Modulating phosphatidic acid metabolism decreases oxidative injury in rat lungs.

We determined that lisofylline, a potent inhibitor of oleate- and linoleate-containing phosphatidic acid formation (half-maximal inhibitory concentration = 40 nM), prevented oxidant-mediated capillary leak in isolated rat lungs given interleukin-8 (IL-8) intratracheally and perfused with human neutrophils. Lung leak was prevented by lung, but not neutrophil, lisofylline pretreatment. Furthermore, although lisofylline inhibited IL-8-stimulated neutrophil production of phosphatidic acid in vitro, it did not prevent IL-8-stimulated neutrophil adherence, chemotaxis, or intracellular calcium mobilization or N-formyl-Met-Leu-Phe (fMLP)-stimulated oxidant production in vitro. Lisofylline also prevented acute capillary leak in isolated rat lungs perfused only with the oxidant generator purine-xanthine oxidase but did not scavenge O2-(+) or H2O2 in vitro. Finally, lisofylline-mediated protection against lung leak in both models was associated with alterations in lung membrane free fatty acid acyl composition (as reflected by the decreased ratio [linoleate + oleate]/[palmitate]). We conclude that lisofylline prevented both neutrophil-dependent and neutrophil-independent oxidant-induced capillary leak in isolated rat lungs and that protection appears to be mediated by blocking intrinsic lung linoleoyl phosphatidic acid metabolism. We speculate that lisofylline, in addition to our previously reported effects on cytokine signaling by intrapulmonary mononuclear cells, alters intrinsic pulmonary capillary membrane composition and renders this barrier less vulnerable to oxidative damage.

Preferential radiosensitization of G1 checkpoint-deficient cells by methylxanthines.

PURPOSE: To develop a checkpoint-based strategy for preferential radiosensitization of human tumors with deficient and/or mutant p53. METHODS AND MATERIALS: A549 human lung adenocarcinoma cell lines differing in their expression of the p53 tumor suppressor gene were produced by transduction with the E6 oncogene from human papilloma virus type 16. The cells expressing E6 (E6+) lack a G1 arrest in response to ionizing radiation, are deficient in p53 and p21 expression, and exhibit a fivefold greater clonogenic survival following 10 Gy radiation. RESULTS: Postirradiation incubation with millimolar concentrations of the methylxanthine pentoxifylline (PTX) results in preferential radiosensitization of the E6+ cells compared to the LXSN+ vector transduced controls. There is a threefold sensitization of the LXSN+ cells and a 15-fold sensitization of the E6+ cells, which results in equal clonogenic survival of the two lines. Flow cytometry reveals PTX abrogation of the radiation induced G2 arrest for both cell lines. PTX also prolongs G1 transit for both cell lines. Preliminary results are presented using a novel methylxanthine, lisofylline (LSF), which has similar cell cycle effects on G1 and G2 and achieves differential radiosensitization at micromolar concentrations that are sustainable in humans. CONCLUSION: This checkpoint-based strategy is a promising approach for achieving preferential radiosensitization of p53- tumors relative to p53+ normal tissues.

Pharmacological inhibition of gelatinase B induction and tumor cell invasion.

The 92 kDa matrix metalloproteinase (gelatinase B, MMP-9) plays a major role in the facilitation of tumor metastasis and in inflammatory disorders characterized by excessive matrix protein destruction. MMP-9 is transcriptionally induced in multiple cell types by exposure to the inflammatory mediators bacterial endotoxin, interleukin-1 (IL-1) or tumor necrosis factor-alpha (TNF-alpha). CT-2519, (1-(5-isothiocyanatohexyl)-3,7-dimethylxanthine), a synthetic small molecule from an anti-inflammatory compound library, was evaluated for its effect on endotoxin and cytokine-induced MMP-9 synthesis by a monocytic leukemic cell line, THP-1, and a monocyte/macrophage line, RAW 264.7. CT-2519 dose-dependently inhibited endotoxin and cytokine-induced synthesis of MMP-9 by these cells. Furthermore, both MMP-9 secretion and matrix invasion by cells of a human fibrosarcoma cell line, HT-1080, were inhibited by CT-2519 in a dose-dependent manner. Northern blot analyses and studies utilizing MMP-9 promoter constructs indicated that the inhibitory action of CT-2519 occurs at the level of transcriptional suppression. Given the observation that cellular activation by endotoxin, IL-1 and TNF-alpha may be mediated, at least in part, through induction of certain species of phosphatidic acid (PA), the effect of CT-2519 on lipid levels was analyzed. CT-2519 effectively reduced endotoxin-mediated increases in particular cellular lipid levels. Pharmacologic modulation of cytokine-dependent gene products, such as MMP-9, may offer an important therapeutic approach to the treatment of neoplastic and inflammatory disorders.

