the action of phospholipase a2 ( pla2 ) on membrane phospholipids produces free fatty acids such as arachidonic acid ( aa ) and the phospholipid backbone . to
the former belongs eicosanoids ( such as prostaglandins , prostacyclin , thromboxane , and leukotrienes ) through the cyclooxygenase ( cox ) and lipoxygenase ( lox ) pathways ; and to the latter , platelet - activating factor ( paf ) ( figure 1 ) [ 1 , 2 ] .
while countless studies have highlighted the actions of eicosanoids and paf on normal human mature myeloid and lymphoid cells ( from hematopoietic progenitors to mature blood cells ) , their effects on leukemic blasts are poorly documented , and furthermore , their putative involvements during leukemic diseases remain almost speculative .
this paper focuses on new results about lipid mediators and human leukemic blast cells from acute myeloid ( aml ) and acute lymphoid ( all ) patients .
the vast majority of results reported previously have been obtained with aml blasts without maturation according to the classification system of the world health organization , thus corresponding to the past aml m0 - 2 nomenclature .
pla2 catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate the eicosanoid precursor aa ( figure 1 ) [ 3 , 4 ] . three distinct families are documented : low molecular weight soluble forms of pla2 ( spla2 ) ; ca - dependent high molecular weight pla2 ( cytoplasmic pla2 , cpla2 ) ; cytoplasmic ca - independent high molecular weight pla2 ( ipla2 ) .
in addition , the spla2 family is implicated in several biological processes such as inflammation and host defence [ 3 , 4 ] .
the cpla2 family consists of four members , with cpla2-iva being the central regulator of the stimulus - coupled cellular aa release [ 3 , 4 ] .
freshly isolated leukemic blasts from aml and all patients express mrna from four out of five cpla2 ( pla2-iva , pla2-ivb , pla2-ivc , and pla2-vi ) and six out of nine spla2 ( pla2-ib , pla2-iia , pla2-iid , pla2-v , pla2-x , and pla2-xii ) and that transcript levels exhibit wide variations as compared to control blood mononuclear cells .
one of the most notable findings is that aml and all blasts express high amounts of pla2-vi and pla2-x .
this could be extremely significant as these two enzymatic activities play a major role in aa release for the generation of cox- and lox - derived lipid mediators .
thus , aml and all blasts have the potential to express multiple isoforms of cpla2 and spla2 which could be of importance given the role of these enzymes in inflammation , generation of lipid mediators , anticoagulant activity , and bacterial infection .
biological activities of pla2 are attributed to their enzymatic capacity to hydrolyze membrane phospholipids . however , in addition spla2 exerts various biological proinflammatory responses through the binding to the cell surface pla2 receptor ( pla2-r ) .
of interest is the functional membrane pla2-r found on aml and all blasts strengthening a role for pla2 signalling in these cells ( denizot and coll .
the concept of anti - inflammation is currently evolving with the discovery of endogenous inhibitory circuits , such as the annexin ( anx ) system , that are important in the control of the host inflammatory response .
anx-1 ( also termed lipocortin ) is a well - known cpla2 inhibitory protein produced by and acting on several blood cell types such as monocytes / macrophages and polymorphonuclear leukocytes .
the anx-1 protein level is markedly elevated in aml blasts , where anx-1 is not only considered as an anti - inflammatory and tumor suppressor molecule ( through its inhibiting cpla2 activity ) but also as one of the  eat - me  signals on apoptotic cells to be recognised and ingested by phagocytes .
it is , thus , tempting to speculate that pla2-r and anx-1 might take an important place in the  yin  and the  yang  of the inflammatory reaction occurring in aml blasts . during the past decade
, considerable research has been directed towards the identification of new biological targets for aml treatment .
it is tempting to suggest that pla2-r antagonists might be one of them especially with respect to the emerging roles for pla2 enzymes in cancer .
in the cox pathway , aa is converted to pgh2 by cox-1 or cox-2 enzymes .
