Bombyx Adipokinetic Hormone receptor Activates Extracellular Signal-Regulated Kinase 1 and 2 via G Protein-Dependent PKA and PKC but β-Arrestin-Independent Pathways

Abstract

Neuropeptides of the adipokinetic hormone (AKH) family are among the best-studied hormone peptides. They play important roles in insect hemolymph sugar homeostasis, larval lipolysis and storage-fat mobilization. Mechanistic investigations have shown that upon AKH stimulation, adipokinetic hormone receptor (AKHR) couples to a Gs protein and enhances adenylate cyclase activity, leading to intracellular cAMP accumulation. However, the underlying molecular mechanism by which this signaling pathway connects to extracellular signal-regulated kinase1/2 (ERK1/2) remains to be elucidated. Using HEK293 cells stably or transiently expressing AKHR, we demonstrated that activation of AKHR elicited transient phosphorylation of ERK1/2. Our investigation indicated that AKHR-mediated activation of ERK1/2 was significantly inhibited by H-89 (Protein kinase A inhibitor), Go6983, and GF109203X (Protein kinase C inhibitors), but not by U73122 (PLC inhibitor) or FIPI (PLD inhibitor). Moreover, AKHR-induced ERK1/2 phosphorylation was blocked by the calcium chelators EGTA and BAPTA-AM. Furthermore, ERK1/2 activation in transiently AKHR expressing HEK293 cells was found to be sensitive to pretreatment of pertussis toxin, whereas AKHR-mediated ERK1/2 activation was insensitive to siRNA-induced knockdown of β-arrestins and to pretreatment of inhibitors of EGFR, Src and PI3K. Based on our data, we propose that activated AKHR signals to ERK1/2 primarily via PKA- and calcium-involved PKC-dependent pathways. Our current study provides the first in-depth study defining the mechanisms of AKH-mediated ERK activation through the Bombyx AKHR.

Graphical Abstract

Insects, the largest group of animals on earth, play very important roles in their ecosystems through plant pollination, nutrient recycling, and maintenance of plant community composition and structure. In addition, they provide us with many useful materials, such as honey, silk, and varnish. The importance of insects as biomedical models is evident by the fact that many discoveries in digestion, muscle contraction, and important metabolic and developmental pathways in insects are applicable to vertebrate systems (1). Adipokinetic hormones (AKHs) produced by the insect corpora cardiaca are among the most extensively characterized peptide hormones, with almost 40 family members from most of the major insect orders (2–5). The structure of AKH was first identified from the locusts Locusta migratoria and Schistocerca gregaria (6). AKH is normally 8–10 amino acids long with a pyroglutamate at the N-terminus and an amidated C-terminus. In addition to the essential role of mobilization of metabolites during energy-expensive activities, such as flight and locomotion, AKH is involved in the control of carbohydrate homeostasis in the hemolymph of Drosophila and Bombyx larvae (7–8). Furthermore, it plays important roles in behavior, diapause, development and reproduction (9–10).

The receptor of AKH was first identified as a typical G protein-coupled receptor from the fruit fly Drosophila melanogaster and the silkworm Bombyx mori in 2002 (11) and later from the cockroach Periplaneta americana (12) and African malaria mosquito Anopheles gambiae (13). Previous biochemical characterization of isolated fat bodies suggested that AKH binds to its receptor and activates adenylyl cyclase via G proteins, which results in an increase of intracellular cAMP levels. In addition, AKH activates phospholipase C (PLC) to induce the release of Ca²⁺ from intracellular Ca²⁺ stores (14–15). However, the mechanistic details of AKHR signaling remain to be further elucidated. A Drosophila mutant with a knock-out of AKHR demonstrated that the adipokinetic hormone signaling pathway is important for chronic accumulation and acute mobilization of storage fat, along with the Brummer lipase, the homolog of mammalian adipose triglyceride lipase (ATGL) (16). Determination of the detailed regulation of energy storage and mobilization in insects will lead to a better understanding of the dysfunction of body fat regulation that results in obesity and lipodystrophy in humans.

In our previous study, we demonstrated that AKH induces phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) through AKHR in a dose-dependent manner (17). However, the mechanistic details of AKHR signaling remain largely unknown. We have therefore in this current study investigated the mechanistic basis of AKH-mediated ERK1/2 activation using HEK293 cells stably and transiently expressing AKHRs. The results demonstrate the involvement of multiple signaling pathways in signaling of AKH receptors to ERK1/2. These findings provide a foundation for future studies of the physiological role of AKHR-mediated ERK1/2 activation in energy homeostasis and mobilization in diapause, development and reproduction of Bombyx.

