Uncoupling of hepatic, epidermal growth factor-mediated mitogen-activated protein kinase activation in the fetal rat

Hepatic Gene Expression During the Perinatal Transition in the Rat

During the immediate postnatal (PN) period, the liver, with its role in energy metabolism and macromolecule synthesis, plays a central role in the perinatal transition. Using RNA microarrays and several complementary computational analyses, we characterized changes in hepatic gene expression in the rat across a developmental period starting with the late gestation fetus (embryonic day 21), and including 30 min PN, 4 h PN, 12 h PN, 1 day PN, and 1 week after birth. Following subtle changes in gene expression at the earliest PN time point, there were marked changes that occurred between 4 and 12 h after birth. These reflected changes in multiple metabolic pathways, with expression of enzymes involved in glycolysis and cholesterol synthesis showing the greatest change. Over 50% of nuclear-encoded mitochondrial genes changed in the first 7 days of PN life, with 25% changing within the first 24 h. We also observed changes coinciding with a transient period of synchronous hepatocyte proliferation that we had observed previously, which occurs during the first PN week. Analysis for upstream regulators of gene expression indicated multiple initiating factors, including cell stress, hormones, and cytokines. Also implicated were multiple canonical transcription factor networks. We conclude that changes in gene expression during the early phases of the perinatal transition involve a complex, choreographed network of signaling pathways that respond to a variety of environmental stimuli. This transcriptomic response during the immediate PN period reflects a complex metabolic adaptive response that incorporates a panoply of signaling pathways and transcriptional regulators.

Overexpression of Tpl2 is linked to imatinib resistance and activation of MEK-ERK and NF-κB pathways in a model of chronic myeloid leukemia

The introduction of tyrosine kinase inhibitors (TKI) has transformed chronic myeloid leukemia (CML) into a chronic disease with long-term survival exceeding 85%. However, resistance of CML stem cells to TKI may contribute to the 50% relapse rate observed after TKI discontinuation in molecular remission. We previously described a model of resistance to imatinib mesylate (IM), in which K562 cells cultured in high concentrations of imatinib mesylate showed reduced Bcr-Abl1 protein and activity levels while maintaining proliferative potential. Using quantitative phosphoproteomic analysis of these IM-resistant cells, we have now identified significant upregulation of tumor progression locus (Tpl2), also known as cancer Osaka thyroid (COT1) kinase or Map3k8. Overexpression of Tpl2 in IM-resistant cells was accompanied by elevated activities of Src family kinases (SFKs) and NF-κB, MEK-ERK signaling. CD34+ cells isolated from the bone marrow of patients with CML and exposed to IMin vitro showed increased MAP3K8 transcript levels. Dasatinib (SFK inhibitor), U0126 (MEK inhibitor), and PS-1145 (IκB kinase (IKK) inhibitor) used in combination resulted in elimination of 65% of IM-resistant cells and reduction in the colony-forming capacity of CML CD34+ cells in methylcellulose assays by 80%. In addition, CML CD34+ cells cultured with the combination of inhibitors showed reduced MAP3K8 transcript levels. Overall, our data indicate that elevated Tpl2 protein and transcript levels are associated with resistance to IM and that combined inhibition of SFK, MEK, and NF-κB signaling attenuates the survival of IM-resistant CML cells and CML CD34+ cells. Therefore, combination of SFK, MEK, and NF-κB inhibitors may offer a new therapeutic approach to overcome TKI resistance in CML patients.

Engraftment and Repopulation Potential of Late Gestation Fetal Rat Hepatocytes

BACKGROUND

The limited availability of donor organs has led to a search for alternatives to liver transplantation to restore liver function and bridge patients to transplantation. We have shown that the proliferation of late gestation (embryonic day 19) fetal rat hepatocytes is mitogen-independent and that mechanisms regulating mRNA translation, cell cycle progression, and gene expression differ from those of adult rat hepatocytes. In the present study, we investigated whether E19 fetal hepatocytes can engraft and repopulate an injured adult liver.

METHODS

Fetal hepatocytes were isolated using a monoclonal antibody against a hepatic surface protein, leucine amino peptidase (LAP). LAP+ and LAP- fractions were analyzed by immunofluorescence and microarray. Immunopurified E19 liver cells from DPPIV+ rats were transplanted via splenic injection into partial hepatectomized DPPIV- rats that had been pretreated with mitomycin C.

RESULTS

More than a third of LAP+ fetal hepatocytes expressed ductal markers. Transcriptomic analysis revealed that these dual-expressing cells represent a population of less well-differentiated hepatocytes. Upon transplantation, LAP+ late gestation fetal hepatocytes formed hepatic, endothelial, and ductal colonies within 1 month. By 10 months, colonies derived from LAP+ cells increased so that up to 35% of the liver was repopulated by donor-derived cells.

CONCLUSIONS

Late gestation fetal hepatocytes, despite being far along in the differentiation process, possess the capacity for extensive liver repopulation. This is likely related to the unexpected presence of a significant proportion of hepatocyte marker-positive cells maintaining a less well-differentiated phenotype.

