Improved coupling between proliferation-arrest and differentiation-induction in ML-1 human myeloblastic leukemia cells

N-terminal truncation of antiapoptotic MCL1, but not G2/M-induced phosphorylation, is associated with stabilization and abundant expression in tumor cells

The antiapoptotic BCL2 family member MCL1 is normally up- and down-modulated in response to environmental signals and conditions, but is constitutively expressed in cancer where it promotes cell survival and drug resistance. A post-translational modification identified here, truncation at the N terminus, was found to act along with previously described ERK- and GSK3-induced phosphorylation events to regulate the turnover of the MCL1 protein and thus its availability for antiapoptotic effects. Although both N-terminally truncated and full-length MCL1 contain sequences enriched in proline, glutamic acid, serine, and threonine and were susceptible to proteasomal degradation, the truncated form decayed less rapidly and was maintained for an extended period in the presence of ERK activation. This was associated with extended cell survival because the truncated form of MCL1 (unlike those of BCL2 and BCLX) retained antiapoptotic activity. N-terminal truncation slightly increased the electrophoretic mobility of MCL1 and differed from the phosphorylation/band shift to decreased mobility, which occurs in the G2/M phase and was not found to affect MCL1 turnover. The N-terminally truncated form of MCL1 was expressed to varying extents in normal lymphoid tissues and was the predominant form present in lymphomas from transgenic mice and human tumor lines of B-lymphoid origin. The degradation versus stabilized expression of antiapoptotic MCL1 is thus controlled by N-terminal truncation as well as by ERK- and GSK3 (but not G2/M)-induced phosphorylation. These modifications may contribute to dysregulated MCL1 expression in cancer and represent targets for promoting its degradation to enhance tumor cell death.

Short nucleotide polymorphic insertions in the MCL-1 promoter affect gene expression

We have recently reported novel short nucleotide (six and eighteen) polymorphic insertions, in the MCL-1 promoter and their association with higher mRNA and protein levels. The aim of the present study was to test the hypothesis that these insertions directly affect MCL-1 gene expression. Haematopoietic and epithelial human cell lines were transfected with +0, +6, or +18 MCL-1 promoter fragments positioned upstream of the Firefly luciferase reporter gene. The cells were stimulated with phorbol 12-myristate 13-acetate (PMA) and granulocyte macrophage colony-stimulating factor (GM-CSF). Compared to +0, both polymorphic insertions (+6 and +18) were associated with increased promoter activity. Although chromatin immunoprecipitation assay showed that there are Sp1/Sp3 binding sites in the MCL-1 promoter, electrophoretic mobility shift assay showed that it is unlikely that these sites are in the region harboring these insertions. These results provide further evidence for the biological effect of MCL-1 promoter polymorphisms on gene expression.

Inducer-and cell type-specific regulation of antiapoptotic MCL1 in myeloid leukemia and multiple myeloma cells exposed to differentiation-inducing or microtubule-disrupting agents

The antiapoptotic BCL2 family member MCL1 is rapidly upregulated upon exposure of ML-1 myeloid leukemia cells to either differentiation-inducing phorbol 12'-myristate 13'-acetate (PMA) or chemotherapeutic microtubule disrupting agents (MTDAs). This report examined how signaling for MCL1 upregulation is coupled to these two different phenotypic changes, and tested for upregulation in other hematopoietic cancers. With PMA, ERK stimulated MCL1 mRNA expression and ML-1 cell differentiation, and ERK additionally stabilized expression of the MCL1 protein. However, with MTDAs, transient ERK and ensuing JNK activation contributed to initial MCL1 upregulation and viability-retention, but sustained JNK activation eventually resulted in cell death. MCL1 was upregulated by PMA in THP-1 and U937 myeloid leukemia cells, but by MTDAs only in THP-1 cells. MCL1 expression was constitutively elevated in multiple myeloma cell lines, and was not affected by PMA/ERK or MTDAs. Thus, MCL1 expression level and sensitivity to regulation are important considerations in selecting approaches for targeting this antiapoptotic gene product to kill cancer cells.