pgh2 is subsequently metabolised to generate different prostanoids , depending on the enzymes expressed in the cell .
cox-1 and cox-2 transcripts are documented in aml and all blasts , but only the cox-1 protein is found .
similarly cox-1 , but not the cox-2 protein , is detected in human primary promyelocytic blasts during differentiation .
in fact , the aml and all blasts can express the cox-2 protein in response to lipopolysaccharide ( lps ) but only in the subsets of patients .
the ability of all blasts to express cox-2 is consistent with its presence in stimulated normal b - cells and in chronic lymphocytic leukaemia ( cll ) b - cells [ 14 , 15 ] .
the production of cox-2 in response to lps by aml blasts is consistent with data reporting that lps is a potent inductor of cox-2 in mature monocytes / macrophages   and that stimulated hl-60 cells ( an aml cell line with an m2/3 subtype ) express cox-2 .
the heterogeneity in the lps - stimulated cox-2 expression by aml blasts is not linked to a different toll - like receptor ( tlr2 or tlr4 ) expression   and remains an open question that requires further evaluation .
following the action of the cox pathway , pgh2 is subsequently metabolized to generate different prostanoids , depending on the enzymes expressed in the cell .
prostanoids include prostacyclin ( pgi2 ) , thromboxane a2 ( txa2 ) , and prostaglandin e2 ( pge2 ) , synthesized by a pgi2 synthase , a txa synthase , and a pge synthase , respectively .
three pge synthase isoforms exist : inducible membrane - bound pge synthase-1 ( mpges-1 ) , constitutive membrane - bound pge synthase-2 , and cytosolic pge synthase .
in addition , the ability of pge2 to regulate the immune system has been widely explored .
data reporting the ability of pge2 to modulate several functions in mature blood cells such as monocyte - macrophages , dendritic cells ,   and t and b lymphocytes can be readily found .
human aml and all blasts spontaneously release pge2 , with pge2   synthesis being inhibited by indomethacin .
transcripts for mpges-1 are detected in aml and all blasts suggesting its role in pge2 synthesis ( denizot and coll . , unpublished results ) .
pge2 effects are well known and are mediated through interactions with four distinct membrane - bound g - protein - coupled ep receptors : ep1 , ep2 , ep3 ,   and ep4 . ep2 and ep4 are coupled to gs and stimulate camp production which leads to gene regulation .
ep1 is coupled to gq / p , and ligand binding induces intracellular calcium level variations .
functional ep2 receptors are present on aml and all blasts [ 19 , 20 ] .
in contrast to ep2 receptors , no functional ep1 , ep3 ,   and ep4 receptors are found . in view of the potentially important role of pge2 in processes of cancer and leukocyte maturation and function ,
pge2 enhances the spontaneous and lps - stimulated growth of aml blasts without affecting their apoptosis . in summary , aml and
all blasts secrete pge2 . a role for pge2 as a compound contributing to aml cell proliferation ( via an ep2 receptor - mediated pathway ) can be hypothesized .
txa2 is produced abundantly by platelets upon exposure to injured blood vessels and thus exhibits potent platelet - aggregating and vessel - contracting activities .
pgi2 is the major cox - derived product of aa formed in the macrovascular endothelium and is a potent inhibitor of platelet aggregation activity and vessel vasodilatation activity .
aml and all blasts express low levels of tx synthase transcripts compared to normal blood mononuclear cells ( denizot and coll .
, unpublished results ) and additionally produce very low amounts of txa2 in response to a calcium ionophore stimulation .
hl-60 cells have also been shown to release txa2 , but only after induction of differentiation [ 23 , 24 ] .
pgi synthase transcripts are absent in aml and all blasts , a result similar to that found in control blood mononuclear cells ( denizot et coll .
