Absence of natural intracellular retinoids in mouse bone marrow cells and implications for PML-RARA transformation

Cytokine exposure mediates transcriptional activation of the orphan nuclear receptor Nur77 in hematopoietic cells

The orphan nuclear receptor Nur77 is an immediate-early response gene that based on tissue and cell context is implicated in a plethora of cellular processes, including proliferation, differentiation, apoptosis, metabolism, and inflammation. Nur77 has a ligand-binding pocket that is obstructed by hydrophobic side groups. Naturally occurring, cell-endogenous ligands have not been identified, and Nur77 transcriptional activity is thought to be regulated through posttranslational modification and modulation of protein levels. To determine whether Nur77 is transcriptionally active in hematopoietic cells in vivo, we used an upstream activating sequence (UAS)-GFP transgenic reporter. We found that Nur77 is transcriptionally inactive in vivo in hematopoietic cells under basal conditions, but that activation occurs following cytokine exposure by G-CSF or IL-3. We also identified a series of serine residues required for cytokine-dependent transactivation of Nur77. Moreover, a kinase inhibitor library screen and proximity labeling-based mass spectrometry identified overlapping kinase pathways that physically interacted with Nur77 and whose inhibition abrogated cytokine-induced activation of Nur77. We determined that transcriptional activation of Nur77 by G-CSF or IL-3 requires functional JAK and mTor signaling since their inhibition leads to Nur77 transcriptional inactivation. Thus, intracellular cytokine signaling networks appear to regulate Nur77 transcriptional activity in mouse hematopoietic cells.

RXRA DT448/9PP generates a dominant active variant capable of inducing maturation in acute myeloid leukemia cells

RARA and RXRA contribute to myeloid maturation in both mice and humans, and deletion of Rxra and Rxrb augments leukemic growth in mice. While defining the domains of RXRA that are required for anti-leukemic effects in murine KMT2A-MLLT3 leukemia cells, we unexpectedly identified RXRA DT448/9PP as a constitutively active variant capable of inducing maturation and loss of their proliferative phenotype. RXRA DT448/9PP was associated with ligand-independent activity in reporter assays, with enhanced co-activator interactions, reduced engraftment in vivo, and activation of myeloid maturation transcriptional signatures that overlapped with those of cells treated with the potent RXRA agonist bexarotene, suggestive of constitutive activity that leads to leukemic maturation. Phenotypes of RXRA DT448/9PP appear to differ from those of two other RXRA mutations with forms of constitutive activity (F318A and S427F), in that DT448/9PP activity was resistant to mutations at critical ligand-interacting amino acids (R316A/L326A) and was resistant to pharmacological antagonists, suggesting it may be ligand-independent. These data provide further evidence that activated retinoid X receptors can regulate myeloid maturation and provide a novel constitutively active variant that may be germane for broader studies of retinoid X receptors in other settings.

Endogenous and combination retinoids are active in myelomonocytic leukemias

Retinoid therapy transformed response and survival outcomes in acute promyelocytic leukemia (APL) but has demonstrated only modest activity in non-APL forms of acute myeloid leukemia (AML). The presence of natural retinoids in vivo could influence the efficacy of pharmacologic agonists and antagonists. We found that natural RXRA ligands, but not RARA ligands, were present in murine MLL-AF9-derived myelomonocytic leukemias in vivo and that the concurrent presence of receptors and ligands acted as tumor suppressors. Pharmacologic retinoid responses could be optimized by concurrent targeting of RXR ligands (e.g., bexarotene) and RARA ligands (e.g., all-trans retinoic acid), which induced either leukemic maturation or apoptosis depending on cell culture conditions. Co-repressor release from the RARA:RXRA heterodimer occurred with RARA activation, but not RXRA activation, providing an explanation for the combination synergy. Combination synergy could be replicated in additional, but not all, AML cell lines and primary samples, and was associated with improved survival in vivo, although tolerability of bexarotene administration in mice remained an issue. These data provide insight into the basal presence of natural retinoids in leukemias in vivo and a potential strategy for clinical retinoid combination regimens in leukemias beyond APL.

