (LT) and small T (ST) antigen. MCPyV isolated from MCC typically contain wild type ST but truncated forms of LT that retain the N-terminus but delete the C-terminus and render LT incapable of supporting virus replication. To determine the oncogenic activity of MCC tumor-derived T antigens in vivo, a conditional, tissue-specific mouse model was developed. Keratin 14-mediated Cre recombinase expression induced expression of MCPyV T antigens in stratified squamous epithelial cells and Merkel cells of the skin epidermis. Mice expressing MCPyV T antigens developed hyperplasia, hyperkeratosis, and acanthosis of the skin with additional abnormalities in whisker pads, footpads and eyes. Nearly half of the mice also developed cutaneous papillomas. Evidence for neoplastic progression within stratified epithelia included increased cellular proliferation, unscheduled DNA synthesis, increased E2F-responsive genes levels, disrupted differentiation, and presence of a DNA damage response. These results indicate that MCPyV T antigens are tumorigenic in vivo, consistent with their suspected etiological role in human cancer.