From the
Departments of Pharmacology and ||Medicine, University of California, San Diego, La Jolla, California 92093-0636 and the ¶Department of Physiology, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio 44272
Received for publication, December 12, 2002 , and in revised form, March 25, 2003.
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Two major pro-fibrotic signals in the heart are cytokines, such as transforming growth factor-, and the peptide hormone ANG II, both of which increase collagen synthesis by cardiac fibroblasts (6, 7). ANG II also inhibits matrix metalloproteinase expression by cardiac fibroblasts, thereby attenuating degradation of ECM proteins (8). An anti-fibrotic role for cAMP is supported by evidence that adenosine and prostacyclin inhibit cardiac fibroblast proliferation and collagen synthesis through activation of A2b and prostanoid receptors, respectively, which each couple via Gs, to enhanced cAMP production (9, 10).
-Adrenergic receptors have been shown to stimulate fibroblast proliferation via epidermal growth factor receptor transactivation (11) but their role in regulating collagen synthesis in cardiac fibroblasts is poorly documented.
Adenylyl cyclase (AC) catalyzes the synthesis of cAMP and its expression limits the ability of a cardiac cell to maximally produce this second messenger (12, 13). Nine different transmembrane AC isoforms exist, each with different amino acid sequence, tissue and chromosomal distribution, and regulation (14). Differences in regulation include stimulation or inhibition by G, Ca2+, and various protein kinases. AC5 and AC6, which are the predominant isoforms expressed in cardiac myocytes (15), represent a subfamily of ACs that are related in structure and regulation. These isoforms are inhibited by protein kinase A, Ca2+, Gi, G
, and nitric oxide (14, 16, 17). By contrast, AC1, AC3, and AC8 are stimulated by Ca2+/calmodulin (yet AC3 can also be inhibited by calmodulin kinase-II) (14, 18, 19). AC2 and AC4 are activated by G
and AC2 and AC7 can be activated by phosphorylation by protein kinase A and/or PKC (14). Thus, the AC isoform expression in a cell determines the interaction between cAMP production and other signal transduction cascades.
Meszaros et al. (20) have recently described a signaling "cross-talk" between two key GPCR signal transduction pathways in cardiac fibroblasts. Isoproterenol (ISO), a -adrenergic receptor (
AR) agonist, activates Gs and stimulates cAMP production 10-fold over basal levels. ANG II activates Gq-coupled angiotensin receptors that, by themselves, do not alter cAMP production in cardiac fibroblasts, but in combination with ISO potentiate the
AR response, resulting in a 2-fold potentiation of ISO-stimulated cAMP production. The goal of the present study was to identify the molecular mechanism and physiological consequence of the cross-talk between these two signaling pathways in cardiac fibroblasts. The current data indicate that Gq-mediated elevation of intracellular Ca2+ induced by ANG II potentiates Gs-stimulated cAMP formation, probably via stimulation of AC3. Moreover, we show that the potentiation is functionally relevant: combined treatment of cardiac fibroblasts with ANG II and ISO inhibits ANG II-promoted collagen synthesis more so than ISO alone. Thus, we identify an endogenous signaling pathway by which intracellular Ca2+ enhances cAMP production and limits collagen synthesis, and perhaps fibrosis, in the heart.
![]() |
EXPERIMENTAL PROCEDURES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Preparation and Culture of Adult Rat Cardiac FibroblastsCardiac fibroblasts were prepared from adult male 250300-g Sprague-Dawley rat hearts. Following rapid excision of the hearts, the ventricles were isolated, minced, pooled, and placed in a collagenase/pancreatin digestion solution. After sequential digestions, the fibroblasts were pelleted and resuspended in Dulbecco's modified Eagle's medium (DMEM) supplemented with penicillin, streptomycin, fungizone, and 10% fetal bovine serum (FBS, Gemini Bio-Products). After a 30-min period of attachment to uncoated culture plates, cells that were weakly attached or unattached were rinsed free and discarded. After 23 days, confluent cultures were amplified by trypsinization and seeding onto new dishes. For signaling assays, only early passage (≤3) cells grown to 8090% confluency were used. The purity of these cultures was greater than 95% cardiac fibroblasts as determined by positive staining for vimentin and negative staining for smooth muscle actin and von Willebrand factor, as previously described (21).
