Abstract B121: AIB1 is a transcriptional and translational mediator of breast cancer cell growth and migration targets
Authors: Ochnik Aleksandra M. , Peterson Mark S. , Avdulov Svetlana V. , Yee Douglas 
Published: May 2015

The nuclear p160 steroid receptor co-activator, AIB1 is a downstream regulator of the insulin-like growth factor (IGF-I) tyrosine kinase signaling axis [1]. Since AIB1 is over-expressed in 31-64% of breast cancers cell several mechanisms have been described to explain its function in breast cancer [2]. AIB1 transcriptionally regulates genes involved in carcinogenesis via its co-partnership with estrogen receptor alpha (ERα) and E2F1 [3]. Interestingly the transcriptional mediators, c-myc and E2F1 induce mRNA translation potentially via the transcriptional regulation of ribosome biogenesis factors [4]. However, whether AIB1 acts in similarly is not well known [3]. Furthermore, since IGF-I signaling stimulates mRNA cap-dependent translation and also affects AIB1, we hypothesize that AIB1 acts as both a transcriptional and translational regulator of breast cancer cell growth and migration in an IGF-dependent manner. ER positive (MCF7L) wild-type, stably expressing AIB1 and scrambled control short hairpin RNA (shAIB1 and shCON) cells were created. Cells were treated for 24h in serum free media plus and minus IGF-I (5nM). Polyribosomal mRNA was stratified using sucrose gradients and fractionated into ten individual fractions and total-RNA samples were obtained. qRT-PCR was performed to determine IGF-I and AIB1 mediated transcriptional and translational effects on AIB1, markers of proliferation (i.e. KI67 and PCNA), cell cycle (i.e. p21), survival (i.e. BCL2), migration (i.e. VEGFA) and ribosome biogenesis factors (i.e. eIF3A and eIF5) in MCF7L shCON and shAIB1 cells. Statistical significance was determined by one-way ANOVA (p&lt;0.05). Total-RNA expression of VEGFA, Ki67, PCNA and eIF5 was induced by IGF-I in the shCON and shAIB1 cells (p&lt;0.05), apart from eIF5 where no induction by IGF-I was detected in the shAIB1 cells. No change in total-RNA expression of AIB1, BCL2, p21 or eIF3A was detected by IGF-I treatment. However, AIB1 and p21 total-RNA expression was reduced under both basal and IGF-I treated conditions in the shAIB1 vs. shCON cells (p&lt;0.05). The percentage of ribosome-bound Ki67, PCNA, p21, VEGFA, and eIF3A mRNA was increased by IGF-I in all ten pooled fractionated RNA samples combined (range: 11% to 340%, mean: 110%), compared to control cells. This effect was either reversed (i.e. Ki67, PCNA and eIF3A), slightly increased (i.e. p21) or unaffected (i.e. VEGFA) in the shAIB1 IGF-I treated vs. control cells. IGF-I treatment reduced ribosome-bound BCL2 (23%) and eIF5 (20%) mRNA in the shCON cells, which was further reduced to 68% and 46% respectively in the shAIB1 cells. We observed a basal reduction of ribosome-bound mRNA for AIB1 (63%), VEGFA (24%), p21 (43%) and eIF3A (37%) and an increase for Ki67 (26%), PCNA (12%), eIF5 (32%) and BCL2 (59%) in the shAIB1 vs. shCON cells. Moreover, all genes had reduced ribosome-bound mRNA following IGF-I treatment in the shAIB1 vs. shCON fractions, except for eIF5 which increased. Our studies demonstrate that IGF-I transcriptionally induces Ki67, PCNA, VEGFA and eIF5 in an AIB1-dependent and independent manner and p21 was only transcriptionally regulated by AIB1. Importantly, IGF-I regulated mRNA ribosome recruitment was predominantly reduced in the AIB1 knock-down cells, indicative of a reduced malignant potential. These findings give preliminary evidence of AIB1 acting as a novel mediator of ribosomal recruitment, which may lead to novel combinatorial therapeutic approaches using AIB1 and translational targeted inhibitors in AIB1 over-expressing breast cancers.

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Citation Format: Aleksandra M. Ochnik, Mark S. Peterson, Svetlana V. Avdulov, Douglas Yee. AIB1 is a transcriptional and translational mediator of breast cancer cell growth and migration targets. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr B121.