Phorbol 12-myristate 13-acetate prevents isoproterenol-induced morphological change in cultured vascular smooth muscle cells

The impact of blunted beta-adrenergic responsiveness on growth regulatory pathways in hypertension

The effects of vasodilator hormones acting through receptors linked to adenylyl cyclase are impaired in the hypertensive state. This has been ascribed to impaired receptor-G protein coupling. However, these receptors also act via effectors not linked to adenylyl cyclase activation. These "alternate" mechanisms may be especially important in growth regulation and might be unaffected (or enhanced) with G protein-coupled receptor-G protein uncoupling. Therefore, we assessed the effects of beta-adrenergic activation on 1) regulation of phosphatidylinositol 3-kinase (PI3 kinase) and extracellular signal-regulated kinase (ERK) activation-two tyrosine kinase-dependent enzymes linked to cell growth-and 2) microarray analysis in vascular smooth muscle cells from spontaneously hypertensive rats (SHR). Isoproterenol-stimulated phosphorylation of ERK1/2 was impaired in SHR. The effect of forskolin was unaltered. In contrast, both vasopressin and angiotensin 2-mediated stimulation of ERK activation was enhanced in SHR. In addition, beta-adrenergic-mediated inhibition of PI3 kinase activity was attenuated in SHR (whereas the effect of forskolin remained intact). In microarray studies, the effect of isoproterenol to regulate transcription was significantly impaired in SHR (as was the effect of forskolin). Together, these data support the hypothesis that the blunted vasodilator effects of hormones linked to adenylyl cyclase activation are an index of a more generalized impairment in modulating growth regulatory pathways. Furthermore, this study supports the hypothesis that the blunting of beta-adrenergic responses relating to increased G protein-coupled receptor kinase 2 expression reflects a "generalized uncoupling" of beta-adrenergic-mediated responses and do not support the concept of "enhanced coupling" of "alternate" pathways of beta-adrenergic growth regulatory pathways in the hypertensive state.

Cyclic AMP inhibits production of interleukin-6 and migration in human vascular smooth muscle cells

BACKGROUND

Gene expression induced by tumor necrosis factor-alpha (TNF-alpha) is involved in the regulation of vascular smooth muscle cell (VSMC) proliferation and migration, two events critical to formation of stenotic vascular lesions. In some systems, elevating adenosine 3',5'-cyclic monophosphate (cyclic AMP) inhibits TNF-alpha induced gene transcription. We recently demonstrated that interleukin-6 (IL-6) was chemotactic to VSMC. Therefore, we tested the hypothesis that elevating cyclic AMP would inhibit TNF-alpha-mediated IL-6 expression and VSMC migration.

MATERIALS AND METHODS

VSMC were cultured from saphenous vein remaining after coronary artery bypass grafting. Migration of VSMC through a porous membrane was determined. Intracellular cyclic AMP was elevated by exposing the cells to forskolin or 8-Br-cyclic AMP and was measured by radioimmunoassay. IL-6 was measured by enzyme-linked immunosorbent assay.

RESULTS

TNF-alpha induced migration of VSMC in a concentration-dependent manner. Incubation of cells with forskolin significantly increased cyclic AMP. Co-incubation of cells with TNF-alpha in combination with 8-Br-cyclic AMP or forskolin inhibited migration by approximately 25 and 70%, respectively. Incubation with TNF-alpha increased release of IL-6 from VSMC 18-fold over basal. This stimulated release was inhibited by either 8-Br-cyclic AMP or forskolin. In cells stimulated with TNF-alpha, addition of an antibody to IL-6 reduced migration by 25%.

CONCLUSIONS

These data show that IL-6 produced by VSMC contributes to cell migration induced by TNF-alpha. Further, elevating cyclic AMP inhibited TNF-alpha-induced release of IL-6, and migration of VSMC. These results are consistent with the notion that mechanisms that increase intracellular cyclic AMP, such as activation of beta-adrenergic receptors on VSMC, act as a brake on cell migration.

