The hepatocyte growth factor/scatter factor (HGF/SF) receptor, met, transduces a morphogenetic signal in renal glomerular fibromuscular mesangial cells

Targeting BET proteins inhibited the growth of non-small cell lung carcinoma through downregulation of Met expression

Hepatocyte growth factor receptor (HGFR or Met) upregulation has been proven to play important roles in non-small cell lung carcinoma (NSCLC). Interestingly, chemoresistance against epidermal growth factor receptor (EGFR) inhibitors including erlotinib and gefitinib was also related to Met. Targeting bromodomain and extra terminal domain (BET) proteins, especially BRD4, has shown inhibitory effects on lung cancer, but the mechanism is unclear. Herein, we found that JQ1 (BET inhibitor) suppressed NSCLC cell growth, reduced the Met expression, and contributed to inactivation of PI3K/Akt and MAPK/ERK pathways. Moreover, another BET protein inhibitor I-BET151, or BRD4 depletion, also inhibited NSCLC cell growth and downregulated Met. JQ1 inhibited HGF-induced cell growth and Met/PI3K/Akt activation, also inhibited A549 tumor growth in xenograft mouse models, in parallel with Met downregulation. Moreover, JQ1 inhibited the growth of paired erlotinib-sensitive and resistant HCC827 cells in parallel with Met downregulation and PI3K/Akt signaling inactivation. JQ1 also exerted inhibitory influences on the growth of erlotinib-sensitive and resistant HCC827 tumors in xenograft mouse models. These results suggested that targeting BET proteins inhibited NSCLC via downregulating Met and inactivating PI3K/AKT pathway. Our findings reveal a novel mechanism of BET proteins implicated in NSCLC progression with Met taken into consideration.

Scatter Factors in renal disease: Dr. Jeckyll and Mr. Hyde?

The Scatter Factors are two homologous proteins, named Scatter Factor/Hepatocyte Growth Factor and Macrophage Stimulating Protein. Their receptors are the products of two oncogenes, Met and Ron, respectively. The Scatter Factors induce movement, stimulate proliferation, regulate apoptosis and are morphogenic, i.e. operate an integrated program that seems tailored to drive organ development and to regenerate injured tissues. On the other hand, Scatter Factors may be responsible for pathologic tissue remodeling, infiltration of inflammatory cells, and tumor growth and diffusion. The review describes the involvement of Scatter Factors in renal disease, including acute renal failure, glomerulonephritis, chronic fibrosing nephropathies, dialysis, renal transplantation and renal tumors, and discusses the double-faced role of Scatter Factors, that play either a protective or a pathogenic role.

Hepatocyte growth factor gene therapy for hypertension

Hepatocyte growth factor (HGF) has mitogenic, motogenic, and morphogenic biological activities as well as helps in regenerating various tissues. In cardiovascular organs, HGF was reported to have anti-apoptotic, anti-fibrotic, and vasodilating effects. HGF has close relationships with hypertension, arteriosclerosis, and heart failure. HGF enhances renal regeneration and suppresses the progression of hypertension. Intramuscular electroporation of the therapeutic gene is a simple, economic, and low toxic method compared with systemic administration of the purified proteins or peptides. We outline the technique of intramuscular electroporation of HGF gene as a remedy for hypertension.

Met and Hgf signaling controls hypaxial muscle and lateral line development in the zebrafish

Somites give rise to a number of different embryonic cell types, including the precursors of skeletal muscle populations. The lateral aspect of amniote and fish somites have been shown to give rise specifically to hypaxial muscle, including the appendicular muscle that populates fins and limbs. We have investigated the morphogenetic basis for formation of specific hypaxial muscles within the zebrafish embryo and larvae. Transplantation experiments have revealed a developmentally precocious commitment of cells derived from pectoral fin level somites to forming hypaxial and specifically appendicular muscle. The fate of transplanted somites cannot be over-ridden by local inductive signals, suggesting that somitic tissue may be fixed at an early point in their developmental history to produce appendicular muscle. We further show that this restriction in competence is mirrored at the molecular level, with the exclusive expression of the receptor tyrosine kinase met within somitic regions fated to give rise to appendicular muscle. Loss-of-function experiments reveal that Met and its ligand, hepatocyte growth factor, are required for the correct morphogenesis of the hypaxial muscles in which met is expressed. Furthermore, we demonstrate a requirement for Met signaling in the process of proneuromast deposition from the posterior lateral line primordia.