Lisofylline inhibits transforming growth factor beta release and enhances trilineage hematopoietic recovery after 5-fluorouracil treatment in mice.

The effectiveness of endogenous or exogenously administered colony-stimulating factors may be modulated by the presence of hematopoietic inhibitory molecules. Cytotoxic therapy may result in the induction of hematopoietic inhibitors contributing to prolonged myelosuppression, whereas preventing the induction of such inhibitors may accelerate multilineage recovery. Lisofylline [LSF; (R)-1-(5-hydroxyhexyl)-3,7, dimethyl-xanthine], inhibits the signaling and/or release of certain hematopoietic inhibitory molecules such as tumor necrosis factor alpha, macrophage inflammatory protein 1 alpha, transforming growth factor beta, and IFN-gamma. Treatment of murine bone marrow cells with the cytotoxic agent 5-fluorouracil (5-FU) results in the release of a nondialyzable inhibitor of progenitor (colony-forming unit-granulocyte macrophage; CFU-GM) proliferation. When murine bone marrow cells were treated with 5-FU plus LSF, release of this inhibitor of CFU-GM proliferation was blocked. Neutralizing antibody and Western blot analysis indicated that the inhibitor was TGF-beta. We tested the effect of LSF (100 mg/kg i.p., b.i.d.) on multilineage regeneration after high-dose 5-FU or thiotepa treatment in BALB/c mice. In 4 of 5 experiments, LSF significantly accelerated neutrophil recovery (P < or = 0.05, Wilcoxon paired-signed test). In addition, platelet, reticulocyte, and CFU-GM regeneration were significantly accelerated in mice treated with LSF compared to control mice (P < or = 0.05). LSF had no significant effects on the ability of 5-FU to kill hematopoietic progenitor cells, nor did LSF stimulate or inhibit proliferation of CFU-GM. LSF had no effect on chemotherapy-induced killing of tumor cells in vitro, nor on the antitumor activity of 5-FU or thiotepa in BALB/c mice implanted with P388 leukemia cells. Inhibition of hematopoietic inhibitor release may accelerate multilineage recovery after cytotoxic therapy and, as such, may represent an alternative or additional therapy to the use of positively acting lineage specific colony-stimulating factors.

Potential role for phosphatidic acid in mediating the inflammatory responses to TNF alpha and IL-1 beta.

Tumor necrosis factor alpha (TNF alpha), interleukin 1 beta (IL-1 beta), and endotoxin (LPS) are potent pro-inflammatory mediators which induce multiple and diverse biological responses in a wide variety of cell types. However, these pro-inflammatory mediators also have significant overlap and redundancy in their biological effects. This suggests that there is significant diversity in second messenger signal transduction systems induced by these stimuli to explain the diversity in biological responses, as well as significant redundancy. Here we show that one such second messenger common to several proinflammatory stimuli may be phosphatidic acid (PA). Intracellular PA species, which may have intracellular signaling functions, are rapidly induced in P388 monocytic leukemia cells by TNF alpha, IL-1 beta, or LPS. These PA species vary according to the bond type (i.e., sn-1 ester vs. ether vs. vinyl ether), acyl chain length, and the degree of saturation in the sn-1 and sn-2 positions. Although PA itself may have direct second messenger activities, many of the PA species induced are converted to diacylglycerol species (DG), which are structurally distinct from the DGs generated by phosphatidylcholine-specific phospholipase C (PC-PLC). Lisofylline [(R)-1-(5-hydroxyhexyl)-3,7-dimethylxanthine; LSF] selectively inhibits generation of selected species of PA in P388 cells induced by TNF alpha, IL-1 beta or LPS. TNF alpha-induced sphingomyelin hydrolysis, PLC-mediated PC hydrolysis, and DG kinase-mediated PA formation or TNF alpha-induced NF-kappa B activation and apoptosis are not inhibited by LSF. LSF has a marked protective effect in a variety of acute inflammatory animal models that may be due to inhibition of this shared second messenger pathway involving PA.

CT-1501R selectively inhibits induced inflammatory monokines in human whole blood ex vivo.