Assessment of rapid morphological changes associated with elevated cAMP levels in human orbital fibroblasts

Orbital fibroblasts exhibit a phenotype distinct from that of other types of fibroblasts. Addition of prostaglandin E2 (PGE2) to culture medium elicits a dramatic change in orbital fibroblast morphology. That response is mediated through the generation of cAMP. Orbital fibroblasts can generate high levels of PGE2 through induction by proinflammatory cytokines of prostaglandin endoperoxide H synthase-2 (PGHS-2). Here we compare the influence on fibroblast morphology of exogenous PGE2, forskolin, and 8-br-cAMP to that mediated through PGHS-2 induction by a lymphocyte-derived cytokine. Within a few hours, orbital fibroblasts treated with any of these test compounds appear under phase-contrast microscopy to exhibit a stellate morphology. When these changes were assessed quantitatively by electric cell-substrate impedance sensing (ECIS), it became evident that 8-br-cAMP, forskolin, and PGE2 initiated shape changes within 30 min of addition to the culture medium, while effects of the cytokine were first evident after approximately 3.5 h. Dermal fibroblasts failed to respond to any of these compounds with regard to changes in cellular morphology. Analysis of micromotion, manifested as small impedance fluctuations, revealed that orbital fibroblasts treated with 8-br-cAMP exhibit less motion than did untreated cells. These results suggest that orbital fibroblast shape can be altered by several compounds known to alter intracellular cAMP levels. They demonstrate the utility of ECIS in the assessment of very rapid and dynamic cellular events associated with changes in cell morphology.

cAMP and bFGF negatively regulate tropomyosin expression in rat cultured astroblasts

We have examined the expression of tropomyosin (TM) messenger RNAs (mRNAs) and protein isoforms in primary cultures of rat astroblasts during morphological changes. Three messenger RNA bands of 2.5, 1.8 and 1.2 kilobase pairs (kb) were detected by Northern blot. Using an antibody cross-reacting with all tropomyosin isoforms, we found that rat cerebellar neonatal astroblasts expressed three tropomyosin protein isoforms termed TM-As1, TM-As2 and TM-As3 (As for Astroblast) with respective molecular masses of 38,000, 33,000 and 31,000. Treatment of cells with agents which promote or mimick the action of cyclic AMP, or with growth factors, is known to induce astroblast morphological alteration from flat, polygonal epitheloid cells into star-shaped, process-bearing cells. In the presence of dibutyryl cAMP (dBcAMP), forskolin or basic fibroblast growth factor (bFGF), these morphological changes were found to be associated with dramatic decreases of the three mRNA transcripts and also of the three protein isoforms. This decrease was reversed upon removal of the drugs. The pattern of the tropomyosin protein isoforms in cultured astroblasts showed that TM-Asl, the most immunoreactive isoform recovered in the cytoskeletal insoluble cell fraction, had a developmental profile similar to that of F-actin. Therefore this isoform, which belongs to the high-molecular-mass family of proteins known to interact strongly with F-actin, could specifically be involved in the regulation/control of F-actin stability and thus be associated with the plasticity of astroblasts.

Expression of myosin regulatory light-chain isoforms and regulation of phosphorylation in smooth muscle

