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HEMATOPOIESIS
From the Department of Veterans Affairs Medical
Service, Department of Medicine, Division of
Hematology/Oncology; and the Vanderbilt-Ingram Cancer Center,
Vanderbilt University, Nashville, TN.
The retinoblastoma (Rb), cyclin-dependent kinase (CDK), and CDK
inhibitor genes regulate cell generation, and deregulation can produce
increased cell growth and tumorigenesis. Polycythemia vera (PV) is a
clonal myeloproliferative disease where the mechanism producing
increased hematopoiesis is still unknown. To investigate possible
defects in cell-cycle regulation in PV, the expression of Rb and CDK
inhibitor gene messenger RNAs (mRNAs) in highly purified human
erythroid colony-forming cells (ECFCs) was screened using an
RNase protection assay (RPA) and 11 gene probes. It was found that RNA
representing exon 2 of p16INK4a and p14ARF was
enhanced by 2.8- to 15.9-fold in 11 patients with PV. No increase of
exon 2 mRNA was evident in the T cells of patients with PV, or in the
ECFCs and T cells from patients with secondary polycythemia. p27 also
had elevated mRNA expression in PV ECFCs, but to a lesser degree.
Because the INK4a/ARF locus encodes 2 tumor suppressors,
p16INK4a and p14ARF with the same exon 2 sequence, the increased mRNA fragment could represent either one. To
clarify this, mRNA representing the unique first exons of INK4a and ARF
were analyzed by semiquantitative reverse transcription-polymerase
chain reaction. This demonstrated that mRNAs from the first exons of
both genes were increased in erythroid and granulocyte-macrophage cells
and Western blot analysis showed that the INK4a protein
(p16INK4a) was increased in PV ECFCs. Sequencing revealed
no mutations of INK4a or ARF in 10 patients with PV.
p16INK4a is an important negative cell-cycle regulator, but
in contrast with a wide range of malignancies where inactivation of the
INK4a gene is one of the most common carcinogenetic events,
in PV p16 INK4a expression was dramatically increased
without a significant change in ECFC cell cycle compared with normal
ECFCs. It is quite likely that p16INK4a and
p14ARF are not the pathogenetic cause of PV, but instead
represent a cellular response to an abnormality of a downstream
regulator of proliferation such as cyclin D, CDK4/CDK6, Rb, or E2F.
Further work to delineate the function of these genes in PV
is in progress.
(Blood. 2001;97:3424-3432) Polycythemia vera (PV) is a member of the clonal
myeloproliferative disorders, a spectrum of diseases resulting from the
transformation of a pluripotent hematopoietic stem cell, and carrying a
very high incidence of secondary acute leukemia.1-5 PV is
characterized by trilineage marrow hyperplasia with increased
production of red cells, granulocytes, and platelets, but it does not
involve the immune system. Patients with PV have erythroid progenitors that develop in vitro without the addition of erythropoietin (EPO), displaying either an exquisite sensitivity to EPO or EPO-independent growth, and their blood EPO levels are generally quite
low.6-8
Past studies have shown that PV hematopoietic progenitors display
hypersensitive responses to a variety of growth factors and cytokines.
The burst-forming units-erythroid (BFU-E) are hypersensitive to
interleukin-3 (IL-3), stem cell factor (SCF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and insulinlike growth
factor.9-14 Despite these abnormal responses, the number
of receptors for the growth factors and their dissociation constants
are normal.10-16 High levels of Bcl-x were identified in
PV cells by one group of investigators and the investigators postulated
that this might be responsible for increased survival of PV cells
without EPO.17 Whereas a culture of normal erythroid
progenitors with orthovanadate, an inhibitor of protein tyrosine
phosphatases (PTPs), resulted in an increased number of erythroid
colonies and enhanced protein tyrosine phosphorylation, little
enhancement was evident with PV cells, suggesting that patients with PV
may have an abnormal PTP activity which promotes increased cell
proliferation.18 Further investigations revealed that the
total PTP activity in PV erythroid cells was 3-fold higher than in
normal cells.19 However, the increase in PTP may represent
an effect of the disease rather than a cause. SHP-1 was completely
characterized in PV cells and was found to be normal,19,20
although one group provided evidence for its
underexpression.21 Despite the development of much new
knowledge on the control of erythropoiesis, the precise molecular
defect that leads to enhanced hematopoiesis in PV is still not
apparent. Unlike the situation with chronic myelogenous leukemia (CML),
where the Philadelphia chromosome and an abnormal, specific
BCR-ABL gene arising from a translocation have been
identified, no pathognomonic chromosomal or gene abnormality has been
found in PV.
