P1-05-01: PELP1 Oncogenic Functions Involve Regulation of Histone Arginine Methylation.

Abstract

BACKGROUND: Cancer is caused by abnormalities of the genome and epigenome including the frequently occurring epigenetic changes of histone modifications. Specifically, estrogen-induced breast carcinogenesis is characterized by alterations in histone modifications. Reader proteins that recognize these modifications facilitate modulation of genes and their resulting biological actions. In addition to regulation by several coregulators, estrogen receptor alpha (ER) transcriptional activation or repression depends on the modulation of histone methylation at target promoters. The ER coregulator PELP1 plays an important role in ER signaling, is a proto-oncogene with aberrant expression in hormone- related cancers and a prognostic indicator of decreased survival in breast cancer patients. The molecular mechanisms by which PELP1 promotes oncogenesis remain unknown; however, previous studies from our lab have shown that PELP1 has the potential to function as a reader of chromatin modifications. MATERIALS AND METHODS: A histone peptide array (Active Motif) that contains 384 unique combinations of histone modifications was used to explore the PELP1 epigenetic interactome using the manufacturer's protocol. Purified full length and various deletions of epitope tagged PELP1 were used for the screening. Using MCF7 and ZR75 model cells with stable expression of PELP1 or PELP1 shRNA, we examined the significance and role of PELP1 on the histone arginine methylation using ERE reporter gene assays, confocal microscopy and ChIP assays. Pre-clinical nude mice based models were used to validate the PELP1 effects on arginine methylation in vivo and PELP1-siRNA liposomes were used to downregulate PELP1 expression in vivo. IHC studies were performed to examine the status of histone arginine methylation in murine breast tumors. RESULTS: Our results show that PELP1 uniquely recognizes histones modified by arginine dimethylation, arginine citrullination and lysine dimethylation. Phosphorylation of residues adjacent to a methyl modification affects the ability of PELP1 to recognize histone methylation. Using various deletions of PELP1 peptides, we have found that PELP1 acts as a module for recognition of a specific histone modification through the carboxyl-terminal glutamic acid rich region. Reporter gene assays showed that PELP1 functionally interacts with arginine methyltransferases including CARM1 and PRMT6, both shown to be dysregulated in human cancers, and synergistically enhances ER-transactivation. Chromatin immunoprecipitation assays revealed that PELP1 has the potential to alter histone H3 arginine methylation status at ER target gene promoters. PELP1 knockdown via PELP1-sRNA liposomes using xenograft based assays resulted in decreased arginine dimethylation with concomitant reduction in tumor volume. Conclusions: Our findings suggest that PELP1 is a reader of histone methyl modifications and deregulation of PELP1 may have implications on tumor proliferation via epigenetic alterations at ER target promoters. Targeting these epigenetic alterations through inhibition of PELP1 and the arginine methyltransferases could be a promising cancer therapeutic. This study was funded by CPRIT pre-doctoral fellowship grant and Komen KG090447. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P1-05-01.