Adenoviral Gene Transfer to Cardiac FibroblastsWild-type Gq was cloned into the PACCMVpLpA shuttle vector for production of adenovirus, as previously described (3). We found that a titer of 200 virus particles per cell was optimal for Gq expression without cytotoxicity, as determined by trypan blue exclusion and morphological examination with phase-contrast light microscopy. We used 1000 viral particles per cell of an adenovirus construct to maximally overexpress the murine adenylyl cyclase type 6 (AC6) gene, produced as described previously (22). Cardiac fibroblasts were infected with the appropriate virus and incubated at 37 °C 1824 h in serum-free DMEM prior to the assays. Preliminary experiments using LacZ expression and -galactosidase staining indicated that the efficiency of the adenoviral construct to increase gene expression was 40% (data not shown).
Quantitation of cAMP ProductionCardiac fibroblasts cultured on 24-well plates were washed three times with 37 °C serum-free and NaHCO3-free DMEM supplemented with 20 mM HEPES, pH 7.4. Cells were equilibrated for 30 min, then assayed for cAMP accumulation by incubation with drugs of interest in the presence of 0.2 mM isobutylmethylxanthine for 10 min. When antagonists or inhibitors were used, these agents were equilibrated with cells for 15 min before addition of agonists. Assay medium was aspirated and 250 µl of ice-cold trichloroacetic acid (7.5% w/v) was immediately added to each well to terminate reactions. Trichloroacetic acid extracts were assayed for cAMP content by either Direct ELISA kit (Assay Designs) or radioimmunoassay as described previously (22). Data were corrected for the amount of total acid-precipitable protein per well. In other studies, AC activity was measured in cardiac fibroblast membranes as previously described (23). Briefly, membranes were prepared by rinsing cells twice in ice-cold phosphate-buffered saline then scraping cells into hypotonic homogenizing buffer (30 mM Na-HEPES, 5 mM MgCl2, 1 mM EGTA, 2 mM dithiothreitol, pH 7.5) and homogenizing with 20 strokes in a Dounce homogenizer. Homogenate was spun at 300 x g for 5 min at 4 °C. Supernatant was then transferred to a clean centrifuge tube and spun at 5,000 x g for 10 min. Pellet was suspended in membrane buffer (30 mM Na-HEPES, 5 mM MgCl2, 2 mM dithiothreitol, pH 7.5) to attain 1 mg/ml total protein concentration. Assay was conducted by adding 30 µl of membranes into tubes containing assay buffer (30 mM Na-HEPES, 100 mM NaCl, 1 mM EGTA, 10 mM MgCl2, 1 mM isobutylmethylxanthine, 1 mM ATP, 10 mM phosphocreatine, 5 µM GTP, 60 units/ml creatine phosphokinase, and 0.1% bovine serum albumin, pH 7.5) and drugs of interest. The mixture was incubated for 15 min at 30 °C and reactions were stopped by boiling for 5 min. cAMP content of each tube was assayed for cAMP content by radioimmunoassay.
Intracellular Ca2+ MeasurementsFibroblasts (0.25 x 105 cells) were plated on 22-mm glass coverslips. The cells were washed once in phosphate-buffered saline and incubated in 1 ml of DMEM containing 1 µM Fura-2/AM at 37 °C for 30 min. Cells were then washed once with DMEM and placed in a 37 °C chamber containing 1.5 ml of HEPES-buffered saline (HBS: 130 mM NaCl, 5 mM KCl, 10 mM glucose, 1 mM MgCl2, 1.0 mM CaCl2, 25 mM HEPES, pH 7.4), such that groups of 58 cells could be viewed using an inverted Olympus IX-70 microscope. Spectrofluorometric measurements were collected using Delta Scan System spectofluorometer (Photon Technology), where the field was excited at 380 and 340 nm and the emission ratio was collected at 511 nm. Agonists were administered from x1000 stocks to maintain a constant volume of 1.5 ml.
