CDK6 directly regulates PIM1 kinase. (A-B) ChIP assays were performed in (A) murine HPC7 hematopoietic progenitor lines and in (B) indicated human AML cell lines as described in Figure 3H-I. (C) PIM1 gene expression was analyzed by quantitative RT-PCR in primary CD34⁺ cells bearing FLT3-ITD after palbociclib (#1, 1 µM; #2, 0.3 µM) administration. Relative PIM1 expression levels were normalized to RPLP0 mRNA. (D) Combined effects of palbociclib with PIM1 inhibitor SGI-1776 free base exceeds Bliss prediction indicating synergy. Dose-response surfaces are centered on the EC₅₀ of each compound in the MOLM-14 cells (upper panel). Analysis was carried out in triplicate. Values depicted represent absolute deviations. Observed values were divided through SDs plus 15th percentile. Needle graphs indicate deviation from Bliss-predicted additivity in FLT3-ITD–expressing AML cells (MOLM-14) (lower panel). (E) Dose-response curve with PIM1 inhibitor SGI-1776 free base alone or in the presence of 10 nM palbociclib (based on the Bliss prediction) in the MOLM-14 cell line. *P < .05; **P < .01.