The effect of (R)-1-(5-hydroxyhexyl)-3,7-dimethylxanthine (CT-1501R; the nonproprietary name for CT-1501R approved by the United States Name Council is lisofylline), an inhibitor of second messenger signaling through phosphatidic acid, on release of endogenous mediators important in the systemic inflammatory response syndrome (SIRS) was studied using the human whole blood ex vivo assay system. Human blood was stimulated with various endotoxin preparations, zymosan, or protein A, and the levels of secreted monokines were measured by enzyme-linked immunosorbent assay. CT-1501R inhibited tumor necrosis factor alpha (TNF-alpha), interleukin 1 beta (IL-1 beta), and IL-6 release in a dose-dependent manner and was active with all stimuli tested including Salmonella and Escherichia coli-derived endotoxin, endotoxin from both rough and smooth E. coli strains, as well as zymosan and protein A. CT-1501R inhibited monokine release by approximately 50% at 200 microM and 30% at 50 microM and was independent of the relative potency of stimulus. CT-1501R also inhibited IL-1 alpha or IL-1 beta induction of either TNF-alpha or IL-1 beta and inhibited the synergistic effects of stimulation with both human IL-1 beta and murine TNF-alpha on release of human TNF-alpha. Inhibition of monokine release following stimulation with monokine(s) was, in general, greater than that achieved with lipopolysaccharide (LPS) stimulation. Northern blot analysis showed decreased mRNA accumulation of TNF-alpha and IL-1 beta in CT-1501R-treated samples following LPS stimulation suggesting that CT-1501R acts at least in part, at the pretranslational level. In contrast, CT-1501R does not inhibit LPS-stimulated IL-8 or IL-1 receptor antagonist (IL-1ra) release in human whole blood or IL-1 alpha-induced release of PGE2 in human foreskin fibroblast cells. These data suggest that CT-1501R may be of use for clinical intervention in SIRS.

Protection from endotoxic shock in mice by pharmacologic inhibition of phosphatidic acid.

Certain phosphatidic/plasmanic/plasmenic acid (PA) species function as lipid intermediates in cell activation and may function directly as intracellular signaling molecules. PA can also be dephosphorylated to 1,2-diradyl-sn-glycerol by phosphatidate phosphohydrolase. Treatment of various cell types, including murine P388 monocytic leukemia cells, with bacterial lipopolysaccharide rapidly stimulates large increases in PA and PA-derived diradylglycerol. Pentoxifylline, 1-(5-oxohexyl)-3,7-dimethylxanthine, inhibits lipopolysaccharide-stimulated formation of PA in P388 cells at high concentrations (IC50 = 500 microM). Lisofylline [1-(5R-hydroxyhexyl)-3,7-dimethylxanthine] is a unique metabolite of pentoxifylline in humans and is > 800-fold more active as an inhibitor of PA formation than pentoxifylline (IC50 = 0.6 microM). Lisofylline does not inhibit lipopolysaccharide-induced activation of phosphatidylinositol-specific phospholipase C and generation of phosphatidylinositol-derived diradylglycerol. Lisofylline but not pentoxifylline protects BALB/c mice from endotoxin lethality when administered 4 hr after lipopolysaccharide. This protective effect is independent of either agent's effect on suppression of plasma tumor necrosis factor alpha. These data suggest that inhibitors of PA formation may have significant clinical potential in the treatment of sepsis and septic shock.

Characterization of two high affinity human interleukin-8 receptors.

Interleukin 8 (IL-8) and melanocyte growth-stimulatory activity/gro (MGSA) are structurally related proinflammatory cytokines that are chemoattractants and activators of neutrophils. Recently, cDNA clones encoding a high affinity IL-8 receptor (IL-8R-A) and a "low affinity" IL-8 receptor (IL-8R-B) have been isolated from human cDNA libraries. These two receptors have 77% amino acid identity and are members of the G protein-coupled superfamily of receptors with seven transmembrane domains. We have expressed these two receptors in mammalian cells and find that in this system both receptors bind IL-8 with high affinity (Kd approximately 2 nM). The receptor affinities differ for MGSA, however. IL-8R-A binds MGSA with low affinity (Kd approximately 450 nM); IL-8R-B binds MGSA with high affinity (Kd approximately 2 nM). The transfected cells respond to ligand binding with a transient increase in the intracellular Ca2+ concentration. A Ca2+ response is found for IL-8R-A following the binding of IL-8; no response is found for MGSA. A Ca2+ response for IL-8R-B follows the binding of both ligands. Blot hybridization with oligonucleotide probes specific for the two receptors shows that mRNA for both receptors is present in human neutrophils. Analysis of IL-8 and MGSA binding data on neutrophils as well as Ca2+ response and desensitization data shows that the presence of these two IL-8 receptors on the cell surface can account for the profile of these two ligands on neutrophils.