Because the proliferation of normal cells is regulated by a combination
of stimulatory and inhibitory factors that can respond to external
signals in a coordinated manner, a permanent alteration within this
regulatory system can lead to abnormal proliferation resulting in tumor
formation.22-26 Inactivation of various cell proliferation-regulating proteins (such as the Rb, CDK, and CDK inhibitor proteins), due to gene deletions or mutations, results in
enhanced cell generation.24 The INK4a/ARF locus provides important CDK inhibitors that include p16INK4a and
p14ARF (p19ARF in the mouse).26-28
p16INK4a inhibits CDK4 and CDK6 by directly blocking cyclin
D-dependent kinase activity and, thereby, functions to suppress tumor
cell growth.26,28 p16INK4a is inactivated by
deletions, mutations, and methylation of a CpG island in many primary
tumor types such as familial melanomas,29 esophageal
carcinosarcomas,30 liver and lung
carcinomas,31-33 and lymphomas and
leukemias.34-37 Whereas p19ARF also inhibits
cell proliferation, it is without direct inhibition of human CDKs and
the process is p53-dependent as demonstrated by gene knockouts in
mice.26,38 Other CDK inhibitors such as p27 may have a
critical role in regulating entry into and exit from the mitotic cycle
in response to extracellular signals.24
We previously established a method to purify the early-stage erythroid
progenitors (BFU-E) and grow late-stage erythroid colony-forming cells
(ECFCs) from human peripheral blood39 and in past studies have repeatedly noticed that PV BFU-E generate many more ECFCs after 8 days of cell culture than normal BFU-E. Because PV is a proliferative
disease where a principle problem is erythrocytosis, we have used
highly purified erythroid progenitor cells from patients with PV and
healthy donors to study the mechanism for increased cellular
proliferation in this condition.
Generation of ECFCs and colony-forming
units-granulocyte-macrophage
Blood mononuclear cells were highly enriched in colony-forming
units-granulocyte-macrophage (CFU-GM) using the method for BFU-E
except that negative selection was performed without CD45 as previously
described,10 but with the addition of CD20. The average
purity of CFU-GM was 40 ± 6% by the standard colony-forming assay.10 Culture of these cells was performed in a
modified liquid medium for 7 days without EPO.10
Preparation of proliferating T cells
RNA extraction and ribonuclease protection assay Total RNA was prepared from day-8 ECFCs or T cells using Ultraspec RNAzol (Biotecx Laboratories, Houston, TX) and ribonuclease protection assays (RPAs) were performed using the MAXIscript and RPA II Ribonuclease Protection Assay Kit (Ambion, Austin, TX) according to the manufacturers' protocols. The hCC-2 Multi-Probe Template Set and individual, custom probes for expression of p130, p107, p53, p57, p27, p21, p19, p18, p16, p14/15, and Rb RNA were purchased from Pharmingen (San Diego, CA). The protected transcripts were separated on denaturing polyacrylamide gels, quantified with a laser scanning densitometer, and normalized to the amount of total RNA present in the lane by comparison with the housekeeping gene signals. Gels were developed at varying times so that we could scan the L32 and GADPH housekeeping gene RNAs as internal controls, to correct for variations in loading.INK4a and ARF sequence analysis Genomic DNAs from day-8 ECFCs from healthy patients and patients with PV were prepared using the QIAamp Blood Kit (Qiagen, Valencia, CA) according to the manufacturer's instructions. Four hundred nanograms genomic DNA template was used in each reaction. The INK4a/ARF DNA has a high G:C ratio and the specific fragments could not be successfully amplified with the usual PCR kit, so an Advantage-GC complementary DNA (cDNA) PCR kit was purchased from Clontech (Palo Alto, CA). The reaction solution contained GC cDNA PCR buffer, GC melt, 10 mM each of dATP, dTTP, dCTP, dGTP, DNA polymerase, and specific primers for INK4a/ARF exon 1 , exon 1 , exon 2, and exon 3. The intron
