Hypoxia-induced accumulation of erythropoietin mRNA in isolated hepatocytes is inhibited by protein kinase C

Sphk1 and Sphk2 Differentially Regulate Erythropoietin Synthesis in Mouse Renal Interstitial Fibroblast-like Cells

Erythropoietin (Epo) is a crucial hormone regulating red blood cell number and consequently the hematocrit. Epo is mainly produced in the kidney by interstitial fibroblast-like cells. Previously, we have shown that in cultures of the immortalized mouse renal fibroblast-like cell line FAIK F3-5, sphingosine 1-phosphate (S1P), by activating S1P₁ and S1P₃ receptors, can stabilize hypoxia-inducible factor (HIF)-2α and upregulate Epo mRNA and protein synthesis. In this study, we have addressed the role of intracellular iS1P derived from sphingosine kinases (Sphk) 1 and 2 on Epo synthesis in F3-5 cells and in mouse primary cultures of renal fibroblasts. We show that stable knockdown of Sphk2 in F3-5 cells increases HIF-2α protein and Epo mRNA and protein levels, while Sphk1 knockdown leads to a reduction of hypoxia-stimulated HIF-2α and Epo protein. A similar effect was obtained using primary cultures of renal fibroblasts isolated from wildtype mice, Sphk1−/−, or Sphk2−/− mice. Furthermore, selective Sphk2 inhibitors mimicked the effect of genetic Sphk2 depletion and also upregulated HIF-2α and Epo protein levels. The combined blockade of Sphk1 and Sphk2, using Sphk2−/− renal fibroblasts treated with the Sphk1 inhibitor PF543, resulted in reduced HIF-2α and Epo compared to the untreated Sphk2−/− cells. Exogenous sphingosine (Sph) enhanced HIF-2α and Epo, and this was abolished by the combined treatment with the selective S1P₁ and S1P₃ antagonists NIBR-0213 and TY52156, suggesting that Sph was taken up by cells and converted to iS1P and exported to then act in an autocrine manner through S1P₁ and S1P₃. The upregulation of HIF-2α and Epo synthesis by Sphk2 knockdown was confirmed in the human hepatoma cell line Hep3B, which is well-established to upregulate Epo production under hypoxia. In summary, these data show that sphingolipids have diverse effects on Epo synthesis. While accumulation of intracellular Sph reduces Epo synthesis, iS1P will be exported to act through S1P₁₊₃ to enhance Epo synthesis. Furthermore, these data suggest that selective inhibition of Sphk2 is an attractive new option to enhance Epo synthesis and thereby to reduce anemia development in chronic kidney disease.

S1P Stimulates Erythropoietin Production in Mouse Renal Interstitial Fibroblasts by S1P1 and S1P3 Receptor Activation and HIF-2α Stabilization

Erythropoietin (Epo) is the critical hormone for erythropoiesis. In adults, Epo is mainly produced by a subset of interstitial fibroblasts in the kidney, with minor amounts being produced in the liver and the brain. In this study, we used the immortalized renal interstitial fibroblast cell line FAIK F3-5 to investigate the ability of the bioactive sphingolipid sphingosine 1-phosphate (S1P) to stimulate Epo production and to reveal the mechanism involved. Stimulation of cells with exogenous S1P under normoxic conditions (21% O₂) led to a dose-dependent increase in Epo mRNA and protein levels and subsequent release of Epo into the medium. S1P also enhanced the stabilization of HIF-2α, a key transcription factor for Epo expression. S1P-stimulated Epo mRNA and protein expression was abolished by HIF-2α mRNA knockdown or by the HIF-2 inhibitor compound 2. Furthermore, the approved S1P receptor modulator FTY720, and its active form FTY720-phosphate, both exerted a similar effect on Epo expression as S1P. The effect of S1P on Epo was antagonized by the selective S1P₁ and S1P₃ antagonists NIBR-0213 and TY-52156, but not by the S1P₂ antagonist JTE-013. Moreover, inhibitors of the classical MAPK/ERK, the p38-MAPK, and inhibitors of protein kinase (PK) C and D all blocked the effect of S1P on Epo expression. Finally, the S1P and FTY720 effects were recapitulated in the Epo-producing human neuroblastoma cell line Kelly, suggesting that S1P receptor-dependent Epo synthesis is of general relevance and not species-specific. In summary, these data suggest that, in renal interstitial fibroblasts, which are the primary source of plasma Epo, S1P_{1 and 3} receptor activation upregulates Epo under normoxic conditions. This may have a therapeutic impact on disease situations such as chronic kidney disease, where Epo production is impaired, causing anemia, but it may also have therapeutic value as Epo can mediate additional tissue-protective effects in various organs.

