Autocrine growth factors for human tumor clonogenic cells

A human epithelial-derived cell line, SW-13, releases a soluble substance that functions as an autocrine growth factor. SW-13 cells, derived from a human adenocarcinoma of the adrenal cortex, form a few small colonies when suspended in soft agar at low densities. The number of colonies increased significantly when either viable SW-13 cells or serum-free medium conditioned by SW-13 cells (CM) was added to agar underlayers. CM increased colony formation in a dose-dependent fashion. Clonal growth at low cell densities was dependent on the presence of both horse serum and SW-13 CM. Neither activity alone was capable of sustaining growth. Even when cells were plated at high densities CM could not substitute for serum, but could reduce the threshold serum concentration. The results suggest that autocrine and serum-derived factors act in concert to maintain clonal growth of epithelial tumor cells in soft agar.     

Periostin mediates human adipose tissue-derived mesenchymal stem cell-stimulated tumor growth in a xenograft lung adenocarcinoma model

Mesenchymal stem cells stimulate tumor growth in vivo through a lysophosphatidic acid (LPA)-dependent mechanism. However, the molecular mechanism by which mesenchymal stem cells stimulate tumorigenesis is largely elusive. In the present study, we demonstrate that conditioned medium from A549 human lung adenocarcinoma cells (A549 CM) induces expression of periostin, an extracellular matrix protein, in human adipose tissue-derived mesenchymal stem cells (hASCs). A549 CM-stimulated periostin expression was abrogated by pretreatment of hASCs with the LPA receptor 1 (LPA(1)) inhibitor Ki16425 or short hairpin RNA-mediated silencing of LPA(1), suggesting a key role of the LPA-LPA(1) signaling axis in A549 CM-stimulated periostin expression. Using a xenograft transplantation model of A549 cells, we demonstrated that co-injection of hASCs potentiated tumor growth of A549 cells in vivo and that co-transplanted hASCs expressed not only periostin but also α-smooth muscle actin (α-SMA), a marker of carcinoma-associated fibroblasts. Small interfering RNA- or short hairpin RNA-mediated silencing of periostin resulted in blockade of LPA-induced α-SMA expression in hASCs. In addition, silencing of periostin resulted in blockade of hASC-stimulated growth of A549 xenograft tumors and in vivo differentiation of transplanted hASCs to α-SMA-positive carcinoma-associated fibroblasts. Conditioned medium derived from LPA-treated hASCs (LPA CM) potentiated proliferation and adhesion of A549 cells and short interfering RNA-mediated silencing or immunodepletion of periostin from LPA CM abrogated proliferation and adhesion of A549 cells. These results suggest a pivotal role for hASC-secreted periostin in growth of A549 xenograft tumors within the tumor microenvironment.     

Transforming growth factor-beta-like activity is secreted by rabbit renal cortical tubular cells in primary culture.

The activity of an autocrine growth factor in a medium conditioned by cultured rabbit renal cortical tubular cells was investigated. Little stimulatory growth activity for tubular cells was observed in the conditioned medium, and inhibitory activity was seen only in acidified conditioned medium. This factor stimulated the colony formation of NRK 49F cells in soft agar only with epidermal growth factor and inhibited the DNA synthesis of primary cultured rat hepatocytes, and its molecular weight was about 25 kDa. The factor was neutralized by the specific antibody to transforming growth factor (TGF)-beta 1. These results indicate that renal tubular epithelial cells can produce latent TGF-beta in primary culture.     

Effects of interleukin 3, interleukin 4, and fibroblasts on cultures of human lung mast cells in bronchoalveolar lavage fluids

The effects of T cell factors, including interleukin (IL)-3 and IL-4, and fibroblasts on the growth and differentiation of human lung mast cells (MCs) obtained by bronchoalveolar lavage (BAL) were examined. The number of MCs identified by alcian blue-safranin staining was twice that of the control culture without conditioned medium (CM) when BAL cells were cultured for 2 weeks in RPMI 1640 containing 10% fetal calf serum and partially purified CM derived from PHA-stimulated lymphocytes. In the presence of both recombinant (r) IL-3 and rIL-4, the number of MCs was twice as high as the control without increase in the per-cell histamine content after 2 weeks' culture. In umbilical cord blood cultures, IL-3 plus IL-4 augmented basophilic cells about 20-fold more than the control when cultured for 2 weeks. In some cases, the percentage of safranin-positive MCs was about 2-5 fold greater, with 2-7 fold higher histamine content, when cultured for 10 days with CM and fibroblasts derived from human embryonic lung. However, in all BAL experiments, there was no increase in the total number of MCs after culture compared with the initial number of MCs, unlike the umbilical cord blood cultures. These results suggest that T cell factors, including IL-3 and IL-4, and fibroblasts may influence the phenotype and the survival of lung mast cells in BAL, whereas there was no evidence for the presence of MC precursors in BAL fluids.     