Random PCR mutagenesis screening of secreted proteins by direct expression in mammalian cells.

We have developed a general method for screening randomly mutagenized expression libraries in mammalian cells by using fluorescence-activated cell sorting (FACS). The cDNA sequence of a secreted protein is randomly mutagenized by PCR under conditions of reduced Taq polymerase fidelity. The mutated DNA is inserted into an expression vector encoding the membrane glycophospholipid anchor sequence of decay-accelerating factor (DAF) fused to the C terminus of the secreted protein. This results in expression of the protein on the cell surface in transiently transfected mammalian cells, which can then be screened by FACS. This method was used to isolate mutants in the kringle 1 (K1) domain of tissue plasminogen activator (t-PA) that would no longer be recognized by a specific monoclonal antibody (mAb387) that inhibits binding of t-PA to its clearance receptor. DNA sequence analysis of the mutants and localization of the mutated residues on a three-dimensional model of the K1 domain identified three key discontinuous amino acid residues that are essential for mAb387 binding. Mutants with changes in any of these three residues were found to have reduced binding to the t-PA receptor on human hepatoma HepG2 cells but to retain full clot lysis activity.

Stage-specific binding of inhibin and activin to subpopulations of rat germ cells.

Flow cytometry was used to separate and identify Sertoli and germ cell populations in primary rat testicular cultures derived from animals of different ages on the basis of cell size and DNA and lipid content. Multiparameter fluorescent evaluation of each cell preparation resulted in the assignment of specific staining patterns to Sertoli cells (diploid, high lipid content), spermatogonia (diploid, low lipid content), spermatocytes (large, tetraploid, high lipid content), and round spermatids (haploid, low lipid content). Each field was separately analyzed for inhibin and activin binding. Fluorescein isothiocyanate-conjugated activin bound with greatest intensity to spermatogonia, with little binding to leptotene or zygotene spermatocytes. Fluorescein isothiocyanate-conjugated inhibin bound to all stages of germ cells tested. Cross-competition data indicate that at least two and probably three distinct receptors exist for these peptides.

Characterization of complementary DNA clones encoding the rabbit IL-8 receptor.

IL-8 is a proinflammatory cytokine that functions as a chemoattractant for neutrophils. Recently, cDNA clones encoding the human neutrophil IL-8R were isolated by an expression cloning strategy. The amino acid sequence of the human IL-8R was sufficiently similar to a published sequence for an isoform of the rabbit FMLP receptor that we considered the possibility that the rabbit sequence might bind IL-8 as well. In order to establish its ligand specificity, we have isolated and characterized cDNA clones encoding the rabbit receptor. These cDNA clones, when expressed in mammalian cells, confer high affinity IL-8 binding (Kd = 3.6 nM), lack detectable binding of FMLP, and produce a transient increase in the intracellular Ca2+ concentration in response to IL-8 but not to FMLP. These data demonstrate that the reported rabbit FMLP receptor is the rabbit IL-8R, not an isoform of the FMLP receptor. In addition, the amino acid sequence of the rabbit IL-8R encoded by these cDNA clones differs at 23 amino acids (of 355) from that previously published.

Red blood cells are a sink for interleukin 8, a leukocyte chemotaxin.

IL-8 (also known as neutrophil-activating peptide 1) is recognized as a potent effector of neutrophil functions. Several different cell types that contact blood, namely T lymphocytes, monocytes, and endothelial cells, secrete this polypeptide following stimulation by cytokines, or lipopolysaccharide. Here we show that when IL-8 is added to blood it rapidly partitions from the plasma fluid to the blood cells and that erythrocytes account for the vast majority of this binding. Analysis of 125I-IL-8 binding [( ala-IL-8]77 form) to human red cells indicates a single, 5 nM Kd affinity class of binding sites, present at approximately 2,000 per red cell representing approximately 15 nmol of red cell IL-8 binding sites per liter of blood. These sites are protease sensitive. Their binding of IL-8 is rapidly reversible and does not result in receptor internalization, although bound IL-8 is resistant to extraction by pH 3 buffer at 5 degrees C. 125I-IL-8 binding to red cells was not inhibited by epidermal growth factor or interleukin 1, but was inhibited by monocyte chemotactic peptide-1, which is not a neutrophil chemotaxin, but is a member of the same family of polypeptides as IL-8. FACS analysis of IL-8-mediated mobilization of Ca2+ in neutrophils indicates that the IL-8 bound to red cells is incapable of stimulating neutrophils. Thus, red cell absorption of IL-8 may function to limit stimulation of leukocytes by IL-8 released into blood.

