Prenatal exposure to maternal low protein diet suppresses replicative potential of myocardial cells

BACKGROUND AND AIMS

We have previously shown that a maternal low protein (LP) diet during pregnancy results in severe depression of neonatal heart contractility due, in part, to an increase in apoptotic loss of cardiomyocytes. The aim of this study was to examine if maternal LP diet would alter replicative potential of neonatal myocardial cells.

METHODS AND RESULTS

We determined the effect of maternal LP and normal diet (90 and 180 g/casein/kg respectively) on relative numbers of mitotic myocardial cells in male offspring at birth and at 7-28 days post-partum. Myocardial cells undergoing mitosis were identified by dual-immunofluorescence of cardiac sections for cardiac muscle myosin and phosphorylated histone 3, whereas cells within the cell cycle were identified by immunoreactivity for Ki67 at 14-28 days post-partum. Neonates from control dams displayed the expected gradual decline in mitotic cells from birth to 28 days post-partum. Hearts from LP offspring had lower numbers of mitotic cells at birth, compared to controls, suggestive of subnormal muscle cell numbers at that stage. When placed in normal diet, LP offspring developed increased myocardial mitosis at 7 days compared to controls, which normalized to control levels at 21-28 days post-partum. An increase in Ki67-positive myocardial cells was also observed in the LP exposed group at 28 days of age.

CONCLUSION

Maternal LP diet suppresses myocardial replicative potential and this likely contributes to reduced cell numbers at birth. This suppression is lifted by a protein-replete diet which stimulates post-natal replication of myocardial cells and likely results in a catching-up in cell numbers.

PKC-Dependent Phosphorylation May Regulate the Ability of Connexin43 to Inhibit DNA Synthesis

Phosphorylation affects several biological functions of connexin43 (Cx43), although its role on Cx43-mediated inhibition of DNA synthesis is not known. Previous studies showed increased Cx43 phosphorylation on serine in response to growth factor stimulation of cardiomyocytes, mediated by protein kinase C-epsilon (PKCepsilon). Here we report that activation of PKCepsilon is also necessary for stimulation of cardiomyocyte DNA synthesis and mitosis. We have investigated the participation of specific serine residues that are putative PKC targets in producing phosphorylated Cx43 species and also in regulating DNA synthesis in cardiomyocytes. Interference with the PKC signaling system and/or the phosphorylation of specific amino-acids of Cx43 may allow regulation of the mitogenic response.

Protein kinase C-epsilon mediates phorbol ester-induced phosphorylation of connexin-43

We have used adenoviral vectors to express dominant negative variants of protein kinase C epsilon (PKCepsilon) or mitogen kinase kinase 1 (MKK1) to investigate their involvement in phorbol ester-induced connexin-43 (Cx43) phosphorylation in cardiomyocytes. Stimulation of cardiomyocytes with phorbol 12-myristate 13-acetate (PMA) increased the fraction of the slower migrating (> or = 45 kDa) and more extensively phosphorylated Cx43 species. Expression of dominant negative MKK1 did not prevent the effect of PMA on Cx43 phosphorylation. Selective inhibition of PKCE significantly decreased baseline levels of Cx43 phosphorylation and the PMA-induced accumulation of > or = 45 kDa Cx43. Thus, production of the more extensively phosphorylated species of Cx43 in cardiomyocytes by PMA requires activation of PKCepsilon.

CUG-initiated FGF-2 induces chromatin compaction in cultured cardiac myocytes and in vitro

Fibroblast growth factor-2 (FGF-2) is a mitogen found in CUG-initiated 21-25 kDa ("hi") or AUG-initiated 16-18 kDa ("lo") forms. Previously we demonstrated that "hi"-but not "lo"-FGF-2 caused a distinct nuclear phenotype characterized by apparently condensed chromatin present as separate clumps in the nucleus of cardiac myocytes. In this manuscript we investigated whether these effects were related to apoptosis or mitosis and whether they reflected a direct effect of "hi" FGF-2 on chromatin. Myocytes overexpressing "hi" FGF-2 and presenting the clumped chromatin phenotype: (i) were not labeled above background with antibodies to phosphorylated histones H1 and H3 used as indicators of mitotic chromatin condensation; (ii) did not stain positive for TUNEL; (iii) their nuclear lamina, visualized by anti-laminB immunofluorescence, appeared intact; (iv) neither caspase inhibitors, nor Bcl-2 or "lo" FGF-2 overexpression prevented the manifestation of the compacted nuclear phenotype. Purified recombinant "hi" FGF-2 was more potent than "lo" FGF-2 in promoting the condensation/aggregation of chick erythrocyte chromatin partially reconstituted with histone H1 in vitro. We conclude that the DNA phenotype induced by "hi" FGF-2 in cardiac myocytes likely reflects a direct effect on chromatin structure that does not require the engagement of mitosis or apoptosis. By affecting chromatin compaction "hi" FGF-2 may contribute to the regulation of gene expression.

Overexpression of FGF-2 increases cardiac myocyte viability after injury in isolated mouse hearts

We generated transgenic (TG) mice overexpressing fibroblast growth factor (FGF)-2 protein (22- to 34-fold) in the heart. Chronic FGF-2 overexpression revealed no significant effect on heart weight-to-body weight ratio or expression of cardiac differentiation markers. There was, however, a significant 20% increase in capillary density. Although there was no change in FGF receptor-1 expression, relative levels of phosphorylated c-Jun NH(2)-terminal kinase and p38 kinase as well as of membrane-associated protein kinase C (PKC)-alpha and total PKC-epsilon were increased in FGF-2-TG mouse hearts. An isolated mouse heart model of ischemia-reperfusion injury was used to assess the potential of increased endogenous FGF-2 for cardioprotection. A significant 34-45% increase in myocyte viability, reflected in a decrease in lactate dehydrogenase released into the perfusate, was observed in FGF-2 overexpressing mice and non-TG mice treated exogenously with FGF-2. In conclusion, FGF-2 overexpression causes augmentation of signal transduction pathways and increased resistance to ischemic injury. Thus, stimulation of endogenous FGF-2 expression offers a potential mechanism to enhance cardioprotection.

Lysophosphatidylethanolamine acyltransferase activity is elevated during cardiac cell differentiation

We examined if elevation in lysophosphatidylethanolamine acyltransferase activity was associated with elevation in phosphatidylethanolamine content during differentiation of P19 teratocarcinoma cells into cardiac myocytes. P19 cells were induced to undergo differentiation into cardiac myocytes by the addition of 1% dimethylsulfoxide to the medium. Immunofluorescence microscopy revealed the presence of striated myosin at 8 days post-dimethylsulfoxide addition confirming differentiation into cardiac cells. The content of phosphatidylethanolamine was increased 2.1-fold (P<0.05) in differentiated cells compared to undifferentiated cells, whereas the content of phosphatidylcholine was reduced 29% (P<0.05). There were no alterations in the pool sizes of other phospholipids, including cardiolipin. The relative abundance of fatty acids in phospholipids of P19 cells was 18:1 > 18:0 > 16:1 = 18:2 > 16:0 = 14:0 > 20:4 and differentiation did not affect the relative amounts of these fatty acids within individual phospholipids. When cells were incubated with [1,3-(3)H]glycerol, radioactivity incorporated into phosphatidylethanolamine was elevated 5.8-fold, whereas radioactivity incorporated into phosphatidylcholine was unaltered. Ethanolaminephosphotransferase, cholinephosphotransferase and membrane CTP:phosphocholine cytidylyltransferase activities were elevated in differentiated cells compared to undifferentiated cells, whereas membrane and cytosolic phospholipase A2 activities were unaltered. Lysophosphatidylethanolamine acyltransferase activities were elevated 2.4-fold (P<0.05). Lysophosphatidylcholine acyltransferase, monolysocardiolipin acyltransferase, acyl-Coenzyme A synthetase and acyl-Coenzyme A hydrolase activities were unaltered in differentiated cells compared to undifferentiated cells. We postulate that during cardiac cell differentiation, the observed elevation in lysophosphatidylethanolamine acyltransferase activity accompanies the elevation in phosphatidylethanolamine mass, possibly to maintain the fatty acyl composition of this phospholipid within the membrane.

Elevation in phosphatidylethanolamine is an early but not essential event for cardiac cell differentiation

The biosynthesis of phosphatidylethanolamine was examined during differentiation of P19 teratocarcinoma cells into cardiac myocytes. P19 cells were induced to undergo differentiation into cardiac myocytes by the addition of dimethyl sulfoxide to the medium. Immunofluorescence labeling confirmed the expression of striated myosin 10 days postinduction of differentiation. The content of phosphatidylethanolamine increased significantly within the first 2 days of differentiation. [1,3-(3)H]Glycerol incorporation into phosphatidylethanolamine was increased 7.2-fold during differentiation, indicating an elevation in de novo synthesis from 1, 2-diacyl-sn-glycerol. The mechanism for the increase in phosphatidylethanolamine levels during cardiac cell differentiation was a 2.8-fold increase in the activity of ethanolaminephosphotransferase, the 1,2-diacyl-sn-glycerol utilizing reaction of the cytidine 5'-diphosphate-ethanolamine pathway of phosphatidylethanolamine biosynthesis. Incubation of P19 cells with the phosphatidylethanolamine biosynthesis inhibitor 8-(4-chlorophenylthio)-cAMP inhibited the differentiation-induced elevation in phosphatidylethanolamine levels but did not affect the expression of striated myosin. The results suggest that elevation in phosphatidylethanolamine is an early event of P19 cell differentiation into cardiac myocytes, but is not essential for differentiation to proceed.

Fibroblast growth factor-2 stimulates phospholipase Cbeta in adult cardiomyocytes

Although fibroblast growth factor-2 (FGF-2) plays an important role in cardioprotection and growth, little is known about the signals triggered by it in the adult heart. We therefore examined FGF-2-induced effects on phosphoinositide-specific phospholipase C (PI-PLC) isozymes, which produce second messengers linked to the inotropic and hypertrophic response of the myocardium. FGF-2, administered by retrograde perfusion to the isolated heart, induced an increase in inositol-1,4,5-trisphosphate levels in the cytosol, as well as an increase in total PI-PLC activity associated with sarcolemmal and cytosolic fractions. Furthermore FGF-2 induced a time-dependent elevation in cardiomyocyte membrane-associated PLC gamma1 and PLC beta1 activities, assayed in immunoprecipitated fractions, and moreover, increased the membrane levels of PLC beta1 and PLC beta3. Activation of PLC beta is suggestive of FGF-2-induced cross-talk between FGF-receptor tyrosine kinase and G-protein-coupled signaling in adult cardiomyocytes and underscores the importance of FGF-2 in cardiac physiology.

The epsilon subtype of protein kinase C is required for cardiomyocyte connexin-43 phosphorylation

Gap junctions (GJs), composed of connexins, are intercellular channels ensuring electric and metabolic coupling between cardiomyocytes. We have shown previously that an endogenous mitogenic and cardioprotective protein, fibroblast growth factor-2 (FGF-2), decreases cardiomyocyte GJ permeability by stimulating phosphorylation of connexin-43 (Cx43). Identifying the kinase(s) phosphorylating cardiac Cx43 may thus provide a way of modulating cardiac intercellular communication. Because FGF-2 activates receptors linked to protein kinase C (PKC) and mitogen-activated protein kinase, we first investigated participation of these enzymatic systems in Cx43 phosphorylation. The inhibitor PD98059 blocked activation of mitogen-activated protein kinase, but it did not prevent the FGF-2 effects on GJs. In contrast, the PKC inhibitor chelerythrine blocked the effects of FGF-2 on Cx43 phosphorylation and permeability. Because the epsilon-isoform of PKC localizes to plasma membrane sites, we examined whether it is directly involved in the FGF-2-induced Cx43 phosphorylation. In nonstimulated myocytes, PKCepsilon displayed a discontinuous pattern of localization at intercellular contact sites and partial colocalization with Cx43. Treatment with FGF-2 or phorbol 12-myristate 13-acetate induced a more continuous pattern of PKCepsilon distribution, whereas the anti-Cx43 staining appeared to overlap extensively with that of PKCepsilon. In immunoprecipitation experiments using specific anti-Cx43 antibodies, PKCepsilon but not PKCalpha coprecipitated with Cx43. FGF-2 increased levels of coprecipitated PKCepsilon, suggesting increased association between PKCepsilon and Cx43 on stimulation. Transient gene transfer and overexpression of cDNAs coding for truncated or mutated dominant-negative forms of PKCepsilon decreased cardiomyocyte Cx43 phosphorylation significantly. We conclude that PKC mediates the FGF-2-induced effects on cardiac GJs and that PKCepsilon likely interacts with and phosphorylates cardiac Cx43 at sites of intercellular contact.

The subcellular distribution of protein kinase Calpha, -epsilon, and -zeta isoforms during cardiac cell differentiation

There is little information on the molecular events that control the subcellular distribution of protein kinase C during cardiac cell differentiation. We examined protein kinase C activity and the subcellular distribution of representatives of the "classical," "novel," and "atypical" protein kinase C's in P19 murine teratoma cells induced to undergo differentiation into cardiac myocytes by the addition of dimethylsulfoxide to the medium (Grepin et al., Development 124, 2387-2395, 1997). Differentiation was assessed by the presence of striated myosin, a morphological marker for cardiac cells. Addition of dimethyl sulfoxide to the medium resulted in the appearance of striated myosin by 10 days postincubation. Immunolocalization and Western blot studies revealed that a significant proportion of protein kinase Calpha, -epsilon, and -zeta were associated with the particulate fraction in P19 cells prior to differentiation. Differentiation into cardiac cells resulted in a translocation of protein kinase C activity from the particulate fraction to cytosol and localization of most of protein kinase Calpha, -epsilon, and -zeta to the cytoplasmic compartment. The total cellular protein kinase C activity was unaltered during differentiation. The translocation of protein kinase C activity during differentiation of P19 cells into cardiac myocytes was associated with a decrease in the levels of cellular 1, 2-diacyl-sn-glycerol. The cellular levels of phosphatidylserine and phosphatidylinositol did not change during differentiation. Addition of 1,2-dioctanoyl-sn-glycerol, a cell-permeant 1, 2-diacyl-sn-glycerol analog, reversed the differentiation-induced switch in the relative distribution of protein kinase C activity and dramatically increased the association of protein kinase Calpha with the particulate fraction. Addition of 1,2-dioctanoyl-sn-glycerol did not reverse the pattern of distribution for protein kinase Cepsilon or -zeta. The results indicate that protein kinase C activity and protein kinase Calpha, -epsilon and -zeta isoforms are redistributed from the particulate to the cytosolic fraction during differentiation of P19 cells into cardiomyocytes. The mechanism for the redistribution of protein kinase Calpha may be related to the reduction in the cellular 1,2-diacyl-sn-glycerol levels that accompany differentiation.

Immunolocalization of the sarcolemmal Ca2+/Mg2+ ecto-ATPase (myoglein) in rat myocardium

Cardiac plasma membrane Ca2+/Mg2+ ecto-ATPase (myoglein) requires millimolar concentrations of either Ca2+ or Mg2+ for maximal activity. In this paper, we report its localization by employing an antiserum raised against the purified rat cardiac Ca2+/Mg2+ ATPase. As assessed by Western blot analysis, the antiserum and the purified immunoglobulin were specific for Ca2+/Mg2+ ecto-ATPase; no cross reaction was observed towards other membrane bound enzymes such as cardiac sarcoplasmic reticulum Ca(2+)-pump ATPase or sarcolemmal Ca(2+)-pump ATPase. On the other hand, the cardiac Ca2+/Mg2+ ecto-ATPase was not recognized by antibodies specific for either cardiac sarcoplasmic reticulum Ca(2+)-pump ATPase or plasma membrane Ca(2+)-pump ATPase. Furthermore, the immune serum inhibited the Ca2+/Mg2+ ecto-ATPase activity of the purified enzyme preparation. Immunofluorescence of cardiac tissue sections and neonatal cultured cardiomyocytes with the Ca2+/Mg2+ ecto-ATPase antibodies indicated the localization of Ca2+/Mg2+ ecto-ATPase in association with the plasma membrane of myocytes, in areas of cell-matrix or cell-cell contact. Staining for the Ca2+/Mg2+ ecto-ATPase was not cardiac specific since the antibodies detected the presence of membrane proteins in sections from skeletal muscle, brain, liver and kidney. The results indicate that Ca2+/Mg2+ ecto-ATPase is localized to the plasma membranes of cardiomyocytes as well as other tissues such as brain, liver, kidney and skeletal muscle.

Overexpression of long or short FGFR-1 results in FGF-2-mediated proliferation in neonatal cardiac myocyte cultures

OBJECTIVE

The type 1 fibroblast growth factor receptor (FGFR-1) is the only high affinity receptor for fibroblast growth factor-2 (FGF-2) in the rat myocardium, and is essential for normal growth and development of the heart. Levels of FGFR-1 are developmentally regulated, being high in embryonic cardiac myocytes. Also, FGFR-1 exists as both 'long' and 'short' isoforms, and there is a switch from predominant expression of the 'long' isoform in the embryo to the 'short' isoform in the adult heart. Both the decrease in receptor levels and the isoform switch in postnatal cardiac myocytes correlate with a loss of proliferative potential. We investigated whether an increase in either 'long' or 'short' FGFR-1 isoforms could stimulate proliferation in postnatal rat cardiac myocyte cultures.

METHODS AND RESULTS

Previously we cloned cDNAs corresponding to 'long' (L) and 'short' (S) FGFR-1 isoforms from embryonic mouse hearts. Hybrid FGFR-1(L) and (S) genes, directed by a myosin light chain-2 promoter and SV40 enhancer sequences, were generated and used to transiently transfect neonatal rat cardiac myocytes. Overexpression of FGFR-1 mRNA and protein was detected by RNA blotting and immunocytochemistry. Ligand-crosslinking confirmed the presence of specific receptors capable of binding FGF-2 on the cell membrane. Overexpression of either FGFR-1(L) or (S) was associated with stimulation of proliferation as assessed by significant increases in bromodeoxyuridine uptake (DNA synthesis) and cell number. To determine whether this response was FGF-2 specific, the level of FGF-2 was assessed in the culture medium of cardiac myocytes overexpressing FGFR-1 isoforms. A three-fold increase was detected in the media of cardiac myocytes overexpressing either FGFR-1(L) or (S) compared to control levels. Neutralization of this FGF-2 with antibodies inhibited the proliferative response.

CONCLUSION

Overexpression of either FGFR-1(L) or (S) resulted in an increase in FGF-2-mediated proliferation of postnatal rat cardiac myocytes.

alpha1-Adrenergic stimulation of FGF-2 promoter in cardiac myocytes and in adult transgenic mouse hearts

Fibroblast growth factor (FGF-2), a mitogenic, angiogenic, and cardioprotective agent, is reported to be released from the postnatal heart by a mechanism of transient remodeling of the sarcolemma during contraction. This release can be increased with adrenergic stimulation. RNA blotting was used to assess whether FGF-2 synthesis in neonatal rat cardiomyocytes might also be regulated by adrenergic stimulation. FGF-2 RNA levels were increased after treatment with norepinephrine for 6 h or with the alpha-adrenergic agonist phenylephrine for 48 h. To assess an effect on transcription, neonatal rat cardiomyocytes were transfected with a hybrid rat FGF-2 promoter/luciferase gene (-1058FGFp.luc) and treated with norepinephrine or phenylephrine for 6 or 48 h, respectively. FGF-2 promoter activity was increased two- to sevenfold in an alpha1-specific manner. Putative phenylephrine-responsive elements (PEREs) were identified at positions -780 and -761 relative to a major transcription initiation site. However, deletion analysis of -1058FGFp.luc showed that the phenylephrine response was independent of the putative PEREs, cell contraction, and Ca2+ influx. In transgenic mice expressing -1058FGFp.luc, a significant three- to sevenfold stimulation of FGF-2 promoter activity was detected in the hearts of two independent lines 6 h after intraperitoneal administration of phenylephrine (50 mg/kg). This increase was still apparent at 24 h but was not detected at 48 h posttreatment. Analysis of FGF-2 mRNA in normal mouse hearts revealed accumulation of the 6.1-kb transcript at 24 h. Control of local FGF-2 synthesis at the transcriptional level through adrenergic stimulation may be important in the response to injury as well as in the maintenance of a healthy myocardium.

FGF-2-induced negative inotropism and cardioprotection are inhibited by chelerythrine: involvement of sarcolemmal calcium-independent protein kinase C

Fibroblast growth factor-2 (FGF-2), administered to the isolated rat heart by perfusion and under constant pressure, is protective against ischemia-reperfusion (I-R). Here we have investigated whether FGF-2 cardioprotection: (a) is dependent on flow modulation; (b) is linked to effects on contractility; (c) is mediated by protein kinase C (PKC); and (d) is linked to PKC and/or mitogen activated protein kinase (MAPK) associated with the sarcolemma. The isolated rat heart was used as a model. Under conditions of constant flow FGF-2 induced significant improvement in recovery of contractile function during I-R. Under constant perfusion pressure, FGF-2 induced a negative inotropic effect (15% decrease in developed pressure). Chelerythrine, a specific PKC inhibitor, prevented both the FGF-2-induced negative inotropic effect before ischemia, and cardioprotection during I-R. FGF-2 induced a chelerythrine-preventable, five-fold increase in sarcolemmal calcium-independent PKC activity. It also increased the association of PKC subtypes -epsilon and -delta with sarcolemmal membranes, detected by Western blotting, as well as, for PKC delta, by immunolocalization. FGF-2 increased the association of PKC epsilon with the membrane fraction of adult cardiomyocyte in culture, confirming that it can affect PKC signaling in cardiomyocytes directly and in a manner similar to its effects in situ. Finally, FGF-2 induced increased active MAPK at sarcolemmal as well as cytosolic sites. Active sarcolemmal MAPK remained elevated when the FGF-2-induced protection was prevented by chelerythrine. In conclusion, we have provided evidence that cardioprotection by FGF-2 is independent of flow modulation. PKC activation mediates both the FGF-2-induced negative inotropic effect before ischemia and the cardioprotective effect assessed during reperfusion, suggesting a cause and effect relationship. Furthermore, FGF-2 cardioprotection is linked to targeting of sarcolemmal sites by calcium-independent PKC.

Protection against myocardial ischemic/reperfusion injury by inhibitors of two separate pathways of Na+ entry

Previous work has demonstrated that drugs which inhibit Na+ entry through voltage-sensitive Na+ channels, or via Na(+)-H+ exchange protect the heart from ischemic reperfusion damage. The purpose of our study was to determine whether these drugs in combination will have an additive protective effect in Langendorff-perfused hearts. During reperfusion following 30 min of ischemia, developed tension and resting tension were 24 +/- 3 and 162 +/- 5%, respectively, of pre-ischemic values in non-treated ischemic hearts. The administration of HOE-642 to inhibit Na+/H+ exchange increased active developed tension (DT) to 58 +/- 2% of pre-ischemic levels and decreased resting tension (RT) to 111 +/- 3% of pre-ischemic levels. The administration of tetrodotoxin (TTX) to block the Na+ channel increased DT to 56 +/- 3% of the pre-ischemic level and reduced the RT to 126 +/- 12% of the pre-ischemic level. Together, HOE-642 and TTX increased recovery of DT to 63 +/- 2% of pre-ischemic levels and improved RT to 116 +/- 4% of pre-ischemic levels after 30 min of reperfusion. All drug treatment protocols significantly lowered the creatine phosphokinase activity measured in the coronary effluent in comparison to that observed in the non-treated hearts. These data demonstrate that inhibition of Na+ entry through either Na(+)-H+ exchange or the Na+ channel protects the heart from ischemic injury, but there is no additional benefit of blocking both routes of Na+ entry simultaneously. This suggests that a threshold level of Na+i may be a critical factor in ischemic cardioprotection.

Expression of fibroblast growth factor receptor-1 in rat heart H9c2 myoblasts increases cell proliferation

Basic fibroblast growth factor (FGF-2) plays an important role in myocardial growth and development and in particular cardiac myocyte proliferation. FGF-2 exerts its effects by binding to cell surface receptors (FGFR-1) of the tyrosine kinase family. We have detected the presence of both long and short isoforms of FGFR-1 in embryonic and adult mouse heart. In this report, we have examined the ability of long and short FGFR-1 isoforms to signal a mitogenic response. Assessment of RNA from rat myoblast H9c2 cells by reverse transcriptase-polymerase chain reaction and RNA blotting revealed that they were deficient in transcripts corresponding to long and short FGFR-1 species. Hybrid genes containing the cDNAs coding for long and short FGFR-1 isoforms directed by the myosin light chain-2 promoter and simian virus 40 enhancer sequences, were used to transiently transfect H9c2 cells. Total tyrosine phosphorylation was increased 2.0 and 2.6 fold in H9c2 cells transfected with the long and short FGFR-1 isoforms, respectively, compared to 'control' transfected H9c2 cells. This was accompanied by a 2.1 and 2.0 fold increase in DNA synthesis, as measured by tritiated thymidine incorporation, in H9c2 cells expressing the long and short FGFR-1 isoforms, respectively. To assess effects on proliferation, H9c2 cells were stably transfected with the myosin light chain-2/FGFR-1 cDNA genes. The rate of proliferation was increased 1.6 and 3.1 fold in H9c2 cells stably expressing the long and short FGFR-1 isoforms, respectively, compared to 'control' H9c2 cells. In contrast to non transfected H9c2 cells, treatment of H9c2 cells stably expressing long FGFR-1 with FGF-2 for 24 h resulted in a slight increase (1.3 fold, p < 0.02) in cell number. However, a greater response (1.5 fold, p < 0.0005) was observed with H9c2 cells stably expressing short FGFR-1 after treatment with FGF-2. These results suggest that both long and short FGFR-1 isoforms are capable of signalling a mitogenic response.

Cell-cycle dependent anti-FGF-2 staining of chicken cardiac myocytes: movement from chromosomal to cleavage furrow- and midbody-associated sites

Fibroblast growth factor-2 (FGF-2) promotes cardiac myocyte proliferation and has been detected in extracellular as well as cytoplasmic and nuclear compartments. As a first step in examining the participation of intracellular FGF-2 in cardiac myocyte cell cycle we have investigated its localization in proliferative chicken cells during interphase and the various stages of mitosis in culture. We have used a previously characterized and affinity-purified anti-FGF-2 antibody preparation which recognizes the 19-22 kDa variants of chick FGF-2. By immunofluorescence, bright, punctate anti-FGF-2 labelling was observed in 26% of interphase nuclei from myocytes derived from 5 day embryonic heart ventricles; these nuclei were positive for anti-bromodeoxyuridine staining indicating that they are at the S- or G2 phase of the cell cycle. In prophase and metaphase, bright anti-FGF-2 staining was detected in apparent association with chromosomes. During anaphase, however, anti-FGF-2 staining dissociated from chromosomal locations distinctly remaining in strand-like structures in the area of ensuing cleavage furrow formation. In late telophase and cytokinesis, strong staining persisted in the area of the midbody and reappeared in a small fraction of newly formed daughter nuclei. Absorption of the antibody preparation with immobilized FGF-2 eliminated all staining. This dynamic pattern of anti-FGF-2 staining suggests that chick FGF-2 or immunologically related protein(s) not only increase in DNA-synthesizing nuclei but they may play a role in subsequent stages of mitosis and cytokinesis.

Selective monoclonal antibody recognition and cellular localization of an unphosphorylated form of connexin43

A sequence-specific monoclonal antibody directed against the gap junction protein connexin43 (Cx43) is shown here to be specific for the unphosphorylated form of this protein. In tissues and cultured cells containing different phosphorylated and unphosphorylated forms of Cx43, the antibody detected only the latter as shown by Western blotting of native and alkaline phosphatase-treated samples. Immunohistochemically, this monoclonal antibody did not recognize gap junctions in the vast majority of cultured cardiac myocytes, where nearly all detectable Cx43 is phosphorylated. In contrast, it was able to detect some intracellular Cx43 in tracheal smooth muscle cells and an epithelial cell line (Cl-9 cells), producing patterns of labeling consistent with those seen using a polyclonal antibody that recognizes both phosphorylated and unphosphorylated forms of Cx43. Immunostaining of gap junctions in the cultured cells indicates that both phosphorylated and unphosphorylated Cx43 are present in some assembled gap junctions, suggesting that assembled junctions do not contain exclusively the phosphorylated form of the protein. Annular gap junctions, believed to form as part of the pathway for internalization and degradation of gap junctions, were only occasionally and sparsely labeled by the monoclonal antibody, indicating that complete protein dephosphorylation is not required for uptake and degradation of gap junctions. Furthermore, the ability of this antibody to recognize only unphosphorylated Cx43, and not any of the phosphorylated forms present in the tissues and cell types examined, suggests that a unique phosphorylation site, perhaps present in the epitope recognized by this antibody, must be phosphorylated prior to phosphorylation of Cx43 at other sites.

Fibroblast growth factor-2 decreases metabolic coupling and stimulates phosphorylation as well as masking of connexin43 epitopes in cardiac myocytes

Cardiac gap junction (GJ) channels, composed of connexins, allow electrical and metabolic couplings between cardiomyocytes, properties important for coordinated action of the heart as well as tissue homeostasis and control of growth and differentiation. Fibroblast growth factor-2 (FGF-2) is an endogenous growth-promoting protein, believed to participate in the short- and long-term responses of the heart to injury. We have examined short-term effects of FGF-2 on cardiac myocyte GJ-mediated metabolic coupling, using cultures of neonatal rat cardiomyocytes. FGF-2 decreased coupling between cardiomyocytes assessed by scrape dye loading as well as microinjection and dye transfer within 30 minutes of administration. Genistein blocked the effects of FGF-2. To determine the mechanism, we next assessed the effect of FGF-2 on expression, distribution, and phosphorylation of connexin43 (Cx43), which is a major cardiomyocyte connexin. FGF-2 did not affect Cx43 mRNA or protein accumulation and synthesis, and it did not change Cx43 localization at sites of intercellular contact as assessed by immunostaining with a polyclonal anti-Cx43 antibody raised against a synthetic peptide containing residues 346 to 363 of Cx43. FGF-2, however, decreased staining intensity at sites of intermyocyte contact when a monoclonal anti-Cx43 antibody was used, suggesting a localized masking of epitope(s) recognized by the monoclonal but not the polyclonal antibody. These epitopes appear to reside within residues 261 to 270 of Cx43, as indicated by full quenching of monoclonal antibody staining with synthetic peptides. In addition, FGF-2 induced a more than twofold increase in Cx43 phosphorylation. Phosphoamino acid analysis indicated increased phosphorylation of Cx43 on serine residues. Although tyrosine phosphorylation of Cx43 was not detected in either treated or control cells, a fraction of Cx43 was immunoprecipitated with anti-phosphotyrosine-specific antibodies in FGF-2-treated myocytes, suggesting interaction (and hence coprecipitation) with phosphotyrosine-containing protein(s). In conclusion, we have identified Cx43 and intercellular communication as targets of FGF-2-triggered and tyrosine phosphorylation-dependent signal transduction in cardiac myocytes. It is suggested that phosphorylation of Cx43 on serine induced by FGF-2 contributes to decreased metabolic coupling between cardiomyocytes.

High and low molecular weight fibroblast growth factor-2 increase proliferation of neonatal rat cardiac myocytes but have differential effects on binucleation and nuclear morphology. Evidence for both paracrine and intracrine actions of fibroblast growth factor-2

Basic fibroblast growth factor (FGF-2) plays a vital role in the growth and differentiation of cardiac myocytes. It exists in high and low molecular weight forms because of the use of alternative initiation codons in the same mRNA. Higher levels of high molecular weight forms (molecular mass of 22 and 21.5 kD) are present in the rat heart during the neonatal stage, whereas the low molecular weight form (molecular mass of 18 kD) is predominant in the adult heart, suggesting different roles in development. Rat FGF-2 cDNAs that can preferentially express high or low molecular weight forms were introduced into neonatal rat ventricular myocyte cultures. Significant and comparable increases in overall cardiac myocyte DNA synthesis and proliferation were seen with 22/21.5- and 18-kD FGF-2 expression. A significantly higher mitotic index was seen in the vicinity of cardiac myocytes overexpressing high or low molecular weight forms of FGF-2 compared with nonoverexpressing cells. This increase was inhibited in the presence of neutralizing antibodies to FGF-2, pointing to a proximity-dependent paracrine effect of 22/21.5- and 18-kD FGF-2 on mitosis. By contrast, overexpression of high but not low molecular weight FGF-2 was associated with a significant increase in binucleation (approximately 36% of cardiac myocytes overexpressing 22/21.5-kD FGF-2 were binucleated compared with 9% of cardiac myocytes overexpressing 18-kD FGF-2), which was not affected by neutralizing antibodies to FGF-2. These results suggest that 22/21.5-kD FGF-2 and 18-kD FGF-2 have similar paracrine effects on proliferation but that 22-21.5-kD FGF-2 exerts a distinct intracrine effect on binucleation.

Effect of "enhancer" sequences on ventricular myosin light chain-2 promoter activity in heart muscle and nonmuscle cells

Positive (HF-1 and HF-2) and negative (HF-3) elements responsible for cardiac-specificity of the rat ventricular myosin light chain-2 (MLC-2v) promoter are contained in a 250 base pair region. The effect of the simian virus 40 (SV40) enhancer or 3 copies of the HF-1, HF-2 and HF-3 elements on MLC-2v promoter activity and specificity was assessed in neonatal rat cardiac myocytes and cardiac nonmuscle cells as well as rat heart myoblast H9c2 and glioma (nonmuscle) C6 cell lines. The SV40 enhancer increased promoter activity by at least 10-fold in both muscle and nonmuscle cell types; however, there was a decrease in cardiac ventricular myocyte-specificity. In contrast, the 3 copies of HF-1, HF-2 and HF-3 elements stimulated MLC-2v promoter activity approximately 3-fold in neonatal ventricular cardiac myocytes alone and, effectively, displayed about a 5-fold increase in specificity over the wild type MLC-2v promoter.

Adult cardiomyocytes express functional high-affinity receptors for basic fibroblast growth factor

As a first step in addressing the question of function for basic fibroblast growth factor (bFGF) in the adult myocardium, expression of bFGF receptors by adult rat myocytes was investigated. Cross-linking of 125I-labeled bFGF to purified sarcolemmal vesicles from adult hearts indicated specific binding to 90- to 104-kDa proteins, whereas equilibrium binding studies revealed the presence of "low"-affinity (1 nM) and "high"-affinity (115 pM) sites. Adult myocytes were found to express short and long variants of bFGF receptor 1 (FGFR-1, tyrosine kinase) mRNA. Adult heart overall levels of FGFR-1 mRNA were decreased by about one-third of corresponding fetal values. Several lines of evidence indicated that bFGF receptors in adult cardiomyocytes in situ and/or in isolation are functional. Isolated adult myocytes were found to be capable of heparin-resistant internalization of 125I-labeled bFGF, to lose their viability after interaction with bFGF-saporin (a mitotoxin known to kill cells after entry via the bFGF receptor), and to respond to bFGF by activation of mitogen-activated protein kinase. In addition, introduction of exogenous bFGF into the myocardium by Langendorff perfusion resulted in stimulation of tyrosine phosphorylation in association with cardiomyocyte intercalated disks, as assessed by immunofluorescence. It is concluded that adult cardiomyocytes express functionally coupled high-affinity bFGF receptors and that they are capable of a biologic response to bFGF in vivo.

Characterization of fibroblast growth factor receptor 1 RNA expression in the embryonic mouse heart

We used reverse transcriptase-polymerase chain reaction (RT-PCR) to clone fibroblast growth factor receptor (FGFR) 1 isoforms from embryonic mouse heart and as a more sensitive method to characterize FGFR1 RNA expression in embryonic and adult mouse hearts. We describe the cloning of both full-length short (2259 base pairs) and long (2526 base pairs) FGFR1 isoform cDNAs which generated 86 and 102 kilodalton proteins, respectively, following in vitro translation. An assessment of FGFR1 RNA indicates that FGFR1-IIIc is the major form in both the embryonic and adult heart but there is an approximately 8.5-fold decrease in RNA levels in the adult. Differential RNA blotting as well as RT-PCR analyses are consistent with a switch in the relative expression of the short versus long FGFR1 isoforms during heart development. The long isoforms are more abundant in the embryo and the short isoforms predominate in the adult. This may be important in the regulation of growth and development of the heart.

Basic fibroblast growth factor is cardioprotective in ischemia-reperfusion injury

To examine whether basic fibroblast growth factor (bFGF) administered to the heart by perfusion can improve cardiac resistance to injury we employed an isolated rat heart model of ischemia-reperfusion injury and determined the extent of functional recovery in bFGF-treated and control hearts. Global ischemia was simulated by interruption of flow for 60 min. Recovery of developed force of contraction (DF), recorded after reestablishment of flow for 30 min, reached 63.8 +/- 1.5% and 96.5 +/- 3.5% of preischemic levels in control and bFGF-treated hearts (10 micrograms/heart), respectively, indicating that bFGF induced significantly improved recovery of mechanical function. Recoveries of the rates of contraction or relaxation were also significantly improved in bFGF-treated hearts. Extent of myocardial injury, assessed by determination of phosphocreatine kinase in the effluent, was reduced as a result of bFGF treatment. As a first step towards understanding the mechanism and direct cellular target(s) of bFGF-induced cardioprotection, we investigated its fate after perfusion. Perfusion of 10 micrograms bFGF/heart resulted in a 4-fold increase in bFGF associated with the heart compared to control levels, as estimated by biochemical fractionation and immunoblotting. Immunofluorescent staining of the bFGF-perfused hearts revealed intense anti-bFGF staining in association with blood vessels as well as the periphery of cardiomyocytes, suggesting that the latter may be a target for direct bFGF action. In conclusion, our findings of bFGF-induced increases in cardiac resistance to, and improved functional recovery from, ischemia-reperfusion injury indicate that bFGF may have clinical applications in the treatment of ischemic heart disease.

Basic fibroblast growth factor stimulates connexin-43 expression and intercellular communication of cardiac fibroblasts

Gap junctions (GJ) are membrane specializations responsible for intercellular communication and for ensuring electrical and/or metabolic coupling between cells. They are composed of connexins, a family of related proteins. Connexin-43 (Cx43) is a major connexin of the rat heart, expressed by myocytes as well as non-muscle cells. In this communication we have examined expression of Cx43 by cardiac fibroblasts and regulation of its expression by an endogenous mitogen, basic fibroblast growth factor (bFGF). Recombinant human bFGF, administered to cultured cells which had been maintained in 0.5% serum for 48 h, induced dose-dependent and statistically significant increases in Cx43 mRNA as well as protein accumulation, at 6 h after addition. Intercellular communication was also increased at 6 h but not 30 min after bFGF treatment, as assessed using a scrape-loading protocol. It is concluded that the bFGF-induced stimulation of Cx43 expression caused increased coupling between cardiac fibroblasts. This would be of importance in injured myocardium, the increased bFGF content of which might stimulate electrical coupling involving fibroblasts of the scar tissue.

Cloning and expression of fibroblast growth factor receptor-1 isoforms in the mouse heart: evidence for isoform switching during heart development

Basic (b) fibroblast growth factor (FGF) mediates various biological responses including mitogenesis and angiogenesis by binding to specific cell surface receptors of the tyrosine kinase family. The bFGF receptor-1 FGFR1) exists in short and long isoforms due to alternate RNA splicing. Minor alterations in the amino acid sequence have also led to reports of different FGFR1 isoforms in different tissues even in the same species. In the absence of any sequence for heart FGFR1 and accumulating evidence for a role of bFGF in heart growth and differentiation, we cloned FGFR1 from embryonic mouse hearts. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to generate full-length short (2259 base pairs) and long (2526 base pairs) forms of FGFR1 cDNAs which generated 86 and 102 kDa proteins, respectively, following in vitro translation. Embryonic mouse heart FGFR1 differed by seven amino acids from the reported sequence for mouse neuroepithelial FGFR1 and appeared more similar to human placental FGFR1. A single FGFR1 transcript of approximately 4.3 kb was seen in RNA isolated from embryonic as well as adult mouse hearts. There was a decrease (approximately 8.5-fold) in FGFR1 RNA levels in the adult. The majority of FGFR1 transcripts in the adult as well as embryonic heart contained exon IIIc (FGFR1-IIIc) which is associated with isoforms that display the highest affinity for bFGF. However, the relative ratio of short versus long FGFR1 RNA expression was 0.5 in the embryonic heart compared to 5.9 in the adult heart. These results indicate that: (i) structurally distinct short and long FGFR1 isoform RNAs are expressed in the embryonic and adult heart; (ii) FGFR1-IIIc is the major form of receptor expressed in the embryonic as well as adult heart; (iii) the transition from the embryo to the adult stage is associated with a decrease but not absence of FGFR1 RNA expression; and (iv) long FGFR1-isoforms are more abundant in the embryo while short FGFR1 isoforms predominate in the adult.

Over-expression of CUG- or AUG-initiated forms of basic fibroblast growth factor in cardiac myocytes results in similar effects on mitosis and protein synthesis but distinct nuclear morphologies

Initiation of translation from alternate codons in the same mRNA results in multiple forms of basic fibroblast growth factor (bFGF). High molecular weight species of bFGF make use of leucine translation initiation sites located upstream of the methionine residue used to produce the 18 kiloDalton (kDa) form. Although the addition of exogenous 18 kDa bFGF is known to stimulate DNA synthesis and proliferation of several cell types including embryonic chicken cardiac myocytes, little is known about the role of high molecular weight forms of bFGF. We modified the rat bFGF cDNA to yield high (22/21.5 kDa) or low (18 kDa) molecular weight species of bFGF. Expression of 22/21.5 kDa or 18 kDa bFGF in transfected embryonic chicken ventricular myocyte cultures was confirmed by protein blotting. Expression of both high and low molecular weight species of bFGF was associated with (i) a three-fold increase in overall thymidine incorporation as well as cardiomyocyte labelling index (fraction of cardiomyocyte nuclei incorporating tritiated thymidine); (ii) a two- to three-fold increase in cell number; (iii) an eight-fold increase in protein synthesis; and (iv) a three-fold decrease in myosin accumulation. Subcellular localization of bFGF in the transfected myocyte cultures was also assessed by immunofluorescence microscopy. Over-expression of cDNAs yielding high molecular weight bFGF resulted in predominantly nuclear bFGF staining. By contrast, both cytoplasmic and nuclear staining were observed following over-expression of 18 kDa bFGF. Over-expression of 22/21.5 kDa bFGF was associated with the formation of multiple DNA-containing "clumps" resembling condensed chromatin in cardiac myocyte nuclei. These DNA "clumps" were not observed in cardiac myocyte cultures over-expressing 18 kDa bFGF. These data indicate that over-expression of high as well as low molecular weight forms of bFGF can stimulate cardiac myocyte proliferative potential and decrease myosin accumulation. However, these forms possess distinct subcellular localizations and can have different biological functions in the nucleus.

The pituitary-muscle axis in mdx dystrophic mice

As myogenesis, muscle growth and differentiation and growth factor expression are influenced by thyroid and growth hormone (GH) levels, it is important to investigate the possibility that altered activity of the pituitary-muscle axis prevents the lethal progression of mdx dystrophy and/or contributes to the muscle fiber hypertrophy of limb muscles. The ultrastructure of pituitary and thyroid tissues in age-matched control and mdx mice at 2 and 12 months of age was examined. Pituitary GH, and serum thyroid stimulating hormone (TSH), thyroid hormone (T4), and creatine kinase (CK) levels were measured. Mdx thyroid gland structure was similar to age-matched control glands. Mdx thyroid gland weighed significantly more than in age-matched controls, but was unchanged relative to body weight. TSH and T4 levels were not different from levels in control mice. High CK levels reflected the active dystrophy in mdx muscles. Somatotrophs in mdx pituitaries were hypertrophied in comparison to controls, indicating increased secretory activity, and pituitary GH was slightly but significantly greater in old mdx female mice compared to age-matched female controls. These observations rule out hypopituitary or hypothyroid function as a reason for the low impact of dystrophin deficiency in mdx muscles. Results suggest a contribution by raised GH to the fiber hypertrophy in mdx limb and heart muscle, which might also assist the large capacity for limb muscle regeneration in mdx mice.

The effects of hyperthyroidism on muscular dystrophy in the mdx mouse: greater dystrophy in cardiac and soleus muscle

Muscle damage and repair were studied in mdx mice treated with triiodothyronine (T3) for 14 days. Hindlimb and cardiac muscles were examined for the severity of dystrophy, the degree of muscle centronucleation, and fiber size. In control and mdx mice, cardiac hypertrophy and skeletal muscle atrophy were present after T3 treatment. Both cardiac and soleus (but not fast-twitch) muscles had larger, more frequent dystrophic lesions in T3-treated mdx mice, and mdx soleus had an increased area of new myotubes after T3. Skeletal myogenesis in mdx mice may have been delayed by excess T3, possibly related to the general reduction in staining for basic fibroblast growth factor in hyperthyroid mice. These are the first observations of a metabolic perturbation which worsens mdx dystrophy and possibly repair in a muscle-specific manner, and are likely related to T3-induced changes in myosin heavy chain expression, and to increased mechanical strain on dystrophin-deficient muscles.

Perinatal phenotype and hypothyroidism are associated with elevated levels of 21.5- to 22-kDa basic fibroblast growth factor in cardiac ventricles

Control of cardiomyocyte growth and differentiation may be exercised, in part, at the level of expression of endogenous growth factors such as bFGF (basic fibroblast growth factor), believed to act locally, in an autocrine or paracrine fashion. Examination of bFGF accumulation by extract fractionation and immunoblotting indicated a 4-fold increase of an 18-kDa bFGF in adult compared to newborn rat heart ventricles. In contrast, a 22-kDa bFGF species, found to be the predominant form in newborn ventricles, displayed a 2.5-fold decrease in extracts from adult (compared to neonatal) ventricles. Since newborn rats are physiologically hypothyroid, the effect of thyroid status on the accumulation of 22-kDa bFGF in the heart was examined. A 21.5- to 22-kDa bFGF showed a 5-fold increase in extracts from hyperthyroid rat heart ventricles, compared to those from euthyroid controls. The 21.5- to 22-kDa bFGF was essentially unaffected by thyroid status in extracts from brain. Our data point to a correlation between immature or differentiated cardiac phenotype and the predominance of 21- to 22.5-kDa or 18-kDa bFGF species, respectively. Elevated levels of 21.5- to 22-kDa bFGF detected in cardiac ventricles in either physiological or experimentally induced hypothyroidism indicates that thyroid hormone may, directly or indirectly, down-regulate accumulation of the higher molecular mass forms of bFGF in the heart.

Increased basic fibroblast growth factor (bFGF) accumulation and distinct patterns of localization in isoproterenol-induced cardiomyocyte injury

Basic FGF is a multifunctional protein which promotes regeneration in several tissues. To investigate involvement in cardiac injury-repair, bFGF accumulation and localization was examined in hearts of rats injected with a single high dose of isoproterenol. The bFGF content of cardiac extracts was analyzed at 6 and 24 hours as well as 1, 4, and 6 weeks by western blotting of heparin-sepharose-bound fractions. The 18 kilodalton bFGF species showed an approximately 2-fold increase in extracts from treated animals compared to non-treated controls. A transient rise in a 21-23 kilodalton bFGF species was seen at 24 hours after treatment. An induction of bFGF mRNA was also observed in treated animals. To localize bFGF in vivo, immunofluorescent labelling with specific antibodies was used at 4-24 hours and 1-4 weeks after treatment. Simultaneous labelling for the cytoskeletal proteins vinculin or vimentin was employed to identify viable myocytes or non-muscle interstitial cells, respectively. Necrotic myocytes, identified by loss of vinculin, displayed a pronounced increase in cytoplasmic anti-bFGF staining compared to adjacent normal myocytes. This increase occurred prior to and may play a role in promoting mobile cell migration and proliferation in areas of necrosis. Viable cardiomyocytes adjacent to fibrotic regions displayed strong pericellular anti-bFGF staining and, occasionally, were also stained by anti-vimentin antibodies, suggesting reexpression of an embryonic phenotype and thus an attempt for regeneration. These data showing increased accumulation and distinct patterns of localization of bFGF in the hearts of isoproterenol-treated animals suggest that this growth factor plays a role in short-term as well as long term response of the myocardium to injury.

Identification of basic fibroblast growth factor-like proteins in African trypanosomes and Leishmania

Basic fibroblast growth factor (bFGF) is a multifunctional, heparin-binding, mitogenic polypeptide found in all tissues or cells of multicellular organisms so far examined. Here we report that Trypanosoma brucei rhodesiense procyclic culture forms (PCF) and Leishmania donovani promastigotes grown in serum-containing and serum-free medium, contained peptides of 15-34 kDa which bound heparin-sepharose with high affinity and which reacted in immunoblots with several preparations of antibodies specific for bovine brain bFGF. Similar peptides were not detectable in foetal bovine serum. Immunofluorescence studies showed bFGF-like molecules to have a cytoplasmic distribution in both species growing in serum-free media. A nuclear and/or perinuclear distribution of immunoreactivity was also observed in parasites which had been grown in the presence of serum. The data indicate that both species of parasites synthesize their own bFGF-like molecules. Association of an ubiquitous growth factor with parasitic protozoa may play an important role in parasite multiplication and in host-parasite interactions.

Basic fibroblast growth factor in cultured cardiac myocytes

Distribution of basic-fibroblast-growth-factor-like peptides in immature cultured cardiac myocytes was investigated using specific antisera and immunolocalization. Basic FGF was detected in association with the external surface of the cell membrane, with specialized intercellular junctions and with the myofibril Z lines in the cytoplasm. Intense, punctate nuclear anti-bFGF labeling was observed in a fraction of interphase myocytes of near-confluent, proliferating cultures. This staining pattern persisted even after the dissolution of the nuclear envelope in prophase myocytes. The pattern of cellular localization of bFGF indicates a continuous participation of this factor in myocyte physiology as well as a role in the cell cycle. Furthermore, the identification of bFGF not only in cell lysates but also in culture media after gentle mechanical disruption suggests that cardiac myocytes may release bFGF in vivo following tissue damage.

Biochemical and ultrastructural evidence for the association of basic fibroblast growth factor with cardiac gap junctions

Basic fibroblast growth factor (bFGF) is a ubiquitous and multifunctional polypeptide that is believed to have a role in tissue repair and to act as a morphogen in embryonic development. Here, we have used immunohistochemical and biochemical methods with antibodies directed against the amino-terminal domain of bFGF, designated IS2, which recognize native and denatured bFGF, to demonstrate that in addition to its known intracellular and extracellular localization in heart, bFGF is also associated with cardiomyocyte gap junctions. In tissue sections, IS2 labeled regions of intercalated discs, producing an immunofluorescence pattern virtually indistinguishable from that obtained with antibodies against the heart gap junction protein connexin-43. By electron microscopy, gap junctions but not other regions of plasma membrane were heavily immunolabeled with this antibody. By solid phase immunoassay, bFGF was found to be more concentrated in a fraction enriched in cardiac gap junctions than in whole sarcolemmal preparations. Finally, an 18-kDa protein was recognized by several different antibodies specific for bFGF on Western blots of heart subcellular fractions enriched in gap junctions. We suggest that bFGF-like peptides are either an integral part of, or exist in close association with, cardiac gap junctions and thus may play a role in modulating gap junctional intercellular communication.

Distinctive patterns of basic fibroblast growth factor (bFGF) distribution in degenerating and regenerating areas of dystrophic (mdx) striated muscles

Mdx mice uniquely recover from degenerative dystrophic lesions by an intense myoproliferative (regenerative) response. To investigate a potential role of endogenous basic fibroblast growth factor (bFGF) in injury-repair processes, we investigated its localization in several striated muscles of mdx and control mice using immunofluorescence labeling with specific antibodies. Basic FGF was localized consistently to the myofiber periphery and nuclei of intact myofibers, as well as in single, dystrophin-positive cells in close association with the myofibers (potential myosatellite cells). In mdx mice, actively degenerating skeletal or cardiac muscle fibers presented intense cytoplasmic anti-bFGF staining prior to mononuclear infiltration. Small regenerating fibers in mdx skeletal muscle exhibited greater bFGF accumulation than adjacent larger myofibers. Strong nuclear anti-bFGF immunolabeling was frequently observed in mdx cardiac myocytes at the borders of necrotic regions. In agreement with differences in intensity of immunolabeling, extracts from slow-twitch muscles contained higher levels of bFGF compared to those from fast-twitch muscles, in both control and mdx mice. In addition, bFGF levels were consistently higher in extracts from all mdx tissues compared to those derived from their control counterparts. Our data suggest that bFGF participates in the degenerative and regenerative responses of striated muscle to dystrophic injury and also indicate a potential involvement of this factor with the physiology of different striated muscles.

Basic fibroblast growth factor in rat brain: localization to glial gap junctions correlates with connexin43 distribution

Light and electron microscope procedures and antibodies against basic fibroblast growth factor (bFGF) were used to study the immunohistochemical localization of bFGF in rat brain. Throughout all areas of the brain analyzed by LM including grey matter, white matter, ependyma, and leptomeninges bFGF-immunoreactivity consisted of punctate immunolabelling that had an appearance and heterogenous distribution nearly identical to that displayed by the gap junction protein connexin43. By immuno-EM, bFGF was localized to gap junctions between astrocytes. It appears that there is a physical association of bFGF with gap junctions composed of connexin43 and it is suggested that bFGF may exert a regulatory influence on intercellular communication at such junctions.

Immunolocalization of basic fibroblast growth factor (bFGF) in growing and growth-inhibited placental cells: a possible role for bFGF in placental cell development

The distribution of basic (b) fibroblast growth factor (FGF) in growing and growth-arrested human placental tumour cells, as well as normal placental villous trophoblasts, was studied by immunofluorescence microscopy with antibodies to the amino terminus of bFGF. Placenta (FAR, FEG-3), breast (MCF-7, T-47D), cervix (HeLa) and uterine (HEC-1-A) tumour cells showed the same two patterns after immunofluorescent staining with antibodies to bFGF: (i) a perinuclear pattern and (ii) an intense homogeneous staining of the nucleus and cytoplasm. The homogeneous bFGF staining pattern was associated predominantly with actively dividing cells, observed at different stages of mitosis and cytokinesis. Placental (FEG-3) cell division was inhibited with methotrexate (MTX), a chemotherapeutic agent used in the treatment of placental tumours. MTX-treated FEG-3 cells as well as 'normal' non-proliferative placental (syncytiotrophoblast) cells from term placentae, showed perinuclear staining with antibodies to bFGF and immunofluorescence microscopy. The nuclear localization of bFGF in dividing but not non-dividing placental cells, suggests a role for bFGF in cytotrophoblast proliferation in vivo.

Calcium protects pituitary basic fibroblast growth factors from limited proteolysis by co-purifying proteases

Extracts from bovine pituitaries and other tissues contained basic fibroblast growth factor-like peptides of 22-26 kda, co-fractionating with smaller, 16-20 kda bFGFs. Heparin-bound, 22-26 kda bFGFs were converted to smaller, heparin-binding forms by tryptic proteolysis. In solution, 22-26 kda bFGFs were converted to smaller, heparin-binding forms by an activity present in pituitary extracts. Calcium protected higher molecular weight pituitary bFGFs from truncation by the endogenous activity, which was not acid-activated, co-purified with bFGF during heparin-sepharose chromatography, remained operant at high salt concentrations and was inhibited by phenylmethan-sulfonyl fluoride.

Differential expression of human placental growth-hormone variant and chorionic somatomammotropin in culture

Regulation of human placental growth-hormone variant (hGH-V) in the presence of its own promoter has been studied. At term, 10-20% of placental mRNA is specific for chorionic somatomammotropin (hCS-A and -B) compared with 0.05% hGH-V, yet these genes show more than 90% sequence similarity at the nucleotide level. By using stable gene transfer of intact hGH-V and hCS-A genes into rat pituitary (GC) cells, synthesis and release of hGH-V and hCS are detected. This suggests that hGH-V as well as hCS is secreted during pregnancy. The hCS-A mRNA level is higher than that observed from the hGH-V gene in stably transfected GC cells. Also, a hybrid gene containing hGH-V 5'-flanking DNA was less active than a hybrid hCS-A gene containing equivalent sequences after transient transfection of these cells. This correlates with the binding of a known transcription factor to a proximal region (-97/-66) of the hCS-A promoter, and not the equivalent hGH-V gene sequences. These results indicate that differential expression of hGH-V and hCS in GC cells is related, in part, to the strength of their respective promoters, and suggest a similar mechanism may exist in the placenta.

Stimulation and inhibition of cardiac myocyte proliferation in vitro

We have examined the effect of crude cardiac tissue extracts as well as that of several growth factors and triiodothyronin (T3) on DNA synthesis of cardiac myocytes in culture. Extracts from embryonic and adult cardiac tissue stimulated DNA synthesis of myocytes. Atrial myocytes exhibited overall higher degree of stimulation than their ventricular counterparts and extracts from adult atrial tissue had the highest apparent mitogenic activity for atrial myocytes. We have shown that adult heart contains basic fibroblast growth factor (bFGF), especially in the atria. Transforming growth factor beta (TGF beta) and insulin-like growth factors (IGFs) are also accumulated in cardiac tissues. We found that bFGF and the IGFs stimulate myocyte cell proliferation and DNA synthesis. These factors also stimulate cardiac non-muscle proliferation, especially in the presence of serum. TGF beta inhibited proliferation and DNA synthesis and cancelled the effect of bFGF or IGFs on the myocytes. T3 also diminished the bFGF-induced mitogenic stimulation of cardiomyocytes. Our data suggest that these factors may be involved in the regulation of cardiomyocyte proliferation in vivo.

The human placental growth hormone variant is mitogenic for rat lymphoma Nb2 cells

Although there is evidence that human (h) placental GH variant (hGH-V) possesses a growth-promoting function, lactogenic activity by the hormone has not been demonstrated. Rat anterior pituitary tumor (GC) cells stably transfected with the hGH-V gene (GC [hGH-V] cells) synthesize and secrete hGH-V. This hormone shares considerable structural similarity with pituitary growth hormone (hGH-N) and chorionic somatomammotropin (hCS) at the nucleotide (greater than 90%) and amino acid (greater than 80%) levels. As expected, both hGH-N and hCS antibodies detect hGH-V by immunoblotting. However, hGH-V, but not hGH-N or hCS, cross-reacts with human or rat pituitary prolactin (PRL) antibodies. These data indicate that structural features shared by hGH-V and pituitary PRL are not present in hGH-N or hCS. Comparison of amino acid sequences implicates two regions that may account for a common epitope between hGH-V and hPRL, and structural difference from hGH-N and hCS. The possible lactogenic activity by hGH-V was assessed in a rat lymphoma Nb2 cell bioassay. Conditioned medium from GC[hGH-V] cells permitted growth of lactogen-dependent Nb2 lymphoma cells in culture. This activity was blocked by antibodies raised to rat PRL but not hPRL or hGH-N. Comparison of the hGH-V amino acid sequence with those from 14 other lactogenic hormones, including hPRL, hCS and hGH-N, reveals 6 conserved amino acids. These data indicate a lactogenic as well as growth-promoting function for the secreted hGH-V protein in vivo.

A potential new role for bFGF in host-parasite interactions

Basic fibroblast growth factor (bFGF) is a mitogenic polypeptide highly conserved between species, implicated in regenerative processes and present in all tissues examined. In the heart, bFGF is localized in association with nuclei, extracellular matrix and intercalated discs of cardiomyocytes. In this article is reported bFGF association with the intramuscular parasitic protozoan Sarcocystis in situ, in bovine hearts, detected by indirect immunofluorescence. Parasitic cysts appear connected directly to specialized host cell junctions: bFGF provides structural continuity between parasitic cyst wall and myocyte intercalated discs. Other proteins associated with intercalated discs such as desmin or desmoplakin are not detected in the cysts. Association with Sarcocystis suggests a new role for bFGF in the context of parasitic invasion and establishment.

Characterization of two preparations of antibodies to basic fibroblast growth factor which exhibit distinct patterns of immunolocalization

Immunoglobulins reactive against basic fibroblast growth factor (bFGF) were obtained from the serum of a single rabbit immunized against residues [1-24] of bFGF conjugated to keyhole limpet hemocyanin (KLH). Pure immunoglobulin preparations no. 1 and no. 2 were prepared using different affinity chromatography columns and preabsorption to KLH-coupled Sepharose for preparation no. 1. Both preparations no. 1 and no. 2 were specific for bFGF in in vitro assays. Competition with synthetic peptides suggests that preparations no. 1 and no. 2 recognize predominantly epitope(s) within residues [16-24]bFGF or residues [1-10]bFGF, respectively, in situ. Furthermore, no. 2 (but not no. 1) antibodies can react with tissue-(heparin-)-bound antigen. When used in indirect immunofluorescence for bFGF in frozen heart sections, preparation no. 1 stained predominantly muscle intercalated discs (IcDs); muscle nuclei were also stained, in an overall punctate fashion. Preparation no. 2 stained muscle nuclei strongly, in association with the nuclear envelope; it also stained basement-membrane associated bFGF. Differences in immunostaining were also observed in uterine smooth muscle and kidney sections but not in skeletal muscle. It is plausible that accessibility of various epitopes within the amino-terminal region depends strongly on the local interactions of bFGF. Our data illustrate the importance of using several different antibodies to localize bFGF in a tissue.

Basic fibroblast growth factor in atria and ventricles of the vertebrate heart

Extracts from atrial and ventricular heart tissue of several species (chicken, rat, sheep, and cow) are strongly mitogenic for chicken skeletal myoblasts, with the highest apparent concentration of biological activity in the atrial extracts. Using several approaches (biological activity assay and biochemical and immunological analyses), we have established that (a) all cardiac extracts contain an 18,000-D peptide which is identified as basic fibroblast growth factor (bFGF) since it elutes from heparin-Sepharose columns at salt concentrations greater than 1.4 M and is recognized by bFGF-specific affinity-purified antibodies; (b) bFGF is more abundant in the atrial extracts in all species so examined; (c) avian cardiac tissue extracts contain the highest concentration of immunoreactive bFGF; and (d) avian ventricles contain a higher relative molecular mass (23,000-D) bFGF-like peptide which is absent from atrial extracts. Examination of frozen bovine cardiac tissue sections by indirect immunofluorescence using anti-bFGF antibodies shows bFGF-like reactivity associated with nuclei and intercalated discs of muscle fibers. There is substantial accumulation of bFGF around atrial but not ventricular myofibers, resulting most likely from more extensive endomysium in the atria. Blood vessels and single, nonmuscle, connective tissue cells react strongly with the anti-bFGF antibodies. Higher bFGF content and pericellular distribution in atrial muscles suggest a correlation with increased regenerative potential in this tissue. Distribution within the myofibers is intriguing, raising the possibility for an intimate and continuous involvement of bFGF-like components with normal myocardial function.

Effect of butyrate on thyroid hormone-mediated gene expression in rat pituitary tumour cells

These studies correlate the effects of (sodium) butyrate on intranuclear thyroid hormone receptor levels, with influences on both endogenous and transfected rat growth hormone (rGH) gene expression and regulation by L-triiodothyronine (T3). In rat anterior pituitary tumour (GH3) cells, 5.0 mM butyrate elicits a biphasic reduction in the number of nuclear T3 receptors. About 75% are depleted rapidly (t1/2 = 7 h), and the remaining receptors are depleted more slowly (t1/2 = 59 h). GH3 cells were treated with increasing concentrations of butyrate (0-5.0 mM), plus or minus 10 nM T3 for 48 h. Total cytoplasmic RNA, cellular protein and medium were analysed for rGH levels with radiolabelled rGH cDNA or antibodies. A greater than 50-fold increase in rGH mRNA level was seen after T3 treatment in the absence or presence of 0.1 mM butyrate. However, 1.0 and 5.0 mM butyrate decreased the stimulation of rGH mRNA levels by T3 to 10- and less than 2-fold, respectively. Control mRNA levels were decreased slightly by increasing butyrate concentrations; rGH mRNA level was 2- to 3-fold higher in the absence of 5 mM butyrate. The pattern of butyrate/T3 response displayed by both cellular and secreted rGH was similar to that seen with mRNA levels. Thus, the predominant effect of butyrate on T3-mediated regulation of growth hormone gene expression is at the level of transcription or mRNA accumulation. A hybrid gene containing 5'-flanking DNA from the rGH gene fused to the bacterial gene coding for chloramphenicol acetyl transferase (CAT), was used to transfect rat pituitary tumour cells with or without butyrate and T3 treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Heparin inhibits skeletal muscle growth in vitro

Heparin or heparan sulfate proteoglycan (HeSPG), but not chondroitin sulfate or hyaluronic acid, exerts a pronounced inhibitory effect on muscle growth in vitro, as determined by total protein, myosin accumulation or synthesis, and [3H]thymidine incorporation studies. Primary muscle fibroblast culture growth is also inhibited by heparin but to a substantially lesser degree compared to muscle (30% and over 90% inhibition of growth, respectively). Heparin-induced inhibition of skeletal muscle growth is a consequence of its interaction with a growth factor(s) present in the media used to support myogenesis; heparin-Sepharose column absorbed horse serum can support muscle growth only in the presence of added heparin-binding growth factors like fibroblast growth factor (FGF) or chicken muscle growth factor (CMGF). Furthermore, heparin prevents the binding of iodinated FGF to the myoblast surface. We also show that the extent of muscle growth is a function of the relative amounts of heparin and FGF in culture. Finally, we provide evidence indicating that FGF can combine with endogenously occurring heparin-like components: immobilized FGF binds sodium-[35S]sulfate labeled components secreted in muscle culture conditioned medium, an interaction inhibited by anti-HeSPG antibodies or heparin, but not by other sulfated glycosaminoglycans. Since heparin binding growth factors not only stimulate myoblast proliferation but also actively inhibit the onset of muscle differentiation (G. Spitzz, D. Roman, and A. Strauss (1986). J. Biol. Chem. 261, 9483-9488), their interaction with naturally occurring heparin-like components may be an important physiological mechanism for modulating muscle growth and differentiation in development and regeneration.

Selected muscle and nerve extracts contain an activity which stimulates myoblast proliferation and which is distinct from transferrin

Extracts from normal chicken anterior latissimus dorsi and dystrophic pectoralis major muscles and from normal chicken sciatic nerves induce a growth stimulation in chicken and rat myogenic cell cultures. Transferrin is only partially responsible for the observed stimulation since the addition of the extracts to transferrin-saturated cultures induces a further growth response and extracts from which transferrin has been removed by immunoabsorption still retain a substantial portion of their stimulation activity. The active fractions of muscle and nerve extracts display heat, acid, and organic solvent inactivation. Gel filtration of ammonium sulfate fractionated activity from the anterior latissimus dorsi muscle suggests the presence of a growth factor in the molecular weight range of 10,000 to 30,000.