EXPERIMENTAL PROCEDURES

Materials.

Larvae and pupae of the silkworm strain Feng-Yi were kindly provided by Dr. Kerong He (Zhejiang Agricultural Institute). Cell culture media and G418 were purchased from Invitrogen (Carlsbad, CA). Fetal bovine serum was purchased from Hyclone (Beijing, China). The pEGFP-N1 and pCMV-Flag vectors were purchased from Clontech Laboratories, Inc. (Palo Alto, CA) and Sigma (St. Louis, MO), respectively. The kinase inhibitors were purchased from Sigma (St. Louis, MO) and Calbiochem (Cambridge, MA). Pertussis toxin (PTX) was purchased from Tocris (Missouri, MA). Primary antibodies for western blotting were purchased from Cell Signaling (Danvers, MA).

Cell culture and transfection.

The human embryonic kidney cell line (HEK293) was maintained in Dulbecco’s Modified Eagle Medium (DMEM, Invitrogen) supplemented with 10% heat-inactivated fetal bovine serum (Hyclone) and 4 mM L-Glutamine (Invitrogen). The AKHR cDNA plasmid constructs were transfected or co-transfected into HEK293 cells using Lipofectamine 2000 (Invitrogen) according to the manufacturer’s instructions. Forty-eight hours after transfection, selection for stable expression was initiated by the addition of G418 (800 μg/ml).

Molecular cloning, plasmid construction, and mutagenesis of Bombyx AKHR.

Total RNA was isolated from the fat body of B. mori pupae using the TRIzol reagent (Keygen, Nanjing, China), according to the manufacturer’s instructions. The cDNA was prepared with an AMV First Strand cDNA Synthesis Kit (Sangon, Shanghai, China), according to the manufacturer’s instructions. Corresponding PCR systems were performed as described previously (17). AKH receptor deletion mutants were constructed by using overlap extension PCR strategies. All of the constructs were constructed by ligation of the chimeric receptors or mutated receptors into the HindIII and XbaI or HindIII and KpnI sites of the pCMV-Flag or pEGFP-N1 vectors. All constructs were sequenced to verify the sequence and orientation.

Synthesis of small Interfering RNAs and siRNA transfection.

All small interfering RNAs (siRNAs) were chemically synthesized by Dharmacon RNA Technologies (Lafayette, CO). Arrestin2 and 3 siRNAs were purchased as a SMARTpool. The nonspecific control siRNA VIII (5’-AAACUCUAUCUGCACGCUGAC-3’) was used as the control for all siRNA experiments. The transfection protocol for arrestin siRNAs has been described previously(18). Forty-eight hours after transfection, cell were split for the indicated assay to take place the following day.

Immunoblot analysis.

The HEK293 cells stably or transiently expressing AKHR were seeded in 6-well plates and starved in serum-free media overnight. After stimulation with AKH, cells were lysed with RIPA lysis buffer (Beyotime, Haimen, China) on ice for 30 min, and the solubilized cells were pipetted into 1.5 ml-microcentrifuge tubes and centrifuged at 12,500 rpm for 15 min. After denaturing at 95°C for 10 min, the supernatants were removed and frozen at −80°C. Equal amounts of total cell lysate were separated by Tris-glycine SDS-PAGE (12%) and transferred to a PVDF membrane (Millipore) using transfer buffer (192 mM glycine, 25 mM Tris base, 0.01% SDS, 20% methanol, and pH 8.3). Membranes were blocked in TBST (20 mM Tris, pH 8.0, 150 mM NaCl, 0.075% Tween 20) containing 5% nonfat dry milk for 1 h at room temperature (RT) and then probed with rabbit monoclonal anti-p-ERK1/2 (1:2000, Cell Signaling) in TBST containing 5% bovine serum albumin (BSA) with agitation at 4°C overnight. The primary antibody was removed, and the sample was washed three times for 15 min each with TBST. The membranes were then probed with anti-rabbit HRP-conjugated secondary antibody (1:3000, Chemicon) in TBST plus 5% nonfat dry milk for 1 h at RT. Total ERK1/2 was assessed as a loading control after p-ERK1/2 chemiluminescence detection.

Internalization assay and fluorescence microscopy.

For the internalization assay, HEK293 cells stably expressing AKHR-EGFP were seeded onto glass coverslips coated with 0.1 mg/ml of poly-L-lysine and allowed to attach overnight under normal growth conditions. After treatment with AKH peptides at 37°C for 60 min, cells were fixed with 3% paraformaldehyde in PBS for 10 min at room temperature. The cells were mounted in mounting reagent (DTT/PBS/glycerol, 1:8:2) and visualized by fluorescence microscopy on a Zeiss LSM510 laser scanning confocal microscope attached to a Zeiss Axiovert 200 microscope using a Zeiss Plan-Apo 63×1.40 NA oil immersion lens.

