The regenerating liver: a site of erythropoiesis in the adult Long-Evans rat

Erythroid enucleation: a gateway into a "bloody" world

Erythropoiesis is an intricate process starting in hematopoietic stem cells and leading to the daily production of 200 billion red blood cells (RBCs). Enucleation is a greatly complex and rate-limiting step during terminal maturation of mammalian RBC production involving expulsion of the nucleus from the orthochromatic erythroblasts, resulting in the formation of reticulocytes. The dynamic enucleation process involves many factors ranging from cytoskeletal proteins to transcription factors to microRNAs. Lack of optimum terminal erythroid maturation and enucleation has been an impediment to optimum RBC production ex vivo. Major efforts in the past two decades have exposed some of the mechanisms that govern the enucleation process. This review focuses in detail on mechanisms implicated in enucleation and discusses the future perspectives of this fascinating process.

Liver sinusoidal endothelial cells promote B lymphopoiesis from primitive hematopoietic cells

Although the bone marrow (BM) microenvironment is the main inducer niche of early B lymphopoiesis during the adult life, other extramedullar microenvironments, such as the liver, may also have potential for supporting B-cell development. Previously, we reported that murine liver sinusoidal endothelial cells (LSECs) support in vitro and in vivo hematopoietic stem cell (HSC) proliferation and myeloid differentiation. In the present study, we investigated the capacity of LSEC to promote B lymphopoiesis from BM progenitor lineage-negative (Lin(-)) cells. Murine BM Lin(-) cells were co-cultured with LSEC, in the absence of exogenous cytokines. B cells were characterized by flow cytometry and cytokine expression by RT-PCR. We show that BM Lin(-) cells differentiated to early B-lymphoid progenitors (B220(+)) and subsequently to mature (CD19(+)) B cells. Functional studies showed the presence of a high number of non-adherent cells (NACs), collected from lipopolysaccharide (LPS)-treated Lin(-)/LSEC co-cultures, expressing IgM on their surface (sIgM). Colony formation from NAC was observed in the presence of IL-7 (CFU-IL-7). LSEC constitutively express IL-7, Flt-3L, and SCF at the mRNA level, and VCAM-1 on their surface, which may explain the capacity of these cells to promote B lymphopoiesis. These data demonstrate that LSEC promote all stages of B lymphopoiesis. To our knowledge, this is the first report that LSEC constitute an in vitro microenvironment for B lymphopoiesis. Further studies will establish whether LSEC can serve in vivo as a B-lymphopoietic niche under physiological or pathological condition, or when HSC are mobilized.

The combined effect of erythropoietin and granulocyte macrophage colony stimulating factor on liver regeneration after major hepatectomy in rats

Background

The liver presents a remarkable capacity for regeneration after hepatectomy but the exact mechanisms and mediators involved are not yet fully clarified. Erythropoietin (EPO) and Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) have been shown to promote liver regeneration after major hepatectomy.

Aim of this experimental study is to compare the impact of exogenous administration of EPO, GM-CSF, as well as their combination on the promotion of liver regeneration after major hepatectomy.

Methods

Wistar rats were submitted to 70% major hepatectomy. The animals were assigned to 4 experimental groups: a control group (n = 21) that received normal saline, an EPO group (n = 21), that received EPO 500 IU/kg, a GM-CSF group (n = 21) that received 20 mcg/kg of GM-CSF and a EPO+GMCSF group (n = 21) which received a combination of the above. Seven animals of each group were killed on the 1st, 3rd and 7th postoperative day and their remnant liver was removed to evaluate liver regeneration by immunochemistry for PCNA and Ki 67.

Results

Our data suggest that EPO and GM-CSF increases liver regeneration following major hepatectomy when administered perioperatively. EPO has a more significant effect than GM-CSF (p < 0.01). When administering both, the effect of EPO seems to fade as EPO and GM-CSF treated rats have decreased regeneration compared to EPO administration alone (p < 0.01).

Conclusion

EPO, GM-CSF and their combination enhance liver regeneration after hepatectomy in rats when administered perioperatively. However their combination has a weaker effect on liver regeneration compared to EPO alone. Further investigation is needed to assess the exact mechanisms that mediate this finding.

Extramedullary haematopoiesis in massive hepatic necrosis

AIMS

To determine the frequency of extramedullary haematopoiesis (EMH) in massive hepatic necrosis (MHN).

METHODS AND RESULTS

Explanted livers of 11 adult patients transplanted consecutively for MHN were examined histologically and immunohistochemically for the presence of EMH. The aetiology of the liver damage was unknown in seven cases and drug induced in four. The presence of stem cell markers (CD34, c-kit), erythroid precursors (glycophorin A), myeloid precursors (myeloperoxidase) and megakaryocyte precursors (CD31) was investigated by immunohistochemistry. Erythroid, myeloid and megakaryocyte precursors were observed in all cases. Morphologically, haematopoietic blast cells were clustered in areas of collapse, separating islands of regenerating ductules and scattered between ductules, in a similar distribution to immunohistochemically identified c-kit-positive putative stem cells. No CD34+ cells other than endothelial cells were seen. All 11 patients were anaemic at the time of transplantation.

