Follicle-stimulating hormone enhances hepatic gluconeogenesis by GRK2-mediated AMPK hyperphosphorylation at Ser485 in mice
Increased serum follicle-stimulating hormone (FSH) is correlated with fasting hyperglycaemia. However, the underlying mechanism remains unclear. Because excessive hepatic gluconeogenesis is a major cause of fasting hyperglycaemia the present study investigated whether FSH increases hepatic gluconeogenesis in mice.
Ovariectomised mice supplemented with oestradiol (E2) to maintain normal levels of serum E2 (OVX+E2 mice) were injected with low or high doses of FSH. We knocked out Crtc2, a crucial factor in gluconeogenesis, and Fshr to discern their involvement in FSH signalling. To evaluate the role of the G-protein-coupled receptor (GPCR) kinase 2 (GRK2), which could affect glucose metabolism and interact directly with non-GPCR components, a specific GRK2 inhibitor was used. The pyruvate tolerance test (PTT), quantification of PEPCK and glucose-6-phosphatase (G6Pase), key enzymes of gluconeogenesis, GRK2 and phosphorylation of AMP-activated protein kinase (AMPK) were examined to evaluate the level of gluconeogenesis in the liver. A nonphosphorylatable mutant of AMPK Ser485 (AMPK S485A) was transfected into HepG2 cells to evaluate the role of AMPK Ser485 phosphorylation.
FSH increased fasting glucose (OVX+E2+high-dose FSH 8.18 ± 0.60 mmol/l vs OVX+E2 6.23 ± 1.33 mmol/l), the PTT results, and the transcription of Pepck (also known as Pck1; 2.0-fold increase) and G6pase (also known as G6pc; 2.5-fold increase) in OVX+E2 mice. FSH also enhanced the promoter luciferase activities of the two enzymes in HepG2 cells. FSH promoted the membrane translocation of GRK2, which is associated with increased AMPK Ser485 and decreased AMPK Thr172 phosphorylation, and enhanced the nuclear translocation of cyclic AMP-regulated transcriptional coactivator 2 (CRTC2). GRK2 could bind with AMPK and induce Ser485 hyperphosphorylation. Furthermore, either the GRK2 inhibitor or AMPK S485A blocked FSH-regulated AMPK Thr172 dephosphorylation and gluconeogenesis. Additionally, the deletion of Crtc2 or Fshr abolished the function of FSH in OVX+E2 mice.
The results indicate that FSH enhances CRTC2-mediated gluconeogenesis dependent on AMPK Ser485 phosphorylation via GRK2 in the liver, suggesting an essential role of FSH in the pathogenesis of fasting hyperglycaemia.
KeywordsAMPK CRTC2 FSH Gluconeogenesis GRK2 Liver
AMP-activated protein kinase
Calcium/calmodulin-dependent protein kinase
Cyclic AMP response element
Cyclic AMP response element-binding protein
Cyclic AMP-regulated transcriptional coactivator 2
Follicle-stimulating hormone receptor
GPCR kinase 2
Glycogen synthase kinase 3β
Haematoxylin and eosin
Insulin tolerance test
Insulin sensitivity index
Liver kinase B1
Mitogen-activated protein kinase
Protein kinase A
Pyruvate tolerance test
Several metabolic disorders, including bone, glucose and lipid metabolic dysregulation, are observed in postmenopausal women. The postmenopausal state is associated with hyperglycaemia independent of age and certain metabolic factors (such as BMI and triacylglycerol levels) among women without diabetes . Decreased oestrogen levels caused by ovarian failure could partly explain high glucose levels , but oestrogen has also been shown to increase glucose levels [3, 4], demonstrating a controversial effect of oestrogen and indicating the involvement of other mechanisms. In addition to the characteristic reduction in oestrogen levels postmenopause, follicle-stimulating hormone (FSH) levels are elevated. A clinical study of women with primary ovarian insufficiency indicated a positive association between FSH and fasting serum glucose . However, the precise mechanism underlying the association remains unclear. FSH is a gonadotropin secreted by the pituitary gland that acts via the FSH receptor (FSHR), a G-protein-coupled receptor (GPCR) expressed exclusively in sexual glands . Recently, multiple studies have found that extragonadal tissues, such as adipose tissue , the biliary epithelium , liver tissue  and bone , express functional FSHR.