In vitro remodeling of tumor vascular endothelial cells using conditioned medium from various tumor cells and their sensitivity to TNF-alpha

Prevention of tumor-associated blood vessel formation (angiogenesis) is a potentially powerful strategy to treat cancer. We found that tumor vascular endothelial cells were rearranged in vitro with conditioned culture medium derived from tumor cells and compared the sensitivity to the effects of TNF-alpha between normal and tumor endothelial cells. Incubation with tumor (Meth-A, Colon26)-derived conditioned medium showed that no effect was observed on cell growth. Tumor cells (Meth-A, Colon26, and B16BL6) only showed no sensitivity to TNF-alpha. Normal and control endothelial cells in culture showed little cytotoxicity in response to TNF-alpha treatment, but marked cytotoxicity of TNF-alpha was observed in endothelial cells cultured with tumor-derived conditioned medium. Sensitivity to TNF-alpha was different depending on the type of tumor from which the conditioned medium was derived. This difference in sensitivity was assumed to be due to the in vivo sensitivity to TNF-alpha. The results of this study suggested that the sensitivity of tumors to TNF-alpha is controlled by the sensitivity of tumor vasculature.     

Conditions affecting clonal growth of lymphoma cells in a semisolid matrix

Summary This study demonstrated the importance of the methods used in determining the lymphoma cell colony stimulating activity of factors derived from lymphoma cells. The in vitro colony formation in a semisolid matrix of the AKR mouse lymphoma cell line, SL 12, and three cloned derivatives, SL 12.1, SL 12.3, and SL 12.4, was studied. We show that the use of soft agar or methylcellulose as a semisolid matrix results in colony formation by the lymphoma cells only in the presence of serum. The addition of conditioned medium (CM) from lymphoma cells growing in serum-free medium does not stimulate colony growth. However, when purified agarose is used, colonies grow in a dose-dependent manner in the absence of serum and in the presence of CM. These results indicate that the type of semisolid matrix used can influence results in studies of this nature. Purified agarose provides the best environment when colony formation by lymphoma cells is used to measure the presence of growth factors in test-conditioned media. This research was supported by the Department of Energy, contract DE-AM03-76-SF00012 and National Institutes of Health grant CA12386. Dr. Bessho was a Visiting Scientist from the National Institute of Radiological Sciences, Chiba, Japan.     

CHANGES IN THE CAPACITY FOR CLONAL GROWTH AND DIFFERENTIATION IN VITRO OF THE VERTEBRAL CARTILAGE CELLS WITH EMBRYONIC DEVELOPMENT II. VITALIZING EFFECT OF CONDITIONED MEDIUM ON THE CELLS OF YOUNGER EMBRYOS

Most of the younger cartilage cells taken from chick embryos at the stage 35 were known to regress after the initial cell multiplication in in vitro clonal culture. A partial supplementation of conditioned medium (CM) to the standard medium was effective to permit colony formation from many of these younger cells. Most colonies thus derived from the younger cells by the aid of CM expressed differentiative traits as cartilage, as the cells from older embryos did. CM exerted multiple effects on clonal development of the younger cells. It vitalized the cells destined to regress, to promote cell multiplication and to flatten the shape of the colony. The experimental results suggested that these effects were associated with an independent factor each.     

A secreted peptide growth factor, phytosulfokine, acting as a stimulatory factor of carrot somatic embryo formation

Somatic embryogenesis of the carrot (Daucus carota L.) depends on a set of factors, some of which accumulate in culture medium (conditioned medium, CM). When embryogenic cell clusters were transferred to an embryo-inducing medium, addition of CM derived from somatic embryo culture markedly stimulated somatic embryo formation. The active principles were purified using a simple bioassay system and identified to be phytosulfokines (PSKs), sulfated oligopeptide growth factors originally isolated from a CM derived from asparagus (Asparagus officinalis L.) mesophyll culture. Quantification studies using a competition ELISA system employing an anti-PSK-alpha polyclonal antibody showed that PSK production might be related to growth of cells, rather than development of somatic embryos. Thus the stimulatory effect of PSK on somatic embryo formation might be due to promotion of cell proliferation.     

