The influence of postnatal development on drug-induced hepatic porphyria and the synthesis of cytochrome P-450. A biochemical and morphological study.

The mitochondrial enzyme delta-aminolevulinate synthetase (ALAS) controls the rate-limiting step in the synthesis of porphyrins and heme. An experimental form of hepatic porphyria can be readily elicited in laboratory animals, such as the rat, by drugs and foreign chemicals which are known to enhance the de novo formation of this enzyme in the liver. The present study shows that there is a striking refractoriness to the induction of ALAS during the perinatal period in the rat. Chemicals which have potent porphyria-inducing activity in adult animals have no significant inducing effect on hepatic ALAS in neonates. The ultrastructural changes which accompany the induction of ALAS by drugs and chemicals in adult liver also fail to take place in the livers of neonates. A progressive capacity for responding to the action of chemical inducers of hepatic ALAS does, however, develop in neonatal animals so that by approximately 5-6 wk of age experimental porphyria can be elicited as effectively in them as in adults. The reasons for the refractoriness of hepatic ALAS to induction in the perinatal period are not known; but the findings of this study make it clear that ALAS belongs to that increasingly large group of liver enzymes in mammals whose appearance, increase of activity, or inducibility is developmentally determined. The occurrence of developmental changes in the indicibility of ALAS in the liver of neonates also provided an opportunity to study the relationship of this enzyme activity to the drug-mediated induction of the hepatic hemoprotein cytochrome P-450. This inducible hemoprotein serves as the terminal oxygenase in the microsomal mixed-function oxidase system in the liver. The results of this study indicate that, in contrast to the refractoriness of ALAS to induction, significant drug-induced changes of hepatic P-450 content and of hemeprecursor incorporation into this cytochrome do take place in neonates. The synthesis of P-450 thus appears to be under a regulatory control different from that of ALAS in neonates, and the relation between ALAS activity and P-450 formation is not therefore a direct one.

Stimulation of the chemotactic migration of human fibroblasts by transforming growth factor beta.

Transforming growth factor beta (TGF-beta) is a potent chemoattractant in vitro for human dermal fibroblasts. Intact disulfide and perhaps the dimeric structure of TGF-beta is essential for its ability to stimulate chemotactic migration of fibroblasts, since reduction with 2-ME results in a marked loss of its potency as a chemoattractant. Although epidermal growth factor (EGF) appears to be capable of modulating some effects of TGF-beta, it does not alter the chemotactic response of fibroblasts to TGF-beta. Specific polyvalent rabbit antibodies to homogeneously pure TGF-beta block its chemotactic activity but has no effect on the other chemoattractants tested (platelet-derived growth factor, fibronectin, and denatured type I collagen). Since TGF-beta is secreted by a variety of neoplastic and normal cells including platelets, monocytes/macrophages, and lymphocytes, it may play a critical role in vivo in embryogenesis, host response to tumors, and the repair response that follows damage to tissues by immune and nonimmune reactions.

Transforming growth factor beta 1-induced changes in cell migration, proliferation, and angiogenesis in the chicken chorioallantoic membrane.

Application of TGF beta 1 (10-100 ng) to the chicken chorioallantoic membrane (CAM) for 72 h resulted in a dose-dependent, gross angiogenic response. The vascular effects induced by TGF beta 1 were qualitatively different than those induced by maximal doses of basic FGF (bFGF) (500 ng). While TGF beta 1 induced the formation of large blood vessels by 72 h, bFGF induced primarily small blood vessels. Histologic analysis revealed that TGF beta 1 stimulated pleiotropic cellular responses in the CAM. Increases in fibroblast and epithelial cell density in the area of TGF beta 1 delivery were observed as early as 4 h after TGF beta 1 treatment. By 8 h, these cell types also demonstrated altered morphology and marked inhibition of proliferation as evidenced by 3H-thymidine labeling. Thus, the TGF beta 1-stimulated accumulation of these cell types was the result of cellular chemotaxis from peripheral areas into the area of TGF beta 1 delivery. Microscopic angiogenesis in the form of capillary sprouts and increased endothelial cell density first became evident at 16 h. By 24 h, capillary cords appeared within the mesenchyme of the CAM, extending towards the point of TGF beta 1 delivery. 3H-thymidine labeling revealed that the growth of these capillary cords was due to endothelial cell proliferation. Finally, perivascular mononuclear inflammation did not become evident until 48 h of treatment, and its presence correlated spatially and temporally with the gross and histological remodelling of newly formed capillary cords into larger blood vessels. In summary, these data suggest that, in the chicken CAM, TGF beta 1 initiates a sequence of cellular responses that results in growth inhibition, cellular accumulation through migration, and microvascular angiogenesis.

