Fatty Acid Binding Protein FABP5 Inhibition Activates Retinoic Acid Signaling and Induces Differentiation in Acute Myeloid Leukemia

Introduction. Accumulation of undifferentiated immature blood cells is a hallmark of myeloid neoplasia (MN), and indicates decoupling of terminal-differentiation from proliferation. Therefore, reinstating to a programmed state of differentiation (lineage-maturation) is a treatment approach, exemplified by all-trans retinoic acid (ATRA) treatment of acute promyelocytic leukemias that contain t(15;17) (q24;q21). This translocation inactivates retinoic acid receptor alpha (RARA) due to its fusion with promyelocytic leukemia PML resulting in PML-RARA. This fusion gene responsible for cellular transformation confers sensitivity to treatment with ATRA. However, ATRA’s therapeutic-success in APLs has not extended to other AML phenotypes. We and others have shown that RA is a ligand, not just for RARs, but also pro-survival peroxisome proliferator-activated receptor β/δ (PPARβ/δ), a member of the nuclear hormone receptor superfamily transcription factors that regulate several metabolic pathways. RA is transported by two carrier proteins, cellular retinoid-binding protein CRABP2, or fatty-acid-binding protein 5 (FABP5), depending on cellular context. These carriers preferentially deliver RA to RARs versus PPARβ/δ respectively. Somatic lesions such as loss of function TET2 or Tp53 mutation, NPM1 and ASXL1 mutations, are associated with upregulated FABP5 and downregulated CRABP2. In AML cells with these mutations, therefore, the high FABP5 can be expected to deliver RA to PPARβ/δ instead of RAR, to potentially contribute to decoupling of differentiation from proliferation. We therefore investigated this possibility and its treatment implications.

Methods. Overexpression, gene knockdown/knockout (ko/kd) of key regulators of RAR and PPARβ/δ were generated in specific genotypes of AMLs. Small molecule inhibitors of FABP5 were used to further interrogate the pathways in a translationally relevant manner, both in vitro, in both malignant and normal cells, and in pre-clinical in vivo models.

Results. Gene expression analysis of AMLs in the Beat AML and TCGA databases demonstrated upregulation of pro-survival PPARβ/δ target genes including FABP5 and downregulation of differentiation promoting RAR pathway genes including CRABP2 in AMLs¹. In particular, FABP5 was highly expressed in AMLs containing TET2, NPM1, TP53 and ASXL1 mutations. AML cell lines OCIAML3 (NPM1^ins/DNMT3a^R882c), OCIAML5 (ASXL1^Y591*/TET2^S825*), THP1 (TP53^del) and isogenic CRISPR engineered K562 (TET2^+/+/TET2^-/-) demonstrated 4-100 fold higher FABP5 expression compared to normal myeloid precursors. These models did not differentiate in response to pharmacologic concentrations of ATRA. To test if FABP5 is necessary and sufficient to maintain the undifferentiated state of these cells in the presence of ATRA, we genetically inactivated FABP5 either using RNAi (kd) or CRISPR (ko). FABP5 knockdown or knockout cells created sensitivity to ATRA, resulting in terminal-differentiation as observed by flow cytometry for terminal-differentiation markers (See Figure). FABP5 was then inhibited using highly specific small molecule FABP5 inhibitors with a 56-fold higher affinity compared to ATRA¹: combination of iFABP5 and ATRA induced terminal-differentiation with significant upregulation of the monocyte lineage markers CD14 and granulocyte lineage marker CD11b measured by flow cytometry at 72h. To investigate whether knocking down FABP5 and ATRA treatment affects gene expression changes we performed transcriptomic profiling of OCIMAL3 Scr, shFABP5 cells treated with or without ATRA using whole exome RNA sequencing. Treatment of FABP5 kd OCIMAL3 cells with ATRA resulted in the upregulation of RAR pathways genes and downregulation of proliferation promoting genes.

Conclusion. In sum, recoupling of lineage-maturation to proliferation can be achieved in non-APL AMLs that highly express the RA carrier FABP5, by inhibiting FABP5 using small molecules. This candidate treatment modality is distinct from cytotoxic chemotherapy, and importantly, can spare normal hematopoiesis.