Autocrine regulation of erythropoietin gene expression in human hepatocellular carcinoma cells

Progress in detecting cell-surface protein receptors: the erythropoietin receptor example

Testing for the presence of specific cell-surface receptors (such as EGFR or HER2) on tumor cells is an integral part of cancer care in terms of treatment decisions and prognosis. Understanding the strengths and limitations of these tests is important because inaccurate results may occur if procedures designed to prevent false-negative or false-positive outcomes are not employed. This review discusses tests commonly used to identify and characterize cell-surface receptors, such as the erythropoietin receptor (EpoR). First, a summary is provided on the biology of the Epo/EpoR system, describing how EpoR is expressed on erythrocytic progenitors and precursors in the bone marrow where it mediates red blood cell production in response to Epo. Second, studies are described that investigated whether erythropoiesis-stimulating agents could stimulate tumor progression in cancer patients and whether EpoR is expressed and functional on tumor cells or on endothelial cells. The methods used in these studies included immunohistochemistry, Northern blotting, Western blotting, and binding assays. This review summarizes the strengths and limitations of these methods. Critically analyzing data from tests for cell-surface receptors such as EpoR requires understanding the techniques utilized and demonstrating that results are consistent with current knowledge about receptor biology.

The effect of erythropoietin on normal and neoplastic cells

Erythropoietin (Epo) is an essential hormone that binds and activates the Epo receptor (EpoR) resident on the surface of erythroid progenitor cells, thereby promoting erythropoiesis. Recombinant human erythropoietin has been used successfully for over 20 years to treat anemia in millions of patients. In addition to erythropoiesis, Epo has also been reported to have other effects, such as tissue protection and promotion of tumor cell growth or survival. This became of significant concern in 2003, when some clinical trials in cancer patients reported increased tumor progression and worse survival outcomes in patients treated with erythropoiesis-stimulating agents (ESAs). One of the potential mechanisms proffered to explain the observed safety issues was that functional EpoR was expressed in tumors and/or endothelial cells, and that ESAs directly stimulated tumor growth and/or antagonized tumor ablative therapies. Since then, numerous groups have performed further research evaluating this potential mechanism with conflicting data and conclusions. Here, we review the biology of endogenous Epo and EpoR expression and function in erythropoiesis, and evaluate the evidence pertaining to the expression of EpoR on normal nonhematopoietic and tumor cells.

The erythropoietin receptor in normal and cancer tissues

The hormone erythropoietin (EPO) is essential for the survival, proliferation and differentiation of the erythrocytic progenitors. The EPO receptor (EPO-R) of erythrocytic cells belongs to the cytokine class I receptor family and signals through various protein kinases and STAT transcription factors. The EPO-R is also expressed in many organs outside the bone marrow, suggesting that EPO is a pleiotropic anti-apoptotic factor. The controversial issue as to whether the EPO-R is functional in tumor tissue is critically reviewed. Importantly, most studies of EPO-R detection in tumor tissue have provided falsely positive results because of the lack of EPO-R specific antibodies. However, endogenous EPO appears to be necessary to maintain the viability of endothelial cells and to promote tumor angiogenesis. Although there is no clinical proof that the administration of erythropoiesis stimulating agents (ESAs) promotes tumor growth and mortality, present recommendations are that (i) ESAs should be administered at the lowest dose sufficient to avoid the need for red blood cell transfusions, (ii) ESAs should not be used in patients with active malignant disease not receiving chemotherapy or radiotherapy, (iii) ESAs should be discontinued following the completion of a chemotherapy course, (iv) the target Hb should be 12 g/dL and not higher and (v) the risks of shortened survival and tumor progression have not been excluded when ESAs are dosed to target Hb <12 g/dL.

Survival and invasiveness of astrocytomas promoted by erythropoietin

OBJECT

The hypoxia-inducible pleiotropic hormone, erythropoietin (EPO), has recently been found to promote the development and survival of neurons and astrocytes. Since hypoxia has been implicated in the malignant progression of some human cancers, the authors investigated whether EPO signaling influenced the malignant properties of human astrocytoma cells.

