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J I Kreisberg

Publications and source records attributed to J I Kreisberg.

At least 19 recordsLinked to original sources
PubMed2001

Interaction of the betaIV-tubulin isotype with actin stress fibers in cultured rat kidney mesangial cells.

Microtubules and actin filaments are two of the major components of the cytoskeleton. There is accumulating evidence for interaction between the two networks. Both the alpha- and beta-subunits of tubulin exist as numerous isotypes, some of which have been highly conserved in evolution. In an effort to better understand the functional significance of tubulin isotypes, we used a double immunofluorescence labeling technique to investigate the interactions between the tubulin beta-isotypes and the actin stress fiber network in cultured rat kidney mesangial cells, smooth-muscle-like cells from the renal glomerulus. Removal of the soluble cytoplasmic and nucleoplasmic proteins by detergent extraction caused the microtubule network to disappear while the stress fiber network was still present. In these extracted cells, the betaI- and betaII-tubulin isotypes were no longer present in the cytoplasm while the betaIV-isotype co-localized with actin stress fibers. Co-localization between betaIV-tubulin and actin stress fibers was also observed when the microtubule network was disrupted by the anti-tubulin drug colchicine and also by microinjection of the betaIV-tubulin antibody. Our results suggest that the betaIV isotype of tubulin may be involved in interactions between microtubules and actin.

Actins↗
PubMed2001

Mechanism of localization of betaII-tubulin in the nuclei of cultured rat kidney mesangial cells.

Tubulin is an alphabeta heterodimer. Both the alpha and beta polypeptides exist as multiple isotypes. Although tubulin was generally thought to exist only in the cytoplasm, we have previously reported the presence of the betaII isotype of tubulin in the nuclei of cultured rat kidney mesangial cells, smooth-muscle-like cells that reside in the glomerular mesangium; nuclear betaII exists as an alphabetaII dimer, capable of binding to colchicine, but in non-microtubule form [Walss et al., 1999: Cell Motil. Cytoskeleton 42:274-284]. We have now investigated the nature of the process by which alphabetaII enters the nuclei of these cells. By micro-injecting fluorescently labeled alphabetaII into mesangial cells, we found that alphabetaII was present in the nuclei of cells only if they were allowed to go through mitosis. In contrast, there were no circumstances in which microinjected fluorescently labeled abetaII or alphabetaIV dimers entered the nuclei. These findings, together with the absence of any nuclear localization signal in alphabetaII, strongly favor the model that alphabetaII, rather than being transported into the intact nucleus, co-assembles with the nucleus at the end of mitosis. Our results also indicate that the nuclear localization mechanism is specific for alphabetaII. This result raises the possibility that alphabetaII may have a specific function that requires its presence in the nuclei of cultured rat kidney mesangial cells.

Animals↗
PubMed2001

Antisense GLUT-1 protects mesangial cells from glucose induction of GLUT-1 and fibronectin expression.

A stable clone of rat mesangial cells expressing antisense GLUT-1 (i.e., MCGT1AS cells) was developed to protect them from high glucose exposure. GLUT-1 protein was reduced 50%, and the 2-deoxy-[(3)H]glucose uptake rate was reduced 33% in MCGT1AS. MCLacZ control cells and MCGT1 GLUT-1-overexpressing cells were used for comparisons. In MCLacZ, 20 mM D-glucose increased GLUT-1 transcription 90% vs. no increase in MCGT1AS. Glucose (8 mM) and 12 mM xylitol [a hexose monophosphate (HMP) shunt substrate] did not stimulate GLUT-1 transcription. An 87% replacement of the standard 8 mM D-glucose with 3-O-methylglucose reduced GLUT-1 transcription 80%. D-Glucose (20 mM) increased fibronectin mRNA and protein by 47 and 100%, respectively, in MCLacZ vs. no increases in MCGT1AS. Fibronectin synthesis was elevated 48% in MCGT1 and reduced 44% in MCGT1AS. We conclude that 1) transcription of GLUT-1 in response to D-glucose depends on glucose metabolism, although not through the HMP shunt, and 2) antisense GLUT-1 treatment of mesangial cells blocks D-glucose-induced GLUT-1 and fibronectin expression, thereby demonstrating a protective effect that could be beneficial in the setting of diabetes.

ATP-Binding Cassette Transporters↗
PubMed2001

Arginine vasopressin stimulates mesangial cell proliferation by activating the epidermal growth factor receptor.