Structure and functional expression of a human interleukin-8 receptor.

Interleukin-8 (IL-8) is a member of a family of pro-inflammatory cytokines. Although the best characterized activities of IL-8 include the chemoattraction and activation of neutrophils, other members of this family have a wide range of specific actions including the chemotaxis and activation of monocytes, the selective chemotaxis of memory T cells, the inhibition of hematopoietic stem cell proliferation, and the induction of neutrophil infiltration in vivo. A complementary DNA encoding the IL-8 receptor from human neutrophils has now been isolated. The amino acid sequence shows that the receptor is a member of the superfamily of receptors that couple to guanine nucleotide binding proteins (G proteins). The sequence is 29% identical to that of receptors for the other neutrophil chemoattractants, fMet-Leu-Phe and C5a. Mammalian cells transfected with the IL-8 receptor cDNA clone bind IL-8 with high affinity and respond specifically to IL-8 by transiently mobilizing calcium. The IL-8 receptor may be part of a subfamily of related G protein-coupled receptors that transduce signals for the IL-8 family of pro-inflammatory cytokines.

Cloning and expression of cDNAs for two distinct murine tumor necrosis factor receptors demonstrate one receptor is species specific.

Complementary DNA clones encoding two distinct tumor necrosis factor receptors were isolated from a mouse macrophage cDNA library. The cDNA for murine tumor necrosis factor receptor type 1 (mTNF-R1) predicts a mature polypeptide of 425 amino acids that is 64% identical to its human counterpart, whereas the cDNA of murine tumor necrosis factor receptor type 2 (mTNF-R2) predicts a mature protein of 452 amino acids that is 62% identical to human tumor necrosis factor receptor type 2. The two murine tumor necrosis factor receptors have limited sequence homology (approximately 20% identity) in their extracellular regions but no apparent similarity in their cytoplasmic portions. Northern (RNA) analysis indicates a single 2.6-kilobase (kb) transcript for mTNF-R1; a 3.6-kb and a more predominant 4.5-kb transcript are observed for mTNF-R2. A human cell line transfected with either mTNF-R1 or mTNF-R2 expression vectors specifically bound 125I-labeled recombinant murine tumor necrosis factor alpha (TNF-alpha). Although mTNF-R1 had a similar affinity for both recombinant murine TNF-alpha and human TNF-alpha, mTNF-R2 showed strong specificity for recombinant murine TNF-alpha. This result suggests that the various activities of human tumor necrosis factor alpha reported in mice or in murine cell lines are probably mediated by mTNF-R1.

Heterogeneity of heat response in murine, canine and human tumors: influence on predictive assays.

The heterogeneity of response to hyperthermia of cells taken from different regions of tumors was tested in a model tumor system (RIF-1) in the mouse and in specimens from spontaneous tumors taken from dogs and humans at the time of surgical resection. Cell survival was assayed by clonogenic survival in the murine tumor and by dansyl lysine staining in tumors from all three species. Using survival as an endpoint, it was found that the extent of heterogeneity depended on the temperature to which the tumor was heated and the duration of exposure. By increasing either of these factors, the coefficient of variation was increased. The large heterogeneity seen after in vivo heating could not be explained entirely by inhomogeneous heating within the tumor as evidenced by temperature mapping. It is concluded that other microenvironmental factors such as blood flow, pH, O2, and nutrient supply may cause variations in the heat response of the tumor cells in vivo. Little, if any, evidence of cellular heterogeneity was evident for all three species when comparisons were made between samples of 100-200 mg. The canine and human tumors were considerably more heat resistant when dansyl lysine was used as an endpoint. In the RIF-1 tumors, heterogeneity of heat response was greater after in vitro heating than after in vivo heating when small biopsy samples (10-20 mg) were taken, suggesting that some cellular heterogeneity was present.

Interaction of recombinant apolipoprotein(a) and lipoprotein(a) with macrophages.