To identify the mechanism underlying FSH regulation, we focused on GPCR kinase 2 (GRK2). Recent evidence suggests that GRK2 may play a role in regulating insulin signalling, mainly by directly interacting with non-GPCR components [9, 10]. This finding is in contrast to those of the classical definition of GRK2 function of desensitising GPCRs . However, which of the GRK2 functions is dominant in FSHR signalling remains unknown.
The liver is a primary organ involved in gluconeogenesis, a main mechanism through which humans maintain blood glucose levels . Excessive hepatic gluconeogenesis is a major contributor to hyperglycaemia observed in type 2 diabetes mellitus . Nuclear translocation of cyclic AMP-regulated transcriptional coactivator 2 (CRTC2) can stimulate the transcription of two rate-limiting enzymes involved in gluconeogenesis, PEPCK and G6Pase (also known as G6PC), a key step in increasing hepatic gluconeogenesis [14, 15]. AMP-activated protein kinase (AMPK), an important intracellular energy sensor, inactivates CRTC2 and blocks its nuclear translocation . Previous studies have shown that AMPK activity could be regulated by phosphorylation sites, such as Ser485 (inactivation) and Thr172 (activation) [17, 18]. In this study, we investigated whether FSH induces gluconeogenesis dysfunction in the liver and whether AMPK and CRTC2 are involved.
More specifically we explored the effect of FSH on hepatic gluconeogenesis. We hypothesised that FSH, via FSHR, targets GRK2 and thereby induces the CRTC2-mediated transcription of PEPCK and G6Pase, which is dependent on AMPK inactivation by Ser485 hyperphosphorylation.
Eight-week-old C57BL/6N female mice were obtained from Beijing Vital River Laboratory Animal Technology Company (Beijing, China). Mice were kept in a 12:12 h light:dark cycle at 25 ± 0.5°C and 50–60% (vol./vol.) humidity and fed a standard diet with water ad libitum. The Ethics Committee of Shandong Provincial Hospital affiliated to Shandong University approved the procedures for animal experiments.
Eight-week-old female mice were randomly divided into four groups: (1) sham-operation (sham), (2) bilateral ovariectomy (OVX) with diet-supplemented oestradiol (E2) (OVX+E2), (3) OVX+E2 with low-dose FSH (L-FSH; 30 U/kg body weight recombinant human follitropin alfa solution, Merck, Kenilworth, New Jersey, USA) (OVX+E2+L-FSH) and (4) OVX+E2 with high-dose FSH (H-FSH; 60 U/kg body weight) (OVX+E2+H-FSH). Groups (1) and (2) were injected with vehicle and used as controls. To exclude the effect of E2, the mice were placed on a hormone replacement diet supplemented with 2.6 ppm desiccated E2 powder (Bayer, Leverkusen, Germany). After E2 was added for 5 days, FSH was given i.p. daily for 2 weeks, until mice were killed.
Additionally, OVX+E2 mice were randomly divided into four groups (n = 6/group): control (saline [154 mmol/l NaCl]), H-FSH, GRK2 inhibitor (200 μg/kg body weight) and GRK2 inhibitor + FSH (see Electronic supplementary material (ESM) Methods for further details).
Fshr and Crtc2 knockout mice, and Fshr siRNA
Fshr+/− mice (Wuhan Kangweida Gene Technology Company, Wuhan, China) were intercrossed to produce Fshr−/− mice. Female Fshr−/− mice were fed a 2.6 ppm E2-supplemented diet from 3 weeks of age. Female wild-type (Fshr+/+) littermates were used as the controls. At 8–10 weeks, mice were ovariectomised (Fshr+/+ mice were then also fed 2.6 ppm of E2) and injected with H-FSH (see ESM Methods for details).
Additionally, OVX+E2 mice were injected with Fshr siRNA adenovirus and, 7 days later, treated with FSH or vehicle. Mice were killed on day 22 (see ESM Methods for details).