The protein kinase C (PKC) family of proteins in cytokine signaling in hematopoiesis

The members of the protein kinase C (PKC) family of proteins play important roles in signaling for various growth factors, cytokines, and hormones. Extensive work over the years has led to the identification of three major groups of PKC isoforms. These include the classic PKCs (PKCalpha, PKCbeta(I), PKCbeta(II), PKCgamma), the novel PKCs (PKCdelta, PKCepsilon, PKCeta, PKCmu, PKCtheta), and the atypical PKCs (PKCzeta, PKCiota/lambda). All these PKC subtypes have been shown to participate in the generation of signals for important cellular processes and to mediate diverse and, in some cases, opposing biologic responses. There is emerging evidence that these kinases also play key functional roles in the regulation of cell growth, apoptosis, and differentiation of hematopoietic cells. In this review, both the engagement of the various PKC members in cytokine and growth factor signaling and their role in the regulation of hematopoiesis are discussed.

Regulation of erythropoietin production

The glycoprotein hormone erythropoietin (EPO) is an essential growth and survival factor for erythroid progenitor cells, and the rate of red blood cell production is normally determined by the serum EPO concentration. EPO production is inversely related to oxygen availability, so that an effective feedback loop is established, which controls erythropoiesis. Since recombinant EPO became available as an effective therapeutic agent, significant progress has also been made in understanding the basis of this feedback control. The main determinant of EPO synthesis is the transcriptional activity of its gene in liver and kidneys, which is related to local oxygen tensions. This control is achieved by hypoxia-inducible transcription factors (HIF), consisting of a constitutive beta-subunit and one of two alternative oxygen-regulated HIFalpha subunits (HIF-1alpha and HIF-2alpha). In the presence of oxygen (normoxia) the HIFalpha subunits are hydroxylated, which targets them for proteasomal degradation. Under hypoxia, because of the lack of molecular oxygen, HIF cannot be hydroxylated and is thereby stabilized. Although HIF-1alpha was the first transcription factor identified through its ability to bind to an enhancer sequence of the EPO gene, more recent evidence suggests that HIF-2alpha is responsible for the regulation of EPO. Although EPO is a prime example for an oxygen- regulated gene, the role of the HIF system goes far beyond the regulation of EPO, because it operates widely in almost all cells and controls a broad transcriptional response to hypoxia, including genes involved in cell metabolism, angiogenesis and vascular tone. Further evidence suggests that apart from its effect as an erythropoietic hormone EPO acts as a paracrine, tissue-protective protein in the brain and possibly also in other organs.

Inflammatory cytokines stimulate adrenomedullin expression through nitric oxide-dependent and -independent pathways

A body of evidence indicates that the production of adrenomedullin (ADM) in vivo is activated in states of inflammation. Our aim was to characterize the intracellular signaling pathways along which inflammation leads to a stimulation of ADM expression. For this purpose, we characterized the effects of inflammatory cytokines, tumor necrosis factor-alpha (100 microg/L), interleukin-1beta (20 microg/L), and interferon-gamma (0.5 U/L) on ADM gene expression in rat aortic vascular smooth muscle cells (AVSMCs). We found that inflammatory cytokines induced a time-dependent 12-fold upregulation of ADM mRNA in AVSMCs that was paralleled by a substantial increase in inducible NO synthase mRNA expression. The stimulatory effect of cytokines on ADM gene expression was attenuated by NO deprivation induced by Nomega-nitro-L-arginine methyl ester (1 mmol/L) and was in part mimicked by the NO donor S-nitroso-N-acetylpenicillamine (100 micromol/L). The cGMP analog 8-bromo-cGMP (100 micromol/L) had no effect on ADM gene expression, and inhibition of cGMP production by 1H-oxodiazolo-quinoxalin-1 (ODQ, 200 micromol/L) was not able to abrogate the increase of ADM mRNA induced by NO donation using S-nitroso-N-acetylpenicillamine (100 micromol/L). The significant induction of ADM gene expression by inflammatory cytokines and NO donation was also observed in mesangial cells, endothelial cells, and hepatocytes. These findings suggest that NO is a direct activator of ADM gene expression in a variety of cell types and that inflammatory cytokines stimulate ADM expression via both NO-dependent and -independent mechanisms. The stimulatory effect of NO appears to not be related to the classic guanylate cyclase-cGMP pathway.