CONCLUSIONS

EMH is a frequent finding in patients undergoing liver transplantation for MHN. This may be a consequence of the anaemia associated with this condition. Alternatively, the possibility that intrahepatic haematopoiesis is linked with hepatopoiesis is an additional, intriguing possibility that deserves further study.

Extramedullary hematopoiesis in the adult mouse liver is associated with specific hepatic sinusoidal endothelial cells

In previous work, two anatomically distinct-liver sinusoid endothelial cells (LEC): LEC-1 and LEC-2, have been described. We also reported that extramedullary hepatic hematopoiesis occurs only in close contact with LEC-1, suggesting that these cells may provide the microenvironment necessary for the maintenance and growth of hematopoietic cells. In the present work, we studied the capacity of LEC-1 and LEC-2 to maintain in vitro hematopoiesis. LEC-1 and LEC-2 were isolated and cloned from livers of adult mice. Bone marrow cells (BM) enriched with primitive hematopoietic progenitors were isolated from day-2, post-5-FU-treated mice (5-FUBMC). LEC-1 supported the maintenance and differentiation of hematopoietic progenitors for more than 6 weeks in vitro. In contrast, LEC-2 cells poorly supported the proliferation of hematopoietic cells for only two weeks of the co-culture. LEC-1 and 5-FUBMC cocultures showed cobblestone-area formation and the presence of hematopoietic progenitors that are able to form colonies (CFC) in the adhering fraction after six weeks of coculture. LEC-1 co-cultures treated with a cocktail of cytokines (stem cell factor, interleukin [IL]1alpha, IL-3, and Epo) showed that megakaryocyte (CFU-Mk) and erythrocyte progenitors (BFU-e) were present during the entire period of the culture. Granulocyte-macrophage progenitors (CFU-GM) were present only during the first three weeks of the culture. These results suggest that LEC-1, but not LEC-2, provide an appropriate hematopoietic microenvironment for supporting the proliferation and differentiation of primitive hematopoietic cells. This could explain the anatomical restriction of hematopoietic cells for growing in LEC-1 domains during liver extramedullary hematopoiesis.

Differential location of hemopoietic colonies within liver acini of postnatal and phenylhydrazine-treated adult mice

We have measured the location of embryonic and adult hemopoietic foci in the liver tissue of postnatal and adult phenylhydrazine-treated mice. Differentiation of acinar domains in liver tissue was made possible by carrying out succinate dehydrogenase histochemical reactions on liver cryostat sections. To determine the position of hemopoietic foci within the lobular gradient of the hepatocyte succinate dehydrogenase activity, this enzyme was measured in hepatocytes surrounding both portal and central veins and hemopoietic foci. Then, assuming the periportal succinate dehydrogenase activity value to be 1.00 +/- 0.2, succinate dehydrogenase activity around postnatal hemopoietic foci was 0.65 +/- 0.19, around phenylhydrazine-induced hemopoietic foci 0.83 +/- 0.24 and around central veins 0.44 +/- 0.11. Scaling the portal to central vein distance and taking 1 as the portal vein point and 0 as the central vein point, the relative position of hemopoietic foci, indirectly calculated from succinate dehydrogenase activity values, was 0.35 +/- 0.13 in postnatal livers and 0.73 +/- 0.12 in phenylhydrazine-treated adult livers. Hemopoietic foci frequencies varied according to both the origin and the liver acinar domain: in postnatal liver acini, it was 37.1% in zone 1, 22.8% in zone 2 and 40% in zone 3; in phenylhydrazine-treated adult acini, it was 89.4% in zone 1 and 10.6% in zone 2. Postnatal hemopoietic foci mainly occurred extrasinusoidally, between hepatocytes and reticular-like cells, whereas adult hemopoietic foci were mostly intrasinusoidal and closely associated to macrophage-like cells. Adult hemopoietic colonies

Erythropoietin production by macrophages in the regenerating liver

The site of erythropoietin (Ep) production and/or storage in the rat liver was determined. A guinea pig anti-Ep was produced against purified rat Ep (64,096 +/- 4j064 IU/mg). This antibody was found to be highly specific using rocket immunoelectrophoresis, Ouchterlony gel diffusion methods, and immunoprecipitin reactions as well as Ep neutralization tests (capable of completely neutralizing up to 2,000 IU Ep/mg). This anti-Ep was labeled with either fluorescein for light microscopic study or ferritin for electron microscopy. Kupffer cells showed varying degrees of labeling after hepatectomy alone or hepatectomy combined with nephrectomy and/or hypoxia. Greatest labeling was seen in Kupffer cells of rats that were nephrectomized 48 hr posthepatectomy and kept at ambient pressure. No labeling of hepatocytes or vascular and bile duct endothelium was noted.