Regulation of fetal liver growth in a model of diet restriction in the pregnant rat

Limited nutrient availability is a cause of intrauterine growth restriction (IUGR), a condition that has important implications for the well being of the offspring. Using the established IUGR model of maternal fasting in the rat, we investigated mechanisms that control gene expression and mRNA translation in late-gestation fetal liver. Maternal fasting for 48 h during the last one-third of gestation was associated with a 10-15% reduction in fetal body weight and a disproportionate one-third reduction in total fetal liver protein. The fetal liver transcriptome showed only subtle changes consistent with reduced cell proliferation and enhanced differentiation in IUGR. Effects on the transcriptome could not be attributed to specific transcription factors. We purified translating polysomes to profile the population of mRNAs undergoing active translation. Microarray analysis of the fetal liver translatome indicated a global reduction of translation. The only targeted effect was enhanced translation of mitochondrial ribosomal proteins in IUGR, consistent with enhanced mitochondrial biogenesis. There was no evidence for attenuated signaling through the mammalian target of rapamycin (mTOR). Western blot analysis showed no changes in fetal liver mTOR signaling. However, eukaryotic initiation factor 2α (eIF2α) phosphorylation was increased in livers from IUGR fetuses, consistent with a role in global translation control. Our data indicate that IUGR-associated changes in hepatic gene expression and mRNA translation likely involve a network of complex regulatory mechanisms, some of which are novel and distinct from those that mediate the response of the liver to nutrient restriction in the adult rat.

Regulation of liver development: implications for liver biology across the lifespan

The liver serves a spectrum of essential metabolic and synthetic functions that are required for the transition from fetal to postnatal life. Processes essential to the attainment of adequate liver mass and function during fetal life include cell lineage specification early in development, enzymic and other functional modes of differentiation throughout gestation, and ongoing cell proliferation to achieve adequate liver mass. Available data in laboratory rodents indicate that the signaling networks governing these processes in the fetus differ from those that can sustain liver function and mass in the adult. More specifically, fetal hepatocytes may develop independent of key mitogenic signaling pathways, including those involving the Erk mitogen-activated protein kinases MAPK1/3 and the mechanistic target of rapamycin (mTOR). In addition, the fetal liver is subject to environmental influences that, through epigenetic mechanisms, can have sustained effects on function and, by extension, contribute to the developmental origin of adult metabolic disease. Finally, the mitogen-independent phenotype of rat fetal hepatocytes in late gestation makes these cells suitable for cell-based therapy of liver injury. In the aggregate, studies on the mechanisms governing fetal liver development have implications not only for the perinatal metabolic transition but also for the prevention and treatment of liver disorders throughout the lifespan.

Egr-1 and serum response factor are involved in growth factors- and serum-mediated induction of E2-EPF UCP expression that regulates the VHL-HIF pathway

E2-EPF ubiquitin carrier protein (UCP) has been shown to be highly expressed in common human cancers and target von Hippel-Lindau (VHL) for proteosomal degradation in cells, thereby stabilizing hypoxia-inducible factor (HIF)-1alpha. Here, we investigated cellular factors that regulate the expression of UCP gene. Promoter deletion assay identified binding sites for early growth response-1 (Egr-1) and serum response factor (SRF) in the UCP promoter. Hepatocyte or epidermal growth factor (EGF), or phorbol 12-myristate 13-acetate induced UCP expression following early induction of Egr-1 expression in HeLa cells. Serum increased mRNA and protein levels of SRF and UCP in the cell. By electrophoretic mobility shift and chromatin immunoprecipitation assays, sequence-specific DNA-binding of Egr-1 and SRF to the UCP promoter was detected in nuclear extracts from HeLa cells treated with EGF and serum, respectively. Overexpression of Egr-1 or SRF increased UCP expression. RNA interference-mediated depletion of endogenous Egr-1 or SRF impaired EGF- or serum-mediated induction of UCP expression, which was required for cancer cell proliferation. Systemic delivery of EGF into mice also increased UCP expression following early induction of Egr-1 expression in mouse liver. The induced UCP expression by the growth factors or serum increased HIF-1alpha protein level under non-hypoxic conditions, suggesting that the Egr-1/SRF-UCP-VHL pathway is in part responsible for the increased HIF-1alpha protein level in vitro and in vivo. Thus, growth factors and serum induce expression of Egr-1 and SRF, respectively, which in turn induces UCP expression that positively regulates cancer cell growth.

Hepatic translation control in the late-gestation fetal rat

We have investigated the regulation of translation during the period of rapid liver growth that occurs at the end of gestation in the rat. This work was based on our prior observation that fetal hepatocyte proliferation is resistant to the inhibitory effects of rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), a nutrient-sensing kinase that controls ribosome biogenesis and protein translation. We hypothesized that translation control in late-gestation fetal liver differs from that in adult liver. We first examined the ability of rapamycin to inhibit the translation of mRNAs encoding ribosomal proteins. Consistent with the effect of rapamycin on proliferation, the activation of adult liver 5'-terminal oligopyrimidine tracts (5'-TOP) translation that occurred during refeeding after food deprivation was sensitive to rapamycin. Fetal liver 5'-TOP translation was insensitive. We went on to examine the eukaryotic initiation factor (eIF) 4F cap-binding complex that controls global protein synthesis. The molecular weights of the multiple eIF4G1 isoforms present in fetal and adult liver eIF4F complexes differed. In addition, fetal liver expressed the eIF4A1 form of the eIF4A helicase, whereas adult liver contained eIF4A1 and eIF4A2. Rapamycin administration before refeeding in adult rats inhibited formation of the preinitiation complex to a much greater degree than rapamycin administration to fetal rats in situ. We conclude that there are major structural and functional differences in translation control between late-gestation fetal and adult liver. These differences may confer differential sensitivity to the growth inhibitory effects of rapamycin.