The potent vasoconstrictor arginine vasopressin (AVP) is also a mitogen for mesangial cells. Treatment with AVP decreased transit time through the cell cycle. AVP-stimulated mesangial cell growth by activating both the Ras mitogen-activated protein kinase (MAPK) and the phosphatidylinositol 3-kinase (PI3K) cell signaling pathways. Both the selective PI3K inhibitor LY-294002 and the MAPK kinase (MEK) inhibitor PD-98059 inhibited AVP-stimulated mesangial cell proliferation. However, LY-294002 was more potent, indicating an important role for PI3K activation in AVP-stimulated mesangial cell proliferation. AVP appeared to exert its effect on MAPK and PI3K activation, as well as on cell proliferation, by activating the epidermal growth factor receptor (EGF-R). Pretreatment with the tyrphostin-derived EGF-R antagonist AG-1478 inhibited mesangial cell proliferation as well as the activation of extracellular signal-regulated kinase 1/2 (ERK1/2 or p42/p44(MAPK)), and p70S6 kinase, a downstream effector of PI3K, providing evidence that MAPK and PI3K activation, respectively, occurred downstream of EGF-R activation. Treatment with rapamycin, an inhibitor of the p70S6 kinase activator mTOR, also resulted in growth inhibition, further suggesting the importance of the PI3K signaling pathway in AVP-induced proliferation. AVP treatment appeared to transactivate EGF-R by inducing tyrosine phosphorylation of the Ca(2+)/protein kinase C (PKC)-dependent nonreceptor tyrosine kinase, Pyk2, leading to Pyk2/c-Src association and c-Src activation. This was followed by association of c-Src with EGF-R and EGF-R activation. These data suggested that AVP-stimulated Pyk2 tyrosine phosphorylation to activate c-Src, thereby leading to EGF-R transactivation.

Animals↗
PubMed2000

Phosphorylation and nuclear exclusion of the forkhead transcription factor FKHR after epidermal growth factor treatment in human breast cancer cells.

Akt, when activated by IGF/insulin, can phosphorylate forkhead transcription factors. We undertook this study to determine whether epidermal growth factor (EGF) treatment could produce a signaling cascade resulting in phosphorylation of the forkhead transcription factor FKHR in a breast cancer cell line, MDA-MB-231. After establishing ErbB1, cbl, PI3 kinase and Akt were activated in EGF treated MDA-MB-231, we determined by immunoblot with FKHR antiserum that the electrophoretic mobility of FKHR was retarded after EGF treatment. This mobility retardation was reversible by treatment with alkaline phosphatase, and immunoblot with phospho-Ser256 FKHR antibody further confirmed phosphorylation on an Akt consensus site after EGF treatment. EGF stimulated FKHR phosphorylation was blocked by the PI3 kinase inhibitor LY294002, and the ErbB1 inhibitor AG1478. FKHR immunoblotting after purification of nuclear and cytoplasmic proteins showed that EGF induced a simultaneous increase of FKHR in the cytoplasm and decrease in the nucleus. This finding was confirmed by immunofluorescence staining. Treatment of cells with pharmacological inhibitors of PI3 kinase or ErbB1 blocked this effect. Thus, these results demonstrate the phosphorylation and nuclear exclusion of FKHR after EGF treatment by a PI3 kinase dependent mechanism, and represent the first report of growth factor regulation of endogenous FKHR localization.

Alkaline Phosphatase↗
PubMed1999

Effect of cyclin E overexpression on lovastatin-induced G1 arrest and RhoA inactivation in NIH3T3 cells.