oligonucleotide primers were synthesized by Gibco BRL, Life
Technologies (Rockville, MD) or Research Genetics (Huntsville, AL) and
the sequences of the primers are listed in Table
2. Forty cycles were performed and the
conditions and cycles were as follows: 5 minutes at 95°C; then 10 cycles with denaturation at 95°C for 1 minute; annealing at 68°C
for 1 minute and extension at 72°C for 1 minute; then 10 cycles at 95°C for 1 minute, 64°C for 1 minute, and 72°C for 1 minute; then 20 cycles at 95°C for 1 minute, 60°C for 1 minute, and 72°C for 1 minute; and final elongation at 72°C for 10 minutes. The amplified PCR products were electrophoresed in 2% agarose gels to confirm expected single bands. The PCR products were then cloned into the PCR
2.1 TA cloning vector using the Original TA Cloning Kit (Invitrogen,
San Diego, CA) and introduced into competent H5 cells. The plasmid
DNAs were purified using the Qiagen QIAprep Kit. Restriction analyses
were performed to determine the presence of the insert and sequence
analyses were performed at the Vanderbilt Sequencing Center,
Nashville, TN.
RT-PCR and semiquantitative RT-PCR First strand cDNA was synthesized from day-8 normal and PV ECFC RNA using the SuperScript First-Strand Synthesis System for RT-PCR (Gibco BRL). One microgram total RNA was incubated with the components in the kit following the manufacturer's protocol. The final volume of the reaction was 20 µl. Two micrograms of the cDNA and the Advantage-GC cDNA PCR kit (described above) were used for PCR. The specific primers designed to amplify INK4a and ARF are listed in Table 2. First we designed a pair of primers to amplify a 494-bp product, from exon 1 through exons 2 and 3 for INK4a, and a pair of primers
to generate a 571-bp product for ARF from exon 1 through exon 2. The
conditions for INK4a PCR were as follows: 95°C for 5 minutes;
followed by 10 cycles at 95°C for 1 minute, 64°C for 1 minute, and
72°C for 1 minute; then 10 cycles at 95°C for 1 minute, 60°C for
1 minute, and 72°C for 1 minute; then 20 cycles at 95°C for 1 minute, 56°C for 1 minute, and 72°C for 1 minute; then the final
step at 72°C for 10 minutes. The conditions for ARF PCR were: 95°C
for 5 minutes; then 10 cycles at 95°C for 1 minute, 66°C for 1 minute and 72°C for 1 minute; 10 cycles at 95°C for 1 minute,
64°C for 1 minute and 72°C for 1 minute; then 20 cycles at 95°C
for 1 minute, 62°C for 1 minute, and 72°C for 1 minute, followed by
the final step at 72°C for 10 minutes. For semiquantitative RT-PCR,
the cDNAs were diluted 1:4, 1:16, and 1:64 using the PCR reaction
solution and PCR was performed using the same method described above.
Table 2 also shows the sequence of shorter primers for amplifying both INK4a (142 bp), from exon 1 through part of exon 2, and ARF (212 bp), from exon 1 through part of exon 2. The PCR conditions were the
same as those stated above for the ARF PCR. The products were electrophoresed in 2% agarose gels and quantified with a laser scanning densitometer.