For quantification of receptor internalization, ELISA was performed as described previously with some modifications (19). Briefly, forty-eight hours after transfection, cells in 48-well plates were stimulated with agonist for 60 min, fixed with 3.7% formaldehyde for 10 min at room temperature, and blocked for 45 min with 1% bovine serum albumin in TBS (20 mM Tris, 150 mM NaCl, pH 7.5). Cells were then incubated for 1 h with a 1:5000 dilution of a mouse anti-Flag M2 monoclonal antibody. Next, cells were washed three times with TBS and blocked again in 1% BSA/TBS for 15 min followed by incubation with horseradish peroxidase (HRP)-conjugated goat anti-mouse (1:5000 in 1% BSA/TBS) for 60 min. To each well, 200 μl HRP substrate (Sigma) was added, and the samples were incubated at 37°C for 20–30 min. Reactions were stopped by adding an equal volume of 1% SDS, and the sample absorbances were measured at 405 nm using a Bio-Rad microplate reader.

Flow cytometry analysis.

Approximately 2 × 10⁵ cells were washed with phosphate-buffered saline (PBS) supplemented with 0.5% BSA (FACS buffer) and incubated with10 μg·mL⁻¹ of FITC-labelled anti-Flag M2 monoclonal antibody (Sigma) in a total volume of 100 μl. After incubating for 60 min at 4°C, cells were pelleted and washed three times in FACS buffer. The cells were then fixed with 2% paraformaldehyde in FACS buffer and subjected to flow cytometry analysis on a FACScan flow cytometer (Coulter EPICS Elite, Coolten Corp., Hialeah, FL, USA).

Peptide synthesis.

The AKH peptides were prepared by solid-phase synthesis using Fmoc chemistry on a 430A peptide synthesizer (Applied Biosystems, Foster City, CA) and a 9050 Pepsynthesizer Plus (Perceptive Biosystems, Cambridge, MA). Crude peptides were purified by preparative reverse-phase high-performance liquid chromatography using a Dynamax-300 Å C18 25 cm × 21.4 mm ID column with a flow rate of 9 ml/min and two solvent systems of 0.1% TFA/H₂O and 0.1% TFA/acetonitrile. Fractions containing the appropriate peptide were pooled and lyophilized. The purity of the final product was assessed by analytical reverse-phase high-performance liquid chromatography, capillary electrophoresis, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

RESULTS

AKHR signals to ERK1/2 upon AKH stimulation.

To study AKH-stimulated ERK1/2 MAP kinase phosphorylation, HEK293 cells stably expressing the Bombyx AKH receptor were established using G418 selection. We previously reported that AKH stimulation led to a ligand concentration-dependent intracellular cAMP accumulation and Ca²⁺ mobilization, as well as an increase in ERK1/2 phosphorylation, with maximal activity at 5 min and a return to almost basal levels by 15 min. In this study, we quantitatively analyzed the ERK1/2 signaling pathway in AKHR-expressing HEK293 cells. As shown in Fig.1, AKH produces a dose-dependent increase in levels of ERK1/2 phosphorylation with an EC₅₀ of 0.50 nM.

Fig.1. Kinetics of AKHR-induced ERK1/2 activation.

(A) Time course of ERK1/2 activation following AKH stimulation in stable AKHR-expressing HEK293 cells. Cells were incubated with 100 nM AKH for the indicated time. (B) Dose-dependent curve of AKH-induced ERK1/2 activation. AKHR expressing cells were serum-starved for 1 hour and then incubated for 5 min with different concentrations of AKH ranging from 0.001 nM to 100 nM. Cell lysates were immunoblotted with phospho-specific (top lane) and non-specific (bottom lane) anti-ERK1/2 antibody, as described in the Materials and Methods. P-ERK1/2 immunoreactivity was quantified by Quantity One software, and data are normalized to the maximal phospho-ERK1/2 response. Data are expressed as the mean ± S.E.M (n = 3).

Involvement of cAMP/PKA in activation of ERK1/2.