The HMG-CoA reductase inhibitor, lovastatin, blocks targeting of the Rho and Ras families of small GTPases to their active sites by inhibiting protein prenylation. Control NIH3T3 cells, and those overexpressing human cyclin E protein were treated with lovastatin for 24 h to determine the effects of cyclin E overexpression on lovastatin-induced growth arrest and cell rounding. Lovastatin treatment (10 microM) of control 3T3 cells resulted in growth arrest at G1 accompanied by actin stress fiber disassembly, cell rounding, and decreased active RhoA from the membranous protein fraction. By contrast, in NIH3T3 cells overexpressing cyclin E, lovastatin did not cause loss of RhoA from the membrane (active) protein fraction, actin stress fiber disassembly, cell rounding or growth arrest within 24 h. Analysis of cell cycle proteins showed that 24 h of lovastatin treatment in the control cells caused an elevation in the levels of the cyclin-dependent kinase inhibitor p27(kip1), inhibition of both cyclin E- and cyclin A-dependent kinase activity, and decreased levels of hyperphosphorylated retinoblastoma protein (pRb). By contrast, lovastatin treatment of the cyclin E overexpressors did not suppress either cyclin E- or cyclin A-dependent kinase activity, nor did it alter the level of maximally phosphorylated pRb, despite increased levels of p27(kip1). However, by 72 h, the cyclin E overexpressors rounded up but remained attached to the substratum, indicating a delayed response to lovastatin. In contrast with lovastatin, inactivation of membrane-bound Rho proteins (i.e., GTP-bound RhoA, RhoB, RhoC) with botulinum C3 transferase caused cell rounding and G1 growth arrest in both cell types but did not inhibit cyclin E-dependent histone kinase activity in the cyclin E overexpressors. In addition, 24 h of cycloheximide treatment caused depletion of RhoA from the membrane (active) fraction in neo cells, but in the cells overexpressing cyclin E, RhoA remained in the active (membrane-associated) fraction. Our observations suggest that (1) RhoA activation occurs downstream of cyclin E-dependent kinase activation, and (2) overexpression of cyclin E decreased the turnover rate of active RhoA.

3T3 Cells↗
PubMed1999

Role of RhoA activation in the growth and morphology of a murine prostate tumor cell line.

Prostate cancer cells derived from transgenic mice with adenocarcinoma of the prostate (TRAMP cells) were treated with the HMG-CoA reductase inhibitor, lovastatin. This caused inactivation of the small GTPase RhoA, actin stress fiber disassembly, cell rounding, growth arrest in the G1 phase of the cell cycle, cell detachment and apoptosis. Addition of geranylgeraniol (GGOL) in the presence of lovastatin, to stimulate protein geranylgeranylation, prevented lovastatin's effects. That is, RhoA was activated, actin stress fibers were assembled, the cells assumed a flat morphology and cell growth resumed. The following observations support an essential role for RhoA in TRAMP cell growth: (1) TRAMP cells expressing dominant-negative RhoA (T19N) mutant protein displayed few actin stress fibers and grew at a slower rate than controls (35 h doubling time for cells expressing RhoA (T19N) vs 20 h for untransfected cells); (2) TRAMP cells expressing constitutively active RhoA (Q63L) mutant protein displayed a contractile phenotype and grew faster than controls (13 h doubling time). Interestingly, addition of farnesol (FOL) with lovastatin, to stimulate protein farnesylation, prevented lovastatin-induced cell rounding, cell detachment and apoptosis, and stimulated cell spreading to a spindle shaped morphology. However, RhoA remained inactive and growth arrest persisted. The morphological effects of FOL addition were prevented in TRAMP cells expressing dominant-negative H-Ras (T17N) mutant protein. Thus, it appears that H-Ras is capable of inducing cell spreading, but incapable of supporting cell proliferation, in the absence of geranylgeranylated proteins like RhoA.

Actin Cytoskeleton↗
PubMed1999

Presence of the betaII isotype of tubulin in the nuclei of cultured mesangial cells from rat kidney.

Tubulin has generally been considered to be a cytosolic protein whose only function is to form microtubules. This assumption is supported by a great deal of evidence derived from immunohistochemical studies using antibodies directed against whole tubulin or its component polypeptides alpha- and beta-tubulin. We have re-examined the intracellular distribution of tubulin using monoclonal antibodies specific for the betaI, betaII, betaIII, and betaIV isotypes of beta-tubulin. Our test system is the cultured rat kidney mesangial cell. We have found that betaIII is absent from these cells and that beta1 and betaIV are present in microtubules throughout the cytosol. In contrast, betaII is present largely in the nuclei. Immunoblotting of purified nuclear extracts shows that the betaII-reactive antigen co-migrates with beta-tubulin. Extraction of the cytosol and chromatin suggests that betaII is concentrated in the nucleoli and also in a reticulated network in the rest of the nucleoplasm. An antibody to tyrosinated alpha-tubulin shows that alpha is also present in the nucleoli. Treatment of the cells with fluorescent colchicine shows an accumulation of colchicine in the nucleoli. Finally, fluorescently labeled alphabetaII-tubulin dimers, when microinjected into the cells, enter the nuclei and are concentrated in the nucleoli. These results suggest that the betaII isotype of tubulin is present as an alphabetaII dimer in the nuclei of cultured mesangial cells and suggest the possibility that different tubulin isotypes may have specific functions within the cell.