Western blot analysis Cell extracts were prepared by lysing 107 cells in 100 µl RIPA lysis buffer containing 1% Triton X-100, 20 mM Tris-HC1, ph 7.5, 10% glycerol, 140 mM NaCl, 100 mM sodium fluoride, 10 mM EDTA, 2 mM vanadate, 0.2 mM phenylmethylsulfonyl fluoride, 0.5% sodium deoxycholate, 0.1% sodium dodecyl sulfate (SDS), and 0.15 U/mL aprotinin at 4°C. Insoluble materials were removed by centrifugation for 20 minutes at 14 000g and 4°C. The samples were quantitated using a Bio-rad protein assay kit II (Bio-rad, Hercules, CA) and were boiled for 5 minutes in an SDS sample buffer before 15% SDS-polyacrylamide gel electrophoresis (SDS-PAGE). The proteins were transferred to nitrocellulose after SDS-PAGE. The blots were incubated for 2 hours at room temperature in Tris-buffered saline with 0.05% Tween-20 (TBST) and 5% nonfat milk and then with anti-p16INK4a overnight at 4°C. The antibodies to p16INK4a were purchased from Santa Cruz (SC-759) (Santa Cruz Biotechnology, Santa Cruz, CA) and Pharmingen and the antibody to p14ARF (clone 14P02) was purchased from Neomarkers (Fremont, CA).Cell-cycle analysis Day-6 to day-19 cells (1-2 × 106) were washed and the cell pellets were resuspended in propidium iodide solution (0.05 mg/mL in 0.1% sodium citrate) and RNase solution (0.1 mg/mL).42 The cell samples generally were examined within 30 minutes of staining at 4°C by the Vanderbilt Flow Cytometry Lab, but some samples were fixed in 70% ethanol for analysis at a later time.
Screen of cell proliferation regulatory gene expression in PV ECFCs Highly purified day-8 ECFCs were generated from the blood BFU-E of 6 patients with PV and 6 healthy subjects and RNA was extracted. Twenty microgram samples were analyzed for the presence of messenger RNA (mRNA) transcripts using the hCC-2 human cell-cycle Multi-Probe Template Set. This contains templates that can be used for T7 polymerase-directed synthesis of high specific activity [32P]-labeled antisense RNA probes, which can hybridize with target human mRNAs for p130, Rb, p107, p53, p57, p27, p21, p19, p18, p16 exon 2, p14/15, and the housekeeping genes L32 and GAPDH as internal controls. RPAs were performed, and protected transcripts were separated on denaturing polyacrylamide gels and quantified by autoradiography (Figure 1). The unprotected template set probes are shown on the left for comparison with the RNase-protected probes following hybridization with normal and PV total RNAs. All samples from the 6 patients with PV showed a well demarcated band with a strikingly greater density than that of the healthy controls. However, because the set contained multiple probes, it was difficult to be sure which specific gene was expressed very highly in these patients, so we next obtained all of the above individual probes from Pharmingen for further studies.
p16 exon 2 gene expression is much greater in PV ECFs Eleven individual gene probes, including all of those of the above set with internal controls L32 and GAPDH, were used for separate RPAs to measure mRNA expression in PV day-8 ECFCs and normal day-8 ECFCs. No differences were evident with most of the above individual gene probes between normal and PV samples, but the p16 mRNA transcript was significantly enhanced in all 11 studied patients with PV compared with 9 healthy donors. Figure 2 (top panel) shows the results of RPAs for p16 exon 2 mRNA expression in ECFCs from 4 healthy donors and 4 patients with PV. Table 3 summarizes these results and the data show that p16 exon 2 transcripts from the 11 patients with PV were quantitatively increased by 2.8- to 15.9-fold as measured by densitometry. We also measured p16 exon 2 mRNA expression in day-10 ECFCs from 3 healthy patients and 4 patients with PV to determine if this increase was limited to a defined stage of erythroid maturation, and we found that no significant difference in p16 mRNA expression was evident, either in normal or PV ECFCs at this time compared with day-8 cells (data not shown).
p27 gene expression is also increased in PV ECFCs, but is less than p16 expression The amount of p27 mRNA was delineated in the same 4 normal and 4 PV samples by RPA and the results are shown in Figure 2 (bottom panel). The percentage increases of the p27 gene transcripts in comparison with the p16 exon 2 transcripts, in the same healthy and patient samples, are listed in Table 4. p27 mRNA expression also was enhanced in PV ECFCs, but the degree of increase was much lower than that seen with the p16 exon 2 mRNA.