Reverse Transcriptase-PCR to Identify AC IsoformsTotal RNA was extracted from cardiac fibroblasts grown to 8090% confluency on 15-cm plates using Trizol reagent (Invitrogen). A DNase reaction was performed to eliminate DNA contaminants and the RNA was reverse transcribed using Superscript II (Invitrogen) and poly-T priming. Individual isoform-selective primer pairs were used to amplify each isoform of AC. Primers were based on rat or murine sequences as previously described (24). PCR reactions with each primer pair were performed on cDNA template, genomic DNA (positive control), and minus RT (negative control) template using 35 cycles and an annealing temperature of 56 °C. Sequence analysis was used to confirm the identity of all PCR products.
Purification of Caveolin-enriched Membrane FractionsCardiac fibroblasts were fractionated using a detergent-free method adapted from Song et al. (25), and described previously (22). The faint light-scattering band resulting from sucrose density centrifugation was collected from the 535% sucrose interface. The bottom 4 ml of the gradient (45% sucrose) was collected as non-caveolar membranes.
Immunoblot AnalysisIndividual fractions or whole cell lysates were separated by SDS-polyacrylamide gel electrophoresis (Nu-PAGE, Invitrogen). Equal volumes of fractions were loaded so that each lane represents similar proportions from the cells, resulting in 10-fold lower amounts of protein loaded in the caveolin-enriched membrane fraction lanes. Proteins were transferred to a polyvinylidene difluoride membrane (Millipore) by electroblotting and probed with primary antibody (see "Materials"). Bound primary antibodies were visualized by chemiluminescence. Images of immunoblot analyses and RT-PCR are representative of at least 3 separate experiments. In some experiments, the optical density of bands was calculated using a digital imaging system and LabWorks software (UVP Bioimaging Systems) and reported as arbitrary optical density units.
Assay of Collagenase-sensitive [3H]Proline Incorporation[3H]Proline incorporation by cardiac fibroblasts was measured according to modified methods of Guarda et al. (26). Briefly, cells were transferred to 12-well plates then were serum starved in 0% FBS for 24 h. Proline incorporation was then assayed by adding 1 µCi/well of [3H]proline (PerkinElmer Life Sciences) along with, where indicated, drugs of interest and 2.5% FBS for 24 h. Cells were removed by trypsinization and protein precipitated overnight with 20% trichloroacetic acid. Protein was pelleted by centrifugation and washed 3 times with 1.0 ml of 5% trichloroacetic acid + 0.01% proline. Pellets were dissolved with 0.2 M NaOH and the solutions titrated to neutral pH with 0.2 M HCl. Collagenase (2 mg/ml: Worthington Biochemical Corp.) in Tris/CaCl2/N-ethymaleimide buffer was added to each tube and samples were allowed to incubate for 1 h at 37 °C. Samples were then placed on ice and proteins were precipitated with 10% trichloroacetic acid for 1 h. Samples were centrifuged at 14,000 rpm for 10 min and the collagenase-sensitive [3H]proline in the supernatant was determined by liquid scintillation counting.
Analysis of DataStatistical comparisons (t tests and one-way analysis of variance) and graphics were performed using Graph Pad Prism 3.0 (GraphPad Software).
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
To assess whether activation of Gq might mediate the effects of ANG II on AR signaling, we used adenoviral-mediated gene transfer of Gq to increase its expression in cardiac fibroblasts (Fig. 1B). Increased expression of Gq enhanced inositol phosphate accumulation by both ANG II (1.6-fold increase over control) and UTP (2.6-fold increase over control) and significantly enhanced the ANG II potentiation of ISO-stimulated cAMP (3.9 ± 0.5-fold over ISO alone) compared with control cells incubated with the null (PACCMVpLpA) virus (2.5 ± 0.3-fold over ISO alone). Thus, results in Fig. 1 are consistent with the conclusion that activation of GPCRs that activate Gq and PLC enhance
AR/Gs-mediated cAMP formation.
To determine whether signaling by G subunits generated by activation of Gq mediates the observed cross-talk, we used an adenovirus to express the C-terminal peptide of GRK2 (
ARK1). This peptide (
ARKct) binds to free G
subunits, and inhibits both G
signaling and GRK2 activation (27). We exposed cardiac fibroblasts to either
ARKct or null virus and measured cAMP accumulation stimulated by ISO with and without ANG II. Expression of
ARKct did not attenuate ANG II-induced potentiation of ISO-stimulated cAMP accumulation in cardiac fibroblasts (null virus: ISO 10.8 ± 0.2-fold over basal, ANG II + ISO 19.4 ± 2.2;
ARKct virus: ISO 9.2 ± 0.5, ANG II + ISO 20.8 ± 0.6) but did induce a 4-fold increase in ISO potency, consistent with its action blocking GRK2 activation (27).