The response of small vessel endothelial cells from fetal rat lung to growth factors

Small vessel pulmonary endothelial cells were obtained from rat fetal lung at day 20 of gestation, and were maintained in culture to passage three for study. Endothelial cells grown on a collagen matrix with Dulbecco's minimal essential medium: Ham's F12 medium (1:1, v/v) supplemented with 20 ml/l fetal bovine serum, bovine pituitary extract (50 mg/l), endothelial cell growth supplement (100 mg/l), hydrocortisone (1 mg/l) and an increased (10 mmol/l) magnesium concentration retained the characteristic endothelial cell marker factor VIII antigen during the third passage in culture. The factors responsible for small vessel growth in the developing fetal lung are unknown. To test the hypothesis that small vessel pulmonary endothelial cells would respond to autocrine or paracrine growth factors the effects of conditioned media from fetal lung endothelial cells, fibroblasts and pneumocytes from lungs of the same gestational age were studied in vitro. None of the tested conditioned media had any effect on endothelial cell DNA synthesis in the presence of 20 ml/l fetal bovine serum. Since no paracrine or autocrine effects of conditioned media were observed, the effect of other growth factors that could be derived from the circulation, or from storage sites in subcellular matrix, were studied for effect. When endothelial cells were studied in the presence of 20 ml/l fetal bovine serum and 100 mg/l endothelial cell growth supplement they had enhanced DNA synthesis in response to the progression-type growth factors insulin (5 mg/l), insulin-like growth factor-I and insulin-like growth factor-II (20 micrograms/l) and epidermal growth factor (10 micrograms/l). In the absence of serum or endothelial growth supplement endothelial cell DNA synthesis was enhanced by the competence-type growth factors acidic and basic fibroblastic growth factors at 100 micrograms/l and platelet derived growth factor at 10 micrograms/l. In the absence of exogenous competence-type growth factors neutralizing antibodies to basic fibroblast growth factor reduce DNA synthesis. Of various cytokines tested only interleukin-1 (1 x 10(3) U/l) and tumor necrosis factor (25 x 10(4) U/l) had an effect on endothelial cell DNA synthesis. Endothelial cell division during fetal lung development may be controlled by progression growth factors present in serum, and by either autocrine release of the competence factor basic fibroblast growth factor or paracrine release of platelet-derived growth factor by other cell types.     

Human endometrial carcinoma cells release factors which inhibit the growth of normal epithelial cells in culture

Autocrine and paracrine interactions between cells are important homeostatic mediators in normal tissues. Alterations to growth factor signalling pathways are likely to play a role in multistep carcinogenesis. In this study normal human endometrial epithelial cells (NHEC) after 3 days in culture were treated with serum-free medium conditioned for 24 h by log phase or confluent cultures of established RL95-2, HEC1A, or AN3CA endometrial carcinoma (EC) cell lines. By day 4, NHEC treated with either log phase or confluent conditioned medium (CM) showed a significant decrease (50–90% of control) in [³H]thymidine ([³H]TdR) incorporation. DNA synthesis was inhibited more by confluent than by log phase CM. By day 7, NHEC treated with CM exhibited fewer colonies per culture, fewer cells per colony, and an increased percentage of single cells. Several growth-regulatory gene products found in the nucleus or at the cell membrane have been shown to be expressed differently in normal and transformed cells. We selected the p53 and c-Ha-ras p21 proteins to further investigate the mechanism of alteration of proliferation in cells treated with carcinoma CM. Thus, by day 7, the percentage of NHEC with nuclear localization of wild type p53 (wt p53) was elevated by treatment with CM. In contrast, CM-treated EC cells continued to proliferate, and showed a decrease in the percentage of cells expressing nuclear wt p53 and an increase in the cytoplasmic expression of c-Ha-ras p21. Our studies show that EC cell lines release factors which inhibit the proliferation of NHEC, thus favoring the proliferation of EC cells.Abbreviations CM conditioned medium - EC endometrial adenocarcinoma - NHEC normal human endometrial epithelial cells     

Breast cancer cell line MDA-231 stimulates osteoclastogenesis and bone resorption in human osteoclasts