The effect of rapamycin on DNA synthesis in multiple tissues from late gestation fetal and postnatal rats

Rapamycin is a potent antiproliferative agent that arrests cells in the G1 phase of the cell cycle through a variety of mechanisms involving the inhibition of the mammalian target of rapamycin (mTOR) pathway. The majority of normal cells in culture are sensitive to the cytostatic effects of rapamycin, whereas the growth of many malignant cells and tumors is rapamycin resistant. We had shown previously that hepatic DNA synthesis in the late gestation rat fetus is rapamycin resistant even though signaling through the mTOR/S6 kinase (S6K) pathway is attenuated. On the basis of this finding, we went on to characterize the response to rapamycin in a spectrum of tissues during late gestation and the early postnatal period in the rat. We found that rapamycin had no effect on DNA synthesis in major organs such as heart, intestine, and kidney in the fetal and early postnatal rat despite a marked attenuation in the phosphorylation of ribosomal protein S6. In contrast, the proliferation of mature hepatocytes during liver regeneration was highly sensitive to rapamycin. These data indicate that basal cellular proliferation in a wide variety of tissues is rapamycin resistant and occurs independently of mTOR/S6K signaling. Furthermore, the well-characterized effects of rapamycin in tissue culture systems are not recapitulated in the asynchronous cell proliferation that accompanies normal growth and tissue remodeling.

Tissue assembly guided via substrate biophysics: applications to hepatocellular engineering

The biophysical nature of the cellular microenvironment, in combination with its biochemical properties, can critically modulate the outcome of three-dimensional (3-D) multicellular morphogenesis. This phenomenon is particularly relevant for the design of materials suitable for supporting hepatocellular cultures, where cellular morphology is known to be intimately linked to the functional output of the cells. This review summarizes recent work describing biophysical regulation of hepatocellular morphogenesis and function and focuses on the manner by which biochemical cues can concomitantly augment this responsiveness. In particular, two distinct design parameters of the substrate biophysics are examined--microtopography and mechanical compliance. Substrate microtopography, introduced in the form of increasing pore size on collagen sponges and poly(glycolic acid) (PGLA) foams, was demonstrated to restrict the evolution of cellular morphogenesis to two dimensions (subcellular and cellular void sizes) or induce 3-D cellular assembly (supercellular void size). These patterns of morphogenesis were additionally governed by the biochemical nature of the substrate and were highly correlated to resultant levels of cell function. Substrate mechanical compliance, introduced via increased chemical crosslinking of the basement membrane, Matrigel, and polyacrylamide gel substrates, also was shown to be able to induce active two-dimensional (2-D, rigid substrates) or 3-D (malleable substrates) cellular reorganization. The extent of morphogenesis and the ensuing levels of cell function were highly dependent on the biochemical nature of the cellular microenvironment, including the presence of increasing extracellular matrix (ECM) ligand and growth-factor concentrations. Collectively, these studies highlight not only the ability of substrate biophysics to control hepatocellular morphogenesis but also the ability of biochemical cues to further enhance these effects. In particular, results of these studies reveal novel means by which hepatocellular morphogenesis and assembly can be rationally manipulated leading to the strategic control of the expression of liver-specific functions for hepatic tissue-engineering applications.

Coordinated regulation of c-Myc and Max in rat liver development

The processes of liver development and regeneration involve regulation of a key network of transcription factors, the c-myc/max/mad network. This network regulates the expression of genes involved in hepatocyte proliferation, growth, metabolism, and differentiation. In previous studies on the expression and localization of c-Myc in the fetal and adult liver, we made the unexpected observation that c-Myc content was similar in the two. However, c-Myc was localized predominantly to the nucleolus in the adult liver. On the basis of this finding, we went on to characterize the expression patterns of the other members of the network, max and mad, comparing their regulation during late fetal development with the proliferation of mature hepatocytes that is seen in liver regeneration. We found that Max content, rather than being constitutive, as predicted by other studies, was elevated in the fetal liver compared with the adult liver. Its content correlated with hepatocyte proliferation during the perinatal transition. In contrast, mad4 expression was decreased in the fetal liver compared with the adult liver. Nucleolar localization of c-Myc coincided with changes in Max content. To explore this relationship, we overexpressed Max in cultured adult hepatocytes. High levels of Max resulted in a shift in c-Myc localization from nucleolar to diffuse nuclear. In contrast, liver regeneration was associated with an increase in c-Myc content but no change in Max content. We conclude that the regulation of Max content during liver development and its potential role in determining c-Myc localization are means by which Max may control the biological activity of the c-Myc/Max/Mad network during liver development.

D-type cyclins and G1 progression during liver development in the rat

Initiation and progression through G1 requires the activity of signaling complexes containing cyclins (D- or E-type) and cyclin-dependent kinases (CDK4/6 and CDK2, respectively). We set out to identify the G1-phase cyclins and CDKs that are operative during late gestation liver development in the rat. This is a period during which hepatocytes show a high rate of proliferation that is, at least in part, independent of the mitogenic signaling pathways that are functional in mature hepatocytes. RNase protection assay and Western immunoblotting indicated that cyclin D1 is expressed at similar levels in fetal and adult liver. When cyclin D1 was induced after partial hepatectomy, its predominant CDK-binding partner was CDK4. In contrast, cyclins D2 and D3 predominated in fetal liver and were complexed with both CDK4 and CDK6. Little CDK6 protein was expressed in quiescent or regenerating adult liver. Cyclins E1 and E2 were both transcriptionally up-regulated in fetal liver. Activity of complexes containing cyclins E1 and E2 was higher in fetal liver, as was content of the cell cycle regulator, Rb. In fetal liver, Rb was highly phosphorylated at both cyclin D- and cyclin E-dependent sites. In conclusion, liver development is associated with a switch from cyclin D2/D3-containing complexes to cyclin D1:CDK4 complexes. We speculate that the switch in D-type cyclins may be associated with the dependence on mitogenic signaling that develops as hepatocytes mature.

Mitochondrial nitric oxide synthase drives redox signals for proliferation and quiescence in rat liver development

Mitochondrial nitric oxide synthase (mtNOS) is a fine regulator of oxygen uptake and reactive oxygen species that eventually modulates the activity of regulatory proteins and cell cycle progression. From this perspective, we examined liver mtNOS modulation and mitochondrial redox changes in developing rats from embryonic days 17-19 and postnatal day 2 (proliferating hepatocyte phenotype) through postnatal days 15-90 (quiescent phenotype). mtNOS expression and activity were almost undetectable in fetal liver, and progressively increased after birth by tenfold up to adult stage. NO-dependent mitochondrial hydrogen peroxide (H(2)O(2)) production and Mn-superoxide dismutase followed the developmental modulation of mtNOS and contributed to parallel variations of cytosolic H(2)O(2) concentration ([H(2)O(2)](ss)) and cell fluorescence. mtNOS-dependent [H(2)O(2)](ss) was a good predictor of extracellular signal-regulated kinase (ERK)/p38 activity ratio, cyclin D1, and tissue proliferation. At low 10(-11)-10(-12) M [H(2)O(2)](ss), proliferating phenotypes had high cyclin D1 and phospho-ERK1/2 and low phospho-p38 mitogen-activated protein kinase, while at 10(-9) M [H(2)O(2)](ss), quiescent phenotypes had the opposite pattern. Accordingly, leading postnatal day 2-isolated hepatocytes to embryo or adult redox conditions with H(2)O(2) or NO-H(2)O(2) scavengers, or with ERK inhibitor U0126, p38 inhibitor SB202190 or p38 activator anisomycin resulted in correlative changes of ERK/p38 activity ratio, cyclin D1 expression, and [(3)H] thymidine incorporation in the cells. Accordingly, p38 inhibitor SB202190 or N-acetyl-cysteine prevented H(2)O(2) inhibitory effects on proliferation. In conclusion, the results suggest that a synchronized increase of mtNOS and derived H(2)O(2) operate on hepatocyte signaling pathways to support the liver developmental transition from proliferation to quiescence.

Targeted hepatic overexpression of human IRS-1: postnatal effects in the developing mouse

Insulin receptor substrate-1 (IRS-1) is an intracellular docking protein involved in insulin and insulin-like growth factor (IGF) signaling. The present studies examine postnatal liver development in transgenic mice with targeted hepatic overexpression of human insulin receptor substrate-1 (hIRS-1). In mature animals, hIRS-1 overexpression augments liver growth. Based on our previous studies that have shown markedly attenuated insulin signaling in the late-gestation and early-postnatal rat, we hypothesized that the liver growth effect of overexpressed hIRS-1 would be attenuated in the neonatal period. Wild-type and heterozygous transgenic mice were studied at 1, 2, 4 or 8 weeks of age. Transgene expression was seen at all ages, albeit at a lower level in the youngest animals. Liver-to-carcass weight ratios were similar in hIRS-1 and wild-type mice at 1 and 2 weeks of age. At 4 and 8 weeks, transgenic mice had larger livers accounted for by increased hepatocyte number, not size. In addition, the transgenic mice had increased liver glycogen content at 8 weeks but not at 1 week. Relative to transgene mRNA expression, hIRS-1 protein levels were restricted in the younger animals. However, IRS-1-associated phosphatidylinositol-3 kinase (PI3K) activity was not similarly suppressed. Downstream from IRS-1, we found activation of the signaling kinase Akt in 8-week-old but not in 1-week-old animals. Our findings indicate that hepatic IRS-1-mediated signaling may be limited in neonatal mice at two levels, post-transcriptional down-regulation of IRS-1 content and attenuated signaling beyond the level of PI3K activation.

Insulin signaling through insulin receptor substrate 1 and 2 in normal liver development

BACKGROUND & AIMS

The insulin growth factor signal transduction pathway is an important regulator of adult hepatocyte proliferation. The purpose of this study was to determine the roles of the insulin receptor substrate (IRS-1 and IRS-2)-mediated growth cascades in rapidly growing fetal rat liver.

METHODS

We determined the expression and tyrosyl phosphorylation of the insulin receptor beta subunit (IRbeta), IRS-1 and IRS-2, the binding of phosphatidylinositol 3-kinase (PI3K), and activation of the mitogen-activated protein kinase (MAPK) pathway in the presence or absence of insulin stimulation in vivo during development and in the adult liver. In addition, activation of other downstream components including PI3K, Akt, GSK3beta, Bad, and p70S6 kinase was studied.

RESULTS

We observed reduced expression and tyrosyl phosphorylation of IRS-1 in the fetal liver compared with the adult liver. These developmental changes resulted in a lack of sensitivity to insulin stimulation and subsequent downstream activation of the PI3K and MAPK cascades until the postneonatal period. In contrast, there was a high level of IRS-2 expression and insulin-stimulated tyrosyl phosphorylation as early as embryonic day 15 with robust PI3K binding and activation, which may enhance hepatocyte survival during the rapid growth phase of the liver.

CONCLUSIONS

The IRS-1 signal transduction pathway does not play a major role in fetal liver growth because IRS-2 functions as the major insulin responsive molecule in early development. However, insulin-mediated IRS-1/MAPK cascade activation contributes to growth in the adult.

Insulin signaling during perinatal liver development in the rat

Insulin has long been assigned a key role in the regulation of growth and metabolism during fetal life. Our prior observations indicated that hepatic insulin signaling is attenuated in the late-gestation fetal rat. Therefore, we studied the perinatal ontogeny of hepatic insulin signaling extending from phosphatidylinositol 3-kinase (PI3K) to the ribosome. Initial studies demonstrated markedly decreased insulin-mediated activation of ribosomal protein S6 kinase 1 (S6K1) in the fetus. We found a similar pattern in the regulation of Akt, a kinase upstream from S6K1. Insulin produced minimal activation of insulin receptor substrate (IRS)-1-associated PI3K activity in fetal liver. A modest IRS-2-associated response was seen in the fetus. However, levels of both IRS-1 and IRS-2 were very low in fetal liver relative to adult liver. IRS-1 content and insulin responsiveness of PI3K, Akt, and S6K1 showed a transition to the adult phenotype during the first several postnatal weeks. Examination of downstream insulin signaling to the translational apparatus showed marked attenuation, relative to the adult, of fetal hepatic insulin-mediated phosphorylation of 4E-BP1, the regulatory protein for the eukaryotic initiation factor eIF4E, and ribosomal protein S6. The mammalian target of rapamycin (mTOR), a key integrator of nutritional and metabolic regulation of translation, was present in low amounts, was hypophosphorylated, and was not insulin sensitive in the fetus. Our results indicate that protein synthesis during late-gestation liver development may be mTOR and insulin independent. Reexamination of the role of insulin in fetal liver physiology may be warranted.

Ribosomal protein S6 phosphorylation and function during late gestation liver development in the rat

The phosphorylation of ribosomal protein S6 is thought to be required for biosynthesis of the cell's translational apparatus, a critical component of cell growth and proliferation. We have studied the signal transduction pathways involved in hepatic S6 phosphorylation during late gestation in the rat. This is a period during which hepatocytes show a high rate of proliferation that is, at least in part, independent of mitogenic signaling pathways that are operative in mature hepatocytes. Our initial studies demonstrated that there was low basal activity of two S6 kinases in liver, S6K1 and S6K2, on embryonic day 19 (2 days preterm). In addition, insulin- and growth factor-mediated S6K1 and S6K2 activation was markedly attenuated compared with that in adult liver. Nonetheless, two-dimensional gel electrophoresis demonstrated that fetal liver S6 itself was highly phosphorylated. To characterize the fetal hepatocyte pathway for S6 phosphorylation, we went on to study the sensitivity of hepatocyte proliferation to the S6 kinase inhibitor rapamycin. Unexpectedly, administration of rapamycin to embryonic day 19 fetuses in situ did not affect hepatocyte DNA synthesis. This resistance to the growth inhibitory effect of rapamycin occurred even though S6K1 and S6K2 were inhibited. Furthermore, fetal hepatocyte proliferation was sustained even though rapamycin administration resulted in the dephosphorylation of ribosomal protein S6. In contrast, rapamycin blocked hepatic DNA synthesis in adult rats following partial hepatectomy coincident with S6 dephosphorylation. We conclude that hepatocyte proliferation in the late gestation fetus is supported by a rapamycin-resistant mechanism that can function independently of ribosomal protein S6 phosphorylation.

Epidermal growth factor induces Egr-1 promoter activity in hepatocytes in vitro and in vivo

Early growth response-1 (Egr-1) is a transcription factor that couples short-term changes in the extracellular milieu to long-term changes in gene expression. Under in vitro conditions, the Egr-1 gene has been shown to respond to many extracellular signals. In most cases, these findings have not been extended to the in vivo setting. The goal of the present study was to explore the role of epidermal growth factor (EGF) in mediating Egr-1 expression in hepatocytes under both in vitro and in vivo conditions. In HepG2 cells, Egr-1 protein and mRNA were upregulated in the presence of EGF. In stable transfections of HepG2 cells, a 1,200-bp Egr-1 promoter contained information for EGF response via a protein kinase C-independent, mitogen-activated protein kinase-dependent signaling pathway. A promoter region containing the two most proximal serum response elements was sufficient to transduce the EGF signal. In transgenic mice that carry the Egr-1 promoter coupled to the LacZ reporter gene, systemic delivery of EGF by intraperitoneal injection resulted in an induction of the endogenous Egr-1 gene and the Egr-1-lacZ transgene in hepatocytes. Together, these results suggest that the 1,200-bp promoter contains information for EGF response in hepatocytes both in vitro and in intact animals.

Beta-adrenergic receptors (betaAR) regulate cardiomyocyte proliferation during early postnatal life

Cardiomyocyte development switches from hyperplasmic to hypertrophic growth between postnatal days 3 and 4 in rats. The mechanisms responsible for this transition have been controversial. beta-Adrenergic receptor (betaAR) activation of mitogenic responses in vitro has been reported. We hypothesized that tonic activation of the betaAR signaling regulates cell division in neonatal cardiomyocytes via effects on signaling kinases known to be important in cell cycle regulation. The purpose of the current study was to elucidate the roles of betaAR in rat cardiomyocyte growth in vivo. We demonstrated that betaAR blockade induced a significant reduction in cardiomyocyte proliferation as measured by the BrdU labeling index. Blockade of betaAR did not affect p38 or p44/42 MAPK activities. We further demonstrated that betaAR blockade induced a prompt deactivation of the p70 ribosomal protein S6 kinase (p70 S6K). To confirm these results, we measured p70 S6K activity directly. Basal activity of p70 S6K in neonatal cardiomyocytes was fourfold higher than that of insulin-treated adult rat liver. The activity of p70 S6K was reduced by 60% within 1 min after betaAR blockade. We conclude that the betaAR are involved in regulation of neonatal cardiomyocyte proliferation and that this mitogenic control may be mediated via the p70 S6K pathway.

Growth regulation via p38 mitogen-activated protein kinase in developing liver

During normal development in the rat, hepatocytes undergo marked changes in the rate of proliferation. We have previously observed transient G(1) growth arrest at term, re-activation of proliferation immediately after birth, and a gradual transition to the quiescent adult hepatocyte phenotype after postnatal day 4. We hypothesized that these changes in proliferation are due in part to growth inhibitory effects mediated by the p38 mitogen-activated protein kinase pathway. p38 kinase activity measurements showed an inverse relationship with hepatocyte proliferation during the perinatal and postnatal transitions, whereas p38 content remained constant. Anisomycin activated the p38 pathway in fetal hepatocyte cultures while inducing growth inhibition that was sensitive to the p38 inhibitor, SB203580. Activation of p38 in these cultures, via transient transfection with a constitutively active form of its upstream kinase MKK6, also inhibited DNA synthesis as well as reducing cyclin D1 content. Transfection with inactive MKK6 did neither. Furthermore, MKK6-induced growth arrest was sensitive to SB203580. Finally, administration of SB203580 to near-term fetal rats in utero abrogated the transient hepatocyte growth arrest that occurs at term. These findings indicate a role for the p38 mitogen-activated protein kinase pathway in the physiological regulation of hepatocyte proliferation during normal development in the rat.

Identification of candidate growth-regulating genes that are overexpressed in late gestation fetal liver in the rat

We have shown previously that hepatocyte proliferation in the late gestation fetal rat is mediated by growth factor-independent mechanisms that are distinct from the signaling pathways that promote proliferation of adult rat hepatocytes. In the present studies, we identified six candidate growth-regulating genes that are overexpressed in fetal rat liver (embryonic day 19, 2 days pre-term) relative to adult rat liver using suppressive subtractive hybridization. These included the following: Grb10, a growth factor receptor binding protein; eps15, a growth factor receptor substrate; nuc2+, a retinoblastoma protein binding protein; cdc25B, a cell cycle tyrosine phosphatase; the peroxisome proliferator-activated receptor PPAR alpha; and a deoxyuridine triphosphatase that functions as a PPAR alpha binding partner. In every case, the ontogeny of the expression of these genes declined postnatally in a manner consistent with the transition from a fetal to an adult hepatocyte phenotype. None were found to be cell cycle-dependent, in that they did not show expression that followed perinatal changes in hepatocyte cell cycle activity. Based on our identification of these genes and previous work characterizing their role in growth regulation, we conclude that they may contribute to the mitogenic signaling phenotype of fetal rat hepatocytes.

Hepatic growth hormone signaling in the late gestation fetal rat

The role of GH in the developing fetus is poorly understood. Several studies have demonstrated a limited role for GH in late fetal life. In fact, few data are available regarding GH signal transduction in the late gestation fetus. We therefore focused on a comparison of hepatic GH signaling in near-term fetal rats [embryonic day 19 (E19)] and adult rats using a combination of in vitro studies employing hepatocytes in primary culture and in vivo studies. We found that GH receptor (GHr) binding was comparable in fetal liver and adult liver. The long isoform of the GHr underwent tyrosine phosphorylation in response to GH stimulation of E19 fetal hepatocytes in a manner similar to that seen in cultured adult hepatocytes. Furthermore, downstream signaling via the Janus kinase-2 tyrosine kinase, STAT1 (signal transducer and activator of transcription), and STAT5 was also intact in both, as demonstrated by the tyrosine phosphorylation of these signaling proteins. To confirm the relevance of these findings to the in vivo situation, GH was directly administered by ip injection to E 19 fetal and adult rats. In both cases, tyrosine phosphorylation of STAT5 was markedly and rapidly induced. Finally, transfection of E19 fetal hepatocytes with GH-responsive reporter elements [Spi2.1(-275/+85)-CAT and 8xGHRE-TKCAT] demonstrated intact transcriptional regulation. Our data indicate that GHr abundance and activity as well as downstream GH signaling are similar in the late gestation fetal rat and in the adult and that these mechanisms appear capable of supporting physiological GH functions in the developing liver.