Another possible pathway by which ANG II could effect cAMP production is via receptor tyrosine kinase signaling. However, cross-talk was not inhibited in cells treated with AG1478 (10 µM), a specific tyrosine kinase inhibitor (ANG II + ISO, 2.0 ± 0.1-fold over ISO alone; ANG II + ISO + AG1478 2.0 ± 0.1) or with PP2 (1 µM), a selective Src inhibitor (data not shown). These findings and those of Fig. 1 indicate that ANG II enhances ISO-mediated cAMP production via the action of Gq and PLC rather than G or tyrosine kinase-associated signaling, an important distinction given that ANG II activates both G protein and tyrosine kinase pathways in cardiac fibroblasts (28, 29).
Requirement for Elevation of Intracellular Ca2+ but Not PKCBecause Gq activation by ANG II elevates intracellular Ca2+ levels and activates PKC in cardiac fibroblasts (29), we examined the role of both responses as potential mechanisms by which ANG II influences cross-talk with Gs. We utilized the Ca2+ ionophore, ionomycin, to elevate intracellular Ca2+ levels in a concentration-dependent manner (Fig. 2A) that synergistically enhanced ISO-induced cAMP production (Fig. 2B) by 2.7 ± 0.4 to 4.0 ± 0.9-fold compared with ISO alone (indicated by the dashed line) using 10 and 100 nM ionomycin, respectively. Ionomycin alone did not alter basal cAMP production. Conversely, pharmacological inhibition of PKC by GF109203X (10 µM) failed to inhibit the ANG II-induced potentiation of the ISO response (ANG II + ISO 2.0 ± 0.1-fold over ISO alone, plus GF109203X 1.9 ± 0.1) but did reduce UTP-stimulated phosphoextracellular signal regulated kinase immunoreactivity (basal, 36 ± 6.1 OD; 100 µM UTP, 1037 ± 41; UTP + GF109203X, 180 ± 14.4). Similar negative results were seen with the PKC inhibitors calphostin C and staurosporine (data not shown). Thus, Gq/Gs cross-talk appears to be mediated by the effect of ANG II to elevate intracellular Ca2+ and not activation of PKC.
|
ANG II and Ionomycin Potentiate Forskolin-induced cAMP ProductionTo test whether the synergistic effects of ANG II on ISO-stimulated cAMP formation might occur via an effect distal to AR, we utilized forskolin, which directly activates AC (albeit with some dependence upon Gs). Forskolin (1 µM) alone stimulated cAMP production by 810-fold over basal levels, a level of response similar to that seen with 0.11 µM ISO. ANG II potentiated forskolin-induced cAMP levels (over forskolin alone) in a concentration-dependent manner in the range tested (1100 nM), reaching statistical significance at 10 and 100 nM (1.6 and 2-fold over forskolin alone, respectively, Fig. 3A). Likewise, ionomycin (100 nM) significantly potentiated forskolin-induced cAMP levels (from 8.3 ± 1.3-fold alone to 25.5 ± 7.4-fold in combination, relative to controls, Fig. 3B). These results indicate that the effect of ANG II is distal to the receptor and that ionomycin can mimic this effect by elevating intracellular Ca2+.
