FBXL16 Promotes Breast Cancer Cell Growth and Diminishes Fulvestrant Responsiveness by Stabilizing ERα Protein
Abstract
Breast cancer is the most prevalent cancer in women. Approximately 75% of breast cancers are estrogen receptor alpha positive (ER+) and are treatable with endocrine therapies and/or CDK inhibitors. However, endocrine therapy (ET) resistance and metastasis are major obstacles in advanced ERα+ breast cancer (ER+ BCa) therapeutics. Upregulated oncogenic ERα activity plays critical role in progression of ER+ BCa. One essential mechanism of regulating ERα signaling is the ubiquitination-dependent proteasomal degradation of ERα. Owing to its direct effect on ERα degradation, fulvestrant is a first-line FDA-approved ET for metastatic and locally advanced breast cancer and a second-line drug for treatment of unresponsive ER+ BCa progression. Unfortunately, intrinsic or acquired resistance to fulvestrant develops in majority of patients with advanced ER+ BCa. The mechanism underlying fulvestrant resistance is still largely unknown. In the current study, we have identified F-Box and Leucine-Rich Repeat Protein 16 (FBXL16) as a novel positive regulator of oncogenic ERα signaling. F-box proteins are major components of the SCF (SKP1-CUL1-F-box) E3 ubiquitin ligases that mediate protein ubiquitination. FBXL16 does not show detectable interaction with CUL1 and is a poorly studied F-box protein. Our lab has recently discovered that FBXL16 upregulates several oncoproteins targeted by SCF-E3 ligases, including c-myc and β-catenin. However, little is known about the roles of FBXL16 in cancer. By data-mining of cancer-related databases and immunohistological analysis of BCa tissue microarrays, we found that FBXL16 is highly upregulated in invasive ductal and lobular carcinomas. There is a strong positive correlation between FBXL16 expression and ERα status, implying its important role in ER+ BCa. Our study reveals that FBXL16 stabilizes ERα and decreases ERα ubiquitination thereby promoting ERα-mediated transcription and breast cancer cell proliferation. Specifically, we identified that FBXL16 decreases estradiol-induced ERα degradation by antagonizing an E3-ubiquitin ligase, FBXO45. Moreover, FBXL16 silencing downregulates protein stability of constitutively active mutant ERα-Y537S and restricts proliferation of cells expressing ERα-Y537S. Silencing of FBXL16 accelerates fulvestrant-mediated ERα degradation and increases fulvestrant efficacy in inhibiting cell growth. In conclusion, our findings identify FBXL16 as a novel regulator of ERα signaling and a potential therapeutic target for treating advanced ER+ BCa.
