Phorbol ester inhibits erythropoietin production in human hepatoma cells (Hep G2)

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.

Protein kinase C alpha protein expression is necessary for sustained erythropoietin production in human hepatocellular carcinoma (Hep3B) cells exposed to hypoxia

Although protein kinase C (PKC) has been implicated as an effector of erythropoietin (EPO) production, its exact role is still uncertain. Hep3B human hepatocellular carcinoma cells were used for this study and were depleted of PKC in three different ways: long-term treatment with phorbol 12-myristate 13-acetate (PMA), selective inhibition with calphostin C, and treatment with PKCalpha antisense oligonucleotides. When EPO-producing Hep3B cells were incubated in 1% O2 (hypoxia) for 24 h, PMA treatment resulted in significant decreases in medium levels of EPO in Hep3B cell cultures at concentrations higher than 10 nM. The specific PKC inhibitor, calphostin C, significantly inhibited medium levels of EPO and EPO mRNA levels in Hep3B cells exposed to 1% O2. Western blot analysis revealed that Hep3B cells express the classical PKCalpha and gamma isoforms, as well as novel PKCepsilon and delta and the atypical zeta isoform. Preincubation with PMA for 6 h specifically down-regulated PKCalpha protein expression. Phosphorothioate modified antisense oligonucleotides specific for PKCalpha also decreased EPO production in Hep3B cells exposed to hypoxia for 20 h when compared to PKCalpha sense treatment. The translocation of PKCalpha from the soluble to particulate fractions was increased in Hep3B cells incubated under hypoxia compared with normoxia (21% O2) controls. These results suggest that the PKCalpha isoform plays an important role in sustaining hypoxia-regulated EPO production.

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.

Secondary polycythemia in male B6C3F1 mice with spontaneously occurring hepatocellular carcinomas

The purpose of this study was to investigate the cause of polycythemia occurring in control male B6C3F1 mice with hepatocellular carcinomas from 2-yr carcinogenicity studies. Erythrocyte counts and plasma erythropoietin levels in these mice were significantly increased compared to those in nontumor-bearing mice. Hepatocellular carcinomas in the mice were well differentiated, and the neoplastic hepatocytes contained either or both of 2 types of intracytoplasmic inclusion bodies; one was relatively large and weakly eosinophilic (pale inclusion body), while the other was relatively small and strongly eosinophilic (globular inclusion body). The pale eosinophilic inclusions but not the globular ones were immunohistochemically positive for erythropoietin. Ultrastructurally, the erythropoietin-positive inclusions were characterized by granular materials in dilated cisternae of rough endoplasmic reticulum, suggesting increased protein synthesis. Erythropoietin-negative inclusions were dense bodies that were not surrounded by a delimiting membrane. These findings indicate that polycythemia in hepatocellular carcinoma-bearing mice occurs secondary to excess synthesis and secretion of erythropoietin by neoplastic hepatocytes.

Hypoxia and mitochondrial inhibitors regulate expression of glucose transporter-1 via distinct Cis-acting sequences

Studies of gene regulation by oxygen have recently defined the existence of a widely operative system that responds to hypoxia but not mitochondrial inhibitors and involves the induction of a DNA-binding complex termed hypoxia-inducible factor 1. This system has been implicated in the regulation of erythropoietin, certain angiogenic growth factors, and particular glycolytic isoenzymes. The glucose transporter Glut-1 is induced by both hypoxia and mitochondrial inhibitors, implying the operation of a different mechanism of oxygen sensing. To explore that possibility, we analyzed the cisacting sequences that convey these responses. An enhancer lying 5' to the mouse Glut-1 gene was found to convey responses both to hypoxia and to the mitochondrial inhibitors, azide and rotenone. However, detailed analysis of this enhancer demonstrated that distinct elements responded to hypoxia and the mitochondrial inhibitors. The response to hypoxia was mediated by sequences that contained a functionally critical, although atypical, hypoxia-inducible factor 1 binding site, whereas sequences lying approximately 100 nucleotides 5' to this site, which contained a critical serum response element, conveyed responses to the mitochondrial inhibitors. Thus, rather than reflecting an entirely different mechanism of oxygen sensing, regulation of Glut-1 gene expression by hypoxia and mitochondrial inhibitors arises from the function of two different sensing systems. One of these responds to hypoxia alone and resembles that involved in erythropoietin regulation, while the other responds to mitochondrial inhibitors and involves activation of a serum response element.

Purification and characterization of hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) is a DNA-binding protein that activates erythropoietin (Epo) gene transcription in Hep3B cells subjected to hypoxia or cobalt chloride treatment. HIF-1 DNA binding activity is also induced by hypoxia or cobalt in non-Epo-producing cells, suggesting a general role for HIF-1 in hypoxia signal transduction and transcriptional regulation. Here we report the biochemical purification of HIF-1 from Epo-producing Hep3B cells and non-Epo-producing HeLa S3 cells. HIF-1 protein was purified 11,250-fold by DEAE ion-exchange and DNA affinity chromatography. Analysis of HIF-1 isolated from a preparative gel shift assay revealed four polypeptides. Peptide mapping of these HIF-1 components demonstrated that 91-, 93-, and 94-kDa polypeptides had similar tryptic maps, whereas the 120-kDa polypeptide had a distinct profile. Glycerol gradient sedimentation analysis suggested that HIF-1 exists predominantly in a heterodimeric form and to a lesser extent as a heterotetramer. Partially purified HIF-1 bound specifically to the wild-type HIF-1 binding site from the EPO enhancer but not to a mutant sequence that lacks hypoxia-inducible enhancer activity. UV cross-linking analysis with purified HIF-1 indicated that both subunits of HIF-1 contact DNA directly. We conclude that in both cobalt chloride-treated HeLa cells and hypoxic Hep3B cells HIF-1 is composed of two different subunits: 120-kDa HIF-1 alpha and 91-94-kDa HIF-1 beta.