Comparison and characterization of nuclear isolation procedures as applied to chick oviduct.

A number of methods for the preparation of chick oviduct nuclei have been compared. Nuclei have been isolated in hypertonic sucrose and citric acid and the product has been characterized with respect to cleanliness, ultrastructure, RNA polymerase activity, RNA integrity, and chromatin composition. The study demonstrates that the choice of oviduct nuclear isolation procedure will depend markedly on the purpose for which the nuclei are required. Thus, nuclei prepared entirely in high-molarity sucrose retain the highest levels of RNA polymerase. Those prepared rapidly in the presence of citric acid retain nuclear RNA in an essentially undegraded state. Finally, a bulk preparation is described which, because of its adaptability and high yield of morphologically intact nuclei using large amounts of tissue, is ideal for use in preparing chromatin. Conditions are described by which isolated nuclei can be stored for up to 6 months and retain their morphology, chemical characteristics, and RNA polymerase activity.

Proteolytic activation of latent transforming growth factor-beta from fibroblast-conditioned medium.

Transforming growth factor-beta (TGF beta) is produced by most cultured cells in an inactive form. Potential activation mechanisms of latent TGF beta were studied using fibroblastic (NRK-49F and AKR-MCA) cell-conditioned medium as a model. Active TGF beta was monitored by radioreceptor and soft agar assays as well as by antibody inhibition and immunoprecipitation. Little or no TGF beta was detected in untreated conditioned medium. Treatment of the medium with extremes of pH (1.5 or 12) resulted in significant activation of TGF beta as shown by radioreceptor assays, while mild acid treatment (pH 4.5) yielded only 20-30% of the competition achieved by pH 1.5. In an effort to define more physiological means of TGF beta activation, the effects of some proteases were tested. Plasmin and cathepsin D were found to generate 25-kD bands corresponding to the active form of TGF beta as shown by immunoprecipitation analysis of radiolabeled cell-conditioned medium. Plasmin treatment of the medium resulted in activity that was quantitatively similar to that of mild acid treatment as measured by radioreceptor and soft agar assays. In addition, the plasmin-generated activity was inhibited by anti-TGF beta antibodies. Sequential treatments of AKR-MCA cell-conditioned medium with mild acid followed by plasmin or plasmin followed by mild acid gave activation comparable to either treatment alone. The data suggest that conditioned medium may contain at least two different pools of latent TGF beta. One pool is resistant to mild acid and/or plasmin and requires strong acid or alkali treatment for activation. A second pool is activated by mild pH change and/or plasmin. Activation of this form of latent TGF beta may take place by dissociation or proteolytic digestion from a precursor molecule or hypothetical TGF beta-binding protein complex.

Immunohistochemical localization of TGF beta 1, TGF beta 2, and TGF beta 3 in the mouse embryo: expression patterns suggest multiple roles during embryonic development.

Isoform-specific antibodies to TGF beta 1, TGF beta 2, and TGF beta 3 proteins were generated and have been used to examine the expression of these factors in the developing mouse embryo from 12.5-18.5 d post coitum (d.p.c.). These studies demonstrate the initial characterization of both TGF beta 2 and beta 3 in mammalian embryogenesis and are compared with TGF beta 1. Expression of one or all three TGF beta proteins was observed in many tissues, e.g., cartilage, bone, teeth, muscle, heart, blood vessels, lung, kidney, gut, liver, eye, ear, skin, and nervous tissue. Furthermore, all three TGF beta proteins demonstrated discrete cell-specific patterns of expression at various stages of development and the wide variety of tissues expressing TGF beta proteins represent all three primary embryonic germ layers. For example, specific localization of TGF beta 1 was observed in the lens fibers of the eye (ectoderm), TGF beta 2 in the cortex of the adrenal gland (mesoderm), and TGF beta 3 in the cochlear epithelium of the inner ear (endoderm). Compared to the expression of TGF beta mRNA transcripts in a given embryonic tissue, TGF beta proteins were frequently colocalized within the same cell type as the mRNA, but in some cases were observed to localize to different cells than the mRNA, thereby indicating that a complex pattern of transcription, translation, and secretion for TGF beta s 1-3 exists in the mouse embryo. This also indicates that TGF beta 1, beta 2, and beta 3 act through both paracrine and autocrine mechanisms during mammalian embryogenesis.