Elevated plasma levels of lipoprotein(a), Lp(a), represent a major, inherited risk factor for coronary heart disease, although the mechanism of its action remains unknown. Lp(a) is distinguished from the related LDL particle by the addition of apolipoprotein(a), apo(a). The presence of this large glycoprotein is likely to affect the binding of the particle to the LDL receptor and/or other receptors which may contribute to the atherogenic potential of Lp(a). Here we demonstrate the binding to macrophages of Lp(a) and pure recombinant apo(a) protein, via a specific, high-affinity receptor. This binding could lead to foam cell formation and the localization of Lp(a) to atherosclerotic plaques.

Measurement of transient cDNA expression in mammalian cells using flow cytometric cell analysis and sorting.

Generalized methods for quantitative and sensitive measurement of transient cDNA expression in mammalian cells using flow cytometry (FCM) are described. The techniques are applicable to a wide variety of cDNAs encoding intracellular or cell surface protein products through the use of immunofluorescence- or nonimmunofluorescence-based detection methods. The methods illustrated have been optimized for sensitive detection of transfectants and efficient recovery of the encoding plasmids from the sorted cells. Expression levels and heterogeneities were compared using four methods of DNA transfer in addition to description of a novel method to optimize single copy transfer probabilities by multiparameter analysis. The overall sensitivities are compared by reconstruction and molecular cloning experiments to other methods of selection, such as immunoselection by panning. Through the measurement of multiple heterologous products per cell, or the measurement of multiple epitopes or binding sites per heterologous protein, expression levels on a single cell basis can be measured and correlated with other endpoints for various purposes. The ability to detect and recover rare clones based on a number of single and multiparameter selection criteria should significantly extend the use of transient mammalian cDNA expression methods for applications involving novel FCM-based reporter cDNA assays and for cloning certain rare surface-bound or secreted proteins using FCM.

Activin stimulates spermatogonial proliferation in germ-Sertoli cell cocultures from immature rat testis.

Activin and inhibin are peptide hormones produced in the gonads which may act as autocrine and/or paracrine regulators of testicular function. Sertoli cells produce inhibin, and it has recently been shown that Leydig cells can produce activin in vitro. To further explore the local actions of activin and inhibin in the testis, Sertoli and germ cells were isolated from immature rats and cocultured in vitro. In these cultures we demonstrate that activin A and activin B, but not inhibin A, stimulated spermatogonial proliferation in vitro. Activin increased [3H]thymidine incorporation 2- to 4-fold in cocultures after 48-72 h of treatment. Using autoradiography, the label was localized in the clusters of spermatogonia adhering to the Sertoli cell monolayer. Additionally, activin stimulated a reaggregation of the cultures into tubule-like structures. Fluorescence-activated cytometry was used to analyze the cell population based on size, DNA content, and lipid content. Sertoli cells were identified using Nile Red staining of intracellular lipid droplets; spermatogonia are Nile Red-negative. Activin treatment caused a marked increase in the fraction of Nile Red-negative cells in the cocultures. Activin also caused an increase in the percentage of these cells having 4C DNA. Lastly, specific binding of activin A to 2C, but not 4C, germ cells was demonstrated. These data demonstrate that activin acts as a regulator of spermatogonial proliferation in the male.

Inhibin and activin locally regulate rat ovarian folliculogenesis.

The role of inhibin and activin in the initiation of follicular development, growth, and atresia was examined. Human recombinant inhibin (1 microgram) was unilaterally injected into the ovarian intrabursal space of 25-day-old rats. The contralateral ovary served as a control. Recruited growing follicles (350-500 microns) were observed 24 h after injection. The accumulation of follicles was greater in the inhibin-treated ovaries than in contralateral control ovaries. Moreover, the size distribution of the follicles was similar to the distribution of follicles recruited by systemic exogenous PMSG treatment. The effect of inhibin plus PMSG on follicular development was not different from that of PMSG treatment alone. Injection of human recombinant activin (1 microgram) into the ovarian bursa caused follicular atresia. Activin therapy blocked the follicular development caused by PMSG treatment. The effect of inhibin and activin on follicular development was further characterized by measuring the incorporation of [3H]thymidine into dividing cells. Inhibin enhanced follicular thymidine incorporation, while activin decreased granulosa cell proliferation. Furthermore, receptors for inhibin-A (6.4 x 10(3) receptors/cell) and activin-A (2.3 X 10(4) receptors/cell) were identified on granulosa cells. The evidence suggests that inhibin and activin act in a paracrine manner to regulate follicular development, inhibin as a follicular growth signal and activin as an atretagenic signal.