Differential Regulation of Expression of the Anti-Leukemogenic Orphan Nuclear Receptor Nur77 (nr4a1) In HSCs, CLPs, and CMPs

Abstract 4782

The molecular basis for the balance between self-renewal and differentiation that regulates hematopoietic stem cell (HSC) development is poorly understood. Recent studies suggest that levels of expression of the orphan nuclear hormone receptor Nur77 (nr4a1), previously implicated in thymic selection, may play a pivotal role in HSC fate decisions. Previously published quantitative PCR data suggest that murine HSCs express relatively high levels of Nur77 mRNA. Genetic deletion of Nur77 and its close family member Nor-1 (nr4a3) in mice resulted in the development of acute myeloid leukemia (Mullican et al. Nat Med, 13:730;2007), consistent with Nur77's role as an anti-leukemogenic transcription factor, but also suggestive of a role in HSC homeostasis. To more closely investigate the role of Nur77 in normal hematopoiesis, we constructed a transgenic reporter mouse in which GFP is driven by the Nur77 promoter (eGFP^Nur77). Unexpectedly, we found that fewer than 3% of freshly isolated bone marrow HSCs (Lin^-c-Kit⁺Sca-1⁺, IL-7R^-) and common lymphoid progenitors (CLP = Lin^-c-Kit⁺Sca-1⁺, IL-7R⁺), and no common myeloid progenitors (CMP) expressed Nur77 in our novel reporter system. Interestingly, culture of Lin- bone marrow cells with serum-containing medium resulted in a marked increase in Nur77 (GFP) expression among the majority of HSC's and CLP's, but not CMP's. Further studies indicate that a heat-insensitive component of serum, also present in plasma, is responsible for this induction. Taken together with published data, our studies suggest that induction and modulation of Nur77 may play a previously unappreciated role in self-renewal and lymphoid versus myeloid lineage commitment decisions of HSCs. We are currently working to understand the developmental difference in HSC/CLP/CMP responsiveness and to identify the serum component(s) responsible for upregulating Nur77.