No abstract

Nicotinamide (NA) is a non-competitive inhibitor of NAD(+)-dependent ADP-ribosyl transferases, of CD38 NADase (a major regulator of cellular NAD levels) and of Sir2 histone-deacetylase. These enzymes are playing a pivotal role in regulation of signal transduction pathways and gene expression. In the present study, we evaluated the effect of NA on the ex-vivo expansion of cord-blood (CB) derived CD34+ cells and their bone-marrow (BM) homing and engraftment potential. Culturing of CD34+ cells for three weeks in the presence of cytokines (SCF, TPO, IL-6, FLT3-ligand) only or cytokines + NA (5mM) resulted in similar expansion of CD34+ cells (40-fold relative to input). However, a remarkable increase in the fraction of CD34+ cells displaying an early progenitor cell phenotype (CD34+Lin−) was observed in the NA-treated cultures as compared with cytokines-only treated cultures (18.6 ± 3% and 0.7 ± 0.06%, n=6, p<0.05, respectively). Tracking the cell-cycle history by PKH2 staining showed fewer division cycles of CD34+ cells cultured with NA. These results may suggest a direct correlation between the rate of proliferation and expansion of CD34+Lin− cells. NA-treated CD34+ cells express similar levels of CXCR4 but display increased migratory activity in response to CXCL12 over CD34+ cells treated with cytokines only (36 ± 19% and 11 ± 4%, n=4, p<0.05, respectively). In order to test their homing potential, similar number of mononuclear cells (MNC), before or following expansion with or without NA, were labeled with CFSE and transplanted into irradiated NOD/SCID mice. Twenty-four hours later the numbers of human cells (CD45+CFSE+) and human progenitor cells (CD34+CFSE+) in the BM were counted. Homing of CD45+CFSE+ cells was comparable in the three groups tested. However, CD34+CFSE+ cells with BM homing potential were 3-fold more numerous in NA-treated cultures relative to cytokines-treated cultures, and 6-fold more than in non-cultured CB cells (n=14, p<0.05). To evaluate engraftment, SCID mice were transplanted with 3x103, 6x103 and 12x103 non-cultured CD34+ cells or their entire progeny following 3-week expansion with cytokines only or cytokines + NA (n = 63). The frequency of SCID repopulating cells (SRC) was estimated by limiting dilution analysis as 1/ 36,756 (non-cultured), 1/19,982 (cytokines), 1/ 2,620 (NA) (SCID engraftment was considered as ≥0.5% human CD45+ cells). We found that, in correlation with homing, NA-treated cells have a 14- and 7.6-fold more SRC than non-cultured cells or cytokine-treated cells, respectively. The marked increase in SCID engraftment potential following culturing with NA may be attributed to both improved homing of CD34+ cells as well as higher proportion of early progenitor cells within the CD34+ cell compartment. Despite their numerical expansion, progenitor cells generated in cytokine-supplemented cultures have reduced homing and engraftment capacity. Our study demonstrates that NA modulates in-vitro expansion and augments the in-vivo homing and engraftment of CB-derived CD34+ cells cultured with cytokines.