Our objectives were to 1) determine how growth state and cell density affect the expression of the smooth muscle (SM) and nonmuscle (NM) isoforms of the 20-kDa myosin regulatory light chains (MLC20) in cultured rat aortic smooth muscle cells (SMC) and 2) to determine whether angiotensin II stimulates differential phosphorylation of SM and NM MLC20 isoforms in an effort to assess whether the SM and NM isoforms may subserve different cellular functions. The results demonstrated that changes in the SM MLC20 isoform content were inversely correlated with cell growth but independent of cell density. MLC20 phosphorylation levels were 20.8 +/- 2.9 and 19.4 +/- 3.7% for SM and NM isoforms, respectively, in unstimulated, substrate-attached SMC. Angiotensin II transiently elevated phosphorylation levels of both the SM and NM MLC20 isoforms to 60-70%. No differences in either the magnitude or the kinetics of phosphorylation were observed for the SM vs. NM isoforms. Forskolin, 3-isobutyl-1-methylxanthine, or isoproterenol treatment led to parallel dephosphorylation of the SM- and NM-specific isoforms followed by depolymerization of stress fibers and cell arborization. The studies provide evidence that growth arrest of cultured SMC enhances expression of cell-specific/-selective proteins characteristic of differentiated SM. However, there was no evidence for differential phosphorylation changes of SM and NM MLC20 isoforms in response to activating or relaxing agents as expected if these isoforms subserve different cellular functions.

Shapes of cells spreading on fibronectin: measurement of the stellation of BHK21 cells induced by raising cyclic AMP, and of its reversal by serum and lysophosphatidic acid

In common with many other animal cells in culture, BHK21, CHO and NIH-3T3 cells adopt bizarre stellate or arborized shapes when exposed, in the absence of serum, to agents which increase cytoplasmic cyclic AMP (cAMP). Dibutyryl cAMP, 3-isobutyl-1-methylxanthine, 5′-deoxy-5′-methylthioadenosine, cholera toxin and the invasive adenylate cyclase from Bordetella pertussis all induce similar shapes. Time lapse video recording of BHK21 cells spreading on fibronectin shows that stellate shapes are generated by outgrowth of neurite-like processes led by small fans of ruffling membrane. These structures stain strongly for F actin, and their outgrowth is completely inhibited by cytochalasin D. Thus if stellation is caused by microfilament depletion, this must be selective for subsets of microfilaments. We have quantified the shape changes of BHK21 cells using the parameter dispersion. They are prevented by low concentrations (1% by volume and below) of bovine sera. The inhibitory component of foetal bovine serum acts humorally, behaves as a macromolecule and is itself inhibited by suramin, but platelet-derived growth factor, insulin, vasopressin and bradykinin are inactive. The inhibitory activity of serum may be due to phospholipids, since it can be replaced by lysophosphatidic acid in the presence of serum albumin.

Phorbol ester-induced reorganization of the cytoskeleton in human fibroblasts during ageing in vitro

Phorbol esters induce drastic morphological alterations in cells of different origin by altering the conformation and the interrelationship of the elements of the cytoskeletal system. Treatment of early passage (young) and late passage (senescent) human fibroblasts MRC-5 with phorbol-12-myristate-13-acetate (PMA) results in the rearrangement of actin and tubulin filaments. PMA brings about the disorientation and diffusion of the heavily criss-crossed network of actin and microtubulin fibres characteristic of senescent cells suggesting thereby an increased sensitivity of senescent cells to phorbol esters. Since phorbol esters are known to be specific activators of protein kinase C (PKC), the PMA-induced modulation of the cytoskeleton patterns in ageing fibroblasts provides further support for the view that the effectiveness of the signalling mechanisms is retained during cellular ageing.

Serotonin produces a configurational change of cultured smooth muscle cells that is associated with elevation of intracellular cAMP