Animals↗
PubMed1999

Expression of osteogenic protein-1 mRNA in cultured kidney cells.

Osteogenic protein-1 (OP-1 or BMP-7) is a multifunctional cytokine that regulates the development of several tissues during embryogenesis, including the skeleton, eye, and kidney. In postnatal life, OP-1 expression is most abundant in the kidney, although the cellular localization of this expression has not been described. In this study, we utilized a cell culture approach to localize OP-1 mRNA expression in various renal cell types and to determine potential target cells for OP-1 effects. OP-1 mRNA expression was demonstrated in several glomerular cell types, such as mesangial, epithelial, and endothelial cells. Distal tubule MDCK cells also expressed OP-1 mRNA but human proximal tubule HK-2 cells did not. Multiple OP-1 transcripts, which ranged in size from 1.6 to 3.8 kb, were observed in both glomerular and tubule cells. Interestingly, the pattern of expression varied among the different cell types, suggesting cell-specific expression of OP-1 mRNA. Analysis of OP-1 receptor expression revealed transcripts for BMP receptors type IA and IB in HK-2 cells and transcripts for BMPR-IA and ALK-2 in mesangial cells. Treatment of HK-2 cells with OP-1 (300 ng/ml) for 24-48 hr increased cellular proliferation whereas treatment of cells with transforming growth factor-beta had no effect. Mesangial cell proliferation was not affected by OP-1. The results suggest that OP-1 is produced in the renal glomerulus and then travels to the proximal tubule to regulate the proliferation of cells in this region of the nephron.

Animals↗
PubMed1997

Lovastatin induces apoptosis by inhibiting mitotic and post-mitotic events in cultured mesangial cells.

Lovastatin, an inhibitor of protein prenylation, was reported to inhibit DNA synthesis and induce apoptosis in cultured cells. This report describes the morphological consequences of lovastatin treatment. Lovastatin (50 microM) induced mesangial cell rounding and disassembly of actin stress fibers within 24 to 48 h. After 48 to 72 h of lovastatin treatment, the cells detached from the substratum and underwent apoptotic cell death as evidenced by condensed nuclear chromatin, nuclear fragmentation, cell blebbing and decrease in cell size. Time lapse cinematography revealed that lovastatin caused cell rounding by either inhibiting cytokinesis or cell spreading following cytokinesis. Lovastatin-induced cell rounding, detachment, and apoptosis were dependent upon cell proliferation. These effects were prevented by serum deprivation to inhibit cell proliferation or by plating cells at densities which resulted in contact inhibition of cell growth. Lovastatin-induced mesangial cell rounding and apoptosis were also prevented by the inclusion of the isoprenoids all-trans-farnesol or all-trans-geranylgeraniol in the incubation medium. These results indicate that the effects of lovastatin were mediated by inhibition of protein isoprenylation because exogenous all-trans-geranylgeraniol can be used only in protein prenylation. The small GTP-binding protein RhoA, which may be important for cell spreading and cytokinesis, accumulated in the cytosol following treatment with lovastatin, suggestive of its inactivation. This effect was also prevented by the inclusion of either farnesol or geranylgeraniol in the incubation medium. Thus, lovastatin-induced apoptosis in mesangial cells occurs by interfering with prenylation dependent mitotic and post-mitotic events.

Actins↗
PubMed1997

Role of Rho and myosin phosphorylation in actin stress fiber assembly in mesangial cells.

