Understanding the functions of the novel oncogene SHON in breast cancer

Abstract

The raising awareness about mammary carcinoma and increasing knowledge of the link between estrogen (E2) and the progression of mammary carcinoma has contributed to the development of anti-estrogens as efficient anticancer drugs over the past few decades. Secreted hominoid-specific oncogene, SHON is an estrogen regulated gene and has been identified as a tumour promoter and a potential biomarker for anti-estrogen therapy. However, it is still not clear how SHON mediates estrogen receptor (ER) signalling. Here I investigated the modulation of ER signalling by SHON. The estrogen response element (ERE)-luciferase reporter assays revealed the stimulatory effect of SHON on ER transcriptional activity in ER positive MCF-7 cells. Upon tamoxifen or fulvestrant treatment, I observed that over-expression of SHON abrogated the inhibitory effects of anti-estrogens. Western blotting showed synergistic up-regulation of ERa and SHON by estrogen or combined estrogen + anti-estrogen treatment. Moreover, evaluation of normal and mutated c-Jun promoter activities using luciferase promoter assays displayed the interaction between c-Jun N-terminal kinase (JNK) signalling and SHON, correlating with protein expression levels. The colony formations of MCF-7 cells with forced expression of SHON in soft agar and Matrigel were significantly restrained by JNK inhibitors. GT27 is another potential novel oncogene identified by the Secreted Protein Discovery Initiative (SPDI) and its functions in breast cancer remain unknown. cDNA expression arrays revealed that GT27 was selectively expressed in human normal tissues and correlated with specific mammary carcinoma stages. To perform the functional characterisation of GT27 in breast cancer, stably transfected cell lines were established using a plasmid expressing GT27. Forced expression of GT27 enhanced oncogenic properties of breast cancer cells in various aspects, including cell proliferation, anchorage-independent growth, 3-dimensional colony formation, wound healing, migration, and invasion. In addition, siRNA or monoclonal antibody mediated depletion of endogenous GT27 significantly abolished the oncogenicity of GT27 in mammary carcinoma. These studies reveal that SHON is involved in ER signalling and may mediate resistance to anti-estrogen treatment through enhanced ER transcriptional activity and elevated expression of c-Jun and JNK. Anti-SHON monoclonal antibody with significant inhibitory effects may therefore be used in combination with hormone therapy to delay the onset of endocrine resistance. Further characterisation of monoclonal antibody against GT27 may support the use of antagonists of GT27 as the therapeutic intervention for human mammary carcinoma.