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Signal Transduction Unit/Laboratory of Cell Biology, Section of Human Anatomy, Department of Morphology and Embryology, University of Ferrara, Ferrara 44100 [V. B., L. M. N., M. M., C. M., S. C.]; and Consiglio Nazionale delle Ricerche Institute of Cytomorphology, Bologna 40100 [L. M. N.], Italy
| ABSTRACT |
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prevented the differentiative process. The data obtained indicate that PI 3-K activity plays an essential role in promoting granulocytic differentiation. | Introduction |
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, p85ß, p55 (termed
or p55PIK), and two alternative splices of p85
(p55
and p50
; Refs. 1
and 2
). All five interact with p110s, of which three distinct isoforms have been identified:
, ß, and
. p110 is responsible for the formation of phosphoinositides with a phosphate group at the D-3 position of the inositol ring (1)
. The 3-phosphoinositides are not substrates of any known PLC, are not components of the canonical phosphoinositide turnover pathway, and may themselves act as intracellular mediators rather than precursors of inositol phosphate second messengers (1)
. Increased levels of PIP3 have been detected during diverse cellular processes, including mitogenesis, indicating that PI 3-K plays a role in cell growth and transformation (1 , 3) . However, PI 3-K is activated also by factors that trigger cellular differentiation, such as nerve growth factor in PC12 cells (4) and DMSO in murine erythroleukemia (MEL) cells (5) , suggesting that PI 3-K and its lipid products may also play a role in cell maturation.
PI 3-K is present in the HL-60 promyelocytic cell line (6) , a well-established model for the study of granulocytic differentiation when the cells are treated with retinoids, which act through a nuclear receptor belonging to the transcription factor family (7) . To determine whether PI 3-K plays a role in HL-60 cell differentiation, we monitored the amount and the activity of the enzyme in whole cells during the differentiation triggered by ATRA. Here, we demonstrate that PI 3-K increases after ATRA treatment and that specific inhibition of PI 3-K activity and down-modulation of the protein prevent the differentiation of this cell line.
| Materials and Methods |
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The experiments of inhibition of PI 3-K activity were performed as follows: the cells were pretreated with 100 nM WT for 24 h and then cultured in the presence of ATRA plus WT for the indicated times. Because WT is unstable at 37°C in culture medium, it was added every 6 h to the cell culture. As controls, cells were incubated in the presence of only WT for the same times.
Preparation of Nuclei and Immunochemical Analysis.
Membrane-depleted nuclei were isolated as described previously (8)
. Nuclear purity was assessed by ultrastructural analysis and marker enzyme assays, as reported previously (8
, 9)
.
Proteins from the purified fractions (50 µg) were separated and blotted as described previously (8) . Polyclonal antibody against the p85 subunit of PI 3-K was purchased from Upstate Biotechnology, Inc. (Lake Placid, NY).
PI-3 K Activity Assay.
The assays of PI 3-K activity were performed on 100 µg of protein of total cells in the presence of 10 mM HEPES, 1 mM EDTA (pH 7.5), and[
-32P]ATP (10 µCi/sample). Incorporation was for 15 min at room temperature, and the reaction was stopped by the addition of chloroform, methanol, and HCl (200:100:0.75, v/v/v), followed by two washes with chloroform, methanol, and 0.6 N HCl (3:48:47, v/v/v).
The lipid-containing organic phase was resolved on oxalate-coated TLC plates (Silica Gel 60; Merck) developed in isopropanol:acetic acid:H2O (65:1:34, v/v/v).
The radiolabeled products were identified by comparison with standard lipids obtained after incubation of purified PI 3-K enzyme (generous gift of S. Volinia, Department of Morphology and Embryology, University of Ferrara) with phosphatidylinositol, phosphatidylinositol 4-phosphate, and phosphatidylinositol 4,5-bisphospate, respectively, in conditions that optimize PI 3-K activity.
Immunofluorescence Staining.
Cells in PBS were plated onto 0.1% poly-L-lysine-coated glass slides, fixed, and processed as previously described (10)
. Samples were reacted with either a polyclonal (dilution, 1:80) or monoclonal (1:40) antibody directed against the Mr 85,000 subunit of PI 3-K (Upstate Biotechnology, Inc.) with identical results. Unless otherwise stated, photographs shown were obtained with monoclonal antibodies. Confocal laser scanning microscopy and image processing analysis were performed as described previously (10)
.
Transfection Experiments.
In the transfection experiments, we used the plasmid pcDNA3-p85
(11)
, containing a fragment of the bovine p85
cDNA (nucleotides 35408) in sense or antisense orientation.