Breast cancers commonly cause osteolytic metastases in bone, a process that is dependent upon osteoclast-mediated bone resorption, but the mechanism responsible for tumor-mediated osteoclast activation has not yet been clarified. In the present study we utilized a well-known human breast cancer cell line (MDA-231) in order to assess its capability to influence osteoclastogenesis in human bone marrow cultures and bone resorption in fully differentiated osteoclasts. We demonstrated that conditioned medium (CM) harvested from MDA-231 increased the formation of multinucleated TRAP-positive cells in bone marrow cultures. Bone resorption activity of fully differentiated human osteoclasts and of osteoclast-like cell lines, from giant cell tumors of bone (GCT), was highly increased by the presence of MDA-231 CM. Moreover, while MDA-231 by themselves did not produce IL-6 tumor cell, CM increased the secretion of IL-6 by primary human osteoclasts and GCT cell lines compared to untreated controls. These data suggest that MDA-231 produce osteoclastic activating factor(s) that increase both osteoclast formation in bone marrow culture and bone resorption activity by mature cells. Moreover, breast cancer cells stimulate IL-6 secretion by osteoclasts that is one of the factors known to supports osteoclastogenesis.     

Therapeutic angiogenesis using tumor cell‐conditioned medium

Stem cell‐conditioned medium (CM), which contains angiogenic factors that are secreted by stem cells, represents a potential therapy for ischemic diseases. Along with stem cells, tumor cells also secrete various angiogenic factors. Here, tumor cells as a cell source of CM for therapeutic angiogenesis was evaluated and the therapeutic efficacy of tumor cell CM in mouse hindlimb ischemia models was demonstrated. CM obtained from a human fibrosarcoma HT1080 cell line culture was compared with CM obtained from a human bone marrow‐derived mesenchymal stem cell (MSC) culture. HT1080 CM contained higher concentrations of angiogenic factors compared with MSC CM, which was attributable to the higher cell density that resulted from a much faster growth rate of HT1080 cells compared with MSCs. For use in in vitro and in vivo angiogenesis studies, HT1080 CM was diluted such that HT1080 CM and MSC CM would have the same cell number basis. The two types of CMs induced the same extent of human umbilical vein endothelial cell (HUVEC) proliferation in vitro. The injection of HT1080 CM into mouse ischemic limbs significantly improved capillary density and blood perfusion compared with the injection of fresh medium. Although the therapeutic outcome of HT1080 CM was similar to that of MSC CM, the preparation of CM by tumor cell line culture would be much more efficient due to the faster growth and unlimited life‐time of the tumor cell line. These data suggest the potential application of tumor cell CM as a therapeutic modality for angiogenesis and ischemic diseases. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:456–464, 2016     

The ErbB4 receptor in fetal rat lung fibroblasts and epithelial type II cells

ErbB receptors are important regulators of fetal organ development, including the fetal lung. They exhibit diversity in signaling potential, acting through homo- and heterodimers to cause different biological responses. We hypothesized that ErbB receptors show cell-specific and stimuli-specific activation, heterodimerization, and cellular localization patterns in fetal lung. We investigated this using immunoblotting, co-immunoprecipitation, and confocal microscopy in primary isolated E19 fetal rat lung fibroblasts and epithelial type II cells, stimulated with epidermal growth factor, transforming growth factor alpha, neuregulin 1beta, or treated with conditioned medium (CM) from the respective other cell type. Fetal type II cells expressed significantly more ErbB1, ErbB2, and ErbB3 protein than fibroblasts. ErbB4 was consistently identified by co-immunoprecipitation of all other ErbB receptors in both cell types independent of the treatments. Downregulation of ErbB4 in fibroblasts initiated cell-cell communication that stimulated surfactant phospholipid synthesis in type II cells. Confocal microscopy in type II cells revealed nuclear localization of all receptors, most prominently for ErbB4. Neuregulin treatment resulted in relocation to the extra-nuclear cytoplasmic region, which was distinct from fibroblast CM treatment which led to nuclear localization of ErbB4 and ErbB2, inducing co-localization of both receptors. We speculate that ErbB4 plays a prominent role in fetal lung mesenchyme-epithelial communication.     

