Page 610 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 39 Megaloblastic Anemias 519

the enterocyte to 5-methyl-tetrahydrofolate. Folate production by transcriptional, translational, and posttranslational mechanisms. 54–59
bacteria in the small intestine can also enter the circulation. 36 Poised in this location facing the external milieu of cells, upregulated
The flux of folate from the basolateral membrane of the enterocyte folate receptor-α can bind all available folate and thereby help to
34
to the portal blood is mediated through MRP3. MRP proteins have restore cellular folate homeostasis.
low affinity but high capacity and are best visualized as cellular “sump The answer to the more fundamental question—how do cells
pumps” that eject excess folates (and antifolates) out of cells. Together sense the existence of folate deficiency in the first place so that
37
with MRP2, which mediates folate transport into the bile, these folate homeostasis can be subsequently restored by upregulating
59
MRPs maintain an efficient enterohepatic circulation, which helps folate receptors?—has finally been discovered. It so happens that
retain folate. 34 the accumulation of intracellular homocysteine during cellular folate
Some bacterially produced folate, especially those produced by deficiency leads to the covalent binding (by homocysteine) of a protein
probiotic bacteria (genus Bifidobacterium), can be absorbed across the known as heterogeneous nuclear ribonucleoprotein-E1 (hnRNP-E1),
39
38
large intestine, sufficient to raise the serum folate levels, but which is already known to mediate the translational upregulation of
normally this accounts for no more than about 5% of the average folate receptor-α. 56,60 Homocysteinylation of hnRNP-E1 at specific
folate requirement. cysteine–cysteine disulfide bonds leads to the unmasking of an
Passive diffusion of folic acid (pteroylmonoglutamate [PteGlu]) is underlying messenger RNA (mRNA)-binding pocket for which folate
probably the primary mechanism of intestinal mucosal folate absorp- receptor-α mRNA has a high affinity. This RNA-protein interaction
15
tion at high pharmacologic concentrations. The small intestine has then triggers the biosynthesis of folate receptors, which soon results in
a large capacity to absorb folate, with peak folate levels in plasma a net increase of cell surface folate receptors that are able to bind more
achieved 1 to 2 hours after oral administration. available folate and thereby normalize cellular folate levels. In this
context, hnRNP-E1 fulfills criteria as a cellular sensor of physiologic
folate deficiency (Fig. 39.6) because this protein is able to sense
Plasma Transport and Enterohepatic Circulation folate deficiency (by interacting with homocysteine) and respond by
increasing RNA-protein interaction that triggers the biosynthesis and
The normal serum folate level is maintained by dietary folate and a upregulation of folate receptors.
substantial enterohepatic circulation that amounts to about 90 µg/ The broader significance of this mechanism is that homocysteinyl-
15
day of folate. Biliary drainage results in a dramatic fall in serum ated hnRNP-E1 actually orchestrates a nutrition-sensitive posttran-
folate (to about 30% of basal levels in 6 hours), whereas abrupt scriptional RNA operon during folate deficiency. Thus during folate
interruption of dietary folate leads to a fall in serum folate levels in deficiency, the mRNA-binding pocket within homocysteinylated-
about 3 weeks. In the plasma, one-third of the folate is free, two- hnRNP-E1 is actually highly promiscuous, in that it allows the
thirds are nonspecifically bound to serum proteins, and a small binding of a variety of very diverse mRNAs (perhaps over 100),
fraction binds soluble folate receptors. However, in contrast to all of which have a common password composed of short RNA
cobalamin uptake, there is no specific serum transport protein that sequences; these RNA-protein interactions can, in turn, trigger the
enhances cellular folate uptake. modulation up or down of a variety of several otherwise entirely
unrelated proteins that contribute to the biologic features of
Cellular Folate Uptake reduced cell proliferation, differentiation, and apoptosis, which are
59
a hallmark of folate deficiency. In the nucleus, folate receptors
also function as a transcription factor by binding to cis-regulatory
Folate Receptors elements at promoter regions of Fgfr4 and Hes1 to regulate their
expression. 62
Plasma 5-methyl-tetrahydrofolate and folic acid are rapidly trans-
ported into proliferating cells by specialized, high-affinity, glycosyl-
phosphatidylinositol-anchored (membrane) folate receptor-α, which Folate Receptors and Placental Folate Transport
takes up these folates at physiologic concentrations found in serum. 40,41
The plasma membrane containing the folate–folate receptor complex Placental folate receptors 63,64 that are abundant and polarized to
then invaginates and forms an endosomal vesicle that moves into the the maternal-facing microvillous membrane of the syncytiotro-
65
42
cytoplasm along microtubules. The perinuclear endosomal compart- phoblast (but not on the basement membrane) are critical to
44
ment then gets acidified to pH 6, which dissociates folate from folate transplacental maternal-to-fetal folate transport. Physiologic
receptors. The released folate then passes across the acidified endo- transplacental folate transport relies on the continued provision of
some into the cytoplasm by a transendosomal pH gradient, which is adequate dietary folate intake by the mother. Following capture of
mediated by either the PCFT or related moiety 33,43 (Fig. 39.5). maternal folate by placental folate receptors, 66,67 the displacement
Folate receptor-α is critical to mediating the cellular uptake of of this pool by incoming dietary folates, results in an intervillous
folates in proliferating malignant and normal cells and in transport blood concentration that is three times that of maternal blood. This
of folate across the placenta to the fetus, 41,44 into the brain, 45–47 and allows for subsequent transfer of the folate to the fetal circulation
44
in renal conservation of folates. 43,48 Folate receptor-α is expressed in along a downhill concentration gradient and ensures continued
several types of cancer cells, whereas folate receptor-β is expressed unidirectional transplacental folate transport. Thus a suboptimum
most in monocytes and macrophages 49–51 ; hence these two forms of intake of folate by the mother can reduce maternal-to-fetal folate
folate receptors are under intense scrutiny for potential clinical use transfer and predispose the embryo/fetus to serious developmental
in detecting (and treating) occult malignancy and inflammation. 49–53 defects. 68–70
The physiologic role of the reduced-folate carrier is less clear; it is Because PCFT colocalizes with folate receptor-α, this suggests
a “low-affinity” but “high-capacity” system that can also mediate the that following binding and internalization of folate into low-pH
uptake of 5-methyl-tetrahydrofolate and pharmacologic folates (like endosomes, the folate dissociates from folate receptors and presum-
methotrexate and folinic acid well, but folic acid poorly) into a variety ably passes via PCFT into the cytoplasm. However, because both
of cells at physiologic pH. 33,43 PCFT and reduced-folate carriers are uniformly distributed in
microvillous membrane, cytoplasm, and the fetal-facing basal plasma
65
Folate Receptor Regulation and Cellular membrane, the following are all still unclear: the precise handover of
folate following endocytosis into the cytoplasm, the potential role of
Folate Homeostasis transcytosis of vesicles containing folate, the transport of folate across
the syncytiotrophoblast basement membrane into the fetal vascula-
Cell surface folate receptor-α is upregulated in response to low ture, and the role of MRPs in net transplacental folate transport to
extracellular and intracellular folate concentrations through the fetus.