Treatment of renal glomerular mesangial cells with adenosine 3',5'-cyclic monophosphate (cAMP)-elevating agents induces actin stress fiber disassembly, myosin light chain (MLC) dephosphorylation, loss of adhesion to the substratum and cell shape change [J. I. Kreisberg and M. A. Venkatachalam. Am. J. Physiol. 251 (Cell Physiol. 20): C505-C511, 1986]. Thrombin and vasopressin block the effects of cAMP. Because these agents are known to promote stress fiber formation via the small GTP-binding protein Rho, we investigated the effect of an activated variant of Rho on the response to cAMP elevation. Microinjecting V14-Rho completely blocked the effect of cAMP elevation on cell shape and the actin cytoskeleton, whereas inactivating Rho with botulinum C3 exoenzyme induced stress fiber disruption and cell retraction that was indistinguishable from that caused by elevations in intracellular levels of cAMP. Disruption of actin stress fibers by cAMP has previously been ascribed to MLC dephosphorylation; however, both C3 and cytochalasin D also caused dephosphorylation of MLC, whereas blocking MLC dephosphorylation failed to block the cAMP-induced loss of actin stress fibers. We conclude that Rho can modulate the effects of cAMP elevation and suggest that MLC dephosphorylation may be a consequence of actin stress fiber disassembly.

1-Methyl-3-isobutylxanthine↗
PubMed1996

Phosphorylation of cAMP responsive element binding protein after treatment of mesangial cells with high glucose plus TGF beta or PMA.

We recently showed that mesangial cells treated with high glucose plus TGF beta or PMA demonstrated activation of a cAMP-response element (CRE) located in the 5' flanking region of the fibronectin gene. Gel shift mobility assays with a CRE oligonucleotide revealed multiple complexes that did not change in mobility or abundance under conditions of high glucose plus TGF beta or PMA. Here we show that treatment with cycloheximide to inhibit protein synthesis also did not change the DNA/protein complexes. These observations led us to conclude that post-translational modification of transcription factors may be responsible for the activation of the fibronectin gene observed under our experimental conditions. We identified the proteins complexed to CRE as CRE binding protein (CREB) and activating factor 1 (ATF1). This was accomplished by supershift assays and immunoblots. Two hours of high glucose plus TGF beta or 30 minutes of PMA caused a twofold elevation in phosphorylated CREB. Neither high glucose nor TGF beta alone caused phosphorylation of CREB. ATF-1 was not phosphorylated. We also show that high glucose plus TGF beta and PMA activated protein kinase C alpha; however, none of the agents tested stimulated intracellular cAMP levels, indicating that phosphorylation of CREB was independent of protein kinase A activation. These results demonstrate cross-talk between the protein kinase C and protein kinase A pathways in that agents which activate the protein kinase C pathway can stimulate phosphorylation of proteins that commonly serve as substrates for protein kinase A.

Activating Transcription Factor 1↗
PubMed1994

High glucose elevates c-fos and c-jun transcripts and proteins in mesangial cell cultures.

It has been previously shown that rat glomerular mesangial cells synthesized increased amounts of fibronectin, laminin, and type IV collagen when grown in medium containing 30 mM glucose. High glucose exerted its effect at the mRNA level since transcripts for all three extracellular matrix (ECM) proteins were similarly elevated. High glucose appeared to exert its effect on ECM mRNA levels through protein kinase C activation. Using quantitative reverse transcription (RT) PCR, we now report that mRNA levels for c-fos and c-jun were increased approximately twofold after treatment with high glucose. The fos levels were elevated 15 minutes after addition of high glucose and were maintained elevated through 30 minutes; by one hour mRNA levels for fos returned to control levels. c-jun, on the other hand, was increased at two hours and remained elevated at 24 and 48 hours. Fibronectin mRNA levels were increased three- to fourfold at 24 and 48 hours. Immunofluorescence studies with polyclonal antibodies to c-fos and c-jun revealed that high glucose treatment for four hours increased nuclear staining intensity two- to threefold for both proteins. Nuclear staining for fos returned to control levels by 24 hours while staining for jun remained elevated. These determinations were made on images obtained on a confocal laser scanning microscope. Thus, high glucose may effect gene expression of ECM proteins by elevating the transcription factors c-fos and c-jun which complex with one another to form activator protein 1 (AP-1).

Animals↗
PubMed1994

High glucose and TGF beta 1 stimulate fibronectin gene expression through a cAMP response element.