METHODS

Reverse transcriptase-polymerase chain reaction, Western blot analysis, and immunohistochemical studies were used to measure EPO and its receptor (EPOR). Cell viability, Matrigel invasion assays, metalloprotease assays, EPO neutralizing antibodies, and EPOR overexpression were used to study the biological actions of EPO. Expression of both EPO and EPOR was observed in the hypoxic regions and invasive margins of glioma specimens obtained at biopsy, and expression of EPOR correlated with the stage of the tumor. The EPOR was also functionally upregulated by hypoxia in cultured glioblastoma multiforme (GBM) cells. Both hypoxia and EPO protected cultured GBM cells from cisplatin cytotoxicity and promoted the invasiveness of GBM cells through Matrigel by potentiating metalloprotease activity. Hypoxia-enhanced cell invasion was attenuated in cells that overexpressed a nonfunctional EPOR.

CONCLUSIONS

Hypoxia-inducible autocrine and paracrine EPO signaling participates in the malignant progression of GBMs.

Preclinical studies of erythropoietin receptor expression in tumour cells: Impact on clinical use of erythropoietic proteins to correct cancer-related anaemia

In vitro and animal model studies have shown erythropoietin receptor (Epo-R) mRNA and/or protein may be present in a range of human tumours and cancer cell lines, and erythropoiesis-stimulating agents (ESAs) have been reported to have tumour cell growth-modulating effects. Following a review of the literature, we conclude that considerations must be made when interpreting data from the preclinical studies. First, supraphysiological doses of ESAs were usually used. Second, there are no well validated, commercially available antibodies for identifying the presence and functionality of Epo-R at the protein level, either intracellularly or on the cell surface. Data from previous studies that used antibodies only for Epo-R detection must therefore be interpreted with caution. Together with diverging results in the literature, these methodological limitations indicate that findings from preclinical studies must not be over-translated in terms of their clinical relevance to patients with cancer.

Erythropoietin signaling promotes invasiveness of human head and neck squamous cell carcinoma

Erythropoietin (Epo) is used for managing anemia in cancer patients. However, recent studies have raised concerns for this practice. We investigated the expression and function of Epo and the erythropoietin receptor (EpoR) in tumor biopsies and cell lines from human head and neck cancer. Epo responsiveness of the cell lines was assessed by Epoetin-alpha-induced tyrosine phosphorylation of the Janus kinase 2 (JAK2) protein kinase. Transmigration assays across Matrigel-coated filters were used to examine the effects of Epoetin-alpha on cell invasiveness. In 32 biopsies, we observed a significant association between disease progression and expression of Epo and its receptor, EpoR. Expression was highest in malignant cells, particularly within hypoxic and infiltrating tumor regions. Although both Epo and EpoR were expressed in human head and neck carcinoma cell lines, only EpoR was upregulated by hypoxia. Epoetin-alpha treatment induced prominent JAK2 phosphorylation and enhanced cell invasion. Inhibition of JAK2 phosphorylation reduced both basal and Epo-induced invasiveness. Our findings support a role for autocrine or paracrine Epo signaling in the malignant progression and local invasiveness of head and neck cancer. This mechanism may also be activated by recombinant Epo therapy and could potentially produce detrimental effects in rhEpo-treated cancer patients.