Egr-1 gene is induced by the systemic administration of the vascular endothelial growth factor and the epidermal growth factor

Egr-1 is a transcription factor that couples short-term changes in the extracellular milieu to long-term changes in gene expression. In cultured endothelial cells, the Egr-1 gene has been shown to respond to a variety of extracellular signals. However, the physiological relevance of these findings remains unclear. To address this question, the growth factor-mediated response of the Egr-1 gene under in vivo conditions was analyzed. To that end, either vascular endothelial growth factor (VEGF) or epidermal growth factor (EGF) was injected into the intraperitoneal cavity of mice. Growth factors were delivered to all tissues examined, as evidenced by the widespread distribution of I125-labeled growth factors and the phosphorylation of their respective receptors. In Western blot analyses of whole-tissue extracts, Egr-1 protein levels were shown to be induced in the heart, brain, liver, and spleen of VEGF-treated mice, and in the heart, lung, brain, liver and skeletal muscle of EGF-treated animals. Changes in Egr-1 levels did not correlate with changes in receptor phosphorylation or ERK1/2 phosphorylation. In Northern blot analyses, VEGF induced Egr-1 mRNA levels in all tissues examined except lung and kidney, whereas EGF led to increased transcripts in all tissues except kidney. In immunofluorescence studies, VEGF induced Egr-1 in microvascular endothelial cells of the heart and liver, and EGF induced Egr-1 in the microvascular bed of skeletal muscle. Taken together, these results suggest that the Egr-1 gene is differentially regulated in response to systemically administered VEGF and EGF.

Egr-1 gene is induced by the systemic administration of the vascular endothelial growth factor and the epidermal growth factor

Egr-1 is a transcription factor that couples short-term changes in the extracellular milieu to long-term changes in gene expression. In cultured endothelial cells, the Egr-1 gene has been shown to respond to a variety of extracellular signals. However, the physiological relevance of these findings remains unclear. To address this question, the growth factor-mediated response of the Egr-1 gene under in vivo conditions was analyzed. To that end, either vascular endothelial growth factor (VEGF) or epidermal growth factor (EGF) was injected into the intraperitoneal cavity of mice. Growth factors were delivered to all tissues examined, as evidenced by the widespread distribution of I125-labeled growth factors and the phosphorylation of their respective receptors. In Western blot analyses of whole-tissue extracts, Egr-1 protein levels were shown to be induced in the heart, brain, liver, and spleen of VEGF-treated mice, and in the heart, lung, brain, liver and skeletal muscle of EGF-treated animals. Changes in Egr-1 levels did not correlate with changes in receptor phosphorylation or ERK1/2 phosphorylation. In Northern blot analyses, VEGF induced Egr-1 mRNA levels in all tissues examined except lung and kidney, whereas EGF led to increased transcripts in all tissues except kidney. In immunofluorescence studies, VEGF induced Egr-1 in microvascular endothelial cells of the heart and liver, and EGF induced Egr-1 in the microvascular bed of skeletal muscle. Taken together, these results suggest that the Egr-1 gene is differentially regulated in response to systemically administered VEGF and EGF.

Treatment of mice with EGF and orthovanadate activates cytoplasmic and nuclear MAPK, p70S6k, and p90rsk in the liver

BACKGROUND/AIMS

Although signal transduction pathways activated by EGF have been extensively studied in cultured cells, few such studies have been done in whole animals. In this study, activation of hepatic kinases, phosphatases, and DNA-binding activity of AP-1 was examined after intraperitoneal injections of either EGF or sodium orthovanadate into mice.

METHODS

Cytoplasmic and nuclear proteins, extracted from isolated hepatocytes or whole liver tissue, were immunoprecipitated with either anti-ERK1/2, anti-70S6k, or anti-p90rsk antibodies and kinase activities were measured using specific substrates. Kinase protein levels was evaluated by Western blot analysis. AP-1 DNA binding activity was measured by electrophoretic mobility shift assay.

RESULTS

Systemic administration of EGF induced simultaneous increase in the activities of cytoplasmic and nuclear MAPK, p70S6k, and p90rsk. MAPK and p70S6k were more potently activated in the cytosol while p90rsk activation was more pronounced in the nucleus. Orthovanadate also activated these kinases but to a much lesser degree than EGF. In vitro phosphatase assays showed that neither EGF nor orthovanadate induced measurable changes in phosphatase activities. EGF, but not orthovanadate, activated nuclear AP-1 DNA-binding activity in intact liver, indicating that activation of MAPK, p70S6k, and p90rsk by orthovanadate is not sufficient to activate this transcription factor.

CONCLUSION

These observations provide groundwork for future studies to examine the role of EGF-induced kinase cascades and transcription factors in liver regeneration and other growth factor-mediated hepatic processes.