|
Effects of Ca2+ Store Release and Ca2+ Buffering on Gq-Gs Cross-talkWe next sought to determine whether Gq-Gs cross-talk might occur via Gq-promoted Ca2+ release from intracellular stores and if we could inhibit the cross-talk by buffering intracellular Ca2+. Thapsigargin, which inhibits the sarcoendoplasmic reticular Ca2+-ATPase pump, elevated intracellular Ca2+ levels in a concentration-dependent manner (Fig. 4A). Thapsigargin at a concentration of 100 nM was maximally efficacious in stimulating rapid Ca2+ release; this concentration also synergistically enhanced forskolin-induced cAMP production (Fig. 4B) with slight but statistically insignificant effects on basal cAMP production. We obtained similar results when these experiments were performed in the absence of extracellular Ca2+ (data not shown), indicating that Ca2+ release from internal stores was sufficient to observe the enhancement in cAMP formation. These results, together with those from the studies with ionomycin, indicate that potentiation of the cAMP signal can occur from elevation of intracellular Ca2+, either by storage release or influx promoted by a Ca2+ ionophore. Conversely, preincubation with 1 µM BAPTA/AM for 30 min prior to agonist stimulation prevented ANG II-induced Ca2+ transients (Fig. 4C) and blocked the synergistic effects of ANG II on forskolin-mediated cAMP production (Fig. 4D). Thus, elevation of intracellular Ca2+ is both necessary and sufficient for potentiation of cAMP production by ANG II. The results seen with forskolin indicate that Ca2+ works at the level of Gs/AC rather than AR.
|
AC Isoform Expression in Cardiac FibroblastsBecause the identity of AC isoforms expressed in cardiac fibroblasts could result in specific signaling characteristics, we sought to define the AC isoforms expressed in these cells and in particular to assess isoforms that are regulated by Ca2+. RT-PCR analysis using isoform-specific primers revealed that cardiac fibroblasts express mRNA for AC2, AC3, AC4, AC5, AC6, AC7, and AC8 (Fig. 5A). Each of the primer pairs amplified appropriate genomic DNA sequence but did not yield PCR products when RNA (no reverse transcriptase) was used as template (data not shown). PCR reactions using primers for AC1 and AC9 yielded products that were not of the expected size or sequence. We also conducted immunoblot analysis to detect expression of AC proteins. Because AC immunoreactivity in cardiac myocytes is enriched in buoyant, caveolin-rich fractions in a manner that improves immunological detection (22, 30), we fractionated cardiac fibroblasts to isolate caveolin-rich fractions and performed immunoblot analyses. As shown in Fig. 5B, caveolin-1 immunoreactivity was detected in buoyant fractions (caveolin-rich fraction) and was excluded from non-buoyant fractions. Immunoreactivity for AC3 and AC5/6 (the antibody used does not distinguish between AC5 and AC6) was detected primarily in caveolin-enriched membrane fractions, whereas immunore-activity of AC2, AC4, and AC7 was detected only in non-caveolin-enriched membrane fractions (Fig. 5B). No immunoreactivity was detected for AC8 or AC9 (Fig. 5B). Thus, cardiac fibroblasts express numerous AC isoforms but the isoforms appear to be differentially localized in caveolin-rich membrane microdomains.
|
AC Isoform Specificity of Gq-Gs Cross-talkWe hypothesized that expression of a Ca2+-calmodulin-stimulable isoform of AC was essential for the cross-talk we observe. Thus, we pretreated cardiac fibroblasts with a specific calmodulin inhibitor (10 µM calmidazolium) and found that it significantly attenuated cross-talk induced by ANG II plus ISO or by ionomycin plus forskolin (Fig. 6, A and B, respectively). We also measured Ca2+ and forskolin-stimulated AC activity in membrane preparations from cardiac fibroblasts with and without exogenously added calmodulin; 3 µM free Ca2+ in the presence of forskolin (10 µM) did not stimulate AC activity unless calmodulin (0.1 µM) was added to the reaction (Fig. 6C). This Ca2+-stimulable AC activity was inhibited by the inclusion of a calmodulin inhibitor (1 µM fluphenazine, which did not alter basal or forskolin-stimulated AC activity). These results indicate that Ca2+-calmodulin activation of AC is a plausible mechanism for the effect of ANG II and ionomycin to enhance cAMP production by AR.