Lack of enhanced p16 exon 2 gene expression in patients with SP Because p16 mRNA expression is greater than p27 gene expression, we focused our study on p16. PV is a clonal disease with increased red cell production and the increased p16 expression could simply be a manifestation of increased erythropoiesis. SP manifests increased erythropoiesis due to increased EPO production and we obtained ECFCs from the 6 patients with SP listed in Table 1. Figure 3 shows a comparison of the amount of the p16 exon 2 transcript from 3 normal, 3 PV, and 3 SP ECFCs and it demonstrates that no increase was present in SP compared with PV. Table 5 summarizes 5 experiments comparing the density of the p16 exon 2 transcripts in normal and SP ECFCs and it shows that no difference was present, indicating that the abnormality of p16 expression in PV does not appear to be related to increased erythropoiesis per se.
Lack of enhanced p16 exon 2 gene expression in activated T cells of patients with PV To determine if the increased p16 expression was a general manifestation of PV, or was restricted to the myeloproliferative clone, we analyzed activated PV T cells because they are not involved in the clonal PV disease. Blood T cells from healthy, PV, and SP donors were isolated and stimulated with phytohemagglutinin followed by IL-2 in order to provide a state of cell proliferation similar to that of the ECFCs. These activated T cells were then analyzed for p16 mRNA expression. We compared the p16 mRNA levels from activated T cells and day-8 ECFCs of the same patient with PV as well as from activated T cells of a second patient with PV and an SP and healthy donor (Figure 4). Activated PV T cells did not demonstrate enhanced expression of p16 exon 2 mRNA. Thus the PV abnormality does not reside in noninvolved tissues, but instead appears to be related to the clonal hematopoietic disease.
RT-PCR demonstrates that both the p16INK4a and p14ARF transcripts are enhanced in PV ECFCs as well as PV myeloid cells p16 is encoded in the INK4a/ARF locus at chromosome 9p21 and described as p16INK4a.26,28 A second transcript, p14ARF, has been identified at the same locus due to an unprecedented use of an alternative reading frame (ARF) and it also provides potent regulation of cell proliferation.26,28,38,43 The 2 transcripts ( for INK4a
and for ARF) arise from different promoters and have different
first exons. They share exons 2 and 3 but, because of distinct reading
frames in exon 2, no amino acid identity exists between the 2 products.26,38 Because the probe for the p16 transcript
provided by Pharmingen was generated from exon 2 and because
p16INK4a and p14ARF share this exon 2 sequence,
the protected fragment that we detected could be a part of either one.
To clarify this, we performed an analysis of p16INK4a and
p14ARF mRNA expression by RT-PCR. Ten normal and 6 PV
samples were studied. No amplification of p16INK4a was seen
in any of the normal samples, but was clearly present in all PV samples
except the sample from patient 4 (Figure
5). The expression of p14ARF
mRNA was equally strong in both the normal and PV ECFCs and was much
greater than p16INK4a expression in both the normal and PV
cells. However, because the amplified fragments were very long and the
mRNA was GC rich, p16INK4a expression could not be
confirmed in the normal ECFCs. Also, because the amplified
p14ARF product was very strong in all samples, it needed to
be quantified. Therefore, we designed another pair of primers for
p16INK4a and p14ARF amplification with specific
forward primers, but only a single reverse primer, for each, because
both p16INK4a and p14ARF share the same exon 2 sequence. The length of the RT-PCR product for p16INK4a was
142 bp and covered almost the entire exon 1 and part of exon 2, whereas the length of p14ARF was 212 bp and covered almost
the entire exon 1 and a part of exon 2. The result of a
semiquantitative assay from a normal ECFC extract and a PV ECFC extract
is shown in Figure 6 and demonstrates that both p16INK4a and p14ARF had enhanced
expression in PV ECFCs compared with the normal control cells. This
same experiment was repeated with 3 different normal and PV collections
of ECFCs and showed the same results. Similar results were evident when
purified CFU-GM were cultured for 7 days and analysis was performed in
the same way (Figure 7).