All-trans retinoic acid in non-promyelocytic acute myeloid leukemia: driver lesion dependent effects on leukemic stem cells

Acute myeloid leukemia (AML) is an aggressive, often fatal hematopoietic malignancy. All-trans retinoic acid (atRA), one of the first molecularly targeted drugs in oncology, has greatly improved the outcome of a subtype of AML, acute promyelocytic leukemia (APL). In contrast, atRA has so far provided little therapeutic benefit in the much larger group of patients with non-APL AML. Attempts to identify genetically or molecularly defined subgroups of patients that may respond to atRA have not yielded consistent results. Since AML is a stem cell-driven disease, understanding the effectiveness of atRA may require an appreciation of its impact on AML stem cells. Recent studies reported that atRA decreased stemness of AML with an FLT3-ITD mutation, yet increased it in AML1-ETO driven or EVI1-overexpressing AML. This review summarizes the role of atRA in normal hematopoiesis and in AML, focusing on its impact on AML stem cells.

Retinoic Acid Receptors in Acute Myeloid Leukemia Therapy

Retinoic acid (RA) signaling pathways regulate fundamental biological processes, such as cell proliferation, development, differentiation, and apoptosis. Retinoid receptors (RARs and RXRs) are ligand-dependent transcription factors. All-trans retinoic acid (ATRA) is the principal endogenous ligand for the retinoic acid receptor alpha (RARA) and is produced by the enzymatic oxidation of dietary vitamin A, whose deficiency is associated with several pathological conditions. Differentiation therapy using ATRA revolutionized the outcome of acute promyelocytic leukemia (APL), although attempts to replicate these results in other cancer types have been met with more modest results. A better knowledge of RA signaling in different leukemia contexts is required to improve initial designs. Here, we will review the RA signaling pathway in normal and malignant hematopoiesis, and will discuss the advantages and the limitations related to retinoid therapy in acute myeloid leukemia.

Accelerated inflammation and oxidative stress induced by LPS in acute lung injury: Ιnhibition by ST1926

Bioavailable and less toxic synthetic retinoids, such as the atypical adamantyl retinoid ST1926, have been well developed and investigated in clinical trials for many diseases. The aim of our study was to explore the role of ST1926 in lipopolysaccharide (LPS)-induced acute lung injury (ALI) and to reveal the possible molecular mechanism. Mice were treated with LPS to induce acute lung injury followed by ST1926 administration. After LPS induction, mice administered with ST1926 showed lower inflammation infiltration in bronchoalveolar lavage (BAL) fluid, and pro-inflammatory cytokines, including interleukin-1β (IL-1β), IL-18, IL-6 and tumor necrosis factor-α (TNF-α) in serum and lung tissue samples obtained from mice. In addition, western blot assays suggested that ST1926 suppressed nuclear factor-κB (NF-κB), inhibitor-κB kinase-α (IκBα) and IκB kinase (IKKα), as well as Toll-like receptor 4 (TLR4) induced by LPS. In addition, reactive oxygen species (ROS) stimulated by LPS was also suppressed for ST1926 through inhibiting p38 and extracellular receptor kinase (ERK) signaling pathway. Taken together, the data here indicated that ST1926 may be of potential value in treating acute lung injury through inflammation and ROS suppression via inactivating TLR4/NF-κB and p38/ERK1/2 signaling pathways.