Crtc2 knockout mice (Crtc2−/−) and their Crtc2+/+ littermates were also ovariectomised, fed 2.6 ppm of E2 and injected with H-FSH or vehicle daily for 2 weeks (see ESM Methods for details).
All the samples were randomised using the random number table method. All group assignments and outcome assessments carried out in this study were blinded. Illnesses including dermatitis, dehydration, inflammation and weight loss were exclusion criteria for animals; however, no animals were excluded. No experimental data was excluded using the Grubbs outlier test.
After treatment, mice underwent OGTTs, insulin tolerance tests (ITTs) and pyruvate tolerance tests (PTTs) (see ESM Methods for details).
Mice were fasted for 8 h or refed and then killed for blood and tissue collection. Blood was collected immediately before mice were killed, and glucose, FSH (Abnova ELISA Kit, #KA2330, Wuhan, China) and E2 (Demeditec Oestradiol-Sensitive ELISA Kit, #DE4399, Kiel, Germany) levels were assessed. Tissues were rapidly obtained after mice were killed and repetitive freeze–thaw cycles were avoided (see ESM Methods for details).
H&E and PAS staining
Paraffin-embedded liver tissues were sectioned and stained with haematoxylin and eosin (H&E). For hepatic glycogen staining, samples were stained with periodic acid–Schiff (PAS) according to the manufacturer’s instructions (Yili Company, Beijing, China).
Optimal cutting temperature compound (OCT)-embedded liver tissues were incubated with rabbit anti-GRK2 antibodies (1:200, Santa Cruz, Dallas, TX, USA) and then with fluorescein (tetramethylrhodamine [TRITC])-conjugated goat anti-rabbit IgG (1:300, Invitrogen, Beijing, China). Nuclei were stained with DAPI. Confocal fluorescence microscopy (LSM 780, Zeiss, Oberkochen, Germany) was used for imaging (see ESM Methods).
Hepatic cell lines, mouse primary hepatocyte cultures
HepG2 cells (Cell Library of the Chinese Academy of Sciences, Shanghai, China), mice hepatocyte NCTC 1469 cells (Xiehe Cell Library, Beijing, China) and mouse primary hepatocytes were used. HepG2 cells were treated with FSH (0, 10, 50 and 100 ng/ml) for 6 h or 24 h, or insulin (100 nmol/l) for 4 h or 6 h. Other regents including glucagon (100 nmol/l), protein kinase A (PKA) inhibitor (PKI; 10 μmol/l) and 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR; 1 mmol/l) were added 2 h before FSH. HepG2 and NCTC 1469 cells were authenticated by the cell library from which they were sourced and were not contaminated with mycoplasma.
NCTC cells were infected with Grk2 shRNA or scrambled shRNA and collected for protein analysis.
HepG2 cells were transfected with AMPK siRNA (targeting human AMPKα1, AMPKα2, AMPKβ1 and AMPKγ1) (see ESM Methods for details).
Glucose production measurements
After treatment with FSH for 22 h or insulin for 2 h, HepG2 cells were incubated for 2 h in glucose-free and phenol red-free DMEM media (Gibco, Grand Island, NY, USA) supplemented with 10 mmol/l lactate and 1 mmol/l pyruvate (and FSH/insulin). Glucose was measured in media using a glucose detection kit (Applygen Technologies, Beijing, China). Data were analysed relative to the controls (see ESM Methods).
Plasmid construction and transfection and dual luciferase activity assays
The plasmid encoding wild-type GRK2 (a gift from J. Staňková ), Firefly luciferase reporter plasmids containing wild-type PEPCK and G6Pase promoters (provided by H.S. Choi ), plasmids encoding green fluorescent protein (GFP)-tagged wild-type AMPK and a nonphosphorylatable mutant of AMPK substituted with alanine (AMPK S485A; Shanghai Genechem, Shanghai, China), and the Firefly luciferase reporter plasmid containing the CRTC2-CREB complex-binding site (CRE)-mutant PEPCK promoter (Hanbio Biotechnology, Shanghai, China) were used for transfection. A Renilla luciferase plasmid (Promega, Madison, WI, USA) served as an internal control. Luciferase activity was measured using the dual luciferase reporter assay (Promega), according to the manufacturer’s protocol. Firefly luciferase activity was normalised to Renilla luciferase activity. Data were analysed relative to controls (see ESM Methods).
