The LRIG (leucine-rich repeats and immunoglobulin-like domains) family of transmembrane proteins contains three vertebrate members (LRIG1, LRIG2, LRIG3) and one member each in flies (Lambik) and worms (Sma-10). LRIGs have stepped into the spotlight as essential regulators of growth factor receptors, including receptor tyrosine and serine/threonine kinases. LRIGs have been found to both negatively (LRIG1, LRIG3) and positively (Sma-10, LRIG3) regulate growth factor receptor expression and signaling, although the precise molecular mechanisms by which LRIGs function are not yet understood. The most is known about LRIG1, which was recently demonstrated to be a tumor suppressor. Indeed, in vivo experiments reinforce the essential link between LRIG1 and repression of its targets for tissue homeostasis. LRIG1 has also been identified as a stem cell marker and regulator of stem cell quiescence in a variety of tissues, discussed within. Comparably less is known about LRIG2 and LRIG3 although studies to date suggest that their functions are largely distinct from LRIG1 and that they likely do not serve as growth/tumor suppressors. Finally, the translational applications of expressing soluble forms of LRIG1 in LRIG1-deficient tumors are being explored and hold tremendous promise.

The receptor ErbB2 is a membrane bound protein that controls important cell functions including cell cycle progression; deregulated expression of this protein is associated with the development of a variety of cancer types. Notably, ErbB2 over-expression is observed in approximately 25% of human breast tumors. Trastuzumab, an FDA approved therapeutic currently used to treat ErbB2 positive breast cancer patients, is a monoclonal antibody that directly binds the ErbB2 extracellular domain. Unfortunately, only a fraction of patients treated with trastuzumab durably respond to the treatment. Moreover, patients that are treated long term with this antibody develop resistance. One of the proposed mechanisms of trastuzumab resistance is the expression of 611-CTF, a C-terminal fragment of ErbB2 which lacks most of its extracellular domain. Remarkably, 611-CTF is hyperactive and has been associated with increased cell migration, tumor progression and metastasis. Currently nothing is known regarding mechanisms which lead to 611-CTF down-regulation. This represents a significant knowledge gap in our understanding of 611-CTF and receptor fragments in general. Since 611-CTF is associated with poor clinical outcome and therapeutic resistance, it is essential to clarify the mechanisms by which this ErbB2 fragment is regulated. This knowledge could ultimately lead to the development of new therapeutics which could improve the response rate of 611-CTF-positive breast cancer. LRIG1 is a tumor suppressor that directly interacts with ErbB2 leading to receptor degradation. In this study we examined whether 611-CTF is susceptible to LRIG1-mediated down-regulation. We find that these proteins interact and that expression of LRIG1 is sufficient to decrease 611-CTF protein expression in three different cell lines. Furthermore, LRIG1 is capable of reducing 611-CTF-driven tumor cell proliferation and migration. Our results are unexpected because LRIG1 is thought to recognize its targets through mutual extracellular domain interactions and 611-CTF lacks a large portion of the ErbB2 extracellular domain. Citation Format: Maria E. Cedano-Prieto. LRIG1 decreases cell proliferation and motility through downregulation of ErbB2 611-CTF. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3337. doi:10.1158/1538-7445.AM2014-3337

The ErbB family of receptor tyrosine kinases is composed of four members: EGFR, ErbB2, ErbB3, and ErbB4; these are membrane bound proteins that control important cell functions through the activation of signaling cascades such as the Ras-mitogen-activated protein kinase (MAPK) and the phosphatidylinoditol3-kinase (PI3K)-Akt pathways. Deregulated expression of the ErbB family by gene amplification and/or protein over-expression is associated with the development and progression of a variety of cancer types. Notably, ErbB2 over expression is observed in approximately 25% of human breast tumors. The negative regulator, LRIG1 is a transmembrane protein that can directly interact with all members of the ErbB family leading to receptor degradation. In addition, in vitro experiments have shown that loss of LRIG1 is sufficient to drive an increase in ErbB2 protein levels and signaling while ectopic expression of LRIG1 decreases ErbB protein expression and signaling. Recently, a group of ErbB2 C-terminal fragments (CTFs) collectively known as “p95HER2”have captured the attention of the scientific community because a) they are capable of inducing more aggressive tumors compared with those expressing full length ErbB2 and because b) they have been directly implicated in therapeutic resistance to Herceptin, the standard of care for ErbB2-positive breast cancer. ErbB2 CTFsare generated by two independent mechanisms; 1) initiation of translation at alternative methionine codons, producing two fragments known as 611-CTF and 687-CTF; 2) Proteolytic shedding of full length ErbB2, generating 648-CTF and 676-CTF fragments. Clinical data indicate that p95HER2 positive patients have lower survival rates compared with those expressing low p95HER2 levels.Among the p95HER fragments, 611-CTF is the most interesting because it is hyperactive and has been associated with increased cell migration, tumor progression and metastasis. Currently, nothing is known of the mechanisms which govern p95HER2 down-regulation. Insight into these mechanisms could ultimately lead to the development of new therapeutics which may improve the response rate of p95HER2-positive breast cancer. In this study we examined whether 611-CTF is susceptible to LRIG1-mediated down-regulation. We find that these proteins interact and that ectopic expression of LRIG1 is sufficient to decrease 611-CTF protein expression in three different cell lines. Furthermore, LRIG1 is capable of reducing 611-driven tumor cell proliferation and migration. Our results are surprising because LRIG1 is thought to recognize its targets through mutual ecto-domain interactions and 611-CTF lacks a large portion of the ErbB2 ecto-domain. Citation Format: Maria E. Cedano-Prieto, Lakmal Kotelawala, Colleen Sweeney. Negative regulation of ErbB2 611-CTF by LRIG1. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3033. doi:10.1158/1538-7445.AM2013-3033 Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.