Although it is well established that fasting blunts GH actions and reduces circulating IGF-1 concentrations in a variety of different mammalian species including man (Thissen et al., 1994
), the molecular underpinnings of this phenomenon have remained obscure. In this report, we show that the fasting-induced hormone, FGF21, elicits the same spectrum of effects as fasting on the GH signaling cascade, including elevated plasma GH concentrations and decreased circulating IGF-1 levels. Moreover, chronic exposure of mice to FGF21 strongly inhibits growth. Taken together, these findings suggest that FGF21 plays a central role in causing growth hormone resistance in response to nutrient deprivation. Since GH exerts powerful effects on carbohydrate and lipid metabolism (Clemmons, 2004
; Davidson, 1987
; LeRoith and Yakar, 2007
), these findings also raise the possibility that inhibition of the GH axis contributes to the diverse metabolic actions of FGF21.
FGF21 caused a marked reduction in hepatic concentrations of phosphorylated STAT5 and corresponding decreases in STAT5-regulated genes including IGF-1
. These changes along with the up-regulation of IGFBP-1
, which is repressed by GH through a STAT5B-dependent mechanism (Ono et al., 2007
; Seneviratne et al., 1990
), are likely to account for the changes that are seen in IGF-1 levels in FGF21-transgenic mice. Notably, the FGF21-transgenic mice closely resemble STAT5A/B-knockout mice in several key respects. First, FGF21-transgenic mice and STAT5A/B-knockout mice have very similar changes in hepatic gene expression including reduced IGF-1, ALS, MUP1, MUP3, SLCO1A1 and HSD3B5 mRNAs and increased SULT1E1 mRNA (Holloway et al., 2007
). Second, the ~50% decreases in serum IGF-1 concentrations and body weight in FGF21-transgenic mice are comparable to the changes reported for STAT5A/B-knockout mice (Teglund et al., 1998
). Finally, female FGF21-transgenic mice are infertile and have few or no corpora lutea as reported for the STAT5A/B-knockout mice (V.Y.L., D.J.M. and S.A.K, unpublished) (Teglund et al., 1998
). Taken together, the striking similarities between the FGF21-Tg and STAT5A/B-knockout mice strongly suggest that many of the effects of FGF21 are mediated through inhibition of STAT5.
How does FGF21 reduce phosphorylated STAT5 levels? We show that FGF21 inhibits the GH signaling pathway downstream of JAK2. FGF21 increased hepatic levels of SOCS2, which functions as a potent negative regulator of GH signaling in vivo (Leroith and Nissley, 2005
; Rico-Bautista et al., 2006
). SOCS2-knockout mice are significantly larger than wild type mice, and this increased growth requires both GH and STAT5B (Greenhalgh et al., 2002
; Greenhalgh et al., 2005
; Metcalf et al., 2000
). Since SOCS2 binds to the tyrosine-phosphorylated GHR, it might blunt GH signaling by competing with STAT5 for access to the GHR. The induction of SOCS2 by FGF21 is surprising in that SOCS2 is also induced by GH and STAT5B (Davey et al., 1999
; Woelfle and Rotwein, 2004
), presumably as part of a feedback loop that attenuates GH signaling. Our results suggest that there is an additional, STAT5-independent mechanism through which SOCS2 can be induced by FGF21. We note that we have been unable to recapitulate the effects of FGF21 on STAT5 activity or SOCS2 expression in primary cultures of mouse or rat hepatocytes treated with FGF21 for periods of 1 to 3 days. One explanation for these data is that FGF21 does not act directly on the liver but instead through an indirect mechanism. In this regard, FGF21 does not efficiently activate FGF receptor 4, the predominant FGF receptor in liver (Ogawa et al., 2007
; Suzuki et al., 2008
). An alternate explanation is that primary hepatocytes have lost their ability to respond to FGF21 during the culturing process.
While FGF21 causes many of the same effects as fasting on the GH axis, there were two notable differences. First, whereas fasting in rats caused resistance to GH-mediated phosphorylation of JAK2 (Beauloye et al., 2002
), hepatic levels of phosphorylated JAK2 were increased in FGF21-transgenic mice. Second, whereas SOCS3 mRNA is increased in livers of fasting rats (Beauloye et al., 2002
), we did not detect any changes in hepatic SOCS3 mRNA levels in FGF21-transgenic mice (data not shown). These differences may be a consequence of either species-specific differences or the contributions of other signaling pathways to the fasting response.
Transcription of the FGF21 gene is stimulated directly by PPARα (Badman et al., 2007
; Inagaki et al., 2007
; Lundasen et al., 2007
). As predicted, treatment of mice with a synthetic PPARα agonist elicited the same spectrum of effects as FGF21 on the GH/IGF-1 pathway, including reduced hepatic levels of phosphorylated STAT5, decreased IGF-1 and ALS mRNA levels, and increased SOCS2 concentrations. Cross-talk between PPARα and STAT5B has been previously described in cell-based assays, with GH-activated STAT5B inhibiting PPARα-regulated gene transcription, and conversely, ligand-activated PPARα inhibiting STAT5B-regulated transcription (Shipley and Waxman, 2004
). Thus, cross-talk between PPARα and STAT5 may occur at multiple levels. Notably, in clinical studies the PPARα agonist bezafibrate significantly lowers IGF-1 levels in patients (Ruotolo et al., 2000
). This finding together with data showing that FGF21 expression is induced by PPARα agonists in primary human hepatocytes (Inagaki et al., 2007
; Lundasen et al., 2007
) suggest that the PPARα/FGF21 pathway may be operative and affect IGF-1 signaling in humans.