To examine in more detail the relationship between AFP and stem cell factor, regression models that estimated stem cell factor over AFP concentrations were fitted. Even though linear regression showed significant links between both variables (R2 = 0.22, P = .002) (Figure 1), the equation that best fit the data was a power model—a nonlinear model whose equation is Y = b0 × (tb1)t or ln(b0) + (b1 × ln(t), with, in our case, b0 = 50.7, b1 = 0.27, Y = stem cell factor, and t = AFP (R2 = 0.54, P < .001) (Figure 2).
It is suspected that the physiologic role of AFP is to modulate various growth-regulatory pathways during fetal development.8 However, the exact biologic role of this abundant protein in fetal life is still unclear.
In a previous study, we found a significant correlation between maternal serum alpha-fetoprotein (MSAFP) and fetal hemoglobin obtained both by prenatal cordocentesis and during elective cesarean delivery before labor.6 In a second part of that study, we also found a significant negative correlation between fetal serum AFP and fetal RBC count, hemoglobin, hematocrit, and erythropoietin. Therefore, it was reasonable to suggest that AFP could have a role in fetal erythropoiesis.
In the late 1970s, several investigators suspected a relationship between AFP and fetal hematopoiesis.9,10 Congote et al11 studied AFP as well as the synthesis of heme associated with hemoglobin in short-term cultures of human fetal liver cells, to correlate the relationship between AFP and erythroid cell function. They concluded that there is no cause-and-effect relationship between AFP production and erythroid cell function in human fetal liver cells and that the two processes occur independently in different cell types.
In vivo hematopoiesis is a complex system regulated by various mechanisms that appear to be under the control of many hemopoietic growth factors. In this context, the synthesis of heme associated with hemoglobin is only one of the last steps in a long chain that includes activation of stem cells, division of progenitors, proliferation, and differentiation.12
We have found a significant correlation between AFP and stem cell factor. Stem cell factor is the ligand for the tyrosine kinase receptor c-kit, which is expressed on both primitive and mature hematopoietic progenitor cells. This cytokine, synergizing with other growth factors, preserves the viability of hematopoietic stem and progenitor cells, preventing their apoptosis, and influences their entry into the cell cycle while it facilitates their proliferation and differentiation.13 In vivo, stem cell factor, in combination with G-CSF, also enhances mobilization of peripheral blood progenitor cells.14 Heidari et al15 found that AF promoted proliferation of both fetal liver cells and adult bone marrow cells, and the proliferation-promoting activity was partially neutralized by the anti–stem cell factor antibody. In another study, the same investigators found that the proliferation-promoting activity of murine AF was present only in mice younger than 2 weeks.16 These results indicate that AF potentially contains multiple growth factors, including stem cell factor, that preferentially affect the early stage of hematopoiesis. Therefore, if AFP has a connection with stem cell factor, it might have a role in fetal hematopoieis, at least at this early stage.
Some relationships between AFP and stem cell factor are well documented. In animal models, bile duct ligation can be used to induce proliferation of a hepatic stem cell compartment that participates in the renewal process of epithelial cell populations in the liver. Stem cell factor is one of the growth factor or receptor systems associated with early activation of this compartment. When this procedure is performed, expression of AFP, used as an indicator of activation of the stem cell compartment, is followed by a consecutive increase in expression of stem cell factor.17 In addition, oval cells, which produce AFP, express the c-kit receptor tyrosine kinase and its ligand, stem cell factor. The stem cell factor/c-kit system might be involved in the early activation of the hepatic stem cells as well as in the expansion and differentiation of oval cells.18 Moreover, both AFP and stem cell factor are genes known to be present in fetal liver.19
Nevertheless, several other factors might influence stem cell factor concentration and action significantly. These factors include cytokines such as tumor necrosis factor α,20 interleukin 1,21 and tumor growth factor β 1,22 and hormones such as FSH23 and glucocorticoids.24 We did not find correlations between AFP and other hematopoietic growth factors. However, there are significant doubts about whether erythropoietin and other classic hematopoietic cytokines such as interleukin 3 are the main promoting factors in fetal liver hematopoiesis. While studying a fetal hepatocyte cell line capable of supporting hematopoiesis, Hata et al25 found that the hematopoietic activities were attributed to granulocyte-macrophage colony-stimulating factor and macrophage colony-stimulating factor, whereas no influences for interleukin 3, interleukin 6, G-CSF, or erythropoietin were identified. Fetuses of preeclamptic women have shown severely reduced hepatic erythropoiesis in spite of an increase in erythropoietin and interleukin 3 levels.26 These results suggested a discrepancy between increased numbers of erythrocytes in peripheral blood and depression of hematopoiesis at the main production site, the fetal liver. Another possible explanation could be that these hematopoietic factors might affect later stages of fetal hematopoiesis.
Ultrastructural differences are described in hepatocytes according to whether the cells produce AFP. These observations led to the hypothesis that AFP might correspond to a specific functional state of the hepatocyte in the human fetal liver.27 Expression of AFP is a constant feature of extramedullary hematopoiesis, occurring not only in physiologic situations but also in pathologic ones. Expression of AFP occurs not only when hepatic tissue is present, such as in cases of hepatoblastoma or hepatic adenoma of the placenta,28 but also in some kinds of leukemia, such as juvenile chronic myeloid leukemia or erythroleukemia, in which there is a major reversion to fetal erythropoiesis related to the phenomenon of fetal protein synthesis.29
We can speculate about three possible mechanisms by which AFP can modulate fetal hematopoiesis. First, it has been reported that AFP might serve as an antioxidant on the basis of amino acid domain and motif structure.1 Given that the main antioxidant enzyme, myeloperoxidase, acts only during the late stages of myeloid differentiation,30 AFP might be useful as an antioxidant in early stages, when cells are too immature to transcribe myeloperoxidase. Second, AFP might enhance the proliferative activity of several growth factors,7 including stem cell factor. The mechanism of the growth-regulatory properties remains unclear, but such modulation might be attributed to transcriptional enhancement and/or suppression activities.31 Third, AFP could act to avoid apoptosis1 in cells participating in early stages of fetal hematopoiesis. Thus, AFP could enhance the aforementioned action of stem cell factor to prevent apoptosis of hematopoietic stem and progenitor cells.
From an obstetric perspective, determining the relationships between AFP and fetal hematopoiesis could be help explain poorly understood clinical phenomena such as increased levels of MSAFP in conditions associated with fetal anemia or decreased levels in clinical situations associated with fetal hemoconcentration (eg, in women at risk of carrying fetuses with chromosomal abnormalities or in pregnant women with diabetes).32 Moreover, unexplained increases in MSAFP levels have been associated with an increased risk of poor perinatal outcome, but the explanation for this phenomenon is still unclear,33,34 and fetal anemia leading to greater hepatic hematopoietic activity could be involved in some of these cases.
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