Because cAMP plays an essential role in AKH-mediated biological function, its impact on ERK1/2 activation in HEK293 cells was investigated. AKHR couples to Gs protein and enhances adenylate cyclase activity, leading to intracellular cAMP accumulation. Increased cAMP levels lead to the activation of PKA, but whether PKA is involved in the activation of ERK1/2 is still unknown. To identify the role of PKA in AKHR-mediated ERK1/2 activation, HEK293 cells stably expressing Flag-AKHR were pretreated with or without PKA inhibitor H89 (10 μM) for 1 hour and then incubated with varying concentrations of AKH (0.1 nM to 10 nM) for 5 minutes. The addition of H89 significantly decreased ERK1/2 phosphorylation (Fig.2A). To determine if H89-mediated inhibition of ERK1/2 activation occurs in a dose-responsive manner, AKHR expressing cells were preincubated with varying concentrations of H89 (1 μM to 20 μM) for 1 hour prior to incubation with AKH (5 nM) for 5 minutes (Fig. 2B). ERK1/2 phosphorylation was detected by western blot. Inhibition of ERK1/2 is dependent on the concentration of H89, in agreement with Fig. 2A. Collectively, these results indicate that AKHR-mediated activation of ERK1/2 is PKA dependent.

Fig. 2. AKHR-mediated ERK1/2 phosphorylation is PKA-dependent.

(A) Effect of PKA inhibitor on AKHR-induced ERK1/2 activation. HEK293 cells stably expressing AKHR were preincubated with PKA inhibitor (H89, 10 μM) or with vehicle (control) at 37°C for 1 hourfollowed by stimulation with AKH (0.1 nM to 10 nM) for 5 min. (B) Dose-dependent effects of PKA inhibitor on AKHR-mediated activation of ERK1/2. Stable cells were pretreated with H89 (1 μM to 20 μM) for 1 hour and were then stimulated with AKH (5 nM, 5 min). Data are expressed as the mean ± S.E.M (n = 3). Statistical analysis was performed by a two-tailed Student’s t-test (*P < 0.05; **P < 0.01;***P < 0.001, versus counterpart control or none).

Ca²⁺ / PKC, but not PLC or PLD, is required for activation of ERK1/2.

Our previous results showed that AKH peptides did not affect Ca²⁺ fluxes in the parental HEK293 cells but elicited a rapid increase of Ca²⁺ in AKHR-expressing cells (17). We next determined whether or not the Ca²⁺-dependent PKC contributed to AKH-mediated activation of ERK1/2. First, calcium chelators EGTA and BAPTA-AM were used to reduce the concentration of extracellular or intracellular calcium, respectively, as shown in Fig. 3A–D, treatment of cells with EGTA and BAPTA-AM resulted in inhibition of AKH-stimulated ERK1/2 phosphorylation in a dose-dependent manner, suggesting that both intracellular and extracellular calcium are necessary for activation of ERK1/2. Next, PKC inhibitor Go6983 or GF109203X was added to cell cultures 60 min prior to stimulation with AKH. As illustrated in Fig. 3(E–H), inhibition of PKC did interfere with ERK1/2 phosphorylation after treatment of stably transfected cells with AKH. Addition of Go6983 or GF109203X led to a decrease in the activation of ERK1/2 stimulated by 5 nM AKH in a dose-dependent manner.

Fig. 3. AKHR-mediated ERK1/2 phosphorylation is via a Ca²⁺/PKC-dependent pathway.

(A,B) Effects of an extracellular calcium chelator on AKHR-mediated ERK1/2 activation. Stable cells were serum-starved for 1 hour and pretreated for 5 min with vehicle or the calcium chelator EGTA in serum-free medium. Cells were then stimulated with AKH for 5min, the concentration as indicated in the figure. (C,D) Effects of an intracellular calcium sequestrant BAPTA-AM on AKHR-mediated ERK1/2 activation. Stable cells were serum-starved with BAPTA-AM for 30 min prior to stimulation with AKH for 5min, the concentration as indicated in the figure. (E,G) HEK293 cells stably expressing AKHR were preincubated at 37°C for 1 hour with vehicle (0.1% DMSO, control) or with PKC inhibitor (Go6983, 1μM; GF109203X, 1μM) before stimulation for 5 min with AKH (0.1 nM to 10 nM). (F,H) Dose-dependent effects of PKC inhibitors on AKHR-mediated activation of ERK1/2. Stable cells were pretreated with Go6983 or GF109203X (0.03 μM to 3 μM) for 1 hour and were then stimulated with AKH (5 nM, 5 min). The data shown are representative of three independent experiments (mean ± S.E.M). Statistical analysis was performed by a two-tailed Student’s t-test (*P < 0.05; **P < 0.01;***P < 0.001, versus counterpart control or none).