Early passaged bovine pulmonary artery smooth muscle cells (SMC) respond to serotonin (5-HT) by developing a reversible change in configuration. (Lee et al. J. Cell. Physiol. 138:145, 1989). This configurational change does not occur in pulmonary artery endothelial cells (EC) subjected to 5-HT and is adenosine triphosphate (ATP) dependent, lost with passage of SMC, and inhibited by various agents that block high-affinity 5-HT uptake. We now report a second configurational change (also dendritic formation) of SMC produced by 5-HT only in the presence of isobutylmethylxanthine (IBMX), an inhibitor of phosphodiesterase. This configurational change was also ATP dependent, but unlike the first response, (Lee et al., 1989), it occurred in both first and later passaged SMC and was not inhibited by blockade of 5-HT uptake. Also, unlike the response with 5-HT alone that failed to elevate cAMP, this one was associated with a large elevation of cAMP (eight fold above control values), similar to the response to the beta-agonist isoproterenol, plus IBMX. The second response was not blocked by a variety of 5-HT receptor antagonists but was reproduced by (+/-)-8-hydroxy-DPAT HBr (8-OH-DPAT), a reputed 5-HT1A agonist. The response was not dependent upon Ca2+ and was blocked by 1-2 mM n-phenylanthranilic acid or anthracene-9-carboxylic acid, electrically conductive Cl- channel inhibitors. Hence, 5-HT in the presence of IBMX causes a marked elevation of cAMP of SMC and this elevation in cAMP likely results in a cellular configurational change through a Cl- channel-dependent mechanism similar to that we previously described for EC in the presence of beta-adrenergic agonist stimulation (Ueda et al. Circ. Res. 66:951, 1990). EC do not show a similar response to 5-HT possibly because cAMP is not adequately elevated, even in the presence of IBMX, to enhance Cl- channel activity. We propose that our observations indicate the presence of two sites of action of 5-HT on the smooth muscle cell, one intracellularly and another at a cell surface receptor.

Stellate cells of aortic intima: II. Arborization of intimal cells in culture

The present study analyzed effects of different cAMP-elevators on cell morphology in primary culture of human intimal and medial cells from grossly normal and atherosclerotic areas. In primary culture of human aortic cells adenylate cyclase activator forskolin and other cAMP elevators induced arborization of cells, i.e. they reversibly changed the shape of cells. This resulted in the formation of thin branching processes and in the concentration of cytoplasm around the nucleus. In the culture, the shape of the arborized cells resembled that of stellate ones detected in the aortic intima in situ. The arborization of cells was accompanied by destruction of myofilaments. Due to cAMP elevators' effect, most of the arborized cells were exhibited in the cultures isolated from the elastic-hyperplastic layer of the intima. The number of arborized cells was significantly less in the cultures isolated from the musculo-elastic layer and still lesser in those isolated from media. We failed to reveal any significant difference in the number of arborized cells cultured from fatty streaks, atherosclerotic plaques and grossly normal aortic areas. Obtained results suggest that the previously revealed polymorphism of human aortic intimal cells may be accounted for by the cell shape transformations underlined by the mechanism similar to that of arborization in vitro.

Cyclic AMP- and cytochalasin B-induced arborization in cultured aortic smooth muscle cells: its cytopharmacological characterization

The present study analyzed effects of dibutyryl cyclic AMP (DB-cAMP) and cytochalasin B (CB) on the morphology of cultured aortic smooth muscle cells (SMC) from rat using phase-contrast microscopy, scanning electron microscopy, and fluorescence staining of actin filaments by the NBD-phallacidin method. The exposure of SMC to each of these agents led to rapid, extensive, and reversible (within 1-2 h of drug withdrawal) changes in their morphology including cytoplasmic arborization (stellation). The latter was preceded by (i) marginal membrane ruffles (DB-cAMP) and (ii) increased zeiotic activity (CB), which were visible within 20 min of the exposure, followed (30-90 min incubation) by a centripetal retraction of the cytoplasm and progressive development of complete or partial arborization. Further, the effects of substances interfering with the assembly-disassembly of microtubules (colchicine, taxol, lidocaine) on DB-cAMP- and CB-induced arborization were studied. None of these agents antagonized CB-induced morphological changes. Colchicine, but not lumicolchicine, taxol, or lidocaine (in a short-term study) prevented DB-cAMP-induced arborization. Taxol added to cell cultures for 24 h promoted DB-cAMP-induced arborization. Both DB-cAMP and CB resulted in the disintegration of actin filaments. The present data suggest that the arborization of cultured aortic SMC is a cytoskeleton-based process involving stabilization of microtubules and disintegration of actin filaments. Our study also suggests that the SMC arborization may represent an in vitro case of SMC stellation found in situ.