Activation of protein kinase C in human hepatocellular carcinoma (HEP3B) cells increases erythropoietin production

Some investigators have reported previously that phorbol esters inhibit in vitro erythropoietin production stimulated by hypoxia; whereas others have reported that phorbol esters enhanced Epo production during exposure to hypoxia. We have demonstrated in the present experiments that hypoxia significantly increased diacylglycerol levels in cultured human hepatocellular carcinoma (Hep3B) cells. 1-oleoyl-2-acetyl-ras-glycerol (OAG) and N-(6-phenylhexyl)-5-chloro-1-naphthalenesulfonamide (SC-9), two well-known protein kinase C activators, significantly increased medium levels of erythropoietin as well as erythropoietin messenger RNA levels in normoxic Hep3B cells. A potent protein kinase C inhibitor, chelerythrine chloride, significantly decreased hypoxia-induced increases in medium levels of erythropoietin as well as erythropoietin messenger RNA levels in Hep3B cells. A cis-unsaturated free fatty acid, oleic acid, significantly enhanced OAG-induced medium levels of erythropoietin in normoxic Hep3B cells, whereas a phospholipase A2 inhibitor, mepacrine, significantly decreased hypoxia-induced erythropoietin production in Hep3B cells. These results provide strong support for a positive role for protein kinase C in the hypoxic regulation of erythropoietin production.

Role of protein-phosphorylation events in the anoxia signal-transduction pathway leading to the inhibition of total protein synthesis in isolated hepatocytes

Incubation of isolated hepatocytes under N2/CO2 (no O2) produced a rapid and strong inhibition of overall polypeptide biosynthesis, which was neither related to cell death nor to the appearance of specific stress proteins. Treatment of the cells with the tyrosine-kinase inhibitor genistein or with the serine/threonine-protein-kinase inhibitor H7 did not modify the impairment of protein synthesis induced by oxygen deprivation, indicating that such signal-transduction pathways are probably not involved in the anoxia-mediated effect. Okadaic acid (100 nM) and Na3VO4 (1 mM) reduced the incorporation of [14C]Leu into proteins of hepatocytes maintained under aerobic conditions (93.3 kPa O2). The effects of oxygen deprivation and okadaic acid were additive, whereas sodium vanadate did not enhance the impairment of protein synthesis induced by anoxia. This observation suggests that a common mechanism, involving the net phosphorylation of protein tyrosine residues, that is insensitive to genistein might participate in the negative control of the translation induced by oxygen deprivation. The effect of anoxia on the synthesis of proteins was fully and rapidly reversible upon the restoration of oxygen supply, thus indicating that hepatocytes are able to sense O2. Although high concentrations of cobalt chloride partially mimic the effect of oxygen deprivation on protein biosynthesis, the nature of such an oxygen sensor remains unknown, and appears unlikely to be a part of a classic haem protein.

Oxygen-dependent regulation of erythropoietin gene expression in rat hepatocytes

The essential role of the glycoprotein hormone erythropoietin (Epo) in the control of red blood cell production is well established. Synthesis of Epo is induced in response to low oxygen (hypoxia). In response to stimulation, increases in serum Epo levels are paralleled by changes in the abundance of Epo mRNA. These changes indicate that the level of Epo mRNA is the major determinant of hormone production rate [1-3]. Studies of the organ distribution of Epo mRNA [4, 5] have confirmed the results of organ ablation studies, which demonstrated that in adults the kidney is the major organ responsible for the Epo production, but the liver is capable of Epo production as well [6, 7]. More sensitive detection methods of Epo mRNA have demonstrated small quantities in testis, brain, lung, and spleen of rodents [3, 8].

Induction of Hypoxia-Inducible Factor-1, Erythropoietin, Vascular Endothelial Growth Factor, and Glucose Transporter-1 by Hypoxia: Evidence Against a Regulatory Role for Src Kinase

The induction by hypoxia of genes such as erythropoietin, vascular endothelial growth factor (VEGF ), and glucose transporter-1 (Glut-1) is mediated in part by a transcriptional complex termed hypoxia-inducible factor-1 (HIF-1). Several lines of evidence have implicated protein phosphorylation in the mechanism of activation of HIF-1 by hypoxia. Recent reports have described the activation of the tyrosine kinase src by severe hypoxia, and a role in the induction of VEGF by severe hypoxia has been proposed. This led us to examine whether src and related kinases operated more widely in the hypoxic induction of HIF-1 and HIF-1–dependent genes regulated by hypoxia. Measurements of src kinase activity in cells exposed to varying severities of hypoxia showed activation by severe hypoxia (0.1% oxygen or catalyst induced anoxia), but not 1% oxygen. This contrasted with the marked induction of HIF-1 by exposure to 1% oxygen. Manipulations of src activity were produced by transient and stable transfection of Hep3B cells. Despite substantial changes in src activity, no alteration was seen in the normoxic or hypoxic expression of erythropoietin, VEGF, or Glut-1, or in the regulation of HIF-1–dependent reporter genes inducible by hypoxia. Similarly, we found that the expression of these genes in src- or c-src kinase-deficient cells did not differ from wild-type cells at either 1% oxygen or more severe hypoxia. These results indicate that src is not critical for the hypoxic induction of HIF-1, erythropoietin, VEGF, or Glut-1.

Hypoxia signalling in the control of erythropoietin gene expression in rat hepatocytes

This study aimed to investigate the sequence of events involved in the stimulation of erythropoietin (EPO) gene expression by hypoxia in hepatocytes. To this end, primary cultures of rat hepatocytes were kept at either high (40% O2) or low (3% O2) oxygen tensions for 2.5 h. Hypoxia increased EPO mRNA about fifteen-fold, whilst the divalent cation cobalt (50-100 microM) or the iron chelator desferrioxamine (10-200 microM) did not increase EPO mRNA levels. Addition of hydrogen peroxide (100-500 microM) to the culture medium did also not change EPO mRNA levels at high or low oxygen tension. Addition of catalase (50-200 micrograms/ml) to the culture medium resulted in a lower level of hypoxia-induced EPO mRNA. Inhibition of protein synthesis by cycloheximide (100 microM) completely abolished the increase of EPO mRNA in response to hypoxia. Hypoxia but not cobalt increased the appearance of the hypoxia-inducible factor 1 (HIF-1), and this increase was blunted by cycloheximide. Taken together, these findings suggest that a classic heme protein and a related oxygen-dependent production of oxygen radicals is less likely to be involved in the regulation of the EPO gene by oxygen in hepatocytes. On the other hand, intact protein synthesis is an absolute requirement for the hypoxia-induced appearance of HIF-1 and for hypoxia-stimulated expression of the EPO gene in hepatocytes.

Differential effects of kinase inhibitors on erythropoietin and vascular endothelial growth factor gene expression in rat hepatocytes

This study sought to investigate whether a common protein kinase activity is involved in the sequence of events by which oxygen controls the expression of the genes for erythropoietin (EPO) and for vascular endothelial growth factor (VEGF) in rat hepatocytes. To this end we examined the influence of the non-specific kinase inhibitor staurosporine and of the tyrosine kinase inhibitor genistein on EPO and VEGF mRNA levels in primary cultures of rat hepatocytes kept at either high (20% O2) or low (1% O2) oxygen tension. We found that 3 h of exposure to the low O2 tension increased EPO mRNA levels about 20-fold and the three VEGF (-180, -164, -120) mRNA levels, on average, about fourfold. Staurosporine did not change EPO and VEGF mRNA levels at 20% O2, but in a concentration-dependent manner, decreased EPO and VEGF mRNA at 1% O2 with IC50 values of 30 nM and 1000 nM, respectively. In the presence of 1% O2, genistein decreased EPO mRNA and VEGF mRNA levels with IC50 values of about 36 and 360 microM, respectively. Although mRNA levels for glycerine aldehyde phosphatehydrogenase (GAPDH) were not changed, staurosporine and genistein inhibited uridine incorporation into total RNA with IC50 values of about 1 microM and 100 microM, respectively. Comparison with the transcription inhibitor actinomycin D suggested that the effects of both kinase inhibitors on VEGF mRNA but not on EPO mRNA levels could be attributed to the non-specific inhibition of transcription in hepatocytes. These findings suggest that a kinase activity is specifically involved in the O2-dependent control of EPO gene expression but not of VEGF gene expression in hepatocytes.

Changes associated with tyrosine phosphorylation during short-term hypoxia in retinal microvascular endothelial cells in vitro

The occlusion of capillary vessels results in low oxygen tension in adjacent tissues which triggers a signaling cascade that culminates in neovascularization. Using bovine retinal capillary endothelial cells (BRCEC), we investigated the effects of short-term hypoxia on DNA synthesis, phosphotyrosine induction, changes in the expression of basic fibroblast growth factor receptor (bFGFR), protein kinase C (PKC alpha), heat shock protein 70 (HSP70), and SH2-containing protein (SHC). The effect of protein tyrosine kinase (PTK) and phosphatase inhibitors on hypoxia-induced phosphotyrosine was also studied. Capillary endothelial cells cultured in standard normoxic (pO2 = 20%) conditions were quiesced in low serum containing medium and then exposed to low oxygen tension or hypoxia (pO2 = 3%) in humidified, 5% CO2, 37 degrees C, tissue culture chambers, on a time-course of up to 24 h. DNA synthesis was potentiated by hypoxia in a time-dependent manner. This response positively correlated with the cumulative induction of phosphotyrosine and the downregulation of bFGFR (M(r) approximately 85 kDa). Protein tyrosine kinase inhibitors, herbimycin-A, and methyl 2,5-dihydroxycinnamate, unlike genistein, markedly blocked hypoxia-induced phosphotyrosine. Prolonged exposure of cells to phosphatase inhibitor, sodium orthovanadate, also blocked hypoxia-induced phosphotyrosine. The expression of HSP70, PKC alpha, and SHC were not markedly altered by hypoxia. Taken together, these data suggest that short-term hypoxia activates endothelial cell proliferation in part via tyrosine phosphorylation of cellular proteins and changes in the expression of the FGF receptor. Thus, endothelial cell mitogenesis and neovascularization associated with low oxygen tension may be controlled by abrogating signaling pathways mediated by protein tyrosine kinase and phosphatases.

Opposite regulation of renin gene expression by cyclic AMP and calcium in isolated mouse juxtaglomerular cells

A quantitative reverse transcriptase-polymerase chain reaction for mouse renin mRNA was utilized to study the influence of classic second messenger molecules on renin mRNA levels in primary cultures of juxtaglomerular (JG) cells isolated from the kidneys of C57/B16 mice. We found that forskolin (3 microM), an activator of adenylate cyclase led to proportional increases of renin secretion and renin mRNA levels. The nitric oxide (NO) donor, sodium nitroprusside (100 microM), stimulated both renin secretion and renin gene expression, the effect on secretion being stronger than that on renin mRNA levels. An increase of the extracellular concentration of calcium from 0.5 to 3 mM led to a transient inhibition of renin secretion, followed by a marked stimulation of secretion and to a continuous suppression of renin mRNA levels. These were also decreased by the calcium ionophore A 23187 (1 microM). The membrane permeable 8-bromo-cyclic GMP (100 microM) inhibited basal renin secretion without an effect on renin mRNA levels. The phorbol ester phorbol-12-myristate-13-acetate (1 to 100 nM), which was used to stimulate protein kinase C activity, had no significant effects on renin secretion and renin mRNA levels, neither alone nor in combination with forskolin. These findings suggest that cAMP, NO and calcium are effective regulators of renin gene expression in renal JG cells, in a way that cAMP and NO are stimulators and calcium acts as an inhibitor. Moreover, in these acute experiments there appears to be no obligatory link between the secretion and the expression of renin, suggesting that both parameters are separately regulated.

Regulation of the expression of the VEGF/VPS and its receptors: role in tumor angiogenesis

Vascular endothelial growth factor (VEGF)/vascular permeability factor (VPS) plays a crucial role for the vascularization of tumors including breast cancers. Tumors produce ample amounts of VEGF, which stimulates the proliferation and migration of endothelial cells (ECs), thereby inducing tumor vascularization by a paracrine mechanism. VEGF receptors (VEGF-Rs) are highly expressed by the ECs in tumor blood vessels. VEGF expression can be induced in various cell types by a number of stimuli including hypoxia, differentiation, growth factors and tumor promoters of the phorbol ester class, such as TPA. The VEGF inductive pathways comprise kinases, oncogenes, tumor suppressor genes, and steroid hormone transcription factors, many of which seem to converge on the activator protein (AP-1) transcription factor. Much less is known about the regulation of VEGF-R expression, which is restricted to ECs. This expression is greatly enhanced in diseased tissue such as solid tumors. So far, it appears that growth factors, cytokines, and tumor promoters are involved in the control of VEGF-R expression. Here we review current knowledge about the regulation of the expression of VEGF and its receptors.