Mechanism of activation of latent recombinant transforming growth factor beta 1 by plasmin.

Medium conditioned by Chinese hamster ovary (CHO) cells transfected with the simian pre-pro-TGF beta 1 cDNA contains high levels of latent TGF beta 1. The amino-terminal region of the TGF beta 1 precursor is secreted and can be detected in the conditioned medium by immunoblotting using peptide antibodies specific for amino-terminal peptides. Chemical cross-linking of CHO-conditioned medium using bis-(sulfosuccinimidyl)-suberate (BS3) followed by immunoblot analyses indicates that latent recombinant TGF beta 1 contains both the cleaved amino-terminal glycopeptide and mature TGF beta 1 polypeptide in a noncovalent association and that this association confers latency. The data presented here do not support the involvement of a unique TGF beta binding protein(s) in latent recombinant TGF beta 1. Plasmin treatment of CHO-conditioned medium resulted in the appearance of TGF beta competing activity. In addition, immunoblot analysis of plasmin-treated CHO-conditioned medium indicates that the amino-terminal glycopeptide is partially degraded and that mature TGF beta 1 is released. Thus, activation of latent TGF beta 1 may occur by proteolytic nicking within the amino-terminal glycopeptide thereby causing a disruption of tertiary structure and noncovalent bonds, which results in the release of active, mature TGF beta 1. Acid activation of latent TGF beta, in comparison, appears to be due to dissociation of the amino-terminal glycopeptide from the mature polypeptide.

Regulation of the synthesis and activity of urokinase plasminogen activator in A549 human lung carcinoma cells by transforming growth factor-beta.

Transforming growth factor-beta (TGF beta) is a regulator of cellular proliferation which can alter the proteolytic activity of cultured cells by enhancing the secretion of endothelial type plasminogen activator inhibitor and affecting the secretion of plasminogen activators (PAs) in cultured fibroblastic cells. We used the TGF beta-responsive malignant human lung adenocarcinoma cell line A549 to study the relationships between the known TGF beta-induced growth inhibition and the effects of TGF beta on the secretion of PA activity by A549 cells. PA activity was quantitated by caseinolysis assays, and characterized by urokinase mRNA analysis, immunoprecipitation, and zymography assays. PA-inhibitor production was observed in autoradiograms of SDS-polyacrylamide gels and reverse zymography assays. It was found that TGF beta enhanced the production of PA activity by these cells, in accordance with an enhancement of urokinase mRNA levels. A concomitant stimulation of type 1 PA-inhibitor production was also observed in A549 cells in response to TGF beta. In contrast to the observations of A549 cells, TGF beta caused a decrease in the expression of both urokinase and the tissue-type PA mRNA in human embryonic WI-38 lung fibroblasts indicating opposite regulation of the expression of PAs in these cells. The results suggest that TGF beta may play a role in the regulation of the invasive, proteolytically active phenotype of certain lung carcinoma cells.

Ultrastructural studies of vasopressin effect on isolated perfused renal collecting tubules of the rabbit.

Isolated cortical collecting tubules from rabbit kidney were studied during perfusion with solutions made either isotonic or hypotonic to the external bathing medium. Examination of living tubules revealed a reversible increase in thickness of the cellular layer, prominence of lateral cell membranes, and formation of intracellular vacuoles during periods of vasopressin-induced osmotic water transport. Examination in the electron microscope revealed that vasopressin induced no changes in cell structure in collecting tubules in the absence of an osmotic difference and significant bulk water flow across the tubule wall. In contrast, tubules fixed during vasopressin-induced periods of high osmotic water transport showed prominent dilatation of lateral intercellular spaces, bulging of apical cell membranes into the tubular lumen, and formation of intracellular vacuoles. It is concluded that the ultrastructural changes are secondary to transepithelial bulk water flow and not to a direct effect of vasopressin on the cells, and that vasopressin induces osmotic flow by increasing water permeability of the luminal cell membrane. The lateral intercellular spaces may be part of the pathway for osmotically induced transepithelial bulk water flow.

