Nucleolin Alterations and ROS Production Associate With Sensitivity of AML Cells to Venetoclax

Acute myeloid leukemia (AML) is a heterogeneous disease with large spectrum of specific mutations and gene aberrations. Recently, the Bcl-2 inhibitor Venetoclax, in combination with hypomethylating agents (HMAs), was approved for older (> 65 years) AML patients, as well as for those unfit for intensive induction chemotherapy. In addition to Bcl-2 inhibition, Venetoclax also induces generation of reactive oxygen species (ROS). We demonstrated that distinct fraction exhibiting specific features arises during 24 h of sample exposure to Venetoclax. This fraction displays characteristic preapoptotic markers as mitochondria depolarization and partial Annexin V surface positivity. Moreover, monitoring of ROS showed negative correlation between signals detected using H₂DCFDA and CellROX probes pointing to dynamic ROS changes induced by Venetoclax. The addition of HMA (Decitabine) had almost no effect on cell viability or ROS production but caused proliferation arrest in sensitive cells. In our panel of AML cell lines and primary AML samples we have found a correlation between ROS production, markers of apoptosis, and attenuation of Bcl-2 activity after Venetoclax treatment. Level of Mcl-1, another antiapoptotic protein from the Bcl-2 family, was reduced in sensitive cells, but increased in the resistant samples in response to Venetoclax. Moreover, the nucleolar protein nucleolin (NCL), which is frequently overexpressed in AML cells, was significantly deregulated in Venetoclax-treated cells. In particular, both NCL protein level and specific phosphorylation decreased in fractions sensitive to Venetoclax. Our findings suggest that Venetoclax targets distinct cell subpopulation, and that ability of a cell to follow increased ROS drives its response to Venetoclax.