Hepatocyte growth factor and c-Met expression in rat and human liver fibrosis

BACKGROUND

Hepatocyte growth factor (HGF) is a potent mitogen for hepatocytes in vitro.

AIMS

Substitution of HGF was suggested for human liver disease on the basis of animal experiments. The cellular sources of HGF and its receptor, c-Met, in liver disease in vivo are not well defined.

METHODS

We characterised HGF and c-Met expression in normal and cirrhotic human livers and rat livers at various time points after CCl4 administration by in situ hybridisation and immunohistology. HGF transcripts were restricted to resting and activated stellate cells in rat and human liver.

RESULTS

In rat liver, HGF showed peak levels 6-12 h following acute intoxication, and remained increased after repeated CCl4 injury. HGF transcript levels were very low in normal human liver, but excessively raised in fibrosis/cirrhosis. In contrast, HGF immunoreactivity was found not only in perisinusoidal/periductular cells but also in cholangiocytes of proliferating ductules. c-Met RNA and protein was expressed in hepatocytes, cholangiocytes, and arteriolar endothelial cells.

CONCLUSIONS

The HGF-specific immunostaining of proliferating cholangioles in the absence of HGF RNA suggests c-Met-mediated uptake of HGF and paracrine stimulation of cholangiocellular proliferation. Mitogenic effects of HGF on hepatocytes may therefore be accompanied by undesired cholangiogenesis and angiogenesis limiting its therapeutic value in chronic liver disease.

Counteractive effects of HGF on PDGF-induced mesangial cell proliferation in a rat model of glomerulonephritis

Activation and proliferation of glomerular mesangial cells play an important role in the development of mesangioproliferative glomerulonephritis. We investigated the role of hepatocyte growth factor (HGF) in regulating activated mesangial cell proliferation. In glomeruli of normal rats, mesangial cells barely expressed the c-Met/HGF receptor. However, when mesangioproliferative glomerulonephritis was induced in rats by the administration of an anti-Thy 1.1 antibody, glomerular HGF expression transiently decreased along with mesangiolysis, and activation of mesangial cells was associated with upregulation of the c-Met receptor. Activated mesangial cells in culture also expressed the c-Met/HGF receptor. Although addition of HGF to cultured mesangial cells did not increase DNA synthesis, HGF did diminish PDGF-induced DNA synthesis. PDGF induced activation of ERK, which continued for at least 48 h. When PDGF and HGF were simultaneously added, HGF inhibited the prolonged activation of ERK, which suggests that early inactivation of PDGF-induced ERK may be involved in the inhibitory effect of HGF on mesangial cell proliferation. Furthermore, administration of HGF to rats with anti-Thy 1.1 nephritis resulted in a selective suppression of activated mesangial cell proliferation, and this suppressive effect was associated with attenuation of phosphorylated glomerular ERK. These results indicate that HGF counteracts PDGF-induced mesangial cell proliferation and functions as a negative regulator of activated mesangial cell proliferation.

HGF/SF induces mesothelial cell migration and proliferation by autocrine and paracrine pathways

Mesothelial repair differs from that of other epithelial-like surfaces as healing does not occur solely by centripetal in-growth of cells as a sheet from the wound margins. Mesothelial cells lose their cell-cell junctions, divide, and adopt a fibroblast-like morphology while scattering across and covering the wound surface. These features are consistent with a cellular response to hepatocyte growth factor/scatter factor (HGF/SF). In this study, we examined the ability of mesothelial cells to secrete HGF/SF and investigated its possible role as an autocrine regulator of mesothelial cell motility and proliferation. We found that human primary mesothelial cells expressed HGF/SF mRNA and secreted active HGF/SF into conditioned medium as determined by ELISA and in a scattering bioassay. These cells also expressed the HGF/SF receptor, Met, as shown by RT-PCR and by Western blot analysis and immunofluorescence. Incubation of mesothelial cells with neutralizing antibodies to HGF/SF decreased cell migration to 25% of controls, whereas addition of HGF/SF disrupted cell-cell junctions and induced scattering and enhanced mesothelial cell migration. Furthermore, HGF/SF showed a small but significant mitogenic effect on all mesothelial cell lines examined. In conclusion, HGF/SF is produced by mesothelial cells and induces both motility and proliferation of these cells. These data are consistent with HGF/SF playing an autocrine role in mesothelial healing.