Expression of a truncated, kinase-defective TGF-beta type II receptor in mouse skeletal tissue promotes terminal chondrocyte differentiation and osteoarthritis.

Members of the TGF-beta superfamily are important regulators of skeletal development. TGF-betas signal through heteromeric type I and type II receptor serine/threonine kinases. When over-expressed, a cytoplasmically truncated type II receptor can compete with the endogenous receptors for complex formation, thereby acting as a dominant-negative mutant (DNIIR). To determine the role of TGF-betas in the development and maintenance of the skeleton, we have generated transgenic mice (MT-DNIIR-4 and -27) that express the DNIIR in skeletal tissue. DNIIR mRNA expression was localized to the periosteum/perichondrium, syno-vium, and articular cartilage. Lower levels of DNIIR mRNA were detected in growth plate cartilage. Transgenic mice frequently showed bifurcation of the xiphoid process and sternum. They also developed progressive skeletal degeneration, resulting by 4 to 8 mo of age in kyphoscoliosis and stiff and torqued joints. The histology of affected joints strongly resembled human osteo-arthritis. The articular surface was replaced by bone or hypertrophic cartilage as judged by the expression of type X collagen, a marker of hypertrophic cartilage normally absent from articular cartilage. The synovium was hyperplastic, and cartilaginous metaplasia was observed in the joint space. We then tested the hypothesis that TGF-beta is required for normal differentiation of cartilage in vivo. By 4 and 8 wk of age, the level of type X collagen was increased in growth plate cartilage of transgenic mice relative to wild-type controls. Less proteoglycan staining was detected in the growth plate and articular cartilage matrix of transgenic mice. Mice that express DNIIR in skeletal tissue also demonstrated increased Indian hedgehog (IHH) expression. IHH is a secreted protein that is expressed in chondrocytes that are committed to becoming hypertrophic. It is thought to be involved in a feedback loop that signals through the periosteum/ perichondrium to inhibit cartilage differentiation. The data suggest that TGF-beta may be critical for multifaceted maintenance of synovial joints. Loss of responsiveness to TGF-beta promotes chondrocyte terminal differentiation and results in development of degenerative joint disease resembling osteoarthritis in humans.

Growth inhibitor from BSC-1 cells closely related to platelet type beta transforming growth factor.

Purified growth inhibitor from BSC-1 cells and type beta transforming growth factor from human platelets are shown to have nearly identical biological activity and to compete for binding to the same cell membrane receptor. These findings suggest that the growth inhibitor and the type beta transforming growth factor are similar molecules. The data also show that the same purified polypeptide can either stimulate or inhibit cell proliferation depending on the experimental conditions.

Adrenal cholesterol: localization by electron-microscope autoradiography.

As determined by electron-microscope autoradiography of adrenal glands containing tritiated cholesterol and by modified differential centrifugation techniques, 70 to 80 percent of adrenal cholesterol is contained within lipid droplets of rat adrenal cortical cells.

TGF-beta receptor II in epithelia versus mesenchyme plays distinct roles in the developing lung.

Transforming growth factor (TGF)-beta signalling plays important roles in regulating lung development. However, the specific regulatory functions of TGF-beta signalling in developing lung epithelial versus mesenchymal cells are still unknown. By immunostaining, the expression pattern of the TGF-beta type II receptor (TbetaRII) was first determined in the developing mouse lung. The functions of TbetaRII in developing lung were then determined by conditionally knocking out TbetaRII in the lung epithelium of floxed-TbetaRII/surfactant protein C-reverse tetracycline transactivator/TetO-Cre mice versus mesenchyme of floxed-TbetaRII/Dermo1-Cre mice. TbetaRII was expressed only in distal airway epithelium at early gestation (embryonic day (E)11.5), but in both airway epithelium and mesenchyme from mid-gestation (E14.5) to post-natal day 14. Abrogation of TbetaRII in mouse lung epithelium resulted in retardation of post-natal lung alveolarisation, with markedly decreased type I alveolar epithelial cells, while no abnormality in prenatal lung development was observed. In contrast, blockade of TbetaRII in mesoderm-derived tissues, including lung mesenchyme, resulted in mildly abnormal lung branching and reduced cell proliferation after mid-gestation, accompanied by multiple defects in other organs, including diaphragmatic hernia. The primary lung branching defect was verified in embryonic lung explant culture. The novel findings of the present study suggest that transforming growth factor-beta type II receptor-mediated transforming growth factor-beta signalling plays distinct roles in lung epithelium versus mesenchyme to differentially control specific stages of lung development.

