Inhibition of the NAMPT-Triggered Deacetylation of HCLS1 Protein: A New Therapeutic Option in Chronic Myeloid Leukemia

G-CSFR-triggered phosphorylation and activation of the hematopoietic-specific lyn-substrate 1 (HCLS1) protein is important for the granulocytic differentiation of hematopoietic stem cells (HSCs) (Skokowa J. et al., Nat Med 2012). G-CSF also activates protein deacetylation via Nicotinamide phosphoribosyltransferase (NAMPT)/NAD⁺/sirtuin 1 (SIRT1) pathway (Skokowa J. et al., Nat Med 2009). SIRT1 is NAD⁺-dependent protein deacetylase playing an important role in the regulation of gene transcription. Interestingly, having strong pro-differentiation effects on myeloid progenitors, both HCLS1 and NAMPT are markedly hyper-activated in myeloid leukemias, inducing proliferation and survival of undifferentiated blasts.

Knowing this, we investigated whether HCLS1 could be activated by NAMPT/NAD⁺/SIRT1 pathway via deacetylation in HSCs. Indeed, we found that HCLS1 is deacetylated in HSCs by NAMPT/SIRT1 and that G-CSF treatment of these cells induced deacetylation and phosphorylation of HCLS1 via NAMPT/SIRT1. Moreover, we found that deacetylation of HCLS1 is important for myeloid differentiation and proliferation of HSCs.

We further demonstrated that HCLS1 protein was activated by deacetylation in blasts of chronic myeloid leukemia (CML) patients, as compared to HSCs of healthy individuals. This was in line with elevated mRNA and protein levels of NAMPT and HCLS1. We further investigated, whether inhibition of NAMPT-triggered deacetylation of HCLS1 has any effect on the survivial and differentiation of CML blasts in vitro. We found that treatment of CML blasts with the specific NAMPT inhibitor FK866 resulted in increased acetylation of HCLS1. In addition, treatment of CML blasts with the FK866 in a dose of 10nM resulted in marked inhibition of proliferation and elevated cell death, which was consistent between different patients and was comparable to the effects of Imatinib treatment. In CFU assay in the presence of FK866 (10nM) no CFU colonies were grown. Even at a dose of 3nM FK866 drastically suppressed total colony counts. This inhibitory effect was further amplified by combination of FK866 with Imatinib. Addition of 10nM of FK866 to CFU culture medium was not toxic for CD34⁺ cells of healthy donors.

Taken together, we demonstrated that NAMPT and HCLS1 have dose-dependent effects on hematopoietic cells and that NAMPT activates HCLS1 functions by SIRT1-triggered deacetylation. Inhibition of NAMPT in CML blasts markedly reduced their survival, proliferation and differentiation. In conclusion, we suggest a possible therapeutic application of NAMPT inhibition in the treatment of CML patients via deactivation of HCLS1 function by acetylation. This therapy could be applied in combination with Imatinib, or in Imatinib-resistant CML patients.