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(Hypertension. 2000;35:292.)
© 2000 American Heart Association, Inc.

Scientific Contributions

Interleukin-1ß Regulates the Human Brain Natriuretic Peptide Promoter via Ca²⁺-Dependent Protein Kinase Pathways

Quan He; Margot C. LaPointe

From the Hypertension and Vascular Research Division, Henry Ford Hospital, Detroit, Mich.

Correspondence to Dr Margot C. LaPointe, Hypertension and Vascular Research Division, Henry Ford Hospital, 2799 W Grand Blvd, Detroit, MI 48202-2689. E-mail mclapointe{at}aol.com

	Abstract

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Abstract—We have shown that interleukin-1ß (IL-1ß) activates the human brain natriuretic peptide (hBNP) promoter via a transcriptional mechanism. Others have reported that changes in intracellular calcium (Ca²⁺) mediate the action of IL-1ß. We questioned whether Ca²⁺ and Ca²⁺-dependent pathways mediate IL-1ß regulation of the hBNP promoter in cardiac myocytes. The hBNP promoter (-1818 to +100) coupled to a luciferase cDNA reporter gene was transferred into neonatal cardiac myocytes. Cells were then treated with agents that modify Ca²⁺ levels or inhibit Ca²⁺-dependent kinases, and luciferase activity was measured as an index of hBNP promoter activity. The Ca²⁺ ionophore A23187 increased hBNP promoter activity; however, neither EGTA nor nifedipine reduced IL-1ß–stimulated promoter activity. Long-term treatment with thapsigargin, which depletes intracellular Ca²⁺ stores, decreased basal promoter activity and blocked the effect of IL-1ß. Inhibition of protein kinase C completely blocked IL-1ß–stimulated hBNP promoter activity, whereas inhibition of Ca²⁺/calmodulin-dependent kinase II decreased promoter activity by 40%. In contrast, inhibition of the Ca²⁺-regulated phosphatase calcineurin by cyclosporin A had no effect. These data suggest that (1) Ca²⁺ activates the hBNP promoter; (2) release of Ca²⁺ from intracellular stores is important to IL-1ß regulation of the hBNP promoter, but transport via voltage-sensitive Ca²⁺ channels is not; (3) protein kinase C and Ca²⁺/calmodulin-dependent kinase II mediate the action of IL-1ß; and (4) the phosphatase calcineurin is not involved in IL-1ß regulation of the hBNP promoter. Thus, Ca²⁺ and Ca²⁺-dependent pathways are critical to IL-1ß regulation of the hBNP promoter.

Key Words: protein kinases • calcium • calcineurin • myocytes

	Introduction

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Interleukin-1ß (IL-1ß) is a potent inflammatory cytokine that is secreted by a variety of cells and affects nearly every tissue and organ system.¹ Increased levels of circulating IL-1ß and IL-1ß mRNA are found in patients with congestive heart failure² and in a rat model of postinfarction heart failure.³ Thus, circulating levels of IL-1ß seem to correlate with myocardial injury and dysfunction. The biological action of IL-1ß is mediated via a specific membrane receptor that couples to multiple signaling cascades and results in altered gene expression.⁴ IL-1ß induces the expression of inducible nitric oxide synthase,^{5 6 7} cyclooxygenase-2,^{6 8 9} metalloproteinases,⁸ interleukin-6,⁸ adhesion molecules,¹⁰ atrial natriuretic peptide (ANP),¹¹ and brain natriuretic peptide (BNP).¹² Upregulation of genes by IL-1ß involves a number of signaling pathways in different types of cells, including the 3 mitogen-activated protein kinases (MAPKs) p42/44 MAPK, c-Jun N-terminal kinase, and p38 MAPK.^{6 8 10 12 13 14}