Hepatocyte growth factor: renotropic role and potential therapeutics for renal diseases

Hepatocyte growth factor (HGF), a ligand for the c-Met receptor tyrosine kinase, has mitogenic, motogenic, anti-apoptotic, and morphogenic (for example, induction of branching tubulogenesis) activities for renal tubular cells, while it has angiogenic and angioprotective actions for endothelial cells. Stromal cells such as mesangial cells, endothelial cells, and macrophages are sources of renal HGF; thus, HGF mediates epithelial-stromal and endothelial-mesangial interactions in the kidney. In response to acute renal injury, the expression of HGF increases in the injured kidney and in distant intact organs such as the lung and spleen. Locally and systemically increased HGF supports renal regeneration, possibly not only by enhancing cell growth but also by promoting morphogenesis of renal tissue. During progression of chronic renal failure/renal fibrosis, the expression of HGF decreases in a manner reciprocal to the increase in expression of transforming growth factor-beta (TGF-beta), a key player in tissue fibrosis. A decrease in endogenous HGF, as well as increase in TGF-beta, augments susceptibility to the onset of chronic renal failure/renal fibrosis. On the other hand, supplements of exogenous HGF have preventive and therapeutic effects in cases of acute and chronic renal failure/renal fibrosis in laboratory animals. HGF prevents epithelial cell death and enhances regeneration and remodeling of renal tissue with injury or fibrosis. A renotropic system underlies the vital potential of the kidney to regenerate, while an impaired renotropic system may confer susceptibility to the onset of renal diseases. Thus, HGF supplementation may be one therapeutic strategy to treat subjects with renal diseases, as it enhances the intrinsic ability of the kidney to regenerate.

Hypoxia up-regulates angiopoietin-2, a Tie-2 ligand, in mouse mesangial cells

BACKGROUND

Angiopoietins are secreted factors modulating endothelial survival and morphogenesis. Our previous studies demonstrated angiopoietin-2 (Ang-2) promoter activity in vivo in maturing kidney vascular smooth muscle and mesangial cells, with Tie-2 expressed by adjacent endothelia, including glomerular capillaries.

METHODS

In this study we investigated Ang-2 expression in immortalized mouse mesangial cell lines and studied the response to hypoxia.

RESULTS

Using reverse transcription-polymerase chain reaction, Ang-2 and Ang-3 mRNA were detected but Ang-1 and Tie-2 transcripts were absent. As assessed by Northern and slot blotting, 8 to 24 hours hypoxia (3% O(2)) significantly increased Ang-2 mRNA levels versus normoxic (21% O(2)) cells and the rate of Ang-2 mRNA degradation was similar in both conditions, consistent with increased transcription. Hypoxia also increased immunoreactive Ang-2 in cell lysates. Hypoxic stimulation of Ang-2 mRNA was significantly reduced by inhibitors of tyrosine kinase (genistein) and protein kinase C (GF109203X), but not by a mitogen-activated protein kinase 1 inhibitor (PD98059). Furthermore, hypoxia coincidentally up-regulated levels of vascular endothelial growth factor (VEGF) mRNA in these cells. Finally, in vivo, immunoreactive Ang-2 was observed in the cores of immature glomeruli of neonatal mice, but immunostaining in this location was absent in four-week postnatal mice.

CONCLUSION

This is the first demonstration that isolated mesangial cells express Ang-2 mRNA and protein and up-regulate Ang-2 in response to hypoxia. We speculate that hypoxia-induced, mesangial-derived Ang-2 and VEGF may have synergistic paracrine roles in the growth of glomerular endothelia during normal development and diseases.

Hepatocyte growth factor in renal regeneration, renal disease and potential therapeutics

Hepatocyte growth factor (HGF) has mitogenic, motogenic, morphogenic, and anti-apoptotic activities on renal cells and is a potential renotropin for renal protection and repair. In chronic renal failure/fibrosis, HGF in the kidney declines in a reciprocal manner to the increase in transforming growth factor-beta (TGF-beta). Neutralization of HGF by the antibody leads to acceleration of renal failure/fibrosis while HGF administration leads to remarkable attenuation, thus indicating the importance of HGF versus TGF-beta counterbalance in both pathogenesis and therapeutics in cases of chronic renal failure. HGF is being strongly considered for potential treatment of acute and chronic renal failure.