Beneficial and ominous aspects of the pleiotropic action of erythropoietin

The primary function of the glycoprotein hormone erythropoietin (Epo) is to promote red cell production by inhibiting apoptosis of erythrocytic progenitors in hemopoietic tissues. However, functional Epo receptors (Epo-R) have recently been demonstrated in various nonhemopoietic tissues indicating that Epo is a more pleiotropic viability and growth factor. Herein, in vitro and in vivo effects of Epo in the brain and the cardiovascular system are reviewed. In addition, the therapeutic impact of Epo in oncology is considered, including the question of whether Epo might promote tumor growth. Convincing evidence is available that Epo acts as a neurotrophic and neuroprotective factor in the brain. Epo prevents neuronal cells from hypoxia-induced and glutamate-induced cell death. Epo-R is expressed by neurons and glia cells in specific regions of the brain. Epo supports the survival of neurons in the ischemic brain. The neuroprotective potential of Epo has already been confirmed in a clinical trial on patients with acute stroke. With respect to the vasculature, Epo acts on both endothelial and smooth muscle cells. Epo promotes angiogenesis and stimulates the production of endothelin and other vasoactive mediators. In addition, Epo-R is expressed by cardiomyocytes. The role of Epo as a myocardial protectant is at the focus of present research. Epo therapy in tumor patients is practiced primarily to maintain the hemoglobin concentration above the transfusion trigger and to reduce fatigue. In addition, increased tumor oxygenation may improve the efficacy of chemotherapy and radiotherapy. However, tumor cells often express Epo-R. Therefore, careful studies are required to fully exclude that recombinant human Epo (rHuEpo) promotes tumor growth.

Molecular biology of erythropoietin

The glycoprotein hormone erythropoietin (EPO) is an essential viability and growth factor for the erythrocytic progenitors. EPO is mainly produced in the kidneys. EPO gene expression is induced by hypoxia-inducible transcription factors (HIF). The principal representative of the HIF-family (HIF-1, -2 and -3) is HIF-1, which is composed of an O2-labile alpha-subunit and a constant nuclear beta-subunit. In normoxia, the alpha-subunit of HIF is inactivated following prolyl- and asparaginyl-hydroxylation by means of alpha-oxoglutarate and Fe(2+)-dependent HIF specific dioxygenases. While HIF-1 and HIF-2 activate the EPO gene, HIF-3, GATA-2 and NFkappaB are likely inhibitors of EPO gene transcription. EPO signalling involves tyrosine phosphorylation of the homodimeric EPO receptor and subsequent activation of intracellular antiapoptotic proteins, kinases and transcription factors. Lack of EPO leads to anemia. Treatment with recombinant human EPO (rHuEPO) is efficient and safe in improving the management of the anemia associated with chronic renal failure. RHuEPO analogues with prolonged survival in circulation have been developed. Whether the recent demonstration of EPO receptors in various non-hemopoietic tissues, including tumor cells, is welcome or ominous still needs to be clarified. Evidence suggests that rHuEPO may be a useful neuroprotective agent.

Immunohistochemical expression of erythropoietin and erythropoietin receptor in breast carcinoma

BACKGROUND

Erythropoietin (Epo), induced by hypoxia, controls the survival, proliferation, and differentiation of Epo receptor (EpoR)-bearing erythroid progenitors and plays a role in the protection of neurons from hypoxic damage. Hypoxia in malignant disease is associated with invasion, metastasis, resistance to therapy, and selection for cells with diminished apoptotic potential. The authors recently demonstrated the basal and hypoxia-stimulated expression of Epo and EpoR in human breast carcinoma cell lines and in breast carcinomas, suggesting a role for autocrine Epo signaling in the hypoxic adaptations of mammary neoplasms.

METHODS

The authors characterized the expression of Epo and EpoR by immunohistochemistry in 184 invasive mammary carcinomas and 158 in situ mammary carcinomas and benign mammary epithelium. They analyzed the correlation of Epo and EpoR immunostaining with clinicopathologic tumor features and the patients' smoking history.

RESULTS

Benign mammary epithelial cells showed weak-to-moderate expression of Epo and EpoR. EpoR immunostaining was increased in carcinomas compared with benign epithelium both in nonsmokers and smokers, and Epo immunostaining was increased in carcinomas compared with benign epithelium in nonsmokers but not in smokers. Prominent Epo staining was seen in tumor cells adjacent to necrotic areas and at the infiltrating edge of tumors. EpoR staining, but not Epo staining, was significantly greater in tumors that showed high histologic grade, tumor necrosis, lymphovascular invasion, lymph node metastases, and loss of hormone receptor expression.