Regulation of differentiation by TGF-beta.

Recent experiments in neural, skeletal, endothelial, and hematopoietic tissues have provided new insights into the way members of the transforming growth factor-beta (TGF-beta) superfamily regulate cellular differentiation. TGF-betas regulate the fate of multipotential stem cells instructively (in the neural crest) by regulating the expression or function of tissue-specific transcription factors, as well as selectively (in the mesenchyme) by regulating the expression of required growth factors and their receptors. During skeletal development, TGF-betas have unique functions and act sequentially to modulate chondrocyte and osteoblast differentiation. Responsiveness to TGF-betas changes as cells differentiate and evidence now suggests that changes in TGF-beta receptor profile may account for some of these differences. Drosophila and transgenic mouse models are now providing useful insights into mechanisms of TGF-beta action in vivo.

Transforming growth factor-beta activity in sheep lung lymph during the development of pulmonary hypertension.

Chronic pulmonary hypertension is associated with extensive structural remodeling of the pulmonary arterial bed. The structural changes in the arterial walls include increased production of extracellular matrix components and smooth muscle cell hypertrophy, changes that have been similarly induced by transforming growth factor-beta (TGF-beta) in culture. In the present study, experiments were performed to determine whether TGF-beta is present in sheep lung lymph, and whether TGF-beta levels were altered in an animal model of chronic pulmonary hypertension induced by continuous air embolization. Several standard biological assays for TGF-beta activity were used for these determinations including soft agar assays, inhibition of epithelial cell proliferation, and a TGF-beta-specific radioreceptor assay. In each case, control lung lymph contained high concentrations of TGF-beta (100 ng/ml) which required transient acidification for detection. Samples of lung lymph from hypertensive sheep showed a transient and early two- to threefold increase in concentrations of latent TGF-beta. This activity could be partially blocked by TGF-beta antibodies. These studies indicate that sheep lung lymph contains TGF-beta and that the level of TGF-beta increases early during the development of pulmonary hypertension. Thus, TGF-beta may contribute to the development of the structural changes in the pulmonary arteries that occur during the onset of chronic pulmonary hypertension.

Hepatic processing of transforming growth factor beta in the rat. Uptake, metabolism, and biliary excretion.

Transforming growth factor beta (TGF beta), a recently discovered polypeptide, modulates growth of normal and neoplastic cells. Since little is known concerning in vivo disposition of TGF beta, we performed studies to examine the hepatic processing of biologically active 125I-TGF beta in the rat. After intravenous injection, 125I-TGF beta disappeared from the plasma with an initial t1/2 of 2.2 min; partial hepatectomy delayed the plasma disappearance of 125I-TGF beta by 80%. 60 min after intrafemoral injection, 63% of the recovered label was present in liver and/or bile; by 90 min, most of the label removed by the liver (83%) had been slowly excreted into bile. Nearly all the label in bile (96%) was soluble in trichloracetic acid and not immunoprecipitable by specific antiserum. Colchicine and vinblastine inhibited cumulative biliary excretion of label by 28 and 37%, respectively; chloroquine and leupeptin each increased the amount of label in bile that was precipitable by trichloracetic acid and that coeluted with authentic 125I-TGF beta on molecular sieve chromatography. There was efficient first-pass hepatic extraction of 125I-TGF beta (36%) in the isolated perfused rat liver, which was inhibited by unlabeled TGF beta (but not by epidermal growth factor, EGF) and by lectins in a dose-dependent manner; prolonged fasting also decreased clearance (26%). After fractionation of liver by differential or isopycnic centrifugation, radiolabel codistributed with marker enzymes for lysosomes. The results indicate rapid, extensive, inhibitable, and organ-selective extraction of TGF beta by the liver. After extraction, TGF beta undergoes efficient transhepatic transport, extensive intracellular metabolism, and slow but complete biliary excretion of its metabolites. Liver fractionation studies and pharmacologic manipulations suggest that these processes are associated with organelles that include microtubules and lysosomes. The data suggest that the liver is a major target tissue or site of metabolism for biologically active TGF beta.

Transforming growth factor-beta increases steady state levels of type I procollagen and fibronectin messenger RNAs posttranscriptionally in cultured human dermal fibroblasts.

Transforming growth factor-beta (TGF beta), when injected subcutaneously into newborn mice, induces a rapid fibrotic response, stimulates chemotaxis, and elevates the rates of biosynthesis of collagen and fibronectin by fibroblasts in vitro. We explored the molecular mechanisms of TGF beta-mediated stimulation of collagen and fibronectin synthesis in cultured human foreskin fibroblasts. TGF beta preferentially stimulated the synthesis of fibronectin and type I procollagen chains 3-5-fold as shown by polypeptide analysis. Concomitant elevation in the steady state levels of messenger RNAs (mRNAs) coding for type I procollagen and fibronectin also occurred but without a net increase in the rate of transcription of either of these genes. The preferential stabilization of mRNAs specifying type I procollagen and fibronectin provides a partial explanation for the mechanisms by which TGF beta enhances the synthesis of type I procollagen and fibronectin in mesenchymal cells.

STRAP and Smad7 synergize in the inhibition of transforming growth factor beta signaling.

Smad proteins play a key role in the intracellular signaling of the transforming growth factor beta (TGF-beta) superfamily of extracellular polypeptides that initiate signaling from the cell surface through serine/threonine kinase receptors. A subclass of Smad proteins, including Smad6 and Smad7, has been shown to function as intracellular antagonists of TGF-beta family signaling. We have previously reported the identification of a WD40 repeat protein, STRAP, that associates with both type I and type II TGF-beta receptors and that is involved in TGF-beta signaling. Here we demonstrate that STRAP synergizes specifically with Smad7, but not with Smad6, in the inhibition of TGF-beta-induced transcriptional responses. STRAP does not show cooperation with a C-terminal deletion mutant of Smad7 that does not bind with the receptor and consequently has no inhibitory activity. STRAP associates stably with Smad7, but not with the Smad7 mutant. STRAP recruits Smad7 to the activated type I receptor and forms a complex. Moreover, STRAP stabilizes the association between Smad7 and the activated receptor, thus assisting Smad7 in preventing Smad2 and Smad3 access to the receptor. STRAP interacts with Smad2 and Smad3 but does not cooperate functionally with these Smads to transactivate TGF-beta-dependent transcription. The C terminus of STRAP is required for its phosphorylation in vivo, which is dependent on the TGF-beta receptor kinases. Thus, we describe a mechanism to explain how STRAP and Smad7 function synergistically to block TGF-beta-induced transcriptional activation.

Modulation of transforming growth factor type beta action by activated ras and c-myc.

Transfection of C3H/10T1/2 cells with a c-myc gene resulted in the acquisition of responsiveness to transforming growth factor type beta. Cells transfected with an activated H-ras gene or an H-ras and c-myc gene, however, exhibited a transformed morphology and spontaneous soft-agar growth, a phenotype induced reversibly by transforming growth factor type beta in responsive fibroblasts.

Selective inhibition of growth-related gene expression in murine keratinocytes by transforming growth factor beta.

Transforming growth factor beta (TGF beta) is a potent inhibitor of epithelial cell proliferation. A nontumorigenic epidermal growth factor (EGF)-dependent epithelial cell line, BALB/MK, is reversibly growth arrested by TGF beta. TGF beta will also abrogate EGF-stimulated mitogenesis of quiescent BALB/MK cells. Increased levels of calcium (greater than 1.0 mM) will induce differentiation in BALB/MK cells; in contrast, TGF beta-mediated growth inhibition does not result in induction of terminal differentiation. In the present study, the effects of TGF beta and calcium on growth factor-inducible gene expression were examined. TGF beta markedly decreased c-myc and KC gene expression in rapidly growing BALB/MK cells and reduced the EGF induction of c-myc and KC in a quiescent population of cells. TGF beta exerted its control over c-myc expression at a posttranscriptional level, and this inhibitory effect was dependent on protein synthesis. TGF beta had no effect on c-fos gene expression, whereas 1.5 mM calcium attenuated EGF-induced c-fos expression in quiescent cells. Expression of beta-actin, however, was slightly increased in both rapidly growing and EGF-restimulated quiescent BALB/MK cells treated with TGF beta. Thus, in this system, TGF beta selectively reduced expression of certain genes associated with cell proliferation (c-myc and KC), and at least part of the TGF beta effect was at a posttranscriptional level.