, unpublished results ) . in accordance with the absence of pgi transcripts in aml and all blasts , calcium ionophore - stimulated blasts do not release pgi2 ( denizot et coll .
txa2 and pgi2 act through membrane receptors ( namely txa2r and ip for txa2 and pgi2 , resp . )
[ 25 , 26 ] . as to whether aml and all blasts release txa2 and pgi2
, they express levels of transcripts for txa2r and ip equal or higher than those found in control blood mononuclear cells .
txa2r and ip receptors belong to the class of gs - protein - coupled receptors [ 25 , 26 ] .  
stimulation of leukemic blasts with u-46619 ,   the txa2 receptor agonist u-46619 , and pgi2 stimulate in a dose - dependant manner camp synthesis from leukemic blasts showing the presence of functional txa2r and ip receptors , respectively . however , simulation of leukemic blast with u-46619 and pgi2 has no effect on their growth and apoptosis rate . at the present time
the physiological meaning of functional txa2r and ip receptors on leukemic blasts remains an open question . in conclusion , among the various cox - derived metabolites of aa only pge2 has , thus , a significant effect on the growth of aml blast cells , and none of them affect their apoptosis rate .
the lox pathway involves the conversion of aa to 5- , 12- , or 15-hydroperoxyeicosatetraenoic acids ( hpete ) by 5- , 12- , or 15-lox , respectively , hpetes being rapidly metabolized to 5- , 12 , or 15-hydroxyeicosatetraenoic acids ( hete ) .
5-hpete could be dehydrated into leukotriene a4 ( lta4 ) , which was enzymatically hydrolyzed to ltb4 ( figure 1 ) .
the ability of ltb4 , 12-hete , and 15-hete to regulate important functions of the immune system has been widely explored .
these compounds activate various blood cell types and stimulate their proinflammatory cytokine productions [ 2830 ] , indicating an ability of ltb4 , 12-hete , and 15-hete to augment and prolong tissue inflammation .
leukemic blasts express 5-lox , 12-lox , and 15-lox transcripts , their expression being in general lower than in blood mononuclear cells from a healthy donor [ 22 , 31 , 32 ] .
leukemic blasts produce in vitro lower amounts of ltb4 than healthy donors [ 22 , 31 , 33 ] . this reduced capacity of aml blasts to produce ltb4
the various lox - derived metabolites of aa regulate a wide spectrum of cellular processes including cell proliferation and apoptosis .
however , ltb4 , 12-hete , and 15 hete have no effect on the growth and apoptosis rate of aml and all blasts in vitro . as to whether receptors for 12-hete and 15-hete remain to be molecularly identified ,
two g - protein - coupled seven transmembrane domain receptors for ltb4 were identified : blt1 and blt2 .
amounts of blt1 transcripts are similar in aml and all blasts as well as control blood mononuclear cells , while amounts of blt2 transcripts are markedly higher . at this time
the physiological meaning ( if any ) of blt1 and blt2 transcripts in aml and all blasts remains an open question .
a similar question exists for the significance of ltb4- and 12-hete - derived leukemic blasts .
one might suggest that these compounds could initiate , augment , and prolong tissue inflammation and damages by affecting the cytokine network , but currently no studies have provided evidences in support of this .
paf is a phospholipid mediator that sparks off a wide range of immunoregulatory activities on blood cells such as polymorphonuclear neutrophils , eosinophils , monocytes , macrophages , and lymphocytes .
paf is released in vitro from several leukemic cell lines of b and t origin   as well as from freshly isolated neoplastic cells of leukemic patients .
however , in spite of experimental evidence reporting its in vitro release from leukemic cells , no clinical studies provide evidences to support this view in vivo .
in contrast , decreased levels of paf are found in the blood of patients with lymphoid and nonlymphoid hematologic malignancies .
blood paf levels are regulated by an acetylhydrolase activity ( aha , also named pla2-viia ) found in serum and plasma .
plasma aha is not altered in leukemic patients   suggesting a lowered paf production by leukemic cells rather than an increased paf catabolism .
paf acts through membrane and nuclear paf receptors ( pafr ) that belong to the g - protein - coupled family [ 40 , 41 ] . as to whether membrane pafr is found on aml and all cells [ 42 , 43 ]
studies report that mature monocytes , macrophages , polymorphonuclear leukocytes , and b lymphocytes produce camp in response to paf [ 44 , 45 ] .