Angiopoietin-2 is a site-specific factor in differentiation of mouse renal vasculature

Angiopoietin-1 (Ang-1) stimulates endothelial and vascular network differentiation through the Tie-2 receptor tyrosine kinase, while Ang-2 modulates this activation in embryo and tumor growth. The nephrogenic pattern of Ang-2 was documented in a mouse strain that expresses the LacZ reporter gene driven by the Ang-2 promoter. Heterozygous animals were healthy with morphologically normal kidneys, and they were examined after X-gal staining. At embryonic days 10.5 (E10.5) and E12.0, transgene expression was absent in the mesonephros and metanephros. At E14.0, expression was noted in the metanephric artery and its major branches. At E19.0 and in neonatal kidneys, expression was maintained in larger renal artery branches, extending to arcuate and smaller cortical vessels. Histologically, transgene expression was located in multiple layers of vessel wall cells, extending further from the endothelium than alpha-smooth muscle actin. The mesangium of immature glomeruli also expressed LacZ. In the first 3 postnatal weeks, a new pattern became evident, with intense X-gal staining in the inner stripe of the outer medulla, where a subset of thin descending limbs of loops of Henle expressed the transgene. This dynamic and developmentally regulated pattern indicates that Ang-2 is an early marker of the renal pericyte and vascular smooth muscle lineage and is also an epithelial-derived growth factor. Because Tie-2 is widely expressed by differentiating renal endothelia, this study is consistent with the hypothesis that Ang-2 has roles in kidney vascular maturation.

Hepatocyte growth factor: a regulator of extracellular matrix genes in mouse mesangial cells

The potential role of hepatocyte growth factor (HGF) in regulating extracellular matrix in mouse mesangial cells (MMC) was evaluated. Functional HGF receptors were deed in MMC by HGF-induced extracellular acidification, a response that was inhibited by the HGF inhibitor HGF/NK2, a splice variant expressing the N-terminal domain through the second kringle domain HGF also increased fibronectin and collagen alpha1 (IV) mRNA levels in these cells; the increases were associated with a concentration-dependent increase in transcriptional activity of the collagen alpha1 (IV) gene. HGF also stimulated fibronectin and collagen alpha1 (IV) mRNA levels in primary rabbit proximal tubule epithelial cells To evaluate the potential consequences of chronic elevation of HGF on renal fuction, HGF was administered continuously for 18 days to normal and diabetic C57BLKS/J lepr(db) mice. In the diabetic mice, HGF reduced creatinine clearance and increased microalbuminuria, indicating that chronic exposure to HGF impairs renal function. Thus, chronically elevated HGF may contribute to the progression of chronic renal disease in diabetes by decreasing the glomerular filtration rate and possibly promoting the accumulation of extracellular matrix.

Expression of angiopoietin-1, angiopoietin-2, and the Tie-2 receptor tyrosine kinase during mouse kidney maturation

The Tie-2 receptor tyrosine kinase transduces embryonic endothelial differentiation, with Angiopoietin-1 (Ang-1) acting as a stimulatory ligand and Ang-2 postulated to be a naturally occurring inhibitor. Expression of these genes was sought during mouse kidney maturation from the onset of glomerulogenesis (embryonic day 14 [E14]) to the end of nephron formation (2 wk postnatal [P2]), and during medullary maturation into adulthood (P8). Using Northern and slot blotting of RNA extracted from whole organs, these three genes were expressed throughout the experimental period with peak levels at P2 to P3. By in situ hybridization analysis at E18, P1, and P3, Ang-1 mRNA was found to localize to condensing renal mesenchymal cells, proximal tubules, and glomeruli in addition to maturing tubules of the outer medulla. In contrast, Ang-2 transcripts were more spatially restricted, being detected only in differentiating outer medullary tubules and the vasa recta bundle area. Using in situ hybridization and immunohistochemistry, Tie-2 was detected in capillaries of the nephrogenic cortex, glomerular tufts, cortical interstitium, and medulla including vessels in the vasa recta. Using Western blotting of protein extracted from whole organs, Tie-2 protein was detected between E14 and P8 with tyrosine phosphorylated Tie-2 evident from E18. These data are consistent with the hypothesis that Tie-2 has roles in maturation of both glomeruli and vasa rectae.

