Artemin: from neurotrophic factor to oncogene in human mammary carcinoma

Abstract

Artemin (ARTN), a member of the glial cell derived neurotrophic factor (GDNF) family of ligands (GFLs), plays a pivotal role in the development and maintenance of the sympathetic and sensory nervous systems. A growing body of evidence supports the oncogenic potential of GFLs in the malignancies within and outside the nervous system. I demonstrate herein that ARTN is ubiquitously expressed in human mammary carcinoma cell lines. Forced expression of ARTN in mammary carcinoma cells increases cell survival, enhances anchorage-independent growth and promotes migration and invasion, with resultant increased tumour growth in vivo. ARTN protein is detectable in 65% of human mammary carcinoma and its expression correlates to decreased overall survival in the cohort of breast cancer patients. Conversely, depletion of endogenous ARTN with small interfering RNA (siRNA), or antibody inhibition of ARTN, decreases the oncogenicity and invasiveness of mammary carcinoma cells. Furthermore, ARTN is an estrogen-inducible gene. ARTN mRNA expression significantly correlates to decreased survival of estrogen receptor (ER)-positive breast cancer patients treated with tamoxifen. Forced expression of ARTN in ER-positive human mammary carcinoma cells increases ER transcriptional activity, promotes estrogen-independent growth and produces resistance to tamoxifen and fulvestrant in vitro and to tamoxifen in xenograft models. Conversely, depletion of endogenous ARTN by siRNA or functional antagonism of ARTN by antibody enhances the efficacy of anti-estrogen treatment. Tamoxifen decreases ARTN expression in tamoxifen-sensitive mammary carcinoma cells whereas ARTN expression is increased in tamoxifen-resistant cells and not affected by tamoxifen treatment. A panel of 27 mouse monoclonal antibodies targeting human ARTN has been generated. One of these antibodies reacts specifically to ARTN with a high affinity. Inhibition of ARTN by this ARTN-monoclonal antibody results in reduced cell growth and invasion, and increased tamoxifen sensitivity of mammary carcinoma cells in vitro. ARTN, therefore, is a functionally and clinically validated molecule involved in growth and metastasis of mammary carcinoma. Functional antagonism of ARTN therefore warrants consideration as a novel adjuvant therapeutic approach for human mammary carcinoma.