|
We next probed whether it was possible to attenuate cross-talk between ANG II and cAMP production by altering the balance of Ca2+-stimulated versus Ca2+-insensitive AC isoforms by using an adenovirus to overexpress an isoform of AC that is not Ca2+-stimulable, AC6 (14). Incubation of cardiac fibroblasts with the AC6 adenovirus for 18 h resulted in a 2-fold increase in maximal forskolin-stimulated cAMP production (Fig. 6D). Based on evidence that the extent of AC overexpression is proportional to the increase in maximal forskolin response (13), the conditions used here approximately doubled the total cellular content of AC and generated cardiac fibroblasts expressing predominantly AC6. Immunoblot analyses indicated an appreciable increase in detectible AC5/6 protein in caveolin-enriched membrane fractions (Fig. 6D, inset). ANG II potentiation of forskolin-stimulated cAMP was unchanged by AC6 overexpression in terms of absolute amount of cAMP stimulated over forskolin alone (14.2 ± 1.4 pmol of cAMP/mg of protein in control cells versus 17.7 ± 8.8 in AC6 cells), but was significantly inhibited in terms of the fractional effect (p = 0.14, Fig. 6D). Thus, the potentiative effect of ANG II appeared not to extend to the overexpressed AC6. The inability of ANG II to potentiate forskolin-stimulated cAMP production to the same degree in AC6-overexpressing cells was not related to a limited capacity of cardiac fibroblasts to synthesize cAMP in this latter condition, as combined treatment of the cells with ISO and forskolin-stimulated cAMP levels to 27.6 ± 3.48 pmol/mg protein, a level nearly 3-fold higher than the combination of forskolin and ANG II. Thus, cross-talk is decreased in cells manipulated to express increased levels of AC6, an AC isoform not activated by Ca2+. Taken together, the data in Figs. 1, 2, 3, 4, 5, 6 implicate the central role of both Ca2+ influx and a Ca2+-calmodulin-stimulable AC isoform in mediating the observed Gq-Gs cross-talk.
Role of Cross-talk on Collagen SynthesisTo investigate the functional relevance of the cross-talk between ANG II signaling and the cAMP pathway, we assessed the synthesis of collagen by cardiac fibroblasts. Increased cellular cAMP levels can inhibit cardiac fibroblast-mediated production of collagen (31). Thus, we tested whether simultaneous incubation with cAMP-elevating agents and ANG II would inhibit collagen synthesis to a greater extent than incubation with cAMP-elevating agents alone. FBS-stimulated collagenase-sensitive [3H]proline incorporation was measured in cardiac fibroblasts as an index of collagen formation in the absence and presence of ANG II and in the absence and presence of ISO. 2.5% FBS stimulated collagen synthesis 2.6 ± 0.2-fold over basal levels. ANG II alone increased collagen synthesis (data not shown) but in the presence of 2.5% FBS induced no significant increase over FBS alone (Fig. 7A). ISO (100 µM) inhibited FBS-stimulated collagen synthesis (31 ± 9% inhibition, Fig. 7, A and B). However, in the presence of ANG II, ISO inhibited proline incorporation 48 ± 8%, an effect that was significantly different (p < 0.01) from serum-stimulated levels (Fig. 7, A and B). Moreover, the ANG II enhancement of the ISO effect was reduced to non-significant levels by chelation of Ca2+ (with 0.1 µM BAPTA/AM) or inhibition of calmodulin (with 0.1 µM calmidazolium). Thus, the data support the conclusion that the enhanced AR-mediated cAMP production that ANG II promotes via elevation of Ca2+ and signaling via calmodulin can blunt collagen synthesis. Taken together, the data suggest that coincidental Gs and Gq signals interact to modulate cardiac fibroblast signaling and function, and thereby promote a Gs-mediated inhibition of collagen synthesis.
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Our studies with forskolin allowed us to test the target at which Ca2+ acts to increase AR signaling. As a direct activator of AC, albeit requiring Gs activation for full effect (32), forskolin-stimulated cAMP formation was also activated by coincident signals that elevate intracellular Ca2+ (e.g. ANG II, ionomycin, or thapsigargin, Figs. 3 and 4). These results led us to focus on the role of AC in Gq-Gs cross-talk. We were surprised to discover that cardiac fibroblasts express mRNA for seven of the nine transmembrane AC isoforms (Fig. 5). This expression of multiple isoforms contrasts with results from cardiac myocytes, which appear to express predominantly 2 isoforms, AC5 and AC6 (15). Of the isoforms expressed in cardiac fibroblasts, the protein for AC3 appears to be predominantly expressed, particularly in caveolin-rich fractions (Fig. 5B).