Early gene expression associated with regeneration is intact after massive hepatectomy in rats

BACKGROUND

Liver regeneration occurs promptly after partial hepatectomy, although the factors regulating this response have not been fully clarified. Molecular events in the regenerative response have been widely characterized after 70% hepatectomy which represents a model of "normal" liver regeneration in rats. More extensive resection results in hepatic failure which has been attributed to a critical loss of hepatic mass. It is not known whether the pattern of genes expressed early in regeneration remains intact after lethal hepatectomy. We hypothesize that the increased expression of selected early response genes remains intact after massive hepatectomy. The aim of this study was to compare the expression of selected genes after 70 and 85% hepatectomy.

MATERIALS AND METHODS

One hundred ten Wistar rats were divided into three groups: control group (sham laparotomy) (n = 30), 70% hepatectomy group (n = 40), and 85% hepatectomy group (n = 40). Animals were sacrificed at intervals. Livers were excised and divided into four equal specimens, snap frozen, and stored at -70 degrees C. RNA was extracted by standard methods and preparations were probed for protooncogenes, c-myc, c-fos, and for hepatocyte growth factor, and its receptor, c-met. After overnight exposure of autoradiographs, quantification was accomplished by densitometry of RNA slot blots.

RESULTS

After 70% hepatectomy, peaks of maximal expression for both c-myc and c-met were observed after 1 and 12 h. For c-fos, peak of maximal expression was observed at 6 h. For HGF, peak was observed between 12 h and Day 2. After 85% hepatectomy, rats demonstrated similar patterns including peak expression of c-myc at 1 h, but altered peak at 12 h. For c-met, the same pattern was observed between 1 and 12 h. For HGF, two peaks were noted: a first peak at 1 h, and a peak similar to the peak observed after 70% hepatectomy at 12 h.

CONCLUSIONS

These results suggest that early molecular events which are part of the regenerative response are largely intact after 85% lethal hepatectomy. We propose that liver dysfunction and the failure of regeneration observed after 85% hepatectomy is not due to alteration of early signaling. Further study will be required to define failure of the regeneration program in this model.

Evidence for a functional hepatocyte growth factor receptor in human mesangial cells

In this study we have investigated both the expression of c-met in cultured human mesangial cells and the proliferative effect of HGF on these cells. RNAse protection analysis using a c-met riboprobe showed c-met to be expressed and further that this expression was unaffected by the glucose concentration or osmolality of the media. Immunofluorescence studies performed using anti-HGF or anti-c-met antibodies clearly showed that both proteins are localised to human mesangial cells. Proliferation of human mesangial cells after 24-h treatment with HGF was also examined. HGF 10 ng/ml and 100 ng/ml stimulated 3-H-Thymidine incorporation 1.35-fold (P = 0.001) and 1.6-fold (P<0.00001) respectively in cells made quiescent for 24 h. A similar dose-dependent stimulation of proliferation was observed in cells made quiescent for 48 h. Finally, using RNAse protection analysis we have shown that HGF (10 ng/ml, 100 ng/ml) induces the expression of c-met in these cells in a dose-dependent manner. Together these results indicate for the first time a potential autocrine role for HGF in the human mesangium.

Activation of glomerular mesangial cells by hepatocyte growth factor through tyrosine kinase and protein kinase C

Hepatocyte growth factor (HGF) induces mitogenesis, chemotaxis, and tubule formation in renal epithelial cells. This study examined the effects of wortmannin and protein kinase C (PKC) inhibitors on HGF-mediated changes in metabolic activity in glomerular mesangial cells and renal epithelial carcinoma A498 cells. The extracellular acidification rate of transformed mouse glomerular mesangial cells and A498 cells was measured as an index of metabolic activity with a microphysiometer. HGF increased the acidification rate of mesangial cells and A498 cells in a concentration-dependent fashion that was inhibited completely by the tyrosine kinase inhibitor tyrophostin-23 (100 microM). The PKC inhibitors RO-32-0432 and SKF-57048 also inhibited HGF-induced acidification. The IC50 values for SKF-57048 were 59 +/- 2 and 20 +/- 10 nM in mesangial cells and A498 cells, respectively (P < 0.05). 12-O-Tetradecanoylphorbol 13-acetate (TPA), a phorbol ester that activates PKC, increased acidification in mesangial and epithelial cells similar to HGF. Wortmannin, an inhibitor of phosphatidylinositol (PI) 3-kinase (IC50 value 1-10 nM), inhibited HGF-induced acidification with an IC50 of 93 +/- 31 and 9 +/- 1 nM in mesangial and A498 cells, respectively (P < 0.05). In contrast, there was no significant difference in the IC50 value of wortmannin for epidermal growth factor (EGF)-induced acidification between mesangial and A498 cells (23 +/- 9 vs 14 +/- 1 nM, respectively). Because the IC50 value for wortmannin in inhibiting HGF but not EGF-induced acidification was an order of magnitude higher in mesangial cells than in epithelial A498 cells, a wortmannin-sensitive PI 3-kinase pathway may not be involved in HGF-mediated acidification in mesangial cells.