RNA isolation and real-time quantitative RT-PCR
Total RNA from cells and mice liver tissue was isolated and real-time quantitative RT-PCR was used to determine relative mRNA expression of mouse Pepck (also known as Pck1), G6pase (also known as G6pc), Crtc2, Pgc1a (also known as Ppargc1a), Gck, Pfkfb1, Pygl, Gys2 and β-actin (Actb), and human PEPCK (also known as PCK1), G6Pase, GLUT2 (also known as SLC2A2), GLUT4 (also known as SLC2A4), AMPK (also known as PRKAA1) and β-actin (ACTB) (see ESM Methods for further details and primers are listed in ESM Table 1).
Protein extraction and western blotting
Protein was extracted from hepatocytes and tissues, separated on 10% (wt/vol.) SDS-PAGE and transferred to polyvinylidene difluoride (PVDF) membranes (Merck Millipore, Darmstadt, Germany). Anti-p-Thr172 AMPKα (1:1000), p-Ser485 AMPKα (1:1000), AMPKα (1:1000), AMPKβ1 (1:1000), p-Ser428 liver kinase B1 (LKB1; 1:1000), p-Ser9 glycogen synthase kinase 3β (GSK3β; 1:1000), GSK3β (1:1000), p-Ser307 IRS1 (1:1000), p-Ser133 cyclic AMP response element-binding protein (CREB; 1:1000), CREB (1:1000), p-Ser473 Akt (1:1000) and Akt (1:1000) antibodies were obtained from Cell Signaling Technology (Boston, MA, USA). Other antibodies used in the study included anti-GLUT2 (1:1000) and anti-GLUT4 (1:1000) (Abcam, Cambridge, UK); anti-LKB1 (1:1000, Merck Millipore); anti-CRTC2 (1:2000, Calbiochem, Merck Millipore); anti-GRK2 (1:200, Santa Cruz); anti-AMPKγ1 (1:1000, Thermo Scientific, Waltham, MA, USA); anti-FSHR (1:500) and anti-glucose-6-phosphatase (G6Pase) (1:1000) (OriGene, Rockville, MD, USA); anti-PEPCK (1:2000), PGC1α (1:1000), GFP (1:5000), LMB1 (1:5000) and GAPDH (1:7500) (Proteintech, Wuhan, China), and anti-p-Ser670 GRK2 (1:200, Bioss, Beijing, China). All the antibodies used in the article were validated by companies from which they were bought respectively, using methods such as, siRNA knock-down, phosphatase and activator treatment, positive/negative cell line and lot-to-lot testing. The appropriate secondary antibodies conjugated to horseradish peroxidase (HRP; ZSGB-Bio, Beijing, China) were used at 1:5000. Immune complexes were detected using the chemiluminescence (see ESM Methods).
Co-immunoprecipitation was carried out to test the binding of GRK2 with AMPK. Protein was extracted from cells and incubated with rabbit anti-AMPKα or rabbit anti-AMPKβ1 antibodies. The pre-immune sample was generated by combining protein extracts with rabbit IgG (Cell Signaling Technology). The following day, Protein A/G plus agarose slurry beads (Santa Cruz) were added to samples for 2 h. The eluted proteins were separated on 10% (wt/vol.) SDS-PAGE and transferred to a PVDF membrane. The membrane was probed with an anti-GRK2 antibody (Santa Cruz) and then with a secondary HRP-conjugated antibody (1:5000). Immune complexes were detected using chemiluminescence (see ESM Methods for details).
The data are expressed as the means ± SEMs. Statistical analyses were performed using either two-tailed unpaired Student’s t tests (two groups) or one-way ANOVA tests (more than two groups). A p < 0.05 is considered to be statistically significant.