The role of protein kinase B and mitogen-activated protein kinase in epidermal growth factor and tumor necrosis factor alpha-mediated rat hepatocyte survival and apoptosis

Perturbation of hepatocyte growth regulation is associated with a number of liver diseases such as fibrosis and cancer. These diseases are mediated by a network of growth factors and cytokines that regulate the induction of hepatocyte proliferation and apoptosis. In this study, we have investigated the role of signaling pathways activated by tumor necrosis factor alpha (TNF-alpha) and epidermal growth factor (EGF) in the regulation of apoptosis induced by transforming growth factor beta(1) (TGF-beta(1)), because this physiological factor is believed to regulate spontaneous apoptosis in the liver. We show that pretreatment with (10 ng/mL) EGF or (25 ng/mL) TNF-alpha can suppress TGF-beta(1)-induced apoptosis by 73% and 50%, respectively, in isolated rat hepatocytes. However, suppression of TGF-beta(1)-induced apoptosis by EGF and TNF-alpha occurs via different protein kinase signaling pathways. Using specific inhibitors, we show that suppression of apoptosis by EGF is dependent on activation of phosphoinositide 3-kinase (PI 3-kinase) and the extracellular signal regulated kinase (ERK) mitogen-activated protein (MAP) kinase pathways, but not p38 MAP kinase. In contrast, suppression of TGF-beta(1)-induced apoptosis by TNF-alpha does not require PI 3-kinase and protein kinase B (PKB or Akt)-mediated pathways, but is dependent on ERK and p38 MAP kinase activity. These data contribute to our understanding of the intracellular survival signals that play a role in normal liver homeostasis and in diverse pathological conditions.

Hepatic epidermal growth factor-regulated mitogen-activated protein kinase kinase kinase activity in the rat: lack of identity with known forms of raf and MEKK

Mitogenic signaling involves protein kinases that phosphorylate the mitogen-activated protein kinase (MAPK) activator, MEK. In rats, basal hepatic MEK kinase activity is low in vivo in both adult rats and late gestation fetal rats, and is markedly stimulated by intraperitoneal administration of epidermal growth factor (EGF). The level of stimulated MEK phosphorylating activity is approximately 15 times higher in fetal liver than in adult liver. To identify regulated forms of the two categories of MEK kinase, Raf and MEKK, Western immunoblotting, immunoprecipitation kinase assays and immunodepletion studies were performed. Western immunoblotting confirmed that Raf-1, A-Raf, B-Raf, MEKK1 and MEKK2 were present at similar levels in E19 and adult liver. However, specific immunoprecipitation kinase assays did not detect any kinases that could account for marked EGF sensitivity or the higher level of activity in E19 fetuses. Immunodepletion studies produced a marked reduction in immunoreactive Raf/MEKK content and activity, but a minimal decrease in the ability of chromatography fractions to phosphorylate and activate recombinant MEK-1. Our results indicate that hepatic, EGF-sensitive MEK kinase activity may reside with a previously unidentified and physiologically relevant form of Raf and/or MEKK.

Mitogen-activated protein kinase regulates transcription of the ApoCIII gene. Involvement of the orphan nuclear receptor HNF4

The transcriptional regulation of the apoCIII gene by hormonal and metabolic signals plays a significant role in determining plasma triglyceride levels. In the current work we demonstrate that the apoCIII gene is regulated by the mitogen-activated protein (MAP) kinase signaling pathway. In HepG2 cells, repression of MAP kinase activity by treatment with the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase inhibitor PD98059 caused a 5-8-fold increase in apoCIII transcriptional activity. Activation of MAP kinase by phorbol ester treatment caused a 3-5-fold reduction in apoCIII transcription. The region of the apoCIII promoter responsible for this regulation was mapped in transiently transfected HepG2 cells to a 6-base pair element located at -740. The major protein binding to this site was identified as the nuclear hormone receptor HNF4. An increase in HNF4 mRNA and protein levels was observed in HepG2 cells after treatment with PD98059, indicating that the MAP kinase pathway regulates the expression of the HNF4 gene. These findings demonstrate that the apoCIII gene can be regulated by signals acting through the MAP kinase pathway and that this regulation is mediated, at least in part, by changes in the amount of HNF4.

Regulation of intestinal tyrosine phosphorylation and programmed cell death by peroxovanadate

Cell suspensions of ileal mucosa undergo a rapid and synchronized form of programmed cell death when cultured in a simple medium at 37 degrees C. Because tyrosine phosphorylation of proteins plays a crucial role in the signal transduction of many cellular processes, we examined its role in intestinal programmed cell death by use of immunoblot and immunohistochemical methods. We observed a 50-70% reduction in tyrosine phosphorylation during the initial 10 min of intestinal epithelial cell culture. We hypothesized that the inhibition of protein tyrosine phosphatases would increase protein tyrosine phosphorylation in these suspensions and decrease programmed cell death. A strong inhibitor of these phosphatases (peroxovanadate) but not a weaker one (sodium orthovanadate) abolished the DNA fragmentation/laddering normally seen in dying enterocytes. Peroxovanadate enhanced protein tyrosine phosphorylation of many intestinal proteins, dramatically increasing the dually phosphorylated and active form of mitogen-activated protein kinase. Immunohistochemistry revealed a particularly high level of increased tyrosine phosphorylation in the intestinal crypts in peroxovanadate-treated mucosa. Kinetic studies indicated that the pivotal time for protein tyrosine phosphatase inhibition occurred within 5 min of ex vivo culture, precisely when protein tyrosine phosphorylation declined. Our data suggest that tyrosine kinase inactivation or tyrosine phosphatase activation may initiate intestinal epithelial cell death.