MCL1 provides a window on the role of the BCL2 family in cell proliferation, differentiation and tumorigenesis

The MCL1 gene (myeloid cell leukemia-1) was discovered serendipitously about a decade ago and proved to be a member of the emerging BCL2 gene family. Ongoing studies of this gene provide an interesting perspective on the role of the BCL2 family in transitions in cell phenotype. Specifically, gene products that influence cell viability as a major effect (eg MCL1, BCL2 and other family members) can act as key determinants in cell proliferation, differentiation and tumorigenesis. Although they do not have a direct role in proliferation/differentiation programs, these genes can either permit these programs to proceed or prevent them. Through such effects, the BCL2 family regulates the normal flow of cells through cycles of proliferation and along various pathways of differentiation. A model is presented suggesting that this is accomplished by sustaining or inhibiting viability at critical points in the cell lifecycle. These critical points represent windows of time during which cell fate transitions are effected. They can also be visualized as windows that open or close to promote or prevent continued progression along various cell fate pathways. The pattern of BCL2 family expression at these points allows for the proliferation differentiation, and continued viability of cell types that are needed, while aborting these processes for cells that are overabundant or no longer needed. The combined action of the various family members can therefore control the fate of cells, tissues and even the organism. This mechanism involving apoptosis-related genes is readily executable, and is poised to respond to external signals through the differential regulation of BCL2 family members. As such, it plays an important role in the maintenance of tissue homeostasis and function. Alterations that affect the BCL2 family impair the capacity to control the flow of cells through these critical points, and thereby 'leave the window open' for cell immortalization and cancer. Targeting this family may thus provide a means of inhibiting cancer development and inducing apoptosis in tumor cells.

MCL1 transgenic mice exhibit a high incidence of B-cell lymphoma manifested as a spectrum of histologic subtypes

Viability-promoting genes such as BCL2 play an important role in human cancer but do not directly cause aggressive tumors. BCL2 transgenic mice develop lymphoma at low frequency, hindering studies of tumorigenesis and its inhibition in the presence of such gene products. MCL1 is a member of the BCL2 family that is highly regulated endogenously and that promotes cell viability and immortalization when introduced exogenously. Mice expressing an MCL1 transgene in hematolymphoid tissues have now been monitored for an extended period and were found to develop lymphoma with long latency and at high probability (more than 85% over 2 years). In most cases, the disease was widely disseminated and of clonal B-cell origin. A variety of histologic subtypes were seen, prominently follicular lymphoma and diffuse large-cell lymphoma. MCL1 thus sets the stage for the development of lymphoma as does BCL2, disease occurring with high probability and recapitulating a spectrum of subtypes as seen in human patients. These findings with the transgene underscore the importance of the normal, highly regulated pattern of MCL1 expression, in addition to providing a model for studying tumorigenesis and its inhibition in the presence of a viability promoting BCL2 family member. (Blood. 2001;97:3902-3909)

Notch1-Induced Delay of Human Hematopoietic Progenitor Cell Differentiation Is Associated With Altered Cell Cycle Kinetics

Hematopoiesis is a balance between proliferation and differentiation that may be modulated by environmental signals. Notch receptors and their ligands are highly conserved during evolution and have been shown to regulate cell fate decisions in multiple developmental systems. To assess whether Notch1 signaling may regulate human hematopoiesis to maintain cells in an immature state, we transduced a vesicular stomatitis virus G-protein (VSV-G) pseudo-typed bicistronic murine stem cell virus (MSCV)-based retroviral vector expressing a constitutively active form of Notch1 (ICN) and green fluorescence protein into the differentiation competent HL-60 cell line and primary cord blood–derived CD34+ cells. In addition, we observed endogenous Notch1 expression on the surface of both HL-60 cells and primary CD34+ cells, and therefore exposed cells to Notch ligand Jagged2, expressed on NIH3T3 cells. Both ligand-independent and ligand-dependent activation of Notch resulted in delayed acquisition of differentiation markers by HL-60 cells and cord blood CD34+ cells. In addition, primary CD34+cells retained their ability to form immature colonies, colony-forming unit–mix (CFU-mix), whereas control cells lost this capacity. Activation of Notch1 correlated with a decrease in the fraction of HL-60 cells that were in G0/G1phase before acquisition of a mature cell phenotype. This enhanced progression through G1 was noted despite preservation of the proliferative rate of the cells and the overall length of the cell cycle. These findings show that Notch1 activation delays human hematopoietic differentiation and suggest a link of Notch differentiation effects with altered cell cycle kinetics.