We also evaluated the effect of the upstream signaling molecules of PKC, PLC and PLD in the AKHR-mediated ERK1/2 signaling pathway. The results showed that PLC inhibitors U73122 (10 μM) (Fig. 4A, S1) or ET-18-OCH₃ (100 μM) (S1) and PLD inhibitor FIPI (1μM) could not block the activation of ERK1/2 stimulated induced by 5 nM AKH (Fig. 4B, S1). Intriguingly, activated AKH receptors signal to ERK1/2 via PKC-dependent but PLC- and PLD- independent pathways, leading us to believe that calcium might play an important role in this process.

Fig.4. PLC and PLD are not involved in AKHR-mediated activation of ERK1/2.

Stable cells were serum-starved with PLC inhibitor (U73122, 10 μM) (A) or with PLD inhibitor (FIPI, 1 μM) (B) for 1 hour before stimulation with AKH (5 nM, 5 min). The data shown are representative of three independent experiments (mean ± S.E.M). Statistical analysis was performed by a two-tailed Student’s t-test (*P < 0.05; **P < 0.01;***P < 0.001, versus counterpart control).

Activation of ERK1/2 in transiently transfected 293 cells is PTX-sensitive.

For several GPCRs, such as the β2-adrenoceptor (20) and the mouse prostacyclin receptor (21), cannabinoid CB1(22) receptor dually coupled to the Gs-mediated cAMP accumulation and also coupled to Gi decreasing the levels of cAMP in forskolin-treated cells or the Gi-induced activation of ERK1/2.To explore the role of Gi proteins in the AKH-stimulated ERK1/2 signaling pathway, we examined the effect of pertussis toxin pretreatment on AKH-stimulated ERK1/2 activation in transiently expressing HEK293 cells. Cells were preincubated at 37°C overnight with or without pertussis toxin (PTX, 100 ng/ml) before stimulation for 5 min with different concentrations (0.1 nM to 10 nM) of AKH, as shown in Fig.5A. PTX(100ng/ml) can decrease ERK1/2 activation significantly. However, only higher concentration PTX(300ng/ml) can partially decreased AKH-stimulated ERK1/2 activation in stably transfected HEK293 cells (Fig.5B). To investigate why the differences between transiently and stably expressed AKHR cells, we detect the quantitative surface expression of transiently or stably expressed AKHR cells, results suggest transiently transfected HEK293 cells led to over-expression of the AKHR(Fig.6), this difference may make the cells sensitive to insensitive to PTX treatment.

Fig.5. Pertussis toxin inhibits AKHR-stimulated ERK1/2 activation in transiently expressing HEK293 cells.

HEK293 cells were transiently (A.) or stably (B.) transfected with AKHR and preincubated at 37°C overnight with or without pertussis toxin (PTX, 100 ng/ml) before stimulation for 5 min with AKH at the indicated concentration. The data shown are representative of three independent experiments (mean ± S.E.M). Statistical analysis was performed by a two-tailed Student’s t-test (*P < 0.05; **P < 0.01;***P < 0.001, versus counterpart control).

Fig. 6. Expression of AKHR in transiently or stably transfected HEK293 cells.

(A) The cell surface expression of the transiently or stably transfected HEK293 cells was analyzed by FACS. HEK293 cells were analyzed for cell surface expression of Flag-AKHR by flow cytometry using the anti-Flag mAb M2. Bars represent the mean fluorescence intensity for cells expressing Flag-AKHR. All data are shown as means ± S.E. from at least three independent experiments. (B) HEK293 cells transiently or stably expressing AKHR–EGFP (GFP) were detected by the confocal microscopy at the same conditions. The pictures shown are representative of most expressed cells.

Arrestins and other factors are not involved in phosphorylation of ERK1/2.

It is well established that GPCRs activate the ERK1/2 cascade via multiple mechanisms such as the β-arrestin-dependent pathway and the EGFR transactivation pathway (23). We therefore investigated whether or not additional pathways are involved in AKH-mediated ERK1/2 activation. Previous studies have shown that arrestins are involved in the ERK1/2 pathway mediated by some GPCRs(24). To examine whether the arrestins were required for the transient activation of ERK1/2 mediated by AKHR, we used siRNA to knock down the expression of arrestins. The siRNAs were transfected into HEK-293 cells, and the cells were evaluated for arrestins expression after three days. The endogenous expression of arrestin was effectively and specifically knocked down by the specific siRNA treatment but was unaffected in cells treated with nonspecific or control siRNA (Fig.7A). As shown in Fig. 7C, silencing of arrestin2 or arrestin3 had no effect on AKHR-mediated ERK1/2 activation, although knockdown of arrestin3 significantly blocked AKH-triggered internalization (Fig.7B). To further examine the role of arrestins in AKHR-activated ERK1/2 phosphorylation, we constructed an AKHR mutant with a deletion in the C-terminal domain from residue 363 to residue 343 (AKHRΔ343–363), which is defective in AKH-induced internalization. HEK293 cells expressing wild-type AKHR or AKHRΔ343–363 mutant were treated with 100 nM AKH, as indicated in Fig.8A, AKHRΔ343–363 mutant exhibited significant impairment of internalization (Fig.8B), but showed similar activation of ERK1/2 compared to wild type AKHR (Fig.8C). These results suggest that AKHR signals to the ERK1/2 pathway via arrestins and an internalization- independent pathway.