Distinct signaling pathways mediate phorbol-ester-induced and cytokine-induced inhibition of erythropoietin gene expression

Hypoxia-induced erythropoietin (Epo) production in vitro is suppressed by interleukin 1 beta (IL-1 beta), tumor necrosis factor alpha (TNF) and phorbol esters. Herein, the Epo-synthesizing human hepatoma cell line HepG2 was used to investigate whether protein kinase C (PKC) is involved in the inhibitory action of the cytokines. Within 1 h after the onset of hypoxia, Epo mRNA levels were markedly increased in untreated HepG2 cells as quantitated by competitive reverse transcription PCR. The cytokines IL-1 beta and TNF prevented this hypoxia-induced increase in Epo mRNA levels. In phorbol-ester-treated cells first inhibitory effects on Epo mRNA levels were observed only after 3 h. Western blot analyses revealed the presence of four isoenzymes of PKC in HepG2 cells. None of these isoenzymes was translocated in response to TNF or IL-1 beta, suggesting that the cytokines do not activate PKC in HepG2 cells. In contrast, phorbol esters translocated and, upon prolonged exposure, down-regulated PKC isoenzymes alpha and epsilon. Activation of protein kinase A by dibutyryl-cAMP partially antagonized the cytokine-dependent inhibition of Epo production but did not influence the inhibitory effect of phorbol esters. Endogenous cAMP levels in HepG2 cells were unchanged by cytokine treatment. Obviously, at least two signaling pathways exist that can confer inhibition of Epo production in HepG2 cells. One of these may be mediated by down-regulation of the PKC alpha or epsilon isoenzyme. The other pathway, however, which is triggered by IL-1 beta and TNF, is independent of PKC.

Activators of protein kinase C selectively mediate cellular cytotoxicity to hypoxic cells and not aerobic cells

PURPOSE

By understanding the signal transduction pathways through which a cell responds to changes in environmental oxygen levels, we may be able to therapeutically exploit this response by manipulating these pathways.

MATERIALS AND METHODS

The human adenocarcinoma cell line A549 was exposed to varying durations of hypoxia alone and then plated for survival, or treated with PKC activating agents for 1 h before plating for survival. Western blots were used to determine the kinetics of PKC epsilon and phospholipase C induction.

RESULTS

The level of hypoxic killing was directly related to the time of exposure and inversely related to the level of oxygen in the environment. Exposure of the cells to protein kinase C (PKC) activators for 1 h after chronic hypoxic exposure increased cell killing by at least an additional three logs beyond that found for hypoxia alone. Treatment of cells with an inactive phorbol ester 4 alpha-phorbol-12,13-didecanoate (PDA) resulted in no increase in hypoxic cell killing, even at the highest concentrations of PDA which produced no detectable toxicity under normal aerobic conditions. Using inhibitors of phospholipases A2 and C, we were able to completely inhibit the additional hypoxic cell killing induced by TPA, but not the uninduced hypoxic cell killing.

CONCLUSION

These studies suggest that accumulation of phospholipid breakdown products may be responsible for TPA induced cell killing, and that hypoxic cells differ from aerobic cells in their ability to tolerate these products.

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

To define the role of protein kinase C (PKC) in oxygen-dependent production of erythropoietin (EPO) in the liver, we have determined EPO messenger ribonucleic acid (mRNA) expression in primary cultures of juvenile rat hepatocytes incubated at different oxygen tensions in the absence and presence of phorbol esters, vasopressin, and structurally different kinase inhibitors. Upon reduction of oxygen concentrations from 40% to 3% EPO mRNA in cultured hepatocytes increased markedly within 1.25 h, reached maximal values after 2.5 h and remained elevated for up to 72 h. Treatment of hepatocytes during 1.25-5 h of hypoxic exposure with phorbol 12-myristate-13 acetate (PMA) attenuated hypoxia-induced EPO mRNA levels dose-dependently by a maximum of approximately 50%. This inhibitory effect of PMA disappeared upon treatment for more than 5 h and was completely lost after incubation for 9 and 18 h in the presence of 10(-6) M and 10(-7) M PMA, respectively. Phorbol 12,13-dibutyrate and vasopressin also inhibited EPO mRNA accumulation, whereas 4 alpha-phorbol 12,13-didecanoate was ineffective. Western blot analysis of PKC isozymes revealed the presence of PKC alpha, beta II, delta, epsilon and zeta and provided no evidence that the PMA-induced inhibition of EPO expression was associated with depletion of any of these isozymes. Conversely, PMA-induced inhibition of EPO mRNA accumulation was paralleled by translocation of PKC alpha from cytosol to membranes and the time- and dose-dependent attenuation of the inhibitory effect of PMA on EPO mRNA levels was paralleled by down-regulation of PKC alpha. A dose-dependent inhibition of EPO mRNA formation, independent of effects on total RNA synthesis, as determined by [3H]uridine incorporation, was also found in the presence of the kinase inhibitor staurosporine (ED50 approximately 2 x 10(-8) M) and three structurally related derivatives with increased selectivity for PKC (RO 317549, ED50 approximately 1 x 10(-6) M; RO 318220, ED50 approximately 1 x 10(-6) M and CGP 41251, ED50 approximately 4 x 10(-6) M). The markedly lower potency of the latter three compounds as compared to staurosporine suggests that this suppression of EPO gene induction was not mediated by inhibition of PKC. In summary the data indicate that PKC alpha is a negative modulator of EPO gene expression in hepatocytes. A kinase other than PKC, however, appears to be an essential element of hypoxic signalling.