CONCLUSIONS

The current findings suggest that increased EpoR expression may play an important role in breast carcinogenesis. The induction of autocrine or paracrine Epo signaling may represent a novel mechanism by which hypoxia can promote breast carcinoma.

Erythrocytosis caused by an erythropoietin-producing hepatocellular carcinoma

A case of erythrocytosis caused by a hepatocellular carcinoma (HCC) that produced erythropoietin (Epo) is described. A 64-year-old man, with a huge HCC tumor in the right lobe of the liver, showed a high concentration of hemoglobin and increased levels of serum Epo, alpha-fetoprotein (AFP), and protein induced by vitamin K absence II (PIVKA-II). Right lobectomy of the liver was performed. Histological findings of the specimen showed a moderately differentiated HCC. The existence of Epo was confirmed immunohistochemically only in the tumor tissue and not in the normal liver tissue. Erythrocytosis disappeared and the serum levels of Epo, AFP, and PIVKA-II returned to the normal range after the operation. Within 2 months after the operation, recurrent tumors appeared in the remnant liver, and the patient died 13 months after the operation.

Erythropoietin stimulates proliferation of human renal carcinoma cells

BACKGROUND

We reported recently that normal human, rat, and mouse tubular cells express authentic erythropoietin-receptors (EPO-R) through which EPO stimulates mitogenesis. The present study examines whether EPO could elicit such a proliferative and thereby potentially detrimental response in cells of human renal-cell carcinoma (RCC).

METHODS

Nephrectomy samples were screened from patients with RCC (one chromophilic, two clear cell) as well as cell lines of human (Caki-2, 786-0) and mouse (RAG) renal adenocarcinomas for expression of EPO-R transcripts and protein. Cells were further tested for specific 125I-EPO binding and mitogenic response to EPO.

RESULTS

Authentic EPO-R transcripts and protein (approximately 72 kD) were detected in renal tumors and cell lines. Tumors showed low-level EPO expression, while cell lines did not. In cells, specific 125I-EPO binding to a single class of EPO-R (apparent Kd 1. 3 to 1.4 nmol/L, Bmax 2.2 to 2.6 fmol/mg protein) was observed. EPO stimulated cell proliferation dose dependently, and the individual mitogenic effects of either EPO or 10% newborn calf serum were markedly amplified when both were coadministered.

CONCLUSION

These data are the first to demonstrate, to our knowledge, that human RCCs express EPO-R message and protein and that receptor activation stimulates their proliferation in vitro. If these mitogenic effects of EPO are also operative in patients with RCC, endogenous EPO or its administration for the treatment of anemia could potentially hasten proliferation of renocellular malignancies.

Human Hematopoietic Progenitors Express Erythropoietin

Erythropoietin (EPO) is a factor essential for erythroid cell proliferation, differentiation, and survival. The production of EPO by the kidneys in response to hypoxia and anemia is well documented. To determine whether EPO is also produced by hematopoietic cells, we analyzed the expression of EPO in normal human hematopoietic progenitors and in their progeny. Undifferentiated CD34+lin− hematopoietic progenitors do not have detectable EPO mRNA. Differentiating CD34+ cells that are stimulated with recombinant human EPO in serum-free liquid cultures express both EPO and EPO receptor (EPOR). Because CD34+ cells represent a heterogeneous cell population, we analyzed individual burst-forming units–erythroid (BFU-E) and nonerythroid colony-forming unit–granulocyte-macrophage colonies for EPO mRNA. Only BFU-E colonies were positive for EPO mRNA. Lysates from pooled BFU-E colonies stained positively for EPO by immunoblotting. To further confirm the intrinsic nature of erythroid EPO, we replaced extrinsic EPO in erythroid colony cultures with EPO-mimicking peptide (EMP). We show EPO expression in the EMP-stimulated BFU-Es at both mRNA and protein levels. Stimulation of bone marrow mononuclear cells (BMMCs) with EMP upregulated EPO expression. Furthermore, we found EPO and EPOR mRNAs as well as EPO protein in K562 cells, a human erythroleukemia cell line. Stimulation of K562 cells with EMP upregulated EPO expression. We suggest that EPO of erythroid origin may have a role in the regulation of erythropoiesis.