Fig. 7. Activated AKHR signals to ERK1/2 via arrestins-independent pathways.

(A) Knockdown of arrestins by siRNA. HEK-293 cells stably expressing AKHR were transfected with specific arrestin siRNAs or nonspecific control siRNA. Seventy-two hours after transfection, cells were harvested, and equal amounts of total cell lysate were separated by 12% SDS-PAGE, transferred to nitrocellulose, and incubated with the indicated antibodies. Blots were stripped and reprobed for tubulin to control for loading. Shown is a representative immunoblot from five independent experiments. (B) Seventy-two hr after transfection with specific arrestin siRNAs or nonspecific control siRNA, cells stably expressed AKHR-EGFP were stimulated with 1 μM AKH for 60 min and examined with fluorescence microscopy. The pictures shown are representative of three independent experiments.(C) Effect of siRNA-induced knockdown of arrestins expression on AKHR-mediated ERK1/2 activation. Activation of ERK1/2 in HEK293 cells transfected with siRNA for three days followed by incubation with 1nM AKH for different time was assessed by western blot as described in the Materials and Methods. Signals were quantified by densitometry and expressed as percentage of phosphorylated ERK1/2 of control obtained at 5 min. Graphs represent mean ± SEM from at least four independent experiments.

Fig. 8. Role of receptor internalization in AKHR-mediated ERK1/2 activation in HEK293 cells.

(A) Cells were transiently transfected with wild type AKHR or AKHR Δ343–363 mutant and, 48 hours after transfection, incubated with 100 nM AKH at 37°C for 30 min. After washing and fixing, cells were examined by confocal microscopy as described in the Materials and Methods. (B) HEK293 cells were transiently transfected with AKHR or AKHR Δ343–363. After 48 hours, cells were transferred to serum-free media, incubated for 1 hour, and subsequently treated with AKH (100 nM) for the indicated time. The data are expressed as fold increase over control p-ERK1/2 levels. The data shown are representative of three independent experiments (mean ±S.E.M).

Previous studies have demonstrated that the EGFR tyrosine kinase, the non-receptor RTK Src and phosphatidylinositol-3-kinase (PI3K) are also involved in GPCR-mediated ERK1/2 activation [22]. To assess the role of EGFR transactivation, Src and PI3K in the AKHR- induced ERK1/2 signaling pathway, MMP inhibitor (GM6001, 5μM), EGFR inhibitor (AG1478, 1μM), PI3K inhibitor (Wortmanmin, 1μM), and Src inhibitor (PP2,10μM) were used to pretreat cells. As shown in Fig. 9, treatment of cells with inhibitors U0126, H89 and Go6983 resulted in a significant decrease of AKH-induced phosphorylation of ERK1/2, whereas inhibitors GM6001, AG1478, Wortmanmin and PP2 showed no effect on ERK1/2 activation in response to AKH (Fig.9 and S2). These results suggest that it is unlikely for EGFR, PI₃K,MMP and Src to be involved in AKHR-mediated phosphorylation of ERK1/2.

Fig.9. Effect of pretreatment of EGFR, Src and PI3K inhibitors on AKHR-mediated ERK1/2 activation.

Serum-starved HEK293 cells stably expressing AKHR were pre-treated or not with the indicated inhibitors at 37°C prior to stimulation with agonist. Cells were pre-treated for 1 h with MEK inhibitor (U0126, 100 nM), PKA inhibitor (H89, 20 μM), PKC inhibitor (Go6983, 3μM), MMP inhibitor (GM6001, 5μM), EGFR tyrosine kinase inhibitor (AG1478, 1μM), PI3K inhibitor (wortmannin, 1μM) and Src-family tyrosine kinase inhibitor (PP2, 10 μM) and then exposed to AKH (5 nM) for 5 min. The data shown are representative of three independent experiments (mean ± S.E.M). Statistical analysis was performed by a two-tailed Student’s t-test (*P < 0.05; **P < 0.01;***P < 0.001, versus none).