CD38, originally described as a differentiation marker, has emerged as an important multifunctional protein. Its most well-characterized function is the ability to catalyze the synthesis of cyclic ADP-ribose (cADPR) from NAD. However, its major enzymatic activity is the hydrolysis of NAD (NADase) implicating it as the major regulator of cellular NAD levels. CD38 expression increases with commitment and differentiation. It is not clear, however, whether such changes in CD38 are merely phenotypic, or reflect an active role for CD38 in the regulation of cell differentiation. The regulation of CD38 gene expression is under the direct control of retinoid receptors (RAR). Antagonists to RAR abolish up-regulation of CD38 gene expression as well as RA induction of granulocytic differentiation down-stream of the myeloid compartment. In the present study we evaluated the involvement of CD38 in the regulation of HPC differentiation by treatment of ex-vivo cultures with LMW antagonists, targeted to either CD38 expression or to its biological activities. CB derived CD34+ cells were cultured with cytokines (S,T,F,6). Treatment of these cultures with an RAR-antagonist (AGN194310) abolished the expression of surface CD38. After 3 weeks in culture, the content of CFC was 3 ±1.1-fold higher, the content of CD34+ cells was 2.4 ± 0.24-fold higher and percentage CD34+ cells displaying CD34+Lin− phenotype was by 35 ± 10-fold higher (p<0.05, n= 14) in RAR-antagonist (10−6M) compared to cytokines-only treated cultures. Colonies derived from RAR-antagonist treated cultures sustained high re-plating capacity, a property that was lost during the first 3-weeks of expansion with cytokines only. In long-term cultures, the peak of CFUc and CD34+ cell expansion of RAR-antagonist treated cultures was 6–10 weeks later than control cultures. At the peak of expansion, cumulative numbers of CD34+ and CFUc were by 130- and 512-fold higher (p<0.05, n=4), respectively, in treated than in control cultures. CFU-MIX colonies were exclusively observed in RAR-antagonist treated cultures (between weeks 7–10). Interestingly, limited (1 week) exposure to the RAR-antagonist was sufficient for this long-term effect. Similarly, we tested the effect of an RXR antagonist (LGN 100754) (10−9 – 10−5 M) on short- and long-term cultures. Treatment with the RXR-antagonist did not down-regulate CD38 expression and only slightly improved ex-vivo expansion parameters over cytokines-only treated cultures. We next evaluated whether inhibition of CD38 enzymatic activities will also modulate in-vitro differentiation of cultured cells. To this end, CD34+ cell cultures were treated with nicotinamide (NA), a non-competitive inhibitor of CD38 NADase, previously demonstrated to abolish its enzymatic activities. 3-week treatment with NA (5mM) resulted in a marked decrease in CD38 expression and a marked increase in the fraction of CD34+Lin− cells as compared to cytokines-only treated cultures (48.0 ± 3.7% vs. 2.8 ± 0.7% and 18.6 ± 3% vs. 0.7 ± 0.06%, n=6, p<0.05, respectively). As with the RAR-antagonist, long-term expansion potential, as determined by CFC and CD34+ cell content, was significantly higher in cultures treated with NA relative to cytokines-only treated cultures. These results demonstrate that both a pan-RAR antagonist and NA inhibit differentiation and promote ex-vivo expansion of progenitor cells, suggesting the possible involvement of CD38 protein in these processes.

CD38, originally described as a differentiation marker, has emerged as an important multifunctional transmembrane protein. Its most intriguing and well-characterized function is its ability to catalyze the synthesis of cyclic ADP-ribose (cADPR) from NAD. Of particular interest is its presence on the inner membrane of the nucleus, suggesting that CD38/cADPR may play a direct role in mediating nuclear activation and gene expression. Our studies on ex vivo expansion of Hematopoietic Stem Cells (HSCs) have led us to test whether alteration of CD38 function carries the potential of affecting cell fate decisions of HSCs. Inhibition of CD38 enzymatic activity was achieved by treating CD34+ cell cultures with nicotinamide (NA), a well-known base-exchange inhibitor demonstrated to inhibit the synthesis of cADPR from NAD. We report here that exogenously added nicotinamide (5–10 mM) to CD34+ cell cultures supplemented with cytokines (SCF, TPO, IL-6, FLt3, +/− IL-3) resulted in significant enrichment of CD34+CD38− (79±9.3%, n=9) and CD34+CD38−Lin− (19±3%, n=8) cells, as compared with control cultures treated only with cytokines (6.3±1.8%, n=9, and 0.7±0.06%, n=8, respectively, p&lt;0.01). The functionality of these early progenitor subsets was demonstrated using the extended LTC-CFC assay, performed in the absence of NA. These results raised the intriguing possibility that cADPR production may have a pivotal role in regulation of CD34+ cell fate. However, inhibition of cADPR downstream signal transduction pathways by its specific antagonist, 8-amino-cADPR did not yield any effect on CD34+ cell cultures, excluding the possibility that nicotinamide modulates CD34+ cell fate solely by inhibition of cADPR synthesis. Nicotinamide is also a well-known potent inhibitor of SIRT2, a unique NAD(+)-dependent type III histone deacetylase (HDAC) with mono-ADP-ribosyltransferase activity involved in gene silencing, metabolism, apoptosis and aging. NA blocks NAD(+) hydrolysis by binding to an adjacent conserved pocket, and is therefore suggested as the physiologically relevant regulator of SIRT2 enzymes. This additional function of nicotinamide raises the intriguing possibility that HSC enrichment achieved by nicotinamide treatment may be related to specific inhibition of SIRT2 deacetylase activity and modulation of chromatin architecture leading to re-activation of previously silenced genes. In line with this hypothesis, Milhem et all. recently reported that addition of trichostatin A, a specific HDAC (type I and II) inhibitor, along with a DNA hypomethylating agent, modulated HSC fate ex vivo resulting in the retention of stem cell phenotype, number, and function (Blood, 2004; 103; 4102). Ongoing work is aimed at elucidating whether inhibition of SIRT2 is specifically involved in NA mechanism of activity leading to modulation of hematopoietic stem cell fate in ex vivo conditions.

