Valproic Acid Accelerates Cell Cycle Progression of Hematopoietic Stem Cells Via the Activation of GSK3beta-Dependent Signaling Pathways.

Histone deacetylase inhibitors (HDI) have attracted considerable attention because of their ability to overcome the differentiation block in leukemic blasts either alone, or in combination with differentiating agents such as all-trans retinoic acid (ATRA). We have previously reported favorable effects of the potent HDI valproic acid (VPA) in combination with ATRA in a small subset of patients with advanced acute myeloid leukemia (AML) leading to blast cell reduction and improvement of hemoglobin. This effect was accompanied by hypergranulocytosis most likely due to an enhancement of non-leukemic myelopoiesis and suppression of malignant hematopoiesis rather than enforced differentiation of leukemic cells. These data prompted us to investigate the impact of VPA on normal hematopoietic stem cells (HSC).

Differentiation of cord blood-derived, purified CD34+ cells was assessed by FACS analysis after a 7-days suspension culture in presence of early acting cytokines and 30–150μg/mL VPA. VPA prevented differentation of CD34+ cells in a dose-dependent manner: concomitant with an increase of CD34+ cells from 17 to 47%, the proportion of monocytic CD14+ cells decreased from 27 to 3% (n=3). In addition, VPA induced a 30-fold amplification of CD34+ bone marrow (BM) cells within 10 days as determined by colony assays (n=3). To evaluate the functional capacity of VPA-treated HSC, murine Sca1+/lin−s cells were harvested from colony assays and replated. VPA treatment allowed up to four cycles of replating in contrast to VPA-naïve control cells. Further analysis demonstrated that the stimulatory effect of VPA on the in vitro growth and colony formation capacity of HSC was mainly due to accelerated cell cycle progression. VPA strongly increased the proportion of cells in S phase compared to untreated controls (38 vs. 17%, resp.), as detected by propidium iodid staining and BRDU incorporation as well as reduced expression of the CDK-inhibitor p21cip-1/waf using murine HSC after 7 days of culture. Downregulation of p21cip-1/waf was confirmed in CD34+ BM cells showing maximum inhibition after 48 hours of VPA treatment and no recovery thereafter. Recent results indicate that VPA exerts inhibitory activity on GSK3beta by phosphorylation on Ser-9 and stimulates Akt in human neuroblastoma cells. GSK3beta is an effector of the Wnt-signaling pathway located upstream of beta-catenin. Wnt-signaling can directly stimulate the proliferation of HSC, expand the HSC pool and lead to upregulation of HoxB4. Here we show that VPA increased the inhibition-associated phosphorylation of GSK3beta on Ser-9 in human CD34+ BM cells after 48 hours as well as in murine Sca1+/lin− cells after 7 days. Exposure to VPA enhanced beta-catenin and Akt activity not only in CD34+ HSC but also in KG-1 and TF-1 cells with maximum activation after 48 hours of VPA stimulation. Moreover, VPA lead to an 8-fold increase of the HoxB4 level in CD34+ BM cells as determined by real time PCR at 48 hours.

In conclusion, we show that VPA i.) expands HSC as assesed by phenotype and function; ii.) accelerates cell cycle progression of HSC accompanied by the down-regulation of p21cip-1waf; iii.) activates the GSK3beta depending beta-catenin pathway and Akt and iv.) up-regulates HoxB4. Our data strongly suggest that VPA is able to influence some of the signaling pathway considered relevant for proliferation and self-renewal which might request reconsideration of their employment for the treatment of AML.