the gq proteins mediate their effects by activating phospholipase c and thus , generating second messengers , inositol-1,4,5-triphosphate ( ip3 ) and diacylglycerol , thereby leading to the activation of protein kinase c and the mobilisation of intracellular calcium .
paf stimulates in a receptor - dependent process ca flux from aml and all blasts showing the presence of functional pafr   and highlighting that pafr signals via the gq instead of the gi / gs protein pathways .
paf has no significant effect on growth and apoptosis rate in these cells   suggesting that paf is not an important modulator of blast cell physiology .
the lack of paf effect is linked to low levels of pafr in aml and all blasts compared to those found in mature leukocytes .
further , strengthening this issue , the differentiation of hl-60 cells towards the macrophage phenotype is associated with the induction of pafr gene expression .
thus , pafr mrna accumulation is correlated to the induction and development of specific paf responsiveness .
recently web-2170 , a pafr antagonist , has been reported to induce apoptosis in aml cells [ 51 , 52 ] .
in fact , web-2170 does not behave as a pure pafr but instead as an inverse agonist leading to a marked cytoplasmic increase of pten proteins ( pten is a protein / phosphoinositide phosphatase regulating the pi3k / akt signaling pathway ) .
consequently , these recent results [ 49 , 51 , 52 ] support the view that paf has probably no significant role in the growth and apoptosis of leukemic blasts .
data reporting our knowledge concerning the enzymatic activities ( such as pla2 , lox , cox ) implicated in lipid mediator synthesis and their receptors on aml and all blasts are schematised in figure 2
. aberrant expression of several pla2 enzymes is common place in tumors derived from many different organ sites .
numerous studies report that altered aa metabolism in a solid tumor microenvironment has a profound impact on the pathogenesis of tumor development .
a multitude of biological activities of paf are evidenced both on the normal cell   as well as on their cancer counterpart .
there is evidence , however , that it is not the case for leukemic blast cells . among the various pro - inflammatory lipid molecules so far tested ( paf , pge2 , pgi2 , txa2 , ltb4 , 12-hete , 15-hete ) , none of them exhibit any role on leukemic blast apoptosis despite the expression of functional receptors ( pafr , ep2 , ip , txa2r ) . among the various compounds
so far tested only pge2 clearly demonstrated a potential role in aml cell growth in vitro . however , it is difficult to compare the m amounts of pge2 used in most of the in vitro studies with the fm amounts of pge2 found in the blood at steady state conditions .
studies showing the effects of continuous addition or infusion of low doses of pge2 ( which seems to be a more relevant protocol of stimulation to obtain information for the in vivo effects of pge2 ) are extremely rare .
, there is absolutely no evidence that pge2 is implicated in the growth of aml blasts in vivo .
thus , in conclusion the biological effects of eicosanoid and paf are particularly important in immunity and inflammation .
though their roles are well known in numerous pathology and cancers , no such role is currently known for leukemic blast growth .
 some of the most potent inflammatory mediators share a lipid origin . 
 they regulate a wide spectrum of cellular processes including cell proliferation and apoptosis . however , 
 the precise roles and ways ( if any ) in which these compounds impact the growth and apoptosis of leukemic blasts remain incompletely resolved . in spite of this , significant advances have been recently made . 
 here we briefly review the current knowledge about the production of lipid mediators ( prostaglandins , leukotrienes , platelet - activating factor ) by leukemic blasts , the enzymatic activities ( phospholipase a2 , cyclooxygenases , lipoxygenases ) involved in their productions and their effects ( through specific membrane bound receptors ) on the growth , and apoptosis of leukemic blasts . 
1. Introduction
2. PLA
3. COX and Human Leukemic Blasts
4. PGE
5. TXA
6. LOX and Human Leukemic Blasts
7. PAF, PAFR, and Human Leukemic Blasts
8. Conclusion