SZF1: a novel KRAB-zinc finger gene expressed in CD34+ stem/progenitor cells

The identification and study of genes expressed in hematopoietic stem/progenitor cells should further our understanding of hematopoiesis. Transcription factors in particular are likely to play important roles in maintaining the set of genes that define the stem/progenitor cell. We report here the identification of a putative KRAB-zinc finger gene (SZF1) from a cDNA library prepared from human bone marrow CD34+ cells. Characterization of SZF1 implicates its role in hematopoiesis. The predicted protein contains a highly conserved KRAB domain at the NH2 terminus and four zinc fingers of the C2H2 type at the COOH terminus. Two alternatively spliced products of SZF1 were isolated, which predict proteins of 421 (SZF1-1) and 361 (SZF1-2) amino acids, differing from each other only at the carboxy terminus. The two transcripts of SZF1 have different expression patterns. SZF1-2 is ubiquitously expressed, as indicated by Northern blot, RNase protection, and reverse transcriptase polymerase chain reaction. SZF1-1 expression, in contrast, was detected only in CD34+ cells. We recently isolated the promoter region for the stem/progenitor cell expressed FLT3/FLK-2/STK-1 gene and used this region to generate a reporter construct to test the effect of SZF1 expression. Cotransfection of the reporter construct with SZF1 constructs showed that SZF1-2 repressed transcription three- to fourfold, whereas SZF1-1 showed a lower level of repression. The expression pattern of SZF1 transcripts and the transcriptional repression of a CD34+-specific promoter demonstrate a possible role for SZF1 in hematopoietic stem/progenitor cell differentiation.

Notch1-Induced Delay of Human Hematopoietic Progenitor Cell Differentiation Is Associated With Altered Cell Cycle Kinetics

Hematopoiesis is a balance between proliferation and differentiation that may be modulated by environmental signals. Notch receptors and their ligands are highly conserved during evolution and have been shown to regulate cell fate decisions in multiple developmental systems. To assess whether Notch1 signaling may regulate human hematopoiesis to maintain cells in an immature state, we transduced a vesicular stomatitis virus G-protein (VSV-G) pseudo-typed bicistronic murine stem cell virus (MSCV)-based retroviral vector expressing a constitutively active form of Notch1 (ICN) and green fluorescence protein into the differentiation competent HL-60 cell line and primary cord blood–derived CD34+ cells. In addition, we observed endogenous Notch1 expression on the surface of both HL-60 cells and primary CD34+ cells, and therefore exposed cells to Notch ligand Jagged2, expressed on NIH3T3 cells. Both ligand-independent and ligand-dependent activation of Notch resulted in delayed acquisition of differentiation markers by HL-60 cells and cord blood CD34+ cells. In addition, primary CD34+cells retained their ability to form immature colonies, colony-forming unit–mix (CFU-mix), whereas control cells lost this capacity. Activation of Notch1 correlated with a decrease in the fraction of HL-60 cells that were in G0/G1phase before acquisition of a mature cell phenotype. This enhanced progression through G1 was noted despite preservation of the proliferative rate of the cells and the overall length of the cell cycle. These findings show that Notch1 activation delays human hematopoietic differentiation and suggest a link of Notch differentiation effects with altered cell cycle kinetics.

Regulation of MCL1 through a serum response factor/Elk-1-mediated mechanism links expression of a viability-promoting member of the BCL2 family to the induction of hematopoietic cell differentiation

Proliferation, differentiation, and apoptosis are tightly regulated during hematopoiesis, allowing amplification along specific lineages while preventing excessive proliferation of immature cells. The MCL1 member of the BCL2 family is up-regulated during the induction of monocytic differentiation (approximately 10-fold with 12-O-tetradecanoylphorbol 13-acetate (TPA)). MCL1 has effects similar to those of BCL2, up-regulation promoting viability, but differs from BCL2 in its rapid inducibility and its pattern of expression. Nuclear factors that regulate MCL1 transcription have now been identified, extending the previous demonstration of signal transduction through mitogen-activated protein kinase. A 162-base pair segment of the human MCL1 5'-flank was found to direct luciferase reporter activity, allowing approximately 10-fold induction with TPA that was suppressible upon inhibition of the extracellular signal-regulated kinase (ERK) pathway. Serum response factor (SRF), Elk-1, and Sp1 bound to cognate sites within this segment, SRF and Elk-1 acting coordinately to affect both basal activity and TPA inducibility, whereas Sp1 affected basal activity only. Thus, the mechanism of the TPA-induced increase in MCL1 expression seen in myelomonocytic cells at early stages of differentiation involves signal transduction through ERKs and transcriptional activation through SRF/Elk-1. This finding provides a parallel to early response genes (e.g. c-FOS and EGR1) that affect maturation commitment in these cells and therefore suggests a means through which enhancement of cell viability may be linked to the induction of differentiation.

