HMGA1 Co-activates Transcription in B Cells through Indirect Association with DNA

The immunoglobulin heavy chain enhancer, or μ enhancer, is required for B cell development. Only the appropriate combination of transcription factors results in B cell-specific enhancer activation. HMGA1 (formerly (HMG-I(Y)) is a proposed co-activator of the ETS transcription factors required for μ enhancer activity. HMGA1 associates with the ETS factor PU.1, resulting in changes in PU.1 structure, and enhanced transcriptional synergy with Ets-1 on the μ enhancer in non-lymphoid cells. New data show HMGA1 directly interacts with Ets-1 in addition to PU.1. In vitro HMGA1/Ets-1 interaction facilitates Ets-1/μ enhancer binding in the absence of an HMGA1·Ets-1·DNA complex. To address whether HMGA1 is present in the transcriptionally active μ nucleoprotein complex, we completed DNA pull-down assays to detect protein tethering in the context of protein/DNA interaction. Results show that HMGA1 is not tightly associated with μ enhancer DNA through PU.1 or Ets-1, despite strong associations between these proteins in solution. However, chromatin immunoprecipitation assays show HMGA1 associates with the endogenous enhancer in B cells. Furthermore, antisense HMGA1 substantially decreases μ enhancer activity in B cells. Taken together, these data suggest that HMGA1 functions as a transcriptional μ enhancer co-activator in B cells through indirect association with DNA.