Another important component of the IL-1ß response is increased intracellular calcium (Ca²⁺) concentration. IL-1ß–induced changes in intracellular Ca²⁺ have been documented in chondrocytes,¹⁵ vascular smooth muscle cells,¹⁶ and neonatal cardiac myocytes¹⁷ and can involve the release of Ca²⁺ from intracellular stores or the modulation of Ca²⁺ channels. These alterations in intracellular Ca²⁺ are linked to changes in kinase activity and gene expression. Studies have shown that IL-1ß can modify the expression of proteins that regulate Ca²⁺ handling by neonatal ventricular myocytes (NVMs).¹¹ This might account for the abnormal cardiac myocyte function in the infarcted heart; myocytes isolated from infarcted rat hearts have a lower peak cytosolic Ca²⁺ concentration during contraction than do control myocytes.¹⁸ IL-1ß signaling has been shown to involve Ca²⁺-regulated kinases, such as protein kinase C (PKC).¹⁹ In addition, PKC,^{20 21} Ca²⁺/calmodulin-dependent kinase II (CaMKII),^{22 23} and the Ca²⁺-activated phosphatase calcineurin^{24 25 26 27} have been implicated in the regulation of cardiac hypertrophy and heart failure.

BNP, 1 of 3 members of the natriuretic peptide family, is a cardiac hormone with diuretic, natriuretic, and vasodilator properties. Circulating levels of BNP are elevated as a result of cardiac infarction, hypertrophy, or heart failure.^{28 29 30} Plasma BNP levels are also used as a biochemical marker of left ventricular dysfunction and a neuroendocrine marker of heart failure.^{31 32} We previously reported that IL-1ß induces human BNP (hBNP) promoter activity in part through the p38 MAPK pathway in cultured NVMs.¹² On the basis of these studies, we questioned whether IL-1ß activation of hBNP promoter activity involves Ca²⁺ and Ca²⁺-dependent kinases and phosphatases. These studies used pharmacological approaches for the inhibition of kinases and phosphatases in transiently transfected NVMs.

	Methods

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Cell Culture
Ventricular myocyte–enriched cultures were generated from 1- to 2-day-old Sprague-Dawley rat pups (Charles River) as described previously,³³ according to a protocol approved by the Henry Ford Hospital Committee for the Care and Use of Experimental Animals. NVMs were cultured in DMEM (GIBCO) containing 100 U/mL penicillin, 100 µg/mL streptomycin, 2 mmol/L glutamine, 0.1 mmol/L bromodeoxyuridine, and 10% FBS (HyClone) for 40 hours and then maintained under serum-free conditions (DMEM plus antibiotics, glutamine, 5 mg/L insulin and transferrin, and 2.5 mg/L selenium).

Transfection and Luciferase Assay
Transfection and luciferase activity were assayed as described previously.³⁴ The chimeric hBNP promoter coupled to a luciferase reporter gene (-1818hBNPLuc) has been described previously.³⁴ -1818hBNPLuc (1 µg/3x10⁶ freshly prepared cells) was transfected by electroporation. After transfection, the cells were aliquoted into 3 wells of a 12-well plate (1x10⁶ cells/well) in medium that contained serum. At 40 hours after transfection, the medium was changed to serum-free DMEM; 24 hours later, cells were treated with IL-1ß (5 ng/mL, 3x10^-10 mol/L). Inhibitors were added 1 hour before IL-1ß, after which the cells were incubated with the inhibitor plus IL-1ß for 24 hours. We tested the specificity of the inhibitors by assaying their effects in the presence of activators of the hBNP promoter in addition to IL-1ß. The PKC inhibitor GF109203X was inactive when used with cAMP-stimulated cells. The CaMKII inhibitor had no effect when cells were treated with endothelin, whereas the calcineurin-specific inhibitor cyclosporin A (CsA) potentiated the effect of endothelin (data not shown).

NVMs were harvested, lysed, and assayed for luciferase activity (Luciferase Assay System; Promega) using an OptoComp 1 luminometer (MGM) according to the manufacturer’s protocol. Duplicate aliquots of cell lysate from triplicate wells were averaged, and luciferase activity was normalized to protein levels as described previously.³⁴ Data were expressed as the mean±SEM and analyzed by t test or 1-way ANOVA with multiple pairwise comparisons according to the Student-Newman-Keuls method. A value of P<0.05 was considered significant.

Chemicals
IL-1ß was obtained from Sigma Chemical Co and Promega. The ionophore A23187, GF109203X, KN-62, thapsigargin, and nifedipine were purchased from BIOMOL. EGTA and CsA were purchased from Sigma Chemical Co. Routine laboratory supplies and chemicals were obtained from Fisher Chemical and Sigma Chemical Co.

	Results

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Effect of Ca²⁺ on the hBNP Promoter
To increase intracellular Ca²⁺ levels, NVMs transfected with -1818hBNPLuc were treated with the Ca²⁺ ionophore A23187 for 24 hours at doses ranging from 1 to 10 µmol/L, resulting in dose-dependent activation of the hBNP promoter (Figure 1A). We next studied possible sources of the Ca²⁺ involved in regulation of the promoter. Thapsigargin inhibits sarcoplasmic reticulum Ca²⁺-ATPase, thus preventing uptake of Ca²⁺ by storage sites. Long-term treatment results in depletion of intracellular Ca²⁺ stores.³⁵ We treated NVMs with 0.1 µmol/L thapsigargin and found that it decreased both basal and IL-1ß–stimulated hBNP promoter activity (Figure 1B). We questioned how Ca²⁺ enters the cell. To determine whether IL-1ß stimulates Ca²⁺ entry through voltage-sensitive Ca²⁺ channels, NVMs were treated with 10 µmol/L nifedipine, but it had no effect on IL-1ß–stimulated hBNP promoter activity (Figure 1C), nor did adding 1 mmol/L EGTA to the medium to chelate extracellular Ca²⁺ (Figure 1C). These data suggest that IL-1ß releases Ca²⁺ from intracellular stores rather than activating sustained Ca²⁺ flux through Ca²⁺ channels.

View larger version (12K): [in this window] [in a new window]   Figure 1. Effect of modulators of Ca2+ on hBNP promoter activity. The y axis represents relative luciferase activity (fold increase versus control [CONT], which is arbitrarily set to 1), and the x axis represents treatment. A, Ionophore A23187 dose-response curve. Each bar represents the mean±SEM of 3 separate experiments. B, Effect of thapsigargin (THAPS). Transfected NVMs were first treated with 0.1 µmol/L THAPS for 1 hour; then IL-1ß (IL) was added for 24 hours. Each bar represents the mean±SEM of 3 separate experiments. **P<0.01, IL-1ß vs THAPS plus IL-1ß. C, Effect of EGTA and nifedipine (NIFED; 10 µmol/L). EGTA was used at a concentration of 1 mmol/L. Each bar represents the mean±SEM of 4 to 6 separate experiments. Values for P=NS, IL-1ß (IL) vs EGTA/IL-1ß or NIFED/IL-1ß.

Ca²⁺-Regulated Kinases in Regulation of the hBNP Promoter
Release of Ca²⁺ from intracellular stores is modulated by lipid products generated by enzymes and kinases at the cytoplasmic surface of the cell membrane. One such enzyme is phospholipase C, which results in the production of 2 important second messengers: inositol triphosphate (IP₃) and 1,2-diacylglycerol (DAG). IP₃ releases Ca²⁺ from intracellular stores, and DAG activates a number of PKC isoforms. Because PKC has been implicated in IL-1ß signaling,¹⁹ we tested its involvement in IL-1ß regulation of the hBNP promoter. The hBNPLuc-transfected NVMs were treated with 1 µmol/L GF109859X, a PKC-specific inhibitor (Figure 2), and then IL-1ß was added for 24 hours. Inhibition of PKC totally prevented IL-1ß–stimulated promoter activity.

View larger version (13K): [in this window] [in a new window]   Figure 2. Role of PKC in IL-1ß regulation of the hBNP promoter. The y axis represents luciferase activity (fold increase versus control [CONT], which is arbitrarily set to 1), and the x axis represents treatment. GF indicates the PKC inhibitor GF109859X (1 µmol/L). Each bar represents the mean±SEM of 4 separate experiments. **P<0.01 vs IL-1ß (IL).

Several studies have implicated a Ca²⁺/calmodulin-dependent kinase, CaMKII, in the regulation of cardiac hypertrophy and the development of heart failure.^{22 36 37} CaMKII has been shown to target transcription factors binding to cAMP- and serum-response elements (SREs) in the promoters of several genes³⁸ and has been implicated in the regulation of ANP gene expression in NVMs.²² We tested the involvement of CaMKII in IL-1ß regulation of the hBNP promoter using the inhibitor KN-62 (1 µmol/L) and found that IL-1ß–stimulated hBNP promoter activity was reduced by 40% (Figure 3).

View larger version (14K): [in this window] [in a new window]   Figure 3. Role of CaMKII in IL-1ß regulation of hBNP promoter activity. The y axis represents luciferase activity (fold increase versus control [CONT]), and the x axis represents treatment. KN indicates CaMKII inhibitor KN-62 (1 µmol/L). Each bar represents mean±SEM of 5 separate experiments. *P<0.05 vs IL-1ß (IL).

Role of Calcineurin
The Ca²⁺-dependent phosphatase calcineurin has been implicated in the regulation of cardiac hypertrophy.^{24 26 27} Calcineurin targets a transcription factor called NF-AT, which has been implicated in the development of cardiac hypertrophy and heart failure^{26 27} as well as phenylephrine (PE) stimulation of the hBNP promoter.²⁷ Thus, we questioned whether IL-1ß regulation of the hBNP promoter involves calcineurin. Treatment of NVMs with the calcineurin inhibitor CsA (10 µmol/L) had no effect on IL-1ß regulation of the hBNP promoter (Figure 4).

View larger version (15K): [in this window] [in a new window]   Figure 4. Role of calcineurin in IL-1ß regulation of the hBNP promoter. The y axis represents luciferase activity (fold increase versus control [CONT]), and the x axis represents treatment. Each bar represents the mean±SEM of 4 separate experiments. P=NS, IL-1ß (IL) vs IL-1ß plus the calcineurin inhibitor cyclosporin A (CsA) (10 µmol/L).

	Discussion

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Previous studies have shown that IL-1ß increases intracellular Ca²⁺ in a variety of cells, including NVMs.¹⁷ We believe our data are the first to show that IL-1ß regulation of the hBNP promoter involves the release of Ca²⁺ from intracellular stores and the activation of several kinases. The action of IL-1ß is mediated by PKC and CaMKII. In contrast to the hypertrophic growth factor PE,²⁷ IL-1ß stimulation of the hBNP promoter does not involve the phosphatase calcineurin.

Increases in intracellular Ca²⁺ in contractile cells occur primarily via 2 mechanisms: release from intracellular compartments (sarcoplasmic reticulum) or entry from the extracellular environment through voltage-dependent L-type Ca²⁺ channels. Ligand/receptor interactions that activate phospholipase C and thereby generate IP₃ result in IP₃-dependent stimulation of Ca²⁺ release from intracellular stores, producing a transient signal. The Ca²⁺ stores are refilled by a thapsigargin-sensitive Ca²⁺-ATPase, which removes Ca²⁺ from the cytosol.³⁹ Our data showing that thapsigargin inhibits IL-1ß–stimulated hBNP promoter activity implicate a mechanism involving the release of Ca²⁺ from an intracellular store. This conclusion is reinforced by the fact that the Ca²⁺ chelator EGTA and the L-type Ca²⁺-channel blocker nifedipine had no effect on IL-1ß–induced hBNP promoter activation. Although it is possible that EGTA did not chelate all of the Ca²⁺ in the culture medium (which contains 1.4 mmol/L), and thus there was some entry through the plasma membrane, on the basis of our experiments with nifedipine, it seems unlikely that the L-type Ca²⁺ channel was involved. Likewise, Luo et al¹⁵ showed that IL-1ß–stimulated increases in Ca²⁺ in chondrocytes are totally abolished with thapsigargin but only partially affected by EGTA.

The results of the present study indicate that PKC is a critical mediator of IL-1ß activation of the hBNP promoter. There are 11 isoforms of PKC, which differ in their requirements for Ca²⁺ and DAG and their subcellular distribution.⁴⁰ The classic PKC isoforms (Ca²⁺ and DAG dependent) are most abundant in the heart and have been cited in the regulation of cardiac-specific genes and myocyte hypertrophy.²⁰ A recent study showed increased PKC activity in the failing human heart.²¹ PKC phosphorylates transcription factors such as c-Jun and alters gene transcription. In addition, it can activate the p42/44 MAPK signaling cascade. We have previously demonstrated that PKC is involved in the regulation of both ANP and BNP mRNA in NVMs.³³ Preliminary data from our laboratory indicate that c-Jun activates the hBNP promoter, but an activator protein-1–like element in the proximal promoter is not involved (Q. He, G. Wu, and M.C. LaPointe, unpublished data). Thus, one of the functions of PKC in IL-1ß regulation of the hBNP promoter could be activation of Jun and Jun family members.

A number of studies have suggested that the Ca²⁺ binding protein calmodulin participates in cardiomyocyte growth regulation.³⁶ The calmodulin-regulated kinase CaMKII appears to be involved in regulation of ANP gene expression through a proximal SRE,²² and a specific isoform is increased in the left ventricle in patients with dilated cardiomyopathy.³⁷ Our data show that the CaMKII-specific inhibitor KN-62 attenuates IL-1ß stimulation of the hBNP promoter. Although the molecular mechanism by which CaMKII targets the promoter remains to be determined, it may involve activator protein-1, SRE, or cAMP-response element sites in the hBNP promoter. Because CaMKII is increased in diseased hearts, it may play a role in upregulation of the BNP gene during hypertrophy and development of heart failure.

The role of the Ca²⁺-regulated phosphatase calcineurin in the regulation of cardiac hypertrophy is controversial, and its role in left ventricular hypertrophy in vivo seems to be dependent on the animal model used.^{24 26 41 42} In NVMs, calcineurin has been implicated in PE regulation of the hBNP promoter through targeting of a distal NF-AT site.²⁷ In contrast, our data indicate that the calcineurin-specific inhibitor CsA has no effect on IL-1ß–induced hBNP promoter activity, and preliminary experiments suggest that it has no effect on endothelin regulation of the promoter (Q. He and M.C. LaPointe, unpublished data). In contrast to hypertrophic growth factors such as PE and endothelin, IL-1ß has atypical effects on NVMs in that it does not induce all aspects of the hypertrophic phenotype (ie, increased protein synthesis, fetal contractile protein gene expression, and ANP and BNP gene expression).^{7 43} In low-density NVM cultures, IL-1ß has been shown to increase protein synthesis but has no effect on skeletal -actin and ß-myosin heavy chain gene expression, which is normally upregulated in hypertrophy.⁴³ In contrast, workers at our laboratory have shown that in high-density NVM cultures, IL-1ß has no effect on protein synthesis⁷ but activates the BNP promoter, a marker gene of hypertrophy, infarction, and heart failure.¹² Because in our laboratory IL-1ß regulates BNP gene expression without concomitant increases in protein synthesis, IL-1ß regulation of the BNP gene likely involves some signaling pathways distinct from hypertrophic growth factors. One distinct difference is the absence of a role for calcineurin. Thus our data, coupled with those from previous studies,^{41 42} suggest that calcineurin may not play a universal role in the upregulation of genes in the heart during hypertrophy and heart failure.

In the cell, the Ca²⁺ level and its spatial and temporal distribution can affect different signaling molecules, and the balance between kinase and phosphatase activation will control the overall cell response to stimuli. In the present study, Ca²⁺ regulated the hBNP promoter and mediated the action of IL-1ß. Our data support a mechanism by which lipid products stimulate the release of intracellular Ca²⁺, which in turn activates PKC and CaMKII. These multiple signals may target cis-elements in the hBNP promoter and activate transcription. The increase in cytokines and signaling molecules such as PKC and CaMKII in diseased hearts may partially explain why BNP gene expression is upregulated in infarction, hypertrophy, and heart failure.

	Acknowledgments

 
This work was supported by NIH grants HL-03188 and HL-28982 (Dr LaPointe). We thank Fangfei Wang for her excellent technical assistance.

Received September 13, 1999; first decision October 21, 1999; accepted October 29, 1999.

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