FSH upregulates hepatic gluconeogenesis in ovariectomised mice
FSH enhances the expression of key enzymes of gluconeogenesis in the liver
CRTC2 is activated by FSH and mediates the regulatory role of FSH in hepatic gluconeogenesis
To determine the involvement of CRTC2 in FSH-induced hepatic gluconeogenesis, we used Crtc2 knockout mice. The results of PCR genotyping and the efficiency of Crtc2 gene deletion by western blotting are presented in Fig. 3f, g. All mice were ovariectomised, supplemented with E2 to maintain the normal E2 levels and then injected with H-FSH (shown in Fig. 3h). Crtc2−/− mice exhibited reduced PTT results compared with Crtc2+/+ littermates; however, Crtc2−/− mice receiving H-FSH exhibited approximately the same PTT results as the Crtc2−/− mice receiving vehicle (Fig. 3i). Changes in hepatic Pepck and G6pase mRNA levels were consistent with the PTT results (Fig. 3j). Moreover, when the CRTC2-CREB complex-binding site (CRE)  in the PEPCK promoter was mutated, the effect of FSH was significantly attenuated (Fig. 3k). To explore the involvement of CREB, we conducted experiments with PKI, a specific pharmacological PKA inhibitor. The results showed that FSH did not alter CREB phosphorylation; however, inhibition of CREB activity by PKI partly blocked FSH-related increases in PEPCK and G6Pase (Fig. 3l). Considered together, these findings suggest that CRTC2 plays an indispensable role in FSH-induced hepatic gluconeogenesis.
AMPK is essential for the FSH-mediated regulation of gluconeogenesis
FSH regulates AMPK-dependent hepatic gluconeogenesis via GRK2
To thoroughly explore the mode of action of GRK2, we performed co-immunoprecipitation assays. Intriguingly, AMPK and GRK2 co-immunoprecipitated, and this was more prominent after FSH stimulation (Fig. 5h). Moreover, the GRK2 inhibitor attenuated the binding of AMPK to GRK2 (Fig. 5i). AMPK consists of three subunits: α, β and γ. Two or three isoforms of each subunit (α1, α2, β1, β2, γ1, γ2 and γ3) are encoded by different genes . The α1- and α2-containing complexes each account for approximately half of the total AMPK activity in liver extracts, β1- and γ1-containing complexes account for over 90%. To identify which AMPK subunit contributes to GRK2 binding, we knocked down AMPK α1, α2, β1 and γ1 by siRNA (ESM Fig. 4b–d). The co-immunoprecipitation results demonstrated that both AMPKα1 and α2, but not β1 or γ1, contributed to the binding (Fig. 5j, k). Knockdown of neither β1 nor γ1 had an effect on AMPK Ser485 and Thr172 phosphorylation, which is consistent with the co-immunoprecipitation results (Fig. 5l).
Fshr knockout blocks the effects of FSH
Compared with their Fshr+/+ littermates, fasting serum glucose levels were decreased (Fig. 6f) and PTT results were significantly reduced in Fshr−/− mice (Fig. 6g). Consistently, the mRNA and protein levels of PEPCK and G6Pase were also decreased in Fshr−/− mice (Fig. 6h, i). Moreover, GRK2, phospho-AMPK (Ser485) and CRTC2 protein levels were attenuated, and phospho-AMPK (Thr172) levels were increased in Fshr −/− mice (Fig. 6i). FSHR deficiency, induced by injecting siRNA targeting Fshr via the tail vein, prevented the FSH-mediated stimulation of Pepck and G6pase transcription (Fig. 6j). These results demonstrate that the effects of FSH on hepatic gluconeogenesis depend on FSHR in the liver.
We have discovered a novel extragonadal role for FSH in regulating hepatic gluconeogenesis via FSHR in the liver. FSH suppresses AMPK activity by stimulating GRK2-mediated AMPK Ser485 phosphorylation, thereby enhancing PEPCK and G6Pase transcription via CRTC2 (Fig. 6k).
