Megaloblastic hematopoiesis in vitro. Interaction of anti-folate receptor antibodies with hematopoietic progenitor cells leads to a proliferative response independent of megaloblastic changes.

Abstract

We tested the hypothesis that anti-placental folate receptor (PFR) antiserum-mediated effects on hematopoietic progenitor cells in vitro of increased cell proliferation and megaloblastic morphology were independent responses. We determined that (a) purified IgG from anti-PFR antiserum reacted with purified apo-and holo-PFR and specifically immunoprecipitated a single (44-kD) iodinated moiety on cell surfaces of low density mononuclear cells (LDMNC); (b) when retained in culture dur-

Translational upregulation of folate receptors is mediated by homocysteine via RNA-heterogeneous nuclear ribonucleoprotein E1 interactions

“…Although HeLa-IU 1 -LF cells (stably propagated in low-folate media) were slightly larger than HeLa-IU 1 -HF cells (stably propagated in folate-replete media), there was no evidence for megaloblastosis by morphologic and morphometric criteria (not shown). However, although the nuclear:cytoplasmic ratio was not increased (24), these cells continued to proliferate, albeit at a slower rate, with a change in doubling time from 22 hours (basal in HeLa-IU 1 -HF cells) to 31 hours in HeLa-IU 1 -LF cells (Table 1). In addition, the basal concentration of total homocysteine in the media, a surrogate marker for intracellular folate deficiency when elevated (29), was 108 µM for HeLa-IU 1 -LF cells and 14 µM for HeLa-IU 1 -HF cells (humans with clinical folate deficiency also have increases in serum total homocysteine; refs.…”

“…Our results could have an effect in several related areas: (a) Homocysteine could also have effects in modulating RNA-protein interactions in the physiology of other proteins involved in folate, cobalamin (vitamin B 12 ), and one-carbon metabolism. Moreover, in erythroid precursors that contain genes for FR and 15-lipoxygenase and α-globin, which encode for RNA cis-elements that can interact with hnRNP E1 (14,45), folate deficiency-related accumulation of homocysteine is likely to lead to functional changes in expression of these genes during erythroid proliferation or differentiation (FRs are active during erythroid proliferation [24,46], whereas 15-lipoxygenase and α-globin are expressed during erythroid differentiation [47]). (b) Earlier studies of iron regulation-related genes implicated the oxidation-reduction state as an important modulator of RNA-protein interactions (48)(49)(50)(51)(52).…”

“…HeLa-IU 1 -HF and HeLa-IU 1 -LF cells at 80% confluency were washed with D-PBS and harvested after exposure to isotonic trypsin-EDTA. Cells (1 × 10 4 ) were then transferred to glass slides, and total cell and nuclear diameters were quantitatively determined (24).…”

Transduction of folate receptor cDNA into cervical carcinoma cells using recombinant adeno-associated virions delays cell proliferation in vitro and in vivo.

“…In the past decade, we and others demonstrated that the mechanism of cellular entry of physiologic folates into normal and several malignant cell lines in-which are anchored to the membrane by glycosyl-phosphatidylinositol (GPI) anchors (reviewed in references 2-4). Earlier, we also showed that the interaction of anti-FR IgG with FRs on hematopoietic erythroid, myeloid, macrophage, and megakaryocytic progenitors led to a profound stimulatory effect on cell proliferation that was independent of anti-FR antibodymediated induction of megaloblastosis and intracellular folate deficiency (5,6). Thus, these data suggested a role for FRs in constitutive control of proliferation from one of two possibilities: either the antibody perturbed a normal inhibitory role of the FRs on cell proliferation, or alternatively, the antibody accentuated a normal function of the FRs in constitutive stimulation of cell proliferation (4).…”

“…For cells cultured in MEM containing 2% FCS, 105 cells were plated in 24-well plates containing 2 ml media. After various days in culture, cells were trypsinized and counted in a Coulter counter (6) and assayed for protein (43). As a routine, before cells were counted, they were visualized under light microscopy to ensure they were single cell suspensions.…”