INK4a and ARF mutation analysis Because a malfunction of p16 could lead to abnormal feedback and increased expression, each of the 3 exons 1, 2, and 3 of INK4a and
the alternatively spliced exon 1 of ARF were amplified by PCR from
genomic DNA of healthy patients and patients with PV using primers
complementary to sequences flanking each exon. Sequence analysis for
each exon was performed, but no mutation was identified in 10 patients
with PV.
Quantitation of INK4a protein (p16INK4a) in PV ECFCs Western blot analysis of p16INK4a was performed on normal and PV ECFC protein. p16INK4a protein was not detectable in 6 normal ECFCs, but was detectable and increased in 4 of 6 PV ECFCs (Figure 8). No stable, reliable results were obtained with the only antihuman ARF protein antibody that was available to us.
Normal and PV ECFC cell-cycle analysis Cell-cycle analyses on normal and PV day-8 ECFCs were performed. In 5 normal samples the percentage of ECFCs in the G0/G1 stage was 42% ± 12%, S phase was 49% ± 9%, and the G2/M phase was 9% ± 4%. In 4 PV ECFC collections the percentage of cells in the G0/G1 phase was 41% ± 5%, S phase was 53% ± 6%, and the G2/M phase was 6% ± 3%. The percentages of PV and normal cells in the S/G2/M phases were not significantly different. The cell-cycle studies also were carried out on 3 normal and 3 PV ECFC samples in different stages of maturation: day-6, day-8, day-10, day-13, and day-19 cells. The average percentage of S/G2/M stage cells on day 6, day 8, day 10, day 13, and day 19 from healthy individuals was 48%, 67%, 50%, 39%, and 9%, respectively, whereas in PV ECFCs it was 54%, 64%, 53%, 42% and 11%, respectively, which were not statistically significant changes.
The INK4a/ARF locus on chromosome 9 is one of the sites mutated
most frequently in human cancer.44 We have demonstrated in
11 different PV ECFCs, compared with normal ECFCs, a marked increase in
the expression of p16INK4a, a unique inhibitor of cell
proliferation and a tumor suppressor. The mRNA also was increased in
highly purified PV myeloid cells, but enhanced expression was not
present in PV T cells and also was not present in SP ECFCs. Thus the
abnormal expression appears to be limited to cells of the
myeloproliferative clone of PV, and it is not due to a mutation of the
structural components of the gene, which might have decreased the
gene's function, as sequence analysis of exons 1 In the normal cell cycle, control of progression from G1 to S phase is executed predominantly by the gatekeeper Rb protein. In G1, hypophosphorylated Rb binds to the E2F transcription factor, which results in transcriptional repression of E2F-responsive genes and blocks G1/S progression.25,26 In response to growth-promoting signals, a complex of cyclin D and CDK4/6 induces Rb phosphorylation with release of Rb from E2F allowing E2F to transactivate many genes important for mitosis. p16INK4a inhibits the kinase activity of CDK4/6 by changing the conformation of the cyclin-binding site, preventing ATP binding, and thereby producing hypophosphorylation of Rb, which in turn decreases the expression of the E2F-dependent genes.26 Thus p16INK4a is able to block passage from G1 into S and is a bona fide tumor suppressor. Ectopic expression of p16INK4a in human cells induces senescence, which can occur independently of p53.26 Although p14ARF also inhibits the cell cycle, it is without
direct inhibition of known CDKs and its effect is p53-dependent as demonstrated by gene knockouts in mice.26,38
p14ARF binds to the C-terminal region of MDM2 through its