Hepatic gluconeogenesis and glycogenolysis are the two main sources of fasting glucose. However, within 2 h of fasting, the glycogenolysis rate returns to the basal level, whereas gluconeogenesis remains active , suggesting the vital importance of gluconeogenesis, particularly in prolonged fasting. In our study, hepatic glycogen and several molecules involved in glycogen metabolism, including GSK3β, glycogen synthase 2 and liver glycogen phosphorylase, were unaltered after FSH injection. Additionally, the transcription of Pfkfb1 and Gck, important genes involved in glycolysis, was unchanged by FSH. These results indicate that FSH does not significantly affect glycogen metabolism and glycolysis in the liver. Approximately 80% of endogenous glucose is produced in the liver, and the remaining 20% in the kidneys . We excluded the effects of FSH on kidney gluconeogenesis. Insulin sensitivity was evaluated using ITTs, OGTTs and by measuring insulin levels during the OGTT. Insulin tolerance was analysed through the insulin sensitivity index (Kitt) which can exclude the interference of changes in basal blood glucose level . After a pyruvate load during a PTT, changes in glucose clearance influence glucose levels. Therefore, we measured p-IRS1, p-Akt and GLUT4 to evaluate glucose degradation and insulin sensitivity in skeletal muscles, and no significant changes were found. The results indicate that FSH does not significantly affect insulin levels and insulin sensitivity. Hence, we focused on the role of FSH in hepatic gluconeogenesis and the underlying mechanism.
GRK2 belongs to the GRK family, GRK1–7 , of which GRK2 is expressed most ubiquitously. In this study, GRK2 upregulated hepatic gluconeogenesis, exhibiting an identical function to FSH. Thus, the results demonstrated that the dominant role of GRK2 is as a signal molecule that helps to mediate FSH, rather than as a GPCR kinase desensitising FSHR and blocking FSHR signalling. Previous research has indicated that both the expression and the translocation of GRK2 can be regulated [11, 35]. Chronic or even acute administration of GPCR agonists can increase GRK2 levels, and the best-known case is the upregulation of GRK2 in heart failure . GRK2 can be regulated by protein–protein interactions, as with many kinases including Akt . When the GPCR is activated, Gβγ subunits bind to GRK2 and recruit it to the membrane, dependent on the presence of anionic phospholipids. GRK2 is also phosphorylated by C-terminal Src (c-Src) and the extracellular signal regulated kinase (ERK), this phosphorylation not only reduces the activity of GRK2 but also promotes its degradation. The phosphorylation of GRK2 (not only by c-Src and ERK) can affect its interaction with Gβγ and membrane recruitment. We hypothesised that after FSH activates FSHR, the Gβγ subunits of the G-protein bind to GRK2 and recruit it to the membrane. A reduction in GRK2 degradation may also occur after chronic FSH stimulation. However, knowledge of the regulation of GRK2 expression and translocation remains incomplete and further exploration is required. These new findings suggest a novel role for GRK2 in hepatic gluconeogenesis regulation and provide promising new insight into the effects of FSH on liver function.
GRK2 can directly interact with proteins , such as Akt  and p38 mitogen-activated protein kinases (p38 MAPK) . AMPK, Akt and p38 MAPK all belong to the Ser/Thr kinases family, indicating that they potentially exhibit a similar phosphorylation pattern. Moreover, GRK2 is necessary for insulin signalling , which implies its involvement in energy regulation, and AMPK is crucial to energy metabolism . Thus, we explored the relationship between GRK2 and AMPK. Based on our data, we hypothesise a possible mechanism in which GRK2 binds to the AMPK α1 and α2 catalytic subunits and phosphorylates AMPK at Ser485, leading to the inhibition of AMPK Thr172 phosphorylation and its inactivation.
