We previously suggested that ASXL1 (additional sex comb-like 1) functions as 
either a coactivator or corepressor for the retinoid receptors retinoic acid 
receptor (RAR) and retinoid X receptor in a cell type-specific manner. Here, we 
provide clues toward the mechanism underlying ASXL1-mediated repression. 
Transfection assays in HEK293 or H1299 cells indicated that ASXL1 alone 
possessing autonomous transcriptional repression activity significantly 
represses RAR- or retinoid X receptor-dependent transcriptional activation, and 
the N-terminal portion of ASXL1 is responsible for the repression. Amino acid 
sequence analysis identified a consensus HP1 (heterochromatin protein 1)-binding 
site (HP1 box, PXVXL) in that region. Systematic in vitro and in vivo assays 
revealed that the HP1 box in ASXL1 is critical for the interaction with the 
chromoshadow domain of HP1. Transcription assays with HP1 box deletion or 
HP1alpha knockdown indicated that HP1alpha is required for ASXL1-mediated 
repression. Furthermore, we found a direct interaction of ASXL1 with histone H3 
demethylase LSD1 through the N-terminal region nearby the HP1-binding site. 
ASXL1 binding to LSD1 was greatly increased by HP1alpha, resulting in the 
formation of a ternary complex. LSD1 cooperates with ASXL1 in transcriptional 
repression, presumably by removing H3K4 methylation, an active histone mark, but 
not H3K9 methylation, a repressive histone mark recognized by HP1. This 
possibility was supported by chromatin immunoprecipitation assays followed by 
ASXL1 overexpression or knockdown. Overall, this study provides the first 
evidence that ASXL1 cooperates with HP1 to modulate LSD1 activity, leading to a 
change in histone H3 methylation and thereby RAR repression.