Differential expression of creatine kinase and phosphoglycerate mutase isozymes during development in aneural and innervated human muscle culture

Several enzymes that occur in multimolecular forms undergo transitions during myogenesis. Studies of such developmentally regulated isozymes (e.g. creatine kinase) indicate that muscle cells, cultured in the absence of neural tissue never develop fully mature isozyme patterns, but continue to express large amounts of 'housekeeping' isozymes that are characteristically present in fetal muscle. We studied two developmentally controlled isozymes, creatine kinase (CK) and phosphoglycerate mutase (PGAM) in normal human muscle, both aneurally cultured and co-cultured with fetal mouse spinal cord complex. Innervated cultures attain a greater degree of maturity than non-innervated cultures, as revealed by light and electron microscopy, showing well-developed sarcomeres and motor endplates after several weeks in vitro. During early stages of muscle regeneration in co-culture, characteristic fetal isozyme patterns of CK-BB and PGAM-BB activity predominate, as in aneural cultures. The muscle-specific isozymes (CK-MM; PGAM-MM) begin to appear as the muscle differentiates, and after 2-3 months in co-culture only, virtually all enzyme activity is due to the muscle-specific forms of CK and PGAM, as is normally observed in mature skeletal muscle in vivo.     

Tissue- and cell type-specific expression of mRNAs for four types of inositol phospholipid-specific phospholipase C

The mRNA levels for four types of inositol phospholipid-specific phospholipase C (PLC) in various tissues and cell cultures have been studied by Northern analysis using cDNA probes for PLC isozyme I, II, and III [Sue, P.-G., Ryu, S.H., Moon, K.H., Sue, H.W., and Rhee, S.G. (1988) Proc. Natl. Acad. Sci. USA 85, 5419-5423 and Cell 54, 161-169], and the recently identified isozyme IV. All four types are ubiquitously expressed in rat tissues, but the levels of the mRNAs vary among tissues and cell lines. PLC-I mRNA levels are extremely high in brain and rat C6 glioma cells with lower levels in other tissues tested. PLC-II and -III have a more widespread distribution, with relatively high levels in brain, lung, spleen, thymus, and testis in the case of PLC-II, and in skeletal muscle, spleen, and testis for PLC-III. PLC-II and -III mRNAs were also detected in all cell lines examined except human promyelocytic HL60 cells. PLC-IV mRNA levels are extraordinarily high in spleen and HL60 cells. These results indicate that rat C6 glioma cells, together with most rat tissues, contain all four PLC isozymes. Other cultured cell types examined also contain two or three PLC isozymes except for HL60 cells, which contain only PLC-IV. The concomitant expression of PLC isozymes in cultured cells suggests a diverse function for PLC isozymes in single cells.     

Differential myogenicity of satellite cells isolated from extensor digitorum longus (EDL) and soleus rat muscles revealed in vitro

Following muscle damage, fast- and slow-contracting fibers regenerate, owing to the activation of their satellite cells. In rats, crush-induced regeneration of extensor digitorum longus (EDL, a fast muscle) and soleus (a slow muscle) present different characteristics, suggesting that intrinsic differences exist among their satellite cells. An in vitro comparative study of the proliferation and differentiation capacities of satellite cells isolated from these muscles is presented there. We observed several differences between soleus and EDL satellite cell cultures plated at high density on gelatin-coated dishes. Soleus satellite cells proliferated more actively and fused into myotubes less efficiently than EDL cells. The rate of muscular creatine kinase enzyme appeared slightly lower in soleus than in EDL cultures at day 11 after plating, when many myotubes were formed, although the levels of muscular creatine kinase mRNA were similar in both cultures. In addition, soleus cultures expressed higher levels of MyoD and myogenin mRNA and of MyoD protein than EDL satellite cell cultures at day 12. A clonal analysis was also carried out on both cell populations in order to determine if distinct lineage features could be detected among satellite cells derived from EDL and soleus muscles. When plated on gelatin at clonal density, cells from both muscles yielded clones within 2 weeks, which stemmed from 3–15 mitotic cycles and were classified into three classes according to their sizes. Myotubes resulting from spontaneous fusion of cells from the progeny of one single cell were seen regardless of the clone size in the standard culture medium we used. The proportion of clones showing myotubes in each class depended on the muscle origin of the cells and was greater in EDL- than in soleus-cell cultures. In addition, soleus cells were shown to improve their differentiation capacity upon changes in the culture condition. Indeed, the proportions of clones showing myotubes, or of cells fusing into myotubes in clones, were increased by treatments with a myotube-conditioned medium, with phorbol ester, and by growth on extra-cellular matrix components (Matrigel). These results, showing differences among satellite cells from fast and slow muscles, might be of importance to muscle repair after trauma and in pathological situations.     

Studies on Calpain Expression during Differentiation of Rat Satellite Cells in Primary Cultures in the Presence of Heparin or a Mimic Compound

We have synthesized dextran derivatives called RGTAs (for regenerating agents) that were designed to mimic some of the properties of heparin or heparan sulfate to interact with and protect heparin binding growth factors. Some of these growth factors have been described to be involved in myogenesis control. In previous studies, we have shown that muscle regeneration in adults could be greatly enhanced in vivo by treatment with RGTA. Since muscle regeneration occurs through the activation of satellite cells, in the present study we have used primary cultures of rat satellite cells and treated them with the heparan sulfate analogue RGTA or heparin in order to stimulate their growth and differentiation. We also studied the effect of these substances on calpain (calcium-activated neutral proteases) expression in these cultures. Indeed, several reports, principally based on fetal myoblast cultures or myogenic cell lines, have suggested that calpains might be involved in myoblast fusion during myogenic differentiation. We therefore studied the expression of microcalpain (mu-calpain), millicalpain (m-calpain), and calpain 3 in the course of differentiation of these satellite cell cultures in the absence or in the presence of heparin or of a mimic compound (the RGTA RG1282). RGTA and heparin were shown to have a dual effect on satellite cell proliferation and differentiation: RGTA stimulated proliferation with a maximum dose effect at 1 microgam/ml. Heparin used at concentrations similar to those of RGTA was less efficient at stimulating proliferation. Both substances were shown, however, to induce precocious and enhanced differentiation of satellite cells. We showed by quantitative RT-PCR analysis that mu-calpain, m-calpain, and calpain 3 mRNAs were expressed in satellite cell cultures in proliferating myoblasts (day 3) and differentiating cultures (days 7 and 12). The level of mu-calpain mRNA was increased by a factor of 3 during differentiation of satellite cells, whereas the level of m-calpain mRNAs was slightly increased at day 12 only, and calpain 3 mRNA was slightly reduced in these differentiating cultures. Interestingly enough, RGTA and heparin, which both strongly increased differentiation, reduced the expression of the mu- and m-calpains and slightly increased that of calpain 3 in differentiating cultures. These results showed that there was no correlation between the extent of myoblast differentiation and the level of calpain expression in satellite cells grown in primary cultures and underscored the differences between these adult cells and fetal myoblasts.     

Differential expression of FGF receptors and of myogenic regulatory factors in primary cultures of satellite cells originating from fast (EDL) and slow (Soleus) twitch rat muscles.

In the rat, the fast and slow twitch muscles respectively Extensor digitorum longus (EDL) and Soleus present differential characteristics during regeneration. This suggests that their satellite cells responsible for muscle growth and repair represent distinct cellular populations. We have previously shown that satellite cells dissociated from Soleus and grown in vitro proliferate more readily than those isolated from EDL muscle. Fibroblast growth factors (FGFs) are known as regulators of myoblast proliferation and several studies have revealed a relationship between the response of myoblasts to FGF and the expression of myogenic regulatory factors (MRF) of the MyoD family by myoblasts. Therefore, we presently examined the possibility that the satellite cells isolated from EDL and Soleus muscles differ in the expression of FGF receptors (FGF-R) and of MRF expression. FGF-R1 and -R4 were strongly expressed in proliferating cultures whereas FGF-R2 and R3 were not detected in these cultures. In differentiating cultures, only -R1 was present in EDL satellite cells while FGF-R4 was also still expressed in Soleus cells. Interestingly, the unconventional receptor for FGF called cystein rich FGF receptor (CFR), of yet unknown function, was mainly detected in EDL satellite cell cultures. Soleus and EDL satellite cell cultures also differed in the expression MRFs. These results are consistent with the notion that satellite cells from fast and slow twitch muscles belong to different types of myogenic cells and suggest that satellite cells might play distinct roles in the formation and diversification of fast and slow fibres.     

Desmin is present in proliferating rat muscle satellite cells but not in bovine muscle satellite cells.

The presence of desmin was characterized in cultured rat and bovine satellite cells and its potential usefulness as a marker for identifying satellite cells in vitro was evaluated. In primary cultures, positive immunohistochemical staining for desmin and skeletal muscle myosin was observed in rat and bovine myotubes. A small number of mononucleated cells (20% of rat satellite cells and 5% of bovine satellite cells) were myosin-positive, indicative of post-mitotic differentiated myocytes. In bovine satellite cell cultures 13% of the mononucleated cells were desmin-positive, while 84% of the mononucleated cells in rat satellite cell cultures were desmin-positive. Rat satellite cell mass cultures and bovine satellite cell clonal density cultures were pulsed with 3H-thymidine, and autoradiographic data revealed that greater than 94% of dividing rat cells were desmin-positive, suggesting that desmin is synthesized in proliferating rat satellite cells. However, no desmin was seen in cells that incorporated labeled thymidine in bovine satellite cell clones. Analysis of clonal density cultures revealed that only 14% of the mononucleated cells in bovine satellite cell colonies were desmin-positive, whereas 98% of the cells in rat satellite cell colonies were desmin-positive. Fibroblast colonies from both species were desmin-negative. In order to further examine the relationship between satellite cell differentiation and desmin expression, 5-bromo-2'-deoxyuridine (BrdU) was added to culture medium at the time of plating to inhibit differentiation. Fusion was inhibited in rat and bovine cultures, and cells continued to divide. Very few desmin-positive cells were found in bovine cultures, but greater than 90% of the cells in rat cultures stained positive for desmin. The presence of desmin and sarcomeric myosin was also evaluated in regenerating rat tibialis anterior five days after bupivacaine injection. In regenerating areas of the muscle many desmin-positive cells were present, and only a few cells stained positive for skeletal muscle myosin. Application of desmin staining to rat satellite cell growth assays indicated that rat satellite cells cultured in serum-containing medium were contaminated with fibroblasts at levels that ranged from approximately 5% in 24 hr cultures to 15% in mature cultures. In defined medium 4 day cultures contain approximately 95% to 98% desmin-positive satellite cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Gene expression patterns of the fibroblast growth factors and their receptors during myogenesis of rat satellite cells.

Satellite cells are the myogenic precursors in postnatal muscle and are situated beneath the myofiber basement membrane. We previously showed that fibroblast growth factor 2 (FGF2, basic FGF) stimulates a greater number of satellite cells to enter the cell cycle but does not modify the overall schedule of a short proliferative phase and a rapid transition to the differentiated state as the satellite cells undergo myogenesis in isolated myofibers. In this study we investigated whether other members of the FGF family can maintain the proliferative state of the satellite cells in rat myofiber cultures. We show that FGF1, FGF4, and FGF6 (as well as hepatocyte growth factor, HGF) enhance satellite cell proliferation to a similar degree as that seen with FGF2, whereas FGF5 and FGF7 are ineffective. None of the growth factors prolongs the proliferative phase or delays the transition of the satellite cells to the differentiating, myogenin(+) state. However, FGF6 retards the rapid exit of the cells from the myogenin(+) state that routinely occurs in myofiber cultures. To determine which of the above growth factors might be involved in regulating satellite cells in vivo, we examined their mRNA expression patterns in cultured rat myofibers using RT-PCR. The expression of all growth factors, excluding FGF4, was confirmed. Only FGF6 was expressed at a higher level in the isolated myofibers and not in the connective tissue cells surrounding the myofibers or in satellite cells dissociated away from the muscle. By Western blot analysis, we also demonstrated the presence of FGF6 protein in the skeletal musle tissue. Our studies therefore suggest that the myofibers serve as the main source for the muscle FGF6 in vivo. We also used RT-PCR to analyze the expression patterns of the four tyrosine kinase FGF receptors (FGFR1-FGFR4) and of the HGF receptor (c-met) in the myofiber cultures. Depending on the time in culture, expression of all receptors was detected, with FGFR2 and FGFR3 expressed only at a low level. Only FGFR4 was expressed at a higher level in the myofibers but not the connective tissue cell cultures. FGFR4 was also expressed at a higher level in satellite cells compared to the nonmyogenic cells when the two cell populations were released from the muscle tissue and fractionated by Percoll density centrifugation. The unique localization patterns of FGF6 and FGFR4 may reflect specific roles for these members of the FGF signaling complex during myogenesis in adult skeletal muscle.     

CircDUS2L (circ_0039908) promotes lung adenocarcinoma progression by upregulating PGAM1 by acting as a miR-590-5p molecular sponge

Lung adenocarcinoma (LUAD) is usually found at the metastatic stage. Circular RNA dihydrouridine synthase 2-like (DUS2L) (circDUS2L) has been discovered to be upregulated in LUAD. Nevertheless, the function of circDUS2L in LUAD has not been verified. Levels of circDUS2L, microRNA-590-5p (miR-590-5p), and phosphoglycerate mutase 1 (PGAM1) mRNA were analyzed using quantitative real-time polymerase chain reaction (RT-qPCR). Cell proliferation, apoptosis, metastasis, and invasion were assessed by 3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide (MTT), colony formation, 5-ethynyl-2′-deoxyuridine (Edu), flow cytometry, and transwell assays. Protein levels were detected by western blotting. Cell glycolysis was analyzed by measuring cell glucose consumption, lactate production, and extracellular acidification rate (ECAR). The regulatory mechanism of circDUS2L in LUAD cells was analyzed by bioinformatics analysis, dual-luciferase reporter, RNA pull-down, and RNA immunoprecipitation (RIP) assays. Xenograft assay was conducted to confirm the function of circDUS2L in vivo. CircDUS2L was highly expressed in LUAD tissues and cells. CircDUS2L silencing constrained xenograft tumor growth in vivo. CircDUS2L knockdown induced apoptosis, repressed viability, colony formation, proliferation, metastasis, invasion, and glycolysis of LUAD cells in vitro by releasing miR-590-5p via functioning as a miR-590-5p sponge. MiR-590-5p was lowly expressed in LUAD tissues and cells, and miR-590-5p mimic curbed malignant behaviors and glycolysis of LUAD cells by targeting PGAM1. PGAM1 was overexpressed in LUAD tissues and cells, and circDUS2L sponged miR-590-5p to regulate PGAM1 expression. CircDUS2L elevated PGAM1 expression through functioning as a miR-590-5p sponge, thus driving malignant behaviors and glycolysis of LUAD cells.     

Characterization of myogenesis from adult satellite cells cultured in vitro

We describe several characteristics of in vitro myogenesis from adult skeletal muscle satellite cells from the rat and several amphibian species. The timing of cell proliferation and fusion into myotubes was determined, and in urodeles, myogenesis from satellite cells was clearly demonstrated for the first time. Growth factors are known to stimulate satellite cell proliferation. Acidic FGF mRNA was present in rat satellite cells during proliferation but it was not detected in myotubes. Fibronectin was synthesized in satellite cells during proliferation and expelled into the extracellular medium when the myotubes differentiated. We suggest that fibronectin plays a part in the formation of myotubes, as this process was inhibited by anti-fibronectin IgG. Adult satellite cells might differ from fetal myoblasts since they were observed to exhibit the opposite response to a phorbol ester (TPA) to that of the myoblasts. We therefore examined the possibility that the different levels of protein kinase C activity and different phorbol ester binding characteristics in the two cell types account for these opposite responses. Our results suggest that the difference is not connected with the phorbol ester receptor but might be caused by events subsequent to protein kinase C activation. Localized extracellular proteolytic activity might have a role in cell mobilization and/or fusion when satellite cells are activated. We showed that the content of plasminogen activators, chiefly urokinase, was larger in tissues from slow twitch muscles which regenerate more rapidly than fast muscles. The urokinase level rose sharply in cultures when cells fused into myotubes, and was twice as high in slow muscle cells as in fast ones. We also found that, in vitro, slow muscle satellite cells displayed greater myogenicity, but that phorbol ester inhibited their mitosis and myogenicity. We conclude that satellite cells acquire characteristics which differentiate them from myoblasts and correspond to the fast and slow muscles from which they originate.     

Effects of thyroid hormone on mRNAs of phosphoglycerate mutase subunits in rat muscle during development

BACKGROUND: We previously showed that triiodothyronine (T3) stimulates muscle phosphoglycerate mutase (PGAM) activity and isozyme transition in rat skeletal and cardiac muscles. METHODS: The effects of T3 on PGAM types B and M subunit expression in rat muscle during development are reported. RESULTS: T3 administration during the first 21 days of rat life more than doubles type M PGAM mRNA levels, but produces minor effects on type B PGAM mRNA levels. The antihormone propylthiouracil (PTU) slightly decreases both type B and M mRNA levels, but this decrease is not statistically significant. CONCLUSION: Thyroid hormone influences PGAM mRNA isozyme levels differently and increases type M mRNA.     

Structure, gene expression, and evolution of primate glutathione peroxidases

Glutathione peroxidases (GPxs) are a family of enzymes that scavenge peroxides generated in cells. We carried out molecular cloning for cDNAs of four GPx isozymes (GPx-1 through 4) in primate species. The essential residues for the function of these isozymes were well conserved. A phylogenetic tree of GPx isozymes of primates and other mammals showed that GPx-4 diverged first, followed by GPx-3, GPx-2, and GPx-1. Expression of mRNAs for GPx-2 through 4 in various tissues of Japanese monkey was analyzed by Northern blot hybridization. GPx-2 mRNA was detected at 1.7 kb, exclusively in the stomach and small intestine. GPx-3 mRNA was detected at 1.8 kb, intensively in the kidney and adrenal gland, and weakly in the cerebellum, heart, and lung. GPx-4 mRNA was detected at 1.1 kb, very intensively in the testis and weakly in lung, heart, and cerebellum. These results showed that GPx isozymes were expressed under tissue-specific regulations.     

Translocation of protein kinase C isozymes in rat pituitary lactotroph-enriched cell cultures by substance P: effects of sex and age

Abstract

It is generally accepted that the phospholipid and calcium-dependent enzyme protein kinase C (PKC) plays a significant role in secretion of hormones from anterior piuitary cells. The present study was undertaken to study age and sex-related changes in 1. levels of immunoreactivity of PKC isozymes and 2. distribution of immunoreactivity of PKC isozymes after stimulation with substance P (SP) in rat lactotroph-enriched cell cultures. The α, β, δ and ζ isozymes were present in both sexes and at all ages. There was a sex-specific differential regulation of the different PKC isozymes as a function of sexual maturation. In male rats there was an up-regulation of the α isozyme throughout the sexual development, while the β subtype showed a small, but significant decrease in immunoreactivity with increasing age. In female rats, on the other hand, the β species was up-regulated with increasing age while the other subtypes remained constant. The concentration of the δ and ζ isozymes was unaffected of sex and age. Stimulation of lactotroph-enriched cell cultures with substance P (SP) resulted in translocation of the α and β isozymes from the soluble to the particulate fraction while the δ and ζ species were left unchanged independently of age and sex. However, a decrease in responsiveness was observed in adult male rats, although a significant degree of translocation of α and β species was still detected. On the basis of these results it is suggested that in lactotroph-enriched cell cultures basal levels of PKC subtype immunoreactivity and distribution of immunoreactivity of PKC isozymes after SP challenge might be regulated as a function of sex and age.     

Single-fiber isolation and maintenance of satellite cell quiescence.

The activity of satellite cells during myogenesis, development, or skeletal muscle regeneration is strongly modelled using cultures of single muscle fibers. However, there are variations in reported features of gene or protein expression as examined with single-fiber cultures. Here, we examined the potential differences in activation of satellite cells on normal mouse muscle fibers produced during a standard isolation protocol, with or without agitation during collagenase digestion. Activation was detected in satellite cells on fibers after 24 and 48 h of culture in basal growth medium using immunodetection of the incorporation of bromodeoxyuridine (BrdU) into DNA and quantification of the number of BrdU-positive cells per fiber. After 24 and 48 h in culture under nonactivating conditions, the number of activated (BrdU+) satellite cells was greater on fibers that had received gentle agitation during collagenase digestion than on those that were subject to digestion without agitation during isolation. The findings are interpreted to mean that at least some of the variation among published reports may derive from the application of various methods of fiber isolation. The information should be useful for maintaining satellite cell quiescence during studies of the regulatory steps that lead to satellite cell activation.     

Emodin-Induced Necroptosis in Prostate Cancer Cells via the Mitochondrial Fission HSP90/MLKL/PGAM Pathway

Currently, prostate cancer is one of the major malignant tumors in males. Recurrence and metastasis are the main obstacles that prevent the effective treatment of prostate cancer. In the present study, we aimed to evaluate emodin (EG) against human prostate cancer PC3 and DU145 cells. Our study showed that EG significantly decreased the cell viability of PC3 and DU145 cells and strikingly induced non-apoptotic cell death via necroptosis that was visualized through colony formation assay, Hoechst 33258 staining, and TEM analysis. Furthermore, RNA-sequencing and KEGG functional enrichment analysis revealed that the necroptosis-related pathway was activated upon EG treatment in PC3 cells. mRNA and protein expression of necroptosis markers were analyzed by qPCR and immunoblotting, which implied that EG-induced cell necroptosis via enhancing the expression of MLKL and HSP90AA1 activating PGAM pathway which is considered as a key mediator of mitochondrial fission and leading to ROS generation in PC3 and DU145 cells. Thus, our findings suggested that EG is a new small molecule agonist that induced necroptosis in prostate cancer cells via the mitochondrial fission HSP90/MLKL/PGAM pathway.     

Identification and functional characterization of protein kinase C isozymes in platelets and HEL cells.

To determine if selective activation of individual isozymes of protein kinase C (PKC) might explain the apparently divergent effects of PKC stimulation on platelets, we purified and characterized the isozymes from both platelets and human erythroleukemia (HEL) cells, a cell line that has many features of megakaryocytes. Two peaks of platelet PKC activity were resolved by hydroxylapatite chromatography; immunoblot analysis revealed that these two peaks represented the alpha and beta isozymes of PKC. In contrast, HEL cells produced only a single peak that contained the beta isozyme. None of the other PKC isozymes were detected in these fractions. The cytosol of platelets and HEL cells, however, were both found to contain the PKC-delta isozyme. Northern hybridization analyses and mRNA amplification by the polymerase chain reaction demonstrated the presence of mRNA encoding the alpha, beta, and delta PKC isozymes in platelets, but only the beta and delta isozymes in HEL cells. Phorbol myristate acetate (PMA), thrombin, or an endoperoxide analog induced the phosphorylation of the 47-kDa substrate of PKC (pleckstrin) found in platelets and HEL cells; preincubation of either HEL cells or platelets with PMA reduced the intracellular Ca2+ rise induced by thrombin. Thus, although both HEL cells and platelets contain PKC-beta and the recently described PKC-delta isozymes, the widely distributed alpha isozyme of PKC is absent in HEL cells; however, isozymes other than PKC-alpha are sufficient for some PMA-mediated functions that are similar to those seen in stimulated platelets.     

Plasminogen carbohydrate side chains in receptor binding and enzyme activation: a study of C6 glioma cells and primary cultures of rat hepatocytes.

The human [Glu1]-plasminogen carbohydrate isozymes, plasminogen type I (Pg 1) and plasminogen type II (Pg 2), were separated by chromatography and studied in cell binding experiments at 4 degrees C with primary cultures of rat hepatocytes and rat C6 glioma cells. In both cell systems, Pg 1 and Pg 2 bound to an equivalent number of receptors, apparently representing the same population of surface molecules. The affinity for Pg 2 was slightly higher. With hepatocytes, the KD for Pg 1 was 3.2 +/- 0.2 microM, and the KD for Pg 2 was 1.9 +/- 0.1 microM, as determined from Scatchard transformations of the binding isotherms. The Bmax was approximately the same for both isozymes. With C6 cells, the KD for Pg 1 was 2.2 +/- 0.1 microM vs. 1.5 +/- 0.2 microM for Pg 2. Again, the Bmax was similar with both isozymes. 125I-Pg 1 and 125I-Pg 2 were displaced from specific binding sites by either nonradiolabeled isozyme. The KI for Pg 2 was slightly lower than the KI for Pg 1 with hepatocytes (0.9 vs. 1.3 microM) and with C6 cells (0.6 vs. 1.1 microM). No displacement was detected with miniplasminogen at concentrations up to 5.0 microM. Activation of Pg 1 and Pg 2 by recombinant two-chain tissue-plasminogen activator (rt-PA) was enhanced by hepatocyte cultures. The enhancing effect was greater with Pg 2. Hepatocyte cultures did not affect the activation of miniplasminogen by rt-PA or the activation of plasminogen by streptokinase. Unlike the hepatocytes, C6 cells did not enhance the activation of plasminogen by rt-PA or streptokinase; however, plasmin generated in the presence of C6 cells reacted less readily with alpha 2-antiplasmin.