Following demonstration that FSH stimulates AMPK Ser485 phosphorylation both in vivo and in vitro, we prepared an AMPK mutant in which Ser485 was altered to determine its role in affecting FSH signalling and biological actions. The overexpression of AMPK S485A blocked FSH-regulated AMPK Thr172 dephosphorylation and PEPCK and G6Pase production. Studies have found that phosphorylation of AMPK at Ser485 reduces the phosphorylation at the activating site Thr172 and decreases AMPK activity [17, 42], which our results confirmed. Although several kinases have been identified, LKB1 is thought to be the predominant upstream kinase responsible for AMPK activation in the liver. LKB1 is activated when phosphorylated at Ser428 . Surprisingly, we found that FSH did not alter LKB1 Ser428 phosphorylation. Although AMPK phosphorylation results partly from the activation of calcium/calmodulin-dependent protein kinase kinase (CaMKK), only trace levels of CaMKK were observed in the liver . Thus, it is unlikely that AMPK activation by CaMKK occurs in hepatocytes .
CRTC2, as a molecule downstream from AMPK, is a key regulator of glucose homeostasis during fasting . In our study, Crtc2−/− mice exhibited reduced PTT results and decreased Pepck and G6pase mRNA levels, consistent with previous research . Additionally, the CRE region mutation in the PEPCK promoter attenuated the enhanced effect of FSH on PEPCK luciferase activity. These results demonstrate the indispensability of CRTC2 in FSH functioning. However, knocking out Crtc2 could not rule out a FSH-mediated CREB activation to stimulate PEPCK and G6Pase production. We found that FSH did not alter CREB phosphorylation; however, inhibiting CREB activity by PKI partly blocked FSH-related increases in PEPCK and G6Pase levels. To explain, when CRTC2 is activated by FSH it must bind to CREB to increase its target gene transcription ; thus, CREB is indispensable in the FSH gluconeogenesis pathway. Therefore, both CRTC2 and CREB were found to participate in FSH signalling in our experiment, and FSH activates CRTC2 but not CREB. Thus, our main focus was CRTC2. Law et al reported that FSH/GPCR/PKA could activate insulin pathway in other cells ; a finding which is of great interest. Whether and how FSH affects the Akt pathway in the liver requires further exploration. While cyclic AMP/PKA could interact with the insulin pathway in the liver, it is widely reported that the cyclic AMP/PKA pathway inactivates insulin signalling, thereby enhancing gluconeogenesis and glycogenolysis . Especially in a fasted state, insulin signalling is inhibited to a large extent. In addition, entirely different biological actions would occur in different cells, although the same proteins are expressed. Landomiel et al reported that activated FSHRs can couple to multiple transduction mechanisms, not just in the cyclic AMP/PKA pathway, including Gαq and the epidermal growth factor receptor .
In conclusion, we present evidence supporting a direct role for FSH in the pathogenesis of unrestrained gluconeogenesis, a surprising example of a previously undescribed metabolic function of FSH. However, the entire mechanism involved in glucose metabolism appears to be highly complex , and additional efforts are necessary to explore the potential involvement of FSH in glycometabolism. In our study, through FSHR and targeting GRK2, FSH suppressed AMPK activation by increasing AMPK Ser485 phosphorylation, then increased hepatic Pepck and G6pase transcription via CRTC2, and consequently enhanced hepatic gluconeogenesis independent of E2. Our findings highlight a novel pathophysiological role for FSH in regulating glucose metabolism in the liver and may provide an additional strategy for treating fasting hyperglycaemia.
We thank Y. Wang (School of Life Sciences, Tsinghua University, China) for providing mice; H. Choi (School of Biological Sciences and Technology, Chonnam National University, Republic of Korea), J. Staňková (Department of Paediatrics, University of Sherbrooke, Canada), F. Mayor (Department of Molecular Biology, Universidad Autónoma de Madrid, Spain) and J. L. Benovic (Department of Biochemistry and Molecular Biology, Thomas Jefferson University, USA) for providing plasmids.
JZ and LG designed and supervised the project. JZ, LG, CY, YS, YG and XQ designed the experiments. XQ, YG and DK performed the experiments. XQ analysed the data and wrote the manuscript. CY, DK, LF and LZ participated in analysis and interpretation of data. All authors revised the manuscript critically and approved the final version for publication. LG is the guarantor of this work.
This work was supported by the National Natural Science Foundation (81670796) and the National Key R&D Programme of China (2017YFC1309800 and 0909600).
Duality of interest
The authors declare that there is no duality of interest associated with this manuscript.
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