Anchorage-independent colony formation of SV40 transformed BALB/c-3T3 cells in serum-free medium: role of cell- and serum-derived factors

The colony-forming response of SV40 transformed BALB/c-3T3 cells in agarose suspension culture was studied in a serum-free medium (with insulin, transferrin and serum albumin as the only macromolecular supplements) that was optimized for colony formation of fibronectin-attached monolayer cultures. In this serum-free medium, the SV3T3 cells fail to form colonies in agarose suspension. However, they can be induced to anchorage-independent colony formation by the growth factors that are additionally required by their untransformed counterparts for proliferation in monolayer culture. The SV3T3 cells are also rendered anchorage-independent for colony formation in serum-free medium by conditioned medium from dense monolayer serum-free SV3T3 cultures. These experiments suggest that it is the cell-substrate interaction that is responsible for the growth factor autonomy of fibronectin-attached transformed cells.     

Requirement of conditioned medium or a high concentration of serum for anchorage-independent growth of adenovirus type 12 E1a-transformed rat cells (HY1)

Adenovirus type 12 (Ad12) Ela-transformed rat cells (HY1) grew in methocel medium containing 9% fetal calf serum (FCS), but the frequency of colony formation was very low (the order of 10(-4)). The addition of conditioned medium or a high concentration of serum (20% FCS) to the methocel medium accelerated colony formation, and plating efficiency increased 10- to 100-fold. In contrast, in stationary culture, HY1 cells grew well even in 1%-FCS medium. These results indicate that HY1 cells require high concentrations of growth factors for anchorage-independent growth. The effects of conditioned medium or FCS also were demonstrated in several transformed cell lines induced by transfection of combined sets of Ad12-transforming genes (E1a, E1b and E4). These growth behaviors suggest that the first step in cell transformation with adenovirus 12 is the acquisition of responsiveness to growth factors in methocel culture, which must be the function of the Ad12-E1a gene products. The function of the other two Ad12-transforming genes was discussed.     

Stimulation of human T cell colony growth by a lymphocyte colony enhancement factor derived from lymphocyte subpopulations

Blood mononuclear cells (MNC) develop into T cell colonies when the cells are sensitized with PHA and seeded in a two-layer soft agar system. Conditioned medium (CM) derived from MNC enhanced lymphocyte colony formation when it was added to the culture system. CFU-TL appear to be stimulated into colony formation by molecules secreted by lymphocyte subpopulations contained in the seeded cells. In this study, human peripheral blood MNC were fractionated by a battery of techniques into adherent, E+, CD4+, CD8+, B and null cells. CM was prepared from each of the subpopulations and its effects on T cell colony growth assayed. All the lymphocyte subpopulations were found to generate lymphocyte colony enhancement factor (LCEF). After several purification procedures, CM prepared from CD4 and CD8+, displayed LCEF activity corresponding to proteins of molecular weight 30-40 and 100-140 kD.     

Changes in the Secretory Profile of NSCLC-Associated Fibroblasts after Ablative Radiotherapy: Potential Impact on Angiogenesis and Tumor Growth

In the context of radiotherapy, collateral effects of ablative doses of ionizing radiation (AIR) on stromal components of tumors remains understudied. In this work, cancer-associated fibroblasts (CAFs) isolated from freshly resected human lung tumors were exposed to AIR (1x 18 Gy) and analyzed for their release of paracrine factors. Inflammatory mediators and regulators of angiogenesis and tumor growth were analyzed by multiplex protein assays in conditioned medium (CM) from irradiated and non-irradiated CAFs. Additionally, the profile of secreted proteins was examined by proteomics. In functional assays, effects of CAF-CM on proliferative and migratory capacity of lung tumor cells (H-520/H-522) and human umbilical vein endothelial cells (HUVECs) and their tube-forming capacity were assessed. Our data show that exposure of CAFs to AIR results in 1) downregulated release of angiogenic molecules such as stromal cell-derived factor-1, angiopoietin, and thrombospondin-2 (TSP-2); 2) upregulated release of basic fibroblast growth factor from most donors; and 3) unaffected expression levels of hepatocyte growth factor, interleukin-6 (IL-6), IL-8, IL-1β, and tumor necrosis factor-α. CM from irradiated and control CAFs did not affect differently the proliferative or migratory capacity of tumor cells (H-520/H-522), whereas migratory capacity of HUVECs was partially reduced in the presence of irradiated CAF-CM. Overall, we conclude that AIR mediates a transformation on the secretory profile of CAFs that could influence the behavior of other cells in the tumor tissue and hence guide therapeutic outcomes. Downstream consequences of the changes observed in this study merits further investigations.     