Exponentially growing HL-60 cells were transfected by Lipofectin carrier (Life Technologies, Inc. Paisley, United Kingdom). Briefly, 10 µg of specific plasmid DNA were mixed with 0.5 µg of pCH110 DNA (Pharmacia, Uppsala, Sweden), a plasmid DNA that constitutively expresses the ß-galactosidase gene as control of transfection efficiency, and incubated in RPMI 1640 at room temperature for 10 min in the presence of Lipofectin reagent. A total of 2 x 106 cells were collected by centrifugation, washed three times in PBS, resuspended in RPMI 1640, and mixed to the Lipofectin-DNA mixture. After 8 h of incubation at 37°C, the cells were collected by centrifugation, washed three times in PBS, resuspended in RPMI 1640 plus 10% fetal bovine serum, and grown for 48 h. After this period, the number of ß-galactosidase-positive cells was counted to determine the efficiency of transfection, which ranged between 15 and 20%. Cells were then treated or not with 1 µM ATRA for 4 days. Daily, 100,000 cells were collected and cytocentrifuged onto a glass slide to determine the differentiation rates.
Statistical Analysis.
The results were expressed as means+ SD of three or more experiments performed in duplicate. The two-tailed Students t test was used for statistical analysis.
| Results |
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| Discussion |
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Here, we provide biochemical and morphological evidence for a specific involvement of PI 3-K activity in HL-60 granulocytic differentiation. PI 3-K is relevant to this process because reduction of PI 3-K activity by WT prevented the effects of ATRA. Accordingly, the decrease of protein expression obtained by antisense transfection inhibited granulocytic differentiation.
Although increased PI 3-K activity was originally associated with activation of receptor tyrosine kinases, it also occurs in response to activation of receptors lacking intrinsic tyrosine kinase activity, such as interleukin 2, interleukin 3, granulocyte macrophage colony-stimulating factor, and T-cell receptors (reviewed in Ref. 3 ). Our data provide the first evidence that also an agonist with a nuclear receptor induces PI 3-K activity, which promotes differentiation.
The action of PI 3-K in ATRA-driven differentiation is conceivably mediated by the downstream targets of PI 3-K products, which include Akt (PKB), p70S6 kinase, cytoskeletal proteins (1)
, and PKC
(15)
. The link between PI 3-K and PKC has been recently elucidated and involves a PDK1 phosphorylation of the activation loop sites of PKC
, and PKC
in a PI 3-K-dependent manner (16)
. It has been recently suggested that PIP3 activates PLC
, revealing a novel mechanism for mutual regulation between these two enzymes that participate in the control of phosphoinositide metabolism (17
, 18)
. Recent data also showed a cross-communication between nuclear PLC and PI 3-K pathways (19)
.
The ATRA-dependent differentiation process of HL-60 induced changes not only in the cytoplasm but also in the nuclear compartment. This observation is related to our previous findings describing the nuclear translocation of PLC ß and
and of specific PKC
and
during granulocytic differentiation of HL-60 cells (8
, 20)
. On the other hand, nuclear PI 3-K has been described in cells other than HL-60, such as PC12 (21)
, Saos-2 (22)
, and rat liver (19)
cells, suggesting that translocation of this enzyme to the nucleus is a widespread occurrence.
It is worthwhile to note that different inositol lipid-modifying enzymes are recruited during granulocytic differentiation, and in particular, PI 3-K may play a relevant role in this process, as demonstrated here by inhibition and down-modulation experiments. It can be informative to explore in future if an additional modulating pathway might rely on cross-talk between different enzymes.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This work was supported by Italian Consiglio Nazionale delle Ricerche (Progetto Finalizzato Biotecnologie), Associazione Italiana Ricerca sul Cancro, and University of Ferrara (60% funding). ![]()
2 To whom requests for reprints should be addressed, at Signal Transduction Unit, Department of Morphology and Embryology, Section of Human Anatomy, University of Ferrara, Via Fossato di Mortara 66, 44100 Ferrara, Italy. Phone: 39-532-291571; Fax: 39-532-207351; E-mail: cps{at}dns.unife.it ![]()
3 The abbreviations used are: PI 3-K, phosphoinositide 3-kinase; p110, catalytic Mr 110,000 protein subunit; PLC, phospholipase C; PIP3, phosphatidylinositol 3,4,5-trisphosphate; ATRA, all-trans-retinoic acid; WT, wortmannin; PKC, protein kinase C. ![]()
Received 3/27/98. Accepted 12/16/98.
| REFERENCES |
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gene. Genomics, 37: 113-121, 1996.[Medline]
and phosphatidylinositol 3-kinase and promotion of macrophage differentiation by insulin-like growth factor-I. J. Immunol., 160: 1393-1401, 1998.
by phosphatidylinositol 3,4,5-trisphosphate. J. Biol. Chem., 273: 4465-4469, 1998.
by PI 3-kinase-induced PH domain-mediated membrane targeting. EMBO J., 17: 414-422, 1998.[Medline]
and
isoforms in HL-60 cells induced to differentiate along the granulocytic lineage by all-trans retinoic acid. Br. J. Haematol., 93: 542-550, 1996.[Medline]
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