exon 1 Our results indicate that unlike several malignant diseases, PV has no mutation of p16INK4a or p14ARF. In contrast, both genes are expressed in an abnormally high amount in PV ECFCs and this is quite paradoxical, as PV is a disease with increased proliferation. We believe that the large increase in p16INK4a expression in a proliferative disease is most likely due to an abnormality of a downstream protein with which this protein interacts, thereby creating a cellular reaction or feedback for an elevated p16INK4a concentration, rather than an intrinsic abnormality of the p16INK4a promoter, as a primary defect of the latter would not account for the increased cell generation in the disease. Because p16INK4a inhibits cell generation by binding CDK4/6, and because our results demonstrate a p16INK4a increase in PV, it is possible that a mutation of CDK4/6 is present in PV which decreases its binding with p16INK4a and may lead to p16INK4a accumulation. A mutation in familial melanoma, arginine-to-cysteine exchange at residue 24, has been reported, which has a specific effect on the p16INK4a binding domain of CDK4/6, but has no effect on its ability to bind cyclin D and form a functional kinase.48,49 This R24C mutation (single-letter amino acid code) of CDK4 generates an oncogene that is resistant to normal physiologic inhibition by p16INK4a. Therefore, analysis of the structure and function of CDK4/6 may be helpful in revealing the mechanism for the increased cell generation in PV. Other alternatives for an abnormal elevation of p16INK4a mRNA in PV also exist. Li et al50 reported that p16INK4a mRNA accumulates in high levels in cells lacking Rb function and that the transcription of p16INK4a is repressed by Rb. Stott et al51 measured the expression of p16INK4a and p14ARF relative to pRb and p53 in 18 tumor cell lines and primary fibroblast strains. All 8 cell lines, which lacked Rb, had a high expression of p16INK4a. An inverse correlation of p14ARF expression and p53 status in human cell lines was also demonstrated. Thus it is possible that Rb has a mutation producing a loss of function in PV ECFCs so that less binding of E2F to Rb is present, which would result in the activation and expression of a constituency of responder genes that increase cell generation either through increased proliferation or reduced apoptosis. These genes are known to encode products necessary for S-phase progression. Because more cells would enter S phase, the molecular mechanisms controlling cell proliferation might then be activated to increase the CDK inhibitors such as p16INK4a, p27, and p14ARF. Further studies are planned along these lines which we hope will enhance our understanding of the pathogenesis of PV as well as the control of normal erythroid proliferation and differentiation.
The authors thank Maurice Bondurant for his discussions relating to this paper which were very helpful; Hong Ao and Judy Luna, for their technical assistance; and Mary Jane Rich for her expert preparation of the manuscript.
Submitted September 1, 2000; accepted January 26, 2001.
Supported by a Veterans Health Administration Merit Review Grant (to S.B.K.), National Institutes of Health (NIH) grants ROI DK-15555 and 2 T32-DK-07186 (to S.B.K.) and CA-68485 (to Vanderbilt-Ingram Cancer Center).
The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked "advertisement" in accordance with 18 U.S.C. section 1734.
Reprints: Sanford B. Krantz, Department of Medicine-Hematology/Oncology, Vanderbilt University, 547 MRB II, 2220 Pierce Ave, Nashville, TN 37232-6305; e-mail: krantz.sanford_b{at}nashville.va.gov.
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Abstract
Introduction
5 M 2-mercaptoethanol; 10 µg/mL insulin; 2 U/mL EPO; 50 U/mL IL-3; 100 ng/mL SCF; and streptomycin plus penicillin
to generate ECFCs. After 7 days of culture the average purity of day-8
ECFCs was 90% or higher as measured by the plasma clot assay. In some
experiments, the BFU-E were incubated for 5 to 18 days.