DISCUSSION

Since the characterization of the first AKH peptide from the locusts Schistocerca gregaria and Locusta migratoria, more than 30 peptide members have been isolated from various species of insects. The essential functions of AKH peptides are analogous to vertebrate glucagons (25). In addition to its role in hyperglycemia and hyperlipidemia, AKH has a diverse array of functions including inhibition of protein synthesis (26), activation of glycogen phosphorylase (27–28), and inhibition of lipid synthesis (29). Recent research has emphasized the value of insects as a powerful model system for the study of human lipometabolic disorders (30). The AKH receptor has been identified to couple to Gs protein, resulting in intracellular cAMP accumulation upon agonist stimulation. In our previous study, we have demonstrated that AKH agonist triggers phosphorylation of ERK1/2 through the AKH receptor. However, the underlying molecular mechanism of its signaling to extracellular signal-regulated kinase1/2 (ERK1/2) remains to be elucidated.

Mitogen-activated protein kinase (MAPK) pathways regulate diverse processes ranging from proliferation and differentiation to apoptosis. Although it is well established that GPCRs play important roles in the regulation of intermediary metabolism, they have only recently been recognized as important mediators of cellular growth and differentiation via the MAPK pathway (23). It is now known that many GPCRs regulate MAPK cascades via distinct G protein, β-arrestin-dependent and EGFR transactivation signaling pathways, leading to activation of the extracellular signal-regulated kinases (ERKs), which function as transcriptional regulators. Therefore, characterization of the signaling pathways that stimulate MAPK activation through a particular receptor is essential to understand its role in physiology and pathology. In this study, the HEK293 cell line was selected as a model system for assessing signaling pathways involved in AKHR-mediated ERK1/2 activation because it is commonly used to characterize GPCR coupling to various signaling pathways. Using HEK293 cells either stably or transiently expressing AKHR combined with the addition of different kinase inhibitors, we demonstrated that upon stimulation with AKH peptides, activated AKH receptors signal to ERK1/2 via PKA- and PKC-dependent, but β-arrestin-independent signaling pathways.

Although the first observation of Gs protein-dependent effects of β₂ARs on ERK1/2 has shown that cAMP-mediated inhibition of ERK1/2 is due to the Gs-dependent activation of PKA, which leads to phosphorylation and inhibition of c-Raf1 (31–32), activation of ERK1/2 via a Gs-dependent pathway by the stimulation of the β₂AR has been demonstrated in HEK293 cells (33) and S49 lymphoma cells (34). The Gs-coupled hormone receptors β₃-adrenergic receptor (35), glucagon receptor (36), follicle-stimulating hormone receptor (37), and parathyroid hormone receptor (38) were found to mediate ERK1/2 phosphorylation via a Gs-dependent cAMP-PKA pathway. Furthermore, the dominant pathway for β₂AR activation of ERK1/2 in HEK293 cells has been shown to require activation of a Src family member that may be downstream of PKA (39). However, in this current study, we did not observe an inhibitory effect of Src inhibitor PP2 on AKHR-mediated ERK1/2 activation, thereby suggesting that Src is unlikely involved in AKH-mediated activation of ERK1/2. Most likely, AKHR activation evokes phosphorylation of ERK1/2 via a Ras-related small G protein, Rap1 and the downstream kinase, B-Raf (40).

Agonist-mediated AKHR activation elicits a rapid increase of intracellular cAMP and Ca²⁺(17). The origin of the AKH-mediated increase in Ca²⁺ has been discussed in detail (41). AKH stimulation causes an entry of Ca²⁺ via channels into the fat body of L. migratoria (42–43). In addition, inhibitors of Ca²⁺ channels, verapamil and nifedipine, effectively impair the adipokinetic effect of AKH in M. sexta, in vivo (44). The influx of extracellular Ca²⁺ plays an essential role in lipid release from fat bodies of locusts, in hyperprolinemia signaling in the fruit beetle (45–46) and exhibits a direct, positive effect on lipid mobilization in moths (44). These previous observations are consistent with the results presented in this report. The AKH-mediated ERK1/2 activation significantly decreased upon treatment of Go6983, a PKC inhibitor, but not with PLC inhibitor U73122 or PLD inhibitor FIPI, suggesting that PKC is involved in activation of ERK1/2 via a PLC- and PLD-independent pathway. Further investigation demonstrated that AKHR-induced ERK1/2 phosphorylation was blocked by the calcium chelators, EGTA and BAPTA-AM. Taken together, our data suggest that the Ca²⁺ influx is likely to play an important role in ERK1/2 activation.

