Insulin-like Growth Factor Binding Protein 7 Activates the Retinoid Acid Differentiation Pathway in Acute Myeloid Leukemia Cells

Acute myeloid leukemia (AML) is characterized by the accumulation of malignant blasts with impaired transcriptional differentiation programs. Despite important advances in AML therapy, the five-year overall survival rate of AML patients remains a disappointing 30-40%. This poor prognosis is mainly caused by survival of chemotherapy resistant leukemic cells, named leukemic stem cells (LSC), re-initiating relapse. However, for AML patients with PML-RARA positive acute promyelocytic leukemia (APL), treatment with all trans retinoic acid (ATRA) results in cure rates of >90%. Upon ATRA treatment, APL cells can restore transcription leading to granulocytic differentiation, and in combination with arsenic trioxide APL cells go into apoptosis. While the success of ATRA treatment has been demonstrated for APL patients, so far it has not proved effective for non-APL AML patients.

Previously, we demonstrated that insulin-like growth factor binding protein 7 (IGFBP7), a negative regulator of IGF1 receptor (IGF1R) activity, induces apoptosis of AML cells and sensitizes AML cells to chemotherapy-induced cell death. Since it has been shown that IGF1R inhibitors can eliminate therapy-resistant cells by modifying their chromatin state, we hypothesized that IGFBP7 may also have the capacity to modify an epigenetic state and unlock the ATRA-driven differentiation response. To this end, we generated APL cell lines with down- or upregulated IGFBP7 levels and demonstrated that knockdown of IGFBP7 in NB4 cells blocked ATRA-induced differentiation, whereas overexpression of IGFBP7 led to an 8-fold increase in differentiation in the presence of low concentrations of ATRA, together suggesting a role for IGFBP7 in ATRA-induced differentiation in APL cells.

Strategies to increase efficacy of ATRA-based therapy might also improve treatment outcomes for non-APL AML, and therefore we investigated the potential of IGFBP7 to induce susceptibility for ATRA-driven differentiation in this group. ATRA and IGFBP7 treatment of non-APL AML cell lines and primary AML cells derived from patients at diagnosis demonstrated an enhanced efficiency of the combination therapy to induce differentiation of myeloid CD45^dimCD33⁺ AML cells (2.5-fold increase in CD11b-expression) and/or to reduce viability of AML CD45^dim cells as compared to single treatments (from 30-39% to 70% reduction upon IGFBP7, ATRA or combination therapy, respectively), in 50% of tested primary AML samples, while this combination therapy did not influence normal hematopoietic cell survival. Remarkably, ATRA-IGFBP7 treatment diminished the in vivo engraftment potential of primary AML cells as compared to treatment with either drug alone in NSG mice (from 28% and 52% to 17% engraftment upon ATRA, IGFBP7 or combination therapy, respectively). Re-transplantation of human AML derived from first transplanted mice into secondary recipients demonstrated that the ATRA-IGFBP7 combination treatment also eliminated LSC more effectively (1.4-fold reduction). Together these data suggest that IGFBP7 enhances sensitivity of AML (stem) cells to ATRA, and is able to induce a transcriptional program sensitizing AML cells for ATRA-induced differentiation and cell death.

To identify factors responsible for IGFBP7-induced ATRA sensitivity, we performed gene expression profiling of primary AML samples treated with IGFBP7, and identified growth factor independent protein 1 (GFI1) as one of the top down-regulated genes upon IGFBP7 stimulation. As overexpression of GFI1 in non-APL AML patient samples resulted in a >2.5-fold reduction in IGFBP7-induced susceptibility to ATRA-driven differentiation, low GFI1 expression is suggested to be associated with susceptibility to ATRA in AML cells.

In conclusion, our results indicate that treatment of AML patient with a combination of ATRA and IGFBP7 might be successful in preventing relapse and improving AML patient survival, which has to be confirmed in further clinical studies.