Previously, we reported that mesangial cells increased fibronectin, laminin and type IV collagen synthesis when cultured in the presence of high glucose (30 mM). Although mRNA levels for all three extracellular matrix (ECM) proteins were also increased in high glucose conditions, the mechanism for this increase was not known. In order to determine whether increased transcription was involved in the observed increase in fibronectin mRNA levels mesangial cells were transfected with a construct containing the 5'-flanking region of the fibronectin (FN) gene [position +69 to -510 base pairs (bp)] fused to the coding region of the chloramphenicol acetyltransferase (CAT) gene [FN-CAT (-510)]. Cells were transiently and stably transfected with this construct. Under serum-free conditions, high glucose increased CAT activity only in the presence of TGF beta 1 (referred to as TGF beta). The experiments were performed without serum because FN-CAT (-510) contains a serum responsive element. The increase in CAT was approximately twofold in transiently transfected cells and threefold in stably transfected cells. TGF beta alone increased CAT activity approximately 30%. Stimulation of fibronectin gene expression appeared to occur at the level of a cAMP response element (CRE) located -170 bp of the FN gene because cells transfected with a construct containing an oligonucleotide encoding for this CRE fused to a minimal fibronectin promoter (-56 bp) and a CAT reporter gene [CRE (-170) FN-CAT] displayed similar increments of CAT activity after treatment with high glucose and TGF beta. Gel shift mobility assays with a CRE oligonucleotide revealed multiple complexes with mesangial cell nuclear proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Interleukin-1 alpha stimulates KC synthesis in rat mesangial cells: glucocorticoids inhibit KC induction by IL-1.

To assess the possible role of the production of chemokines by intrinsic glomerular cells in the generation of inflammation in glomerulonephritis, the chemokine, KC, was cloned from a rat macrophage cDNA library. Transfection of rat KC into COS-7 cells resulted in increased neutrophil chemotactic activity. The KC cDNA was expressed as a fusion protein in Escherichia coli for generation of an antibody. By using a riboprobe derived from the cDNA and the antibody, interleukin-1 (IL-1) was found to induce the expression of KC in rat mesangial cells. The induction of KC by IL-1 could be inhibited by dexamethasone (DEX). The protein synthesis inhibitor cycloheximide reversed the DEX-mediated inhibition, which suggested that new protein synthesis was necessary for the inhibitory effect. A nuclear runoff analysis indicated that DEX inhibited the transcription of KC induced by IL-1. The stability of KC mRNA was not decreased in the presence of DEX. Furthermore, immunoblots showed that DEX also inhibited KC expression at the level of translation. Together the inhibition of transcription and translation of the KC gene by DEX contribute to decreased KC expression in mesangial cells. The finding that mesangial cells express KC in response to proinflammatory cytokines, such as IL-1, points to a central role for the mesangial cell as a chemotactic source in glomerular inflammation.

Animals↗
PubMed1993

Regulation of integrin-mediated adhesion at focal contacts by cyclic AMP.

Cyclic AMP (cAMP) elevation causes diverse types of cultured cells to round partially and develop arborized cell processes. Renal glomerular mesangial cells are smooth, muscle-like cells and in culture contain abundant actin microfilament cables that insert into substratum focal contacts. cAMP elevation causes adhesion loss, microfilament cable fragmentation, and shape change in cultured mesangial cells. We investigated the roles of the classical vitronectin (alpha V beta 3 integrin) and fibronectin (alpha 5 beta 1 integrin) receptors in these changes. Mesangial cells on vitronectin-rich substrata contained microfilament cables that terminated in focal contacts that stained with antibodies to vitronectin receptor. cAMP elevation caused loss of focal contact and associated vitronectin receptor. Both fibronectin and its receptor stained in a fibrillary pattern at the cell surface under control conditions but appeared aggregated along the cell processes after cAMP elevation. This suggested that cAMP elevation caused loss of adhesion mediated by vitronectin receptor but not by fibronectin receptor. We plated cells onto fibronectin-coated slides to test the effect of ligand immobilization on the cellular response to cAMP. On fibronectin-coated slides fibronectin receptor was observed in peripheral focal contacts where actin filaments terminated, as seen with vitronectin receptor on vitronectin-coated substrata, and in abundant linear arrays distributed along microfilaments as well. Substratum contacts mediated by fibronectin receptor along the length of actin filaments have been termed fibronexus contacts. After cAMP elevation, microfilaments fragmented and fibronectin receptor disappeared from peripheral focal contacts, but the more central contacts along residual microfilament fragments appeared intact. Also, substratum adhesion was maintained after cAMP elevation on fibronectin--but not on vitronectin-coated surfaces. Although other types of extracellular matrix receptors may also be involved, our observations suggest that cAMP regulates adhesion at focal contacts but not at fibronexus-type extracellular matrix contacts.

Actin Cytoskeleton↗