Endogenous retinoid X receptor ligands in mouse hematopoietic cells

The retinoid X receptor α (RXRA) has been implicated in diverse hematological processes. To identify natural ligands of RXRA that are present in hematopoietic cells, we adapted an upstream activation sequence-green fluorescent protein (UAS-GFP) reporter mouse to detect natural RXRA ligands in vivo. We observed reporter activity in diverse types of hematopoietic cells in vivo. Reporter activity increased during granulocyte colony-stimulating factor (G-CSF)-induced granulopoiesis and after phenylhydrazine (PHZ)-induced anemia, suggesting the presence of dynamically regulated natural RXRA ligands in hematopoietic cells. Mouse plasma activated Gal4-UAS reporter cells in vitro, and plasma from mice treated with G-CSF or PHZ recapitulated the patterns of reporter activation that we observed in vivo. Plasma from mice with dietary vitamin A deficiency only mildly reduced RXRA reporter activity, whereas plasma from mice on a fatty acid restriction diet reduced reporter activity, implicating fatty acids as plasma RXRA ligands. Through differential extraction coupled with mass spectrometry, we identified the long-chain fatty acid C24:5 as a natural RXRA ligand that was greatly increased in abundance in response to hematopoietic stress. Together, these data suggest that natural RXRA ligands are present and dynamically increased in abundance in mouse hematopoietic cells in vivo.

Hedgehog and retinoid signaling alters multiple myeloma microenvironment and generates bortezomib resistance

Interactions between multiple myeloma (MM) cells and the BM microenvironment play a critical role in bortezomib (BTZ) resistance. However, the mechanisms involved in these interactions are not completely understood. We previously showed that expression of CYP26 in BM stromal cells maintains a retinoic acid-low (RA-low) microenvironment that prevents the differentiation of normal and malignant hematopoietic cells. Since a low secretory B cell phenotype is associated with BTZ resistance in MM and retinoid signaling promotes plasma cell differentiation and Ig production, we investigated whether stromal expression of the cytochrome P450 monooxygenase CYP26 modulates BTZ sensitivity in the BM niche. CYP26-mediated inactivation of RA within the BM microenvironment prevented plasma cell differentiation and promoted a B cell-like, BTZ-resistant phenotype in human MM cells that were cocultured on BM stroma. Moreover, paracrine Hedgehog secretion by MM cells upregulated stromal CYP26 and further reinforced a protective microenvironment. These results suggest that crosstalk between Hedgehog and retinoid signaling modulates BTZ sensitivity in the BM niche. Targeting these pathological interactions holds promise for eliminating minimal residual disease in MM.

Pathways of retinoid synthesis in mouse macrophages and bone marrow cells

In vivo pathways of natural retinoid metabolism and elimination have not been well characterized in primary myeloid cells, even though retinoids and retinoid receptors have been strongly implicated in regulating myeloid maturation. With the use of a upstream activation sequence-GFP reporter transgene and retrovirally expressed Gal4-retinoic acid receptor α in primary mouse bone marrow cells, we identified 2 distinct enzymatic pathways used by mouse myeloid cells ex vivo to synthesize retinoic acid receptor α ligands from free vitamin A metabolites (retinyl acetate, retinol, and retinal). Bulk Kit(+) bone marrow progenitor cells use diethylaminobenzaldehyde-sensitive enzymes, whereas bone marrow-derived macrophages use diethylaminobenzaldehyde-insensitive enzymes to synthesize natural retinoic acid receptor α-activating retinoids (all-trans retinoic acid). Bone marrow-derived macrophages do not express the diethylaminobenzaldehyde-sensitive enzymes Aldh1a1, Aldh1a2, or Aldh1a3 but instead, express Aldh3b1, which we found is capable of diethylaminobenzaldehyde-insensitive synthesis of all trans-retinoic acid. However, under steady-state and stimulated conditions in vivo, diverse bone marrow cells and peritoneal macrophages showed no evidence of intracellular retinoic acid receptor α-activating retinoids, despite expression of these enzymes and a vitamin A-sufficient diet, suggesting that the enzymatic conversion of retinal is not the rate-limiting step in the synthesis of intracellular retinoic acid receptor α-activating retinoids in myeloid bone marrow cells and that retinoic acid receptor α remains in an unliganded configuration during adult hematopoiesis.