Previous studies have demonstrated some receptors can dual couple Gs and Gi protein, led to multiple cAMP or ERK1/2 signalling patways(20–22) In this study, the activation of ERK1/2 by AKH stimulation is PTX-sensitive in transiently AKHR expressing HEK293 cells, but not in cells stably expressing AKHR. It is possible that HEK293 cells transiently expressing AKHR couple poorly to the Gs-protein pathway. Transient, higher-level expression of AKHR on the cell surface could affect coupling efficiency or specificity of receptor activation of Gs versus Gi (39). Additional evidence that AKHR coupled to Gi was provided by the finding that PTX pretreatment of AKHR-expressing cells resulted in an approximately 1.8 fold increase in AKH-induced activation of cAMP production, but only about 1.2 fold increase in stable expressed cells (Fig.S3). Taken together, these studies raised the possibility that both Gs and Gi proteins are involved in AKHR-mediated ERK1/2 signalling.

There is a growing body of evidence that indicates that there are two mechanisms with specialized roles in the activation ERK1/2: EGF receptor transactivation and activation of ERK bound to β-arrestin scaffolds (23). EGFR transactivation can mediate GPCR-induced ERK1/2 activation and therefore contribute to coupling between GPCRs and cell cycle progression (47–48). Previous studies have demonstrated that GPCR-induced EGFR transactivation is mediated by the release of precursor forms of the EGFR ligands HB-EGF, generated by activation of metalloproteinases of the zinc-dependent a disintegrin and metalloproteinase (ADAM) family (49–50). However, in HEK293 cells stably expressing Bombyx AKHR, AKH peptide-mediated ERK1/2 activation was not blocked by EGFR or MMP inhibitors. Moreover, we used RNA interference and an internalization-defective AKHR mutant to show that β-arrestins are not involved in AKH-induced ERK1/2 activation. It is well established that GPCRs employ multiple mechanisms to activate the ERK1/2 cascade. Depending on the receptor and cell type, one mechanism may predominate, or multiple mechanisms may be activated simultaneously (51). It seems likely that the signaling mechanism underlying AKHR-mediated ERK1/2 activation predominantly results from PKA- and PKC-dependent pathways.

Activated Gs-coupled hormone receptors are known to stimulate intracellular cAMP accumulation, leading to the activation of a cAMP dependent protein kinase, PKA. Hormone sensitive lipase (HSL) and perilipin are phosphorylated by PKA, resulting in catalysis in the breakdown of triglycerides and diglycerides (52–53). Experiments have revealed that, in vitro, activated ERK1/2 causes HSL phosphorylation at Ser600 (54), producing a hydrolytic activity similar to that observed when HSL is phosphorylated by PKA (55). Recently, β₃AR-stimulated lipolysis was found to require activation of PKA as well as the ERK1/2 pathway (56). Moreover, magnolol-induced lipolysis was inhibited by PD98059, an inhibitor of mitogen-activated protein kinase kinase (MEK) (57). The details of the hormonal regulation of the lipolytic process that takes place in the insect fat body remain to be elucidated. In our preliminary experiment, we demonstrated that treatment of MEK inhibitor U0126 resulted in a blockade of lipolysis in the fat body of Bombyx (data not shown). Further experiments are required to dissect the role of AKH-activated ERK1/2 in mediating various physiological events. Therefore, the present findings will help clarify the role of AKHR-mediated ERK1/2 activation in the regulation of the molecular events responsible for energy homeostasis and mobilization in insects and will also be helpful to better understand the dysfunction of body fat regulation that results in obesity and lipodystrophy in humans.

Fig.10. Schematic diagram of regulation of agonist-induced AKHR ERK1/2 activation in HEK 293 cells.

AKH binding to AKHR activates the Gs family of heterotrimeric G proteins, leading to the dissociation of G protein subunits. Gαs can enhance adenylate cyclase activity, leading to intracellular cAMP accumulation, and then cAMP will increase the activity of PKA; Activated receptor may contribute to open the calcium channel in the membrane, results in increased intracellular concentration of calcium, which led to activate PKC; Gi protein inhibitor PTX partially decreased activation of the ERK1/2 mediated by AKHR. These pathways will contribute to ERK1/2 activation, but whether ERK1/2 involved in AKHR - mediated fat lipolysis and how the ERK1/2 regulated the lipid metabolism, to be further studied.

Abbreviations :

GPCR

G protein-coupled receptor

AKH

adipokinetic hormone

MAPK

mitogen-activated protein kinase

ERK1/2

extracellular signal-regulated kinase1/2

PTX

pertussis toxin

PI₃K

phosphoinositide 3-kinase

EGFR

epidermal growth factor receptor

PKA

protein kinase A

PKC

protein kinase C

MMP

matrix metalloproteinase