The pp90rsk-CREB Signaling Pathway Regulates Apoptosis In Acute Myelogenous Leukemia

CREB (cAMP Response-Element Binding Protein) is a nuclear transcription factor critical for hematopoietic cell proliferation, differentiation, and survival. We previously demonstrated that 60% of patients with Acute Myelogenous Leukemia (AML) overexpress CREB in leukemic blasts, and CREB overexpression in these patients was associated with an increased risk of relapse and decreased event-free survival. Previous studies have suggested that CREB may play an important role in the regulation of apoptosis in a wide variety of cancers. Specifically, CREB has been shown to up-regulate members the anti-apoptotic protein family such as Bcl-2, Bcl-XL and Mcl-1, leading to chemotherapy resistance in vitro. CREB-mediated resistance to apoptosis may underlie the increased rate of relapse and poor survival of AML patients with CREB overexpression. Thus, we hypothesized that targeted inhibition of CREB in AML cells would promote AML cell apoptosis. To test this hypothesis, we developed a small-molecule inhibitor of CREB function, XX-650-23. This molecule disrupts the interaction between CREB and its binding partner CBP (CREB-Binding Protein), which is required for full activation of CREB-mediated gene transcription. Treatment of primary AML patient bone marrow samples with XX-650-23 induced apoptosis and cell death at a dose of 2 uM. The degree of apoptosis varied with the expression level of CREB in primary AML cells tested. Higher CREB levels correlated with higher sensitivity to XX-650-23. In non-leukemic primary patient bone marrow samples, CREB levels were very low, and XX-650-23 did not induce apoptosis in these cells. AML cell lines (KG-1 and HL-60) also underwent apoptosis following CREB inhibition, in proportion to CREB expression level. CREB knockdown or overexpression in KG-1 cells decreased and increased susceptibility to apoptosis, respectively. Mechanistically, the onset of apoptosis in AML cells occurred simultaneously with down-regulation of Bcl-2, a validated CREB-regulated gene. Inhibition of Bcl-2 function using the specific Bcl-2 inhibitor ABT-737 (100 nM) induced apoptosis similar to XX-650-23, indicating that Bcl-2 inhibition alone is sufficient to cause apoptosis. Thus, targeted inhibition of CREB results in Bcl-2 downregulation and is sufficient to induce apoptosis in AML cells. Proteomic analysis using Mass Cytometry-Time of Flight (CyTOF) revealed that one compensatory cellular response to CREB inhibition is increased phosphorylation of CREB. This phosphorylation decreased in the presence of BI-D1870, a specific inhibitor of the pp90RSK kinase (RSK), but not by pharmacologic inhibition of the p38 or ERK kinases, using SB202190 or U0126, respectively. We therefore examined the role of pp90RSK in the regulation of apoptosis in AML cells. Pharmacologic inhibition of RSK independently lead to AML cell apoptosis (BI-D1870, IC50=3.3 uM), in part due to blockade of CREB phosphorylation. In summary, our data provide the first evidence that inhibition of CREB, or its chief activator RSK, is sufficient to induce apoptosis in AML cells. Current work focuses on defining CREB target genes mediating XX-650-23 response using chromatin-immunoprecipitation with massively parallel DNA sequencing (ChIP-Seq), and defining the RSK kinome in AML cells using 2-dimensional gel phosphoprotein profiling. These studies will more fully define the role of the RSK-CREB signaling axis in AML proliferation, survival, and apoptosis.