The influence of pancreatic hormones and diabetogenic procedures on erythropoietin production

The influence of the pancreas on renal and extrarenal erythropoietin (Ep) production and on the elaboration of the hepatic erythropoietic factor (HEF) was studied in these experiments. Insulin was found to elevate Ep levels in the anephric hypoxic rat when compared to controls, whereas glucagon treatment augmented the hepatic Ep response to hypoxia in the subtotally hepatectomized (hepx) animal while lowering it in the renal intact rat. Production of experimental diabetes either through chemical induction by alloxan or following pancreatectomy diminished the Ep response in all groups tested. Treatment with antiglucagon caused an elevation in the Ep response to hypoxia in the intact rat but lowered Ep levels in the hepx animal. In addition, glucagon and a synthetic hepatotrophic agent (L-histidyl L-lysine acetate) stimulated HEF production in the hepx rat, although none of the agents tested were capable of enhancing HEF levels in the intact rat.

The effect of prostaglandins A2,E1,E2,15 methyl E2, 16, 16 dimethyl E2 and F2 alpha on erythropoiesis

Prostaglandins A2, E1, E2, methylated E2s, and F2 alpha affected erythropoiesis and/or erythropoietin (Ep) production. This action is indicated in the exhypoxic, polycythemic mouse where radioiron incorporations into RBC increased after administration of these compounds. The kidney and liver have been indicated through previous studies, to actively participate in Ep production. The kidney and liver have been indicated through previous studies, to actively participate in Ep production. By the removal of one of these active sites in a murine system treated with prostaglandins it is shown that a response is reflected in Ep levels. Interference of the action of prostaglandins (PG) is altered by the removal of these target sites of Ep production. The erythropoietic responses elicited by PGA2, E1, and perhaps the methylated PGE2s act through the liver whereas PGE2 may operate through a renal pathway for its response. PGF2 alpha reveals no effect on erythropoietic activity and is no different than that observed for vehicle-treated controls. The prostaglandins tested appear to act primarily through the kidney or liver but the possibility exists that some yet undetermined organ site may also be involved.

Hepatic erythropoietin (Ep) production following double partial hepatectomy in the rat

Double partial hepatectomy (hepx) evokes an elevation in serum erythropoietin (Ep) levels in anephric hypoxic animals when compared to non-hypoxic or sham hepx controls. But this Ep response is significantly lower than that found in singly hepx, anephric hypoxic rats. Double hepx also induces numerous cytological changes in the liver. Extravascular accumulation of fat, fibrous scarring, localized necroses, and multiple abscesses, as well as decreased vascularity, occur following the second hepx. A humoral factor was detected in the serum of these animals that is capable of inducing hepatic Ep production when injected into normal rats 18 hours before nephrectomy and hypoxia. This factor, termed hepatopoietin (Hp), was previously demonstrated in the venous serum of singly hepx rats. The serum from animals subjected to double partial hepx is not as potent in inducing Ep production as the serum from singly hepx animals. The discrepancies noted between the single and double hepx groups is attributed to the necrotic cytological changes described above.

The development of the sinusoids of fetal rat liver: morphology of endothelial cells, Kupffer cells, and the transmural migration of blood cells into the sinusoids

The fine structural development of rat fetal liver sinusoids from 10 to 22 days gestation was studied. Colloidal carbon (Pelikan ink) was injected into 14-22 day gestation fetuses via the umbilical vein to assess the continuity of the sinusoidal lining and the phagocytic ability of the developing lining cells. Endothelial cells, devoid of an underlying basal lamina, form the bulk of the vascular lining at all gestational ages. These cells possess typical intercellular junctions and fenestrae with diaphragms before 17 days gestation. Transendothelial open fenestrations, typical of the adult liver, appear around 17 days gestation, increasing in number for the remainder of gestation. Although fenestrae possessing diaphragms are permeable to carbon before 16 days gestation, open fenestrations, first seen at 17 days gestation, allowed large amounts of carbon to reach the extravascular space. Endocytosis of carbon by endothelial cells was accomplished exclusively by large bristle-coated vesicles. Endothelial cells were also seen to be involved in transmural diapedesis of newly formed erythrocytes and megakaryocyte processes from the extravascular space by forming a temporary migration pore allowing these cells and processes to enter the circulation. At the end of gestation, blood-forming activity had nearly ceased, and only the space of Dissé separated the lining cells from the parenchymal cells. Kupffer cells were easily identified as early as 13 days gestation by their content of phagosomes and engulfed erythrocytes. The Kupffer cells are much more avid in the phagocytosis of carbon than are endothelial cells. Toward the end of gestation, some Kupffer cells develop a homogeneous "sticky coat" to carbon.