Human Hematopoietic Progenitors Express Erythropoietin

Erythropoietin (EPO) is a factor essential for erythroid cell proliferation, differentiation, and survival. The production of EPO by the kidneys in response to hypoxia and anemia is well documented. To determine whether EPO is also produced by hematopoietic cells, we analyzed the expression of EPO in normal human hematopoietic progenitors and in their progeny. Undifferentiated CD34+lin− hematopoietic progenitors do not have detectable EPO mRNA. Differentiating CD34+ cells that are stimulated with recombinant human EPO in serum-free liquid cultures express both EPO and EPO receptor (EPOR). Because CD34+ cells represent a heterogeneous cell population, we analyzed individual burst-forming units–erythroid (BFU-E) and nonerythroid colony-forming unit–granulocyte-macrophage colonies for EPO mRNA. Only BFU-E colonies were positive for EPO mRNA. Lysates from pooled BFU-E colonies stained positively for EPO by immunoblotting. To further confirm the intrinsic nature of erythroid EPO, we replaced extrinsic EPO in erythroid colony cultures with EPO-mimicking peptide (EMP). We show EPO expression in the EMP-stimulated BFU-Es at both mRNA and protein levels. Stimulation of bone marrow mononuclear cells (BMMCs) with EMP upregulated EPO expression. Furthermore, we found EPO and EPOR mRNAs as well as EPO protein in K562 cells, a human erythroleukemia cell line. Stimulation of K562 cells with EMP upregulated EPO expression. We suggest that EPO of erythroid origin may have a role in the regulation of erythropoiesis.

Unexpected hypoxia-dependent erythropoietin secretion during experimental conditions not affecting tissue oxygen supply/demand ratio

Although a great deal of evidence supports the hypothesis that plasma erythropoietin (EPO) levels of mammals are related to the oxygen supply to the tissues relative to their oxygen needs, several observation millitate against its inherent simplicity. This study presents our results obtained from in vivo experiments that suggest that hypoxia-dependent EPO production can be altered by conditions which apparently do not modify the tissue oxygen supply/demand ratio. Hypoxia-dependent EPO production rate (EPO-PR), derived from plasma EPO titers and plasma EPO half-lives, were estimated in both transfused-polycythemic and normocythemic mouse models subjected to different treatments. From calculations of the O2 carrying capacity of blood and body O2 consumption, it was assumed that the tissue supply/demand ratios were similar in both experimental and control mice of the same model at the time of induction of EPO production. The following observations were worth noting: (1) EPO-PRs in transfused polycythemic mice whose erythropoietic rates were stimulated by intermittent exposure to hypobaria (0.5 atm, 18 hr/day x 3 weeks), phenylhydrazine administration (40 mg/kg at weekly intervals x 3 weeks) or repeated rh-EPO injections (1500 U/kg 3 times a week x 3 weeks) before transfusion were more than five times high than in comparabily polycythemic mice whose erythropoietic rates were not stimulated previously; and (2) EPO-PR in response to hypobaric hypoxia was 2.08 times normal in normocythemic mice with cyclophosphamide (100 mg/kg) induced depression of erythropoiesis, and 0.33 times normal in normocythemic mice with rh-EPO (400 U/kg x 2) induced enhancement of erythropoiesis. Although the results obtained in polycythemic mice are difficult to explain, those from normocythemic mice suggest the existence of a feedback mechanism between EPO-responsive cells and EPO-producing cells. Both demonstrate the existence of experimental conditions in which modulation of the hypoxia-dependent expression of the EPO gene appears to occur. This modulation would be dependent on factors other than oxygen.