In vitro cell expansion is constrained by default pathways of commitment and differentiation resulting in limited expansion of hematopoietic stem-progenitor cells (HSPCs). Still, several ex vivo manipulations have been reported to achieve expansion of HSPCs by altering cell cycle kinetics and enhancing progression through the G1-S barrier. We have previously shown that addition of tetraethylenepentamine (TEPA), a polyamine copper chelator, to cytokine-supplemented CD34+ cell cultures modulates cytokine-driven hematopoietic cell fate in vitro, resulting in remarkable expansion of a cell population that displays phenotypic and functional characteristics of HSPCs (Exp Hematol.2004;32 (6):547–55). The objective of the present study was to evaluate the mechanism leading to expansion of early progenitor cells following short-term exposure to TEPA. To this end, cell cycle profile, tracking of proliferation history, as well as determination of actual numbers of progenitor subsets were studied. In order to follow the extent of proliferation by tracking the number of cellular divisions, freshly isolated CD34+ cells were labeled with PKH2, a membrane dye that is sequentially diluted during every cell division. Fluorescence intensities of CD34+ and that of a more immature CD34+CD38− cell subset were determined immediately after staining. The cells were then cultured in serum-containing medium and a cocktail of cytokines (SCF, TPO, IL-6, Flt3-ligand, at 50 ng/ml each and IL-3 at 20 ng/ml), with and without TEPA. Total nucleated cells (TNC), purified CD34+ cells and CD34+CD38− cells were analyzed for PKH2 fluorescence intensity during the first two weeks of culture. Cell cycle profile was detected with the DNA intercalating agent propidium iodide, which determines cellular DNA content. FACS analysis of the cultured cells as well as progenitor cell quantification by immuno-affinity purification revealed comparable expansion levels of TNC and CD34+ cells in both TEPA-treated and control cultures during the first two weeks, as previously published. Although similar CD34+ cell numbers were observed, the mean frequency of CD34+CD38− and CD34+CD38-Lin- cells within the CD34+ cell population was significantly higher in TEPA-treated cultures over the control (0.2 vs. 0.04 and 0.07 vs. 0.01, respectively; n=6, p&lt;0.05). Median PKH2 fluorescence intensity of CD34+CD38− subset was two fold higher in TEPA than in control cultures, demonstrating that early progenitor cells derived from TEPA-treated cultures consistently accomplished less proliferation cycles as compared to early progenitor cells derived from control cultures. This effect was not mirrored by a significant alteration of the cell cycle profile (Control (%): G1=26±14, S=2.6±0.1, G2=0.7±0.4; TEPA(%): G1=29±12, S=1.7±0.9, G2=0.4±0.2). Taken together, the data suggest that during cycling, the CD34+CD38− phenotype is preserved more successfully in TEPA-treated than in control cultures, suggesting retention of self-renewing potential of early progenitor cells under these culture conditions. This mechanism also supports a role for TEPA in inhibition of early progenitor cell differentiation. Ongoing work is aimed at further defining whether phenotype reversion or self-renewal (or both) lie at the foundation of TEPA-mediated progenitor cell expansion.