Control of invasive growth by hepatocyte growth factor (HGF) and related scatter factors

Hepatocyte growth factor (HGF) is the prototype of a family of structurally related soluble molecules, named scatter factors (SFs). These control a complex genetic programme leading to cell-dissociation, migration in the extracellular matrix, growth, acquisition of polarity and tubule formation. This programme is pivotal during the embryonic development of epithelial and some mesodermal-derived tissues. In the adult HGF sustains cell survival and regeneration. A structurally related molecule, originally identified as macrophage stimulating protein (MSP), triggers the same complex genetic programme in epithelial and neural cells. The receptors for HGF and MSP are the tyrosine kinases encoded by the homologous genes MET and RON. As a distinctive feature, these receptors act via a two-phosphotyrosine docking site, capable of concomitant activation of multiple intracellular transducers and signalling pathways. In a number of malignant tumours, MET and RON constitutively sustain the genetic programme of scattering, leading to invasive growth and metastatic phenotype. Four MET-related receptors have been recently identified (the SEX protein family). These molecules are predominantly expressed during development and are likely to mediate repelling cues between cells of different type.

Expression of hepatocyte growth factor/scatter factor and its receptor, MET, suggests roles in human embryonic organogenesis

Hepatocyte growth factor/scatter factor (HGF/SF) is secreted by mesenchymal cells and elicits proliferation, motility, differentiation, and morphogenesis of epithelia and other cells. These effects are mediated by binding to MET, a receptor tyrosine kinase. Genetically engineered mice lacking HGF/SF die in utero due to a failure of placental and hepatocyte differentiation, but little information exists regarding the expression of this signaling system in human development. Using reverse transcriptase-polymerase chain reaction, Western blots, and immunohistochemistry, we report that HGF/SF and MET are expressed during critical early periods of human organogenesis from 6 to 13 wk of gestation. Organs that expressed both genes included liver, metanephric kidney, intestine, and lung, each of which develop by inductive interactions between mesenchyme and epithelia. Of all organs studied, the placenta contained the highest levels of HGF/SF protein, and MET was detected in trophoblastic cells of chorionic villi as early as the 5th wk of gestation. Finally, examination of a human multicystic dysplastic kidney demonstrated that malformed, hyperproliferative tubules expressed MET, whereas HGF/SF protein was immunolocalized to the same epithelia and also to the surrounding undifferentiated cells. Hence HGF/SF might be an important growth factor in normal human embryogenesis and may additionally play a role in human organ malformations.

Potassium conductances and proliferation in conditionally immortalized renal glomerular mesangial cells from the H-2Kb-tsA58 transgenic mouse

The effects of the temperature-sensitive, immortalizing Simian Virus 40 T antigen, tsA58, on whole-cell potassium conductances were assessed in renal glomerular mesangial cells from H-2Kb-tsA58 transgenic mice [1]. MTT cell viability assay data indicated that in permissive (33 degrees C, 50 U ml-1 gamma-interferon, IFN+) and non-permissive (37 degrees C, without gamma-interferon, IFN-) culture conditions the oncogene was active and inactive respectively. In IFN+ cells whole-cell currents were inhibited by 10 mM 4-aminopyridine, 1 mM ATP and glibenclamide (glyburide, IC50 = 0.4 microM) and stimulated by cromakalim (EC50 = 40 microM). Furthermore, increases in pipette free calcium activity stimulated the potassium conductance (EC50 = 0.5 microM). Apamin inhibited this conductance (IC50 = 9 nM). None of these effects were observed in IFN- cells. The potassium conductance in IFN- cells was activated by a hyposmotic shock and this was inhibited by Gd3. These data indicate that (1) conductances consistent with ATP-sensitive and small, calcium-activated potassium channels are found in IFN+ cells, (2) an osmotically-sensitive channel is found in IFN- cells and (3) channel expression is dependent upon the activation of tsA58.