Metabolic hormone FGF21 is induced in ground squirrels during hibernation but its overexpression is not sