Complex regulation of CDKs and G1 arrest during the granulocytic differentiation of human myeloblastic leukemia ML-1 cells

Strategies to generate functionally normal neutrophils to reduce infection and infection-related mortality in cancer chemotherapy

Neutrophils form an essential part of innate immunity against infection. Cancer chemotherapy-induced neutropenia (CCIN) is a condition in which the number of neutrophils in a patient's bloodstream is decreased, leading to increased susceptibility to infection. Granulocyte colony-stimulating factor (GCSF) has been the only approved treatment for CCIN over two decades. To date, CCIN-related infection and mortality remain a significant concern, as neutrophils generated in response to administered GCSF are functionally immature and cannot effectively fight infection. This review summarizes the molecular regulatory mechanisms of neutrophil granulocytic differentiation and innate immunity development, dissects the biology of GCSF in myeloid expansion, highlights the shortcomings of GCSF in CCIN treatment, updates the recent advance of a selective retinoid agonist that promotes neutrophil granulocytic differentiation, and evaluates the benefits of developing GCSF biosimilars to increase access to GCSF biologics versus seeking a new mode to fundamentally advance GCSF therapy for treatment of CCIN.

Retinoic acid receptor alpha drives cell cycle progression and is associated with increased sensitivity to retinoids in T-cell lymphoma

Peripheral T-cell lymphomas (PTCLs) are aggressive non-Hodgkin lymphomas with generally poor outcomes following standard therapy. Few candidate therapeutic targets have been identified to date. Retinoic acid receptor alpha (RARA) is a transcription factor that modulates cell growth and differentiation in response to retinoids. While retinoids have been used to treat some cutaneous T-cell lymphomas (CTCLs), their mechanism of action and the role of RARA in CTCL and other mature T-cell lymphomas remain poorly understood. After identifying a PTCL with a RARAR394Q mutation, we sought to characterize the role of RARA in T-cell lymphoma cells. Overexpressing wild-type RARA or RARAR394Q significantly increased cell growth in RARAlow cell lines, while RARA knockdown induced G1 arrest and decreased expression of cyclin-dependent kinases CDK2/4/6 in RARAhigh cells. The retinoids, AM80 (tamibarotene) and all-trans retinoic acid, caused dose-dependent growth inhibition, G1 arrest, and CDK2/4/6 down-regulation. Genes down-regulated in transcriptome data were enriched for cell cycle and G1-S transition. Finally, RARA overexpression augmented chemosensitivity to retinoids. In conclusion, RARA drives cyclin-dependent kinase expression, G1-S transition, and cell growth in T-cell lymphoma. Synthetic retinoids inhibit these functions in a dose-dependent fashion and are most effective in cells with high RARA expression, indicating RARA may represent a therapeutic target in some PTCLs.

Role of cell cycle regulatory molecules in retinoic acid- and vitamin D3-induced differentiation of acute myeloid leukaemia cells

The important role of cell cycle regulatory molecules in all trans-retinoic acid (ATRA)- and vitamin D3-induced growth inhibition and differentiation induction has been intensively studied in both acute myeloid leukaemia primary cells and a variety of leukaemia cell lines. Cyclin-dependent kinases (CDK)-activating kinase has been demonstrated to interact with retinoic acid receptor (RAR)α in acute promyelocytic leukaemia cells, and inhibition of CDK-activating kinase by ATRA causes hypophosphorylation of PML-RARα, leading to myeloid differentiation. In many cases, downregulation of CDK activity by ATRA and vitamin D3 is a result of elevated p21- and p27-bound CDKs. Activation of p21 is regulated at the transcriptional level, whereas elevated p27 results from both (indirectly) transcriptional activation and post-translational modifications. CDK inhibitors (CKIs) of the INK family, such as p15, p16 and p18, are mainly involved in inhibition of cell proliferation, whereas CIP/KIP members, such as p21, regulate both growth arrest and induction of differentiation. ATRA and vitamin D3 can also downregulate expression of G1 CDKs, especially CDK2 and CDK6. Inhibition of cyclin E expression has only been observed in ATRA- but not in vitamin D3-treated leukaemic cells. In vitro, not only dephosphorylation of pRb but also elevation of total pRb is required for ATRA and vitamin D3 to suppress growth and trigger their differentiation. Finally, sharp reduction in c-Myc has been observed in several leukaemia cell lines treated with ATRA, which may regulate expression of CDKs and CKIs.

Retinoic Acid-induced Cell Cycle Arrest of Human Myeloid Cell Lines

Retinoic acid-induced terminal differentiation of myeloid cells involves the sequential regulation of cell cycle regulatory genes, coordinating the process of differentiation with arrest in the G0/G1 phase of the cell cycle. In this review we have summarized changes in expression and activity of cell cycle regulatory proteins associated with retinoic acid induced-growth arrest in human myeloid cell lines. These changes involve: (i) an early down-regulation of c-Myc; (ii) up-regulation of p21CIP1 and p27KIP1 and, in some cases, p15INK4b or p18INK4c; (iii) down-regulation of cyclin E and cyclin D1/D3, and, at later stages, cyclin A and cyclin B; and (iv) decreased CDK activity and dephosphorylation of pRb.

Synergistic induction of gene expression during the differentiation into mature macrophage in human myeloblastic leukemia cells treated with TPA and KH1060

Treatment of human myeloblastic leukemia ML-1 cells with the phorbol ester TPA in combination with the vitamin D(3) analogue KH1060 will induce a synergistic differentiation to mature macrophage with multinuclei. To investigate the mechanism underlying this differentiation and the synergistic effect, a cDNA microarray and Northern blot analysis were used to examine gene expression profiles of ML-1 cells treated with TPA and/or KH1060. Results show that KH1060 enhanced several TPA-induced gene expressions and that TPA enhanced several KH1060-induced gene expressions. Studies with inhibitors of signaling molecules suggested that PKC and MAPK pathways play an important role in the differentiation induced by TPA and KH1060, and that they are associated with the synergistic induction of genes. The results of this study indicate the possibility that the expression of various genes are induced synergistically by cross-talk between TPA and KH1060 signals. It is likely that the synergistic effect on gene expression leads to the synergistic induction of differentiation by both reagents.

Retinoic acid causes cell growth arrest and an increase in p27 in F9 wild type but not in F9 retinoic acid receptor β2 knockout cells

We have previously shown that an F9 teratocarcinoma retinoic acid receptor beta(2) (RARbeta(2)) knockout cell line exhibits no growth arrest in response to all-trans-retinoic acid (RA), whereas F9 wild type (Wt), F9 RARalpha(-/-), and F9 RARgamma(-/-) cell lines do growth arrest in response to RA. To examine the role of RARbeta(2) in growth inhibition, we analyzed the cell cycle regulatory proteins affected by RA in F9 Wt and F9 RARbeta(2)(-/-) cells. Flow microfluorimetry analyses revealed that RA treatment of F9 Wt cells greatly increased the percentage of cells in the G1/G0 phase of the cell cycle. In contrast, RA did not alter the cell cycle distribution profile of RARbeta(2)(-/-) cells. In F9 Wt cells, cyclin D1, D3, and cyclin E protein levels decreased, while cyclin D2 and p27 levels increased after RA treatment. Compared to the F9 Wt cells, the F9 RARbeta(2)(-/-) cells exhibited lower levels of cyclins D1, D2, D3, and E in the absence of RA, but did not exhibit further changes in the levels of these cell cycle regulators after RA addition. Since RA significantly increased the level of p27 protein (approximately 24-fold) in F9 Wt as compared to the F9 RARbeta(2)(-/-) cells, we chose to study p27 in greater detail. The p27 mRNA level and the rate of p27 protein synthesis were increased in RA-treated F9 Wt cells, but not in F9 RARbeta(2)(-/-) cells. Moreover, RA increased the half-life of p27 protein in F9 Wt cells. Reduced expression of RARbeta(2) is associated with the process of carcinogenesis and RARbeta(2) can mediate the growth arrest induced by RA in a variety of cancer cells. Using both genetic and molecular approaches, we have identified some of the molecular mechanisms, such as the large elevation of p27, through which RARbeta(2) mediates these growth inhibitory effects of RA in F9 cells.

End-stage differentiation of neutrophil granulocytes in vivo is accompanied by up-regulation of p27kip1 and down-regulation of CDK2, CDK4, and CDK6

The in vivo expression profiles of cell-cycle proteins regulating G1-to-S-phase transition were determined in three neutrophil precursor populations from human bone marrow: myeloblasts (MBs) and promyelocytes (PMs); myelocytes (MCs) and metamyelocytes (MMs); and band cells (BCs) and segmented neutrophil cells (SCs) and in mature polymorphonuclear neutrophils (PMNs) from peripheral blood. Complete cell-cycle arrest was observed in BCs/SCs and PMNs. Cyclins D1, D2, and D3 were found to be down-regulated during granulopoiesis, whereas a slight increase of cyclin E was seen. In contrast, cyclin-dependent kinase (CDK)2, -4, and -6 were down-regulated from the MC/MM stages and onward. The transcript levels of CDK2, -4, and -6 were concurrently down-regulated. As the only CDK inhibitor, p27kip1 protein and mRNA expression were up-regulated in MCs/MMs and reached peak levels in PMNs. Protein expression of retinoblastoma protein and the related pocket proteins p107 and p130 was down-regulated from the MC/MM stages and onward. This is the first report to describe expression levels of cell-cycle proteins during granulopoiesis in vivo, and it strongly contrasts the observations made in cell-culture systems in vitro.

Relationship between cyclin D1 and p21(Waf1/Cip1) during differentiation of human myeloid leukemia cell lines

Expression of cell cycle-regulating genes was studied in human myeloid leukemia cell lines ML-1, ML-2 and ML-3 during induction of differentiation in vitro. Myelomonocytic differentiation was induced by phorbol ester (12-o-Tetradecanoyl-phorbol-13-acetate, TPA), tumor necrosis factor alpha (TNFalpha) or interferon gamma (INFgamma), or their combination. Differentiation (with the exception of TNFalpha alone) was accompanied by inhibition of DNA synthesis and cell cycle arrest. Inhibition of proliferation was associated with a decrease in the expression of cdc25A and cdc25B, cdk6 and Ki-67 genes, and with increased p21(Waf1/Cip1) gene expression, as measured by comparative RT-PCR. Expression of the following genes was not changed after induction of differentiation: cyclin A1, cyclin D3, cyclin E1 and p27(Kip1). Surprisingly, cyclin D1 expression was upregulated after induction by TPA, TNFalpha with IFNgamma or BA. Cyclin D2 was upregulated only after induction by BA. The results of the expression of the tested genes obtained by comparative RT-PCR were confirmed by quantitative real-time (RQ) RT-PCR and Western blotting. Quantitative RT-PCR showed as much as a 288-fold increase of cyclin D1 specific mRNA after a 24h induction by TPA. The upregulation of cyclin D1 in differentiating cells seems to be compensated by the upregulation of p21(Waf1/Cip1). These results, besides others, point to a strong correlation between the expression of cyclin D1 and p21(Waf1/Cip1) on the one hand and differentiation on the other hand in human myeloid leukemic cells and reflect a rather complicated network regulating proliferation and differentiation of leukemic cells.

Cyclin D3 is a cofactor of retinoic acid receptors, modulating their activity in the presence of cellular retinoic acid-binding protein II

Ligand-induced transcription activation of retinoic acid (RA) target genes by nuclear receptors (retinoic acid (RAR) and retinoid X (RXR) receptors) depends on the recruitment of coactivators. We have previously demonstrated that the small 15-kDa cellular RA-binding protein II (CRABPII) is a coactivator present in the RA-dependent nuclear complex. As identifying cell-specific partners of CRABPII might help to understand the novel control of RA signaling, we performed a yeast two-hybrid screen of a hematopoietic HL-60 cDNA library using human CRABPII as bait and have subsequently identified human cyclin D3 as a partner of CRABPII. Cyclin D3 interacted with CRABPII in a ligand-independent manner and equally bound RAR alpha, but not RXR alpha, and only in the presence of RA. We further show that cyclin D3 positively modulated RA-mediated transcription through CRABPII. Therefore, cyclin D3 may be part of a ternary complex with CRABPII and RAR. Finally, we show that cyclin D3 expression paralleled HL-60 differentiation and arrest of cell growth. These findings led us to speculate that control of cell proliferation during induction of differentiation may directly involve, at the transcriptional level, nuclear receptors, coactivators, and proteins of the cell cycle in a cell- and nuclear receptor-specific manner.

Cell cycle regulators and hematopoiesis

The cell cycle behavior of hematopoietic cells varies from extended quiescence to spectacular proliferation. Cell cycle regulators choreograph these transitions through variation in the makeup of cyclin-dependent kinase (cdk)-containing complexes and through alteration in protein expression levels and subcellular localization. The mechanisms through which cell cycle regulators couple proliferation, differentiation and survival is coming into sharper focus. Cdk-inhibitors, once thought of solely in terms of a checkpoint function on cycling, are now known to interact directly with proteins and pathways central to differentiation and apoptosis. By shuttling between binding partners committed to discrete functional pathways, cell cycle regulators may directly coordinate proliferation with differentiation, migration and apoptosis.

p27(Kip1): regulation and function of a haploinsufficient tumor suppressor and its misregulation in cancer

A major function of p27, also known as Kip1, is to bind and inhibit cyclin/cyclin-dependent kinase complexes, thereby blocking cell cycle progression. As p27 operates at the heart of the cell cycle, it is perhaps not surprising that it is emerging as a key player in multiple cell fate decisions including proliferation, differentiation, and cell death. The central role of p27 makes it important in a variety of disease processes that involve aberrations in cellular proliferation and other cell fates. Most notable among these processes is neoplasia. A large number of studies have reported that p27 expression is frequently downregulated in human tumors. In most tumor types, reduced p27 expression correlates with poor prognosis, making p27 a novel and powerful prognostic marker. In addition to these practical implications, murine and tissue culture models have shown that p27 is a potent tumor suppressor gene for multiple epithelially derived neoplasias. Loss of p27 cooperates with mutations in several oncogenes and tumor suppressor genes to facilitate tumor growth, indicating that p27 may be a "nodal point" for tumor suppression. In contrast to most tumor suppressor genes studied to date, which are recessive at the cellular level, p27 is haploinsufficient for tumor suppression. The fact that tumor suppression by p27 is critically dependent on the absolute level of p27 expression indicates that p27 acts as a rheostat rather than as an on/off switch to control growth and neoplasia.