Identification of immediate early genes during TPA-induced human myeloblastic leukemia ML-1 cell differentiation

Human myeloblastic ML-1 can be induced to differentiate into monocytes/macrophages by 12-0-tetradecanoylphorbol-13-acetate (TPA). In order to understand the molecular mechanism regulating ML-1 cell differentiation, we focused on the characterization of immediate early genes activated by TPA using the mRNA differentiation display polymerase chain reaction (DD-PCR) and Northern analyses. A modified procedure, the reverse dot slot, was developed to confirm upregulated genes during the early stages of TPA-induced ML-1 cell differentiation. DNA sequencing analyses of 10 subcloned cDNA fragments, selected on the basis of the outcome of the reverse dot slot procedure, revealed that eight were derived from distinct genes. Among these clones, one was a novel gene (G07-5), another (A02-1) was highly homologous to the sequence of a fetal brain cDNA fragment, and the remaining six corresponded to jun-D, rantes, ssat, CD 14, ferritin heavy chain (fhc) and transposons Tn10-like transcript, respectively. Although these genes were all upregulated by TPA, the peak time of mRNA expression varied. jun-D, ssat and A02-1 expressions were superinduced in the presence of cycloheximide, which indicates that they belong to the immediate early gene family. On the other hand, TPA-induced rantes expression was not superinduced by cycloheximide, suggesting a protein synthesis-dependent process. As there are no previous reports of expression of these genes in TPA-induced ML-1 cells, little or no information is available concerning their function in mediating myeloblastic cell differentiation. Thus, this study illuminates new avenues of research for elucidating the function of genes regulating terminal differentiation of myeloid progenitors. 1998 Elsevier Science B.V.

Induction of human myeloblastic ML-1 cell G1 arrest by suppression of K+ channel activity

Our previous studies have shown that a voltage-gated K+ channel is highly expressed in proliferating human myeloblastic ML-1 cells and is suppressed in the early stages of 12-O-tetradecanoylphorbol-13-acetate-induced ML-1 cell differentiation. In the present study, we report that inhibition of the K+ channel activity by 4-aminopyridine (4-AP) suppressed ML-1 cell proliferation, as measured by DNA synthesis. Cell cycle mapping indicated that ML-1 cells were arrested in G1 phase after 24-h treatment with 4-AP. Blockade of ML-1 cells at the G1/S boundary of the cell cycle with aphidicolin revealed that ML-1 cells past the G1 checkpoint were capable of entering S phase and synthesizing DNA independently of the channel blockade. ML-1 cell differentiation, measured by CD14 marker protein expression, revealed that the effect of 4-AP was to cause growth arrest and that it did not cause differentiation. Dephosphorylation of retinoblastoma protein accompanied inhibition of ML-1 cell proliferation and suggested that suppression of K+ channel activity by 4-AP is associated with retinoblastoma protein-mediated G1 arrest in ML-1 cells. Moreover, we found that ML-1 cell volume increased 35 +/- 7% after 4-AP treatment, which could be an early event triggering inhibition of ML-1 cell proliferation. These findings suggest that a 4-AP-sensitive K+ channel may play an important role in the transduction of mitogenic signals in ML-1 cells.

MCL-1, a member of the BLC-2 family, is induced rapidly in response to signals for cell differentiation or death, but not to signals for cell proliferation

mcl-1 was identified as an "early-induction" gene that increases in expression during the differentiation of ML-1 human myeloblastic leukemia cells. The mcl-1 gene product proved to be a member of the bcl-2 gene family and, like bcl-2, to have the capacity to promote cell viability. The pattern of expression of mcl-1 has now been characterized, the aim being to determine whether increased expression is consistently associated with differentiation-induction and whether expression is also associated with other changes in proliferative state or cell viability. Expression of the mcl-1 mRNA was found to increase rapidly in ML-1 cells exposed to inducers of monocyte/macrophage differentiation (phorbol esters or lymphocyte conditioned medium), but not cells exposed to an inducer of granulocyte differentiation (retinoic acid). Expression also increased rapidly in response to certain cytotoxic agents (colchicine and vinblastine), but did not increase during serum stimulation or growth-arrest in reduced serum. Increased expression of mcl-1 occurred during the initiation of cell differentiation or death and was not inhibited by cycloheximide, in agreement with the designation of mcl-1 as an early-induction gene. Increased transcription contributed to the increase in expression, and turnover of the mcl-1 mRNA was rapid. These findings suggest that mcl-1 may serve as a modulator of cell viability that can undergo rapid upregulation as well as downregulation, with upregulation harbingering the initiation of cell differentiation or death.

