Dominant-negative HMGA1 blocks mu enhancer activation through a novel mechanism

Frontline Science: Targeted expression of a dominant-negative high mobility group A1 transgene improves outcome in sepsis

High mobility group (HMG) proteins are a family of architectural transcription factors, with HMGA1 playing a role in the regulation of genes involved in promoting systemic inflammatory responses. We speculated that blocking HMGA1-mediated pathways might improve outcomes from sepsis. To investigate HMGA1 further, we developed genetically modified mice expressing a dominant negative (dn) form of HMGA1 targeted to the vasculature. In dnHMGA1 transgenic (Tg) mice, endogenous HMGA1 is present, but its function is decreased due to the mutant transgene. These mice allowed us to specifically study the importance of HMGA1 not only during a purely pro-inflammatory insult of endotoxemia, but also during microbial sepsis induced by implantation of a bacterial-laden fibrin clot into the peritoneum. We found that the dnHMGA1 transgene was only present in Tg and not wild-type (WT) littermate mice, and the mutant transgene was able to interact with transcription factors (such as NF-κB), but was not able to bind DNA. Tg mice exhibited a blunted hypotensive response to endotoxemia, and less mortality in microbial sepsis. Moreover, Tg mice had a reduced inflammatory response during sepsis, with decreased macrophage and neutrophil infiltration into tissues, which was associated with reduced expression of monocyte chemotactic protein-1 and macrophage inflammatory protein-2. Collectively, these data suggest that targeted expression of a dnHMGA1 transgene is able to improve outcomes in models of endotoxin exposure and microbial sepsis, in part by modulating the immune response and suggest a novel modifiable pathway to target therapeutics in sepsis.

The High Mobility Group A1 (HMGA1) Transcriptome in Cancer and Development

BACKGROUND & OBJECTIVES

Chromatin structure is the single most important feature that distinguishes a cancer cell from a normal cell histologically. Chromatin remodeling proteins regulate chromatin structure and high mobility group A (HMGA1) proteins are among the most abundant, nonhistone chromatin remodeling proteins found in cancer cells. These proteins include HMGA1a/HMGA1b isoforms, which result from alternatively spliced mRNA. The HMGA1 gene is overexpressed in cancer and high levels portend a poor prognosis in diverse tumors. HMGA1 is also highly expressed during embryogenesis and postnatally in adult stem cells. Overexpression of HMGA1 drives neoplastic transformation in cultured cells, while inhibiting HMGA1 blocks oncogenic and cancer stem cell properties. Hmga1 transgenic mice succumb to aggressive tumors, demonstrating that dysregulated expression of HMGA1 causes cancer in vivo. HMGA1 is also required for reprogramming somatic cells into induced pluripotent stem cells. HMGA1 proteins function as ancillary transcription factors that bend chromatin and recruit other transcription factors to DNA. They induce oncogenic transformation by activating or repressing specific genes involved in this process and an HMGA1 "transcriptome" is emerging. Although prior studies reveal potent oncogenic properties of HMGA1, we are only beginning to understand the molecular mechanisms through which HMGA1 functions. In this review, we summarize the list of putative downstream transcriptional targets regulated by HMGA1. We also briefly discuss studies linking HMGA1 to Alzheimer's disease and type-2 diabetes.

CONCLUSION

Further elucidation of HMGA1 function should lead to novel therapeutic strategies for cancer and possibly for other diseases associated with aberrant HMGA1 expression.

HMGA1 co-activates transcription in B cells through indirect association with DNA

The immunoglobulin heavy chain enhancer, or mu 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 mu 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 mu enhancer in nonlymphoid cells. New data show HMGA1 directly interacts with Ets-1 in addition to PU.1. In vitro HMGA1/Ets-1 interaction facilitates Ets-1/mu enhancer binding in the absence of an HMGA1.Ets-1.DNA complex. To address whether HMGA1 is present in the transcriptionally active mu 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 mu 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 mu enhancer activity in B cells. Taken together, these data suggest that HMGA1 functions as a transcriptional mu enhancer co-activator in B cells through indirect association with DNA.