Action mechanisms of physiological doses of androgen or pharmacological doses of estrogen in growth stimulation of Shionogi carcinoma 115 in mice

Shionogi carcinoma 115 (SC115) had been accepted for 20 yr as an androgen-dependent mouse mammary tumor. However, we recently found that the growth of SC115 tumors in vivo is also stimulated by pharmacological doses of estrogen through estrogen receptor. In the present study, action mechanisms of androgen or high doses of estrogen in the growth stimulation of SC115 were examined using a cloned cell line (SC-3) derived from the SC115 tumor. In serum-supplemented [2% steroid-free fetal calf serum-Eagle's minimum essential medium (MEM)] and serum-free [HAM F-12: MEM (1:1, v/v) containing 0.1% bovine serum albumin] media, testosterone (Test, 10(-9)-10(-6) M) significantly increased both cell number and DNA synthesis of SC-3 cells (by up to 10-fold), whereas oestradiol-17 beta (10(-12)-10(-6) M) had no such effects; the Test-induced growth was completely inhibited by the addition of a 100-fold molar excess of cyproterone acetate (CA). The serum-free medium cultured with SC-3 cells in the presence or absence of 10(-8) M Test was collected [conditioned medium (CM) or conditioned medium without Test (CM-)], and then Test in CM was removed by Gel filtration using Sephadex G-100 or inactivated by the addition of a 100-fold molar excess of CA. In the serum-free culture system, the addition of the CM without Test activity significantly enhanced both number of SC-3 cells and DNA synthesis in the cells, whereas CM(-) had no such effects. The present findings suggest that growth-stimulatory activities of androgen and high doses of estrogen on SC115 cells are mediated by growth factor(s), secreted from SC115 cells through androgen receptor and from some of nontransformed cells through estrogen receptor, respectively.     

The human lung fibroblast cell line, MRC-5, produces multiple factors involved with megakaryocytopoiesis

The human lung fibroblast cell line MRC-5 constitutively produces a megakaryocyte potentiator activity, identified in murine bone marrow liquid culture assays for acetylcholinesterase and megakaryocyte colony assays in the presence of low concentrations of IL-3. The production levels of this activity were increased after stimulation with the phorbol ester analog, mezerein, and the calcium ionophore, A23187. Complete purification of a protein having this activity from conditioned media of induced MRC-5 cells was achieved using gel filtration and ion exchange chromatography. The first 14 residues of the purified protein identified by amino-terminal sequencing were identical to the first 14 residues of IL-6. Recombinant human IL-6 was tested and found to promote megakaryocyte growth. IL-1 beta, another component detected in MRC-5 conditioned media, was unable to promote megakaryocyte colony formation but did reduce the concentration of IL-6 necessary to support megakaryocyte colony formation. Immunoprecipitation using rabbit antiserum prepared against IL-6 removed the megakaryocyte growth activity found in MRC-5 conditioned media. Thus, connective tissue cells such as fibroblasts in the bone marrow may co-stimulate thrombocytopoiesis via IL-6 and, possibly, via IL-1 production.     

Multilineage, non-species specific hematopoietic growth factor(s) elaborated by a feline fibroblast cell line: enhancement by virus infection

In studies designed to determine the role of feline leukemia virus (FeLV) in the pathogenesis of marrow failure in the cat, we tested medium conditioned by uninfected and FeLV-infected feline embryonic fibroblasts (FEA) for its effect on hematopoietic colony growth in culture. As opposed to an inhibitory effect, we found that the conditioned medium (CM) from FEA or FEA/FeLV increased the in vitro growth of multiple hematopoietic progenitor cell types including erythroid burst-forming cells (BFU-E), granulocyte/macrophage colony-forming cells, megakaryocytic colony-forming cells, and mixed-cell colony-forming cells. Furthermore, CM enhanced the growth of progenitors in cultures of mouse or human marrow cells, as well as cat marrow cells. Stimulation of feline BFU-E was most marked with an increment in growth of 400% over control. The human burst promoting activity (BPA) of the CM was equivalent or better than other CM available in our laboratory. The evidence suggest that the growth-promoting activity is a constitutive product(s) released by FEA which was enhanced eightfold with virus infection. Studies with non-adherent and T-lymphocyte-depleted human marrow cells and human peripheral blood cells suggest that the growth factor(s) acts directly on progenitor cells and not through readily identified accessory cells. These findings are consistent with the concept that mesenchymal cells such as fibroblasts have the capacity to release hematopoietic growth factor(s) capable of acting on primitive hematopoietic progenitors. The results provide an example of how injury of such cells, through virus infection, may enhance growth factor(s) release and influence the hematopoietic microenvironment.