The intracellular distribution and pattern of expression of Mcl-1 overlap with, but are not identical to, those of Bcl-2

A family of genes related to the bcl-2 protooncogene has recently emerged. One member of this family, mcl-1, was cloned from a human myeloblastic leukemia cell line (ML-1) undergoing differentiation. The intracellular localization of mcl-1, as well as the kinetics of its expression during differentiation, have now been studied. These studies show that the intracellular distribution of mcl-1 overlaps with, but is not identical to, that of bcl-2: mcl-1 is similar to bcl-2 in that the mcl-1 protein has a prominent mitochondrial localization, and in that it associates with membranes through its carboxyl hydrophobic tail. mcl-1 differs from bcl-2, however, in its relative distribution among other (nonmitochondrial/heavy membrane) compartments, mcl-1 also being abundant in the light membrane fraction of immature ML-1 cells while bcl-2 is abundant in the nuclear fraction. Similarly, in differentiating ML-1 cells, the timing of expression of mcl-1 overlaps with, but is not identical to, that of bcl-2: the mcl-1 protein increases rapidly as cells initiate differentiation, and mcl-1 is a labile protein. In contrast, bcl-2 decreases gradually as cells complete differentiation. Overall, the mcl-1 and bcl-2 proteins have some properties in common and others tht are distinct. A burst of expression of mcl-1, prominently associated with mitochondria, complements the continued expression of bcl-2 in ML-1 cells differentiating along the monocyte/macrophage pathway.

MCL1, a gene expressed in programmed myeloid cell differentiation, has sequence similarity to BCL2

During their lifespan, immature cells normally pass through sequential transitions to a differentiated state and eventually undergo cell death. This progression is aberrant in cancer, although the transition to differentiation can be reestablished in inducible leukemia cell lines. This report describes a gene, MCL1, that we isolated from the ML-1 human myeloid leukemia cell line during phorbol ester-induced differentiation along the monocyte/macrophage pathway. Our results demonstrate that expression of MCL1 increases early in the induction, or "programming," of differentiation in ML-1 (at 1-3 hr), before the appearance of differentiation markers and mature morphology (at 1-3 days). They further show that MCL1 has sequence similarity to BCL2, a gene involved in normal lymphoid development and in lymphomas with the t(14;18) chromosome translocation. MCL1 and BCL2 do not fall into previously known gene families. BCL2 differs from many oncogenes in that it inhibits programmed cell death, promoting viability rather than proliferation; this parallels the association of MCL1 with the programming of differentiation and concomitant maintenance of viability but not proliferation. Thus, in contrast to proliferation-associated genes, expression of MCL1 and BCL2 relates to the programming of differentiation and cell viability/death. The discovery of MCL1 broadens our perspective on an emerging MCL1/BCL2 gene family and will allow further comparison with oncogene families.

Alterations in a voltage-gated K+ current during the differentiation of ML-1 human myeloblastic leukemia cells

A voltage-gated K+ current has been identified in ML-1 human myeloid leukemia cells, with the use of the whole-cell patch-clamp technique. ML-1 cells proliferate in tissue culture as immature myeloblasts and can be induced to differentiate to nonproliferative monocyte/macrophages. In the myeloblastic cells, activation of the K+ current occurs upon depolarization of the membrane potential to above -40 mV; inactivation of this current is also voltage dependent and follows a simple exponential time course with a time constant (Ti) of 900 msec at 0 mV. The current is inhibited by 4-aminopyridine (IC50 of 80 microM at 0 mV), but is much less sensitive to tetraethylammonium of Ba2+. In cells exposed to the differentiation-inducer 12-O-tetradecanoylphorbol-13-acetate (TPA), dramatic alterations in the K+ current occur: upon exposure to 10 nM TPA during whole-cell recording, the amplitude of the voltage-activated current initially increases (within 4 min) and later decreases (at approximately 30-50 min). Upon addition of 0.5 nM TPA to cells in tissue culture, the current shows suppressed activation and accelerated inactivation in the early stages of differentiation (10-fold decrease in Ti at approximately 7 hr) and is completely suppressed in the later stages (3 days). Thus, this voltage-gated K+ current is suppressed early in the induction of differentiation and associated loss of proliferation in myeloid ML-1 cells exposed to TPA; this parallels the fact that channels of a similar type are activated upon the stimulation of proliferation in lymphoid cells exposed to mitogens.(ABSTRACT TRUNCATED AT 250 WORDS)