Given its level of expression and action as a Ca2+-stimulable isoform, AC3 is likely critical for the cross-talk that we observe in cardiac fibroblasts. The other AC isoforms capable of being activated by Ca2+, AC1 and AC8, were not detected by immunoblot analysis (although AC8 mRNA was detected in PCR studies). Previous data using heterologously expressed protein indicate that AC3 stimulation by Ca2+ is dependent upon coincidental activation of AC by either Gs or forskolin (18), results consistent with what we observe in cardiac fibroblasts. An important role for a Ca2+-stimulable AC isoform in the observed cross-talk is inferred by data from vascular smooth muscle (33) and directly supported by our studies overexpressing AC6. By increasing AC6 expression, we altered the balance of AC isoform expression in favor of an isoform not stimulated by Ca2+ and found that the Gq-Gs cross-talk was fractionally reduced. Taken together, these data provide a clear-cut example of AC isoform expression imparting distinct signaling characteristics and regulation of cell function.
The membrane microdomains in which expression of AC isoforms occurs can influence cell responses (22, 34). Caveolae are a subset of lipid rafts, plasma membrane microdomains enriched in sphingolipid and cholesterol, and are the site of enrichment of many membrane-associated signaling molecules (35). We found that among numerous AC isoforms expressed in cardiac fibroblasts, only AC3 and AC5/6 localize in buoyant, caveolin-rich domains. These data are the first of which we are aware that show that native AC isoforms localize differently with respect to caveolae/lipid rafts. Smith et al. (36) recently reported that AC7 and AC8 heterologously expressed in HEK-293 cells localized differently with respect to caveolin-rich domains (AC8 in caveolar fractions, AC7 in non-caveolar fractions). The reason for the difference between the data from Smith et al. (36) and the present studies (we observe AC8 in non-caveolar fractions, Fig. 5) is unclear, but localization of GPCR and AC isoforms in caveolar fractions can be highly cell-specific and can differ between exogenously and endogenously expressed protein (24). More work is needed to understand the functional importance, if any, of the observed pattern of AC isoform expression in different plasma membrane microdomains of these cells.
Our studies of collagen synthesis (Fig. 7) suggest the physiological importance of Gq-Gs cross-talk. Agents that elevate cellular cAMP attenuate collagen synthesis by cardiac fibroblasts (5). In our studies, ISO alone (in the absence of a phosphodiesterase inhibitor) could significantly inhibit collagen synthesis, and the addition of ANG II, which produced a 2-fold increase in ISO-stimulated cAMP accumulation, led to a greater reduction in collagen synthesis. These results appear to conflict with observations that AR antagonists can reduce fibrosis in heart failure (37, 38). However, the beneficial effects of
AR blockade are likely dominated by improvements in cardiac contractility (which would lead to decreased levels of renin-angiotensin and possibly other hormones) and myocyte survival, which in turn influence ECM formation and turnover as well as fibroblast proliferation. Our findings are consistent with data on certain other Gs-linked agonist action on cardiac fibroblasts (31, 39) and with a previous report that the
AR antagonist, propranolol, inhibits collagen deposition in pulmonary fibroblasts, cells not subjected to the mechanical forces of a contractile organ (40). In addition, our studies examine fibroblasts over a short period (24 h) and in isolation, removed from circulating catecholamines and the mechanical forces of cardiac contraction.
In conclusion, the current results demonstrate that ANG II enhances AR signaling via activation of Gq and elevated intracellular Ca2+. We propose that a Ca2+-stimulable isoform of AC, likely AC3, is the regulatory site of this Ca2+/calmodulindependent potentiation and acts as an integrator of these signaling pathways. Moreover, cross-talk between the Gq and Gs pathways (i.e. enhanced cAMP production) appears to have functional consequences for regulation of the ECM in myocardium, acting to limit collagen synthesis.
![]() |
FOOTNOTES |
---|
Both authors contributed equally to this manuscript.
|| To whom correspondence should be addressed. E-mail: jgmeszar{at}neoucom.com.
1 The abbreviations used are: ECM, extracellular matrix; GPCR, G protein-coupled receptors; ANG II, angiotensin II; AC, adenylyl cyclase; PKC, protein kinase C; ISO, isoproterenol; AR,
-adrenergic receptors; BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid; DMEM, Dulbecco's modified Eagle's medium; FBS, fetal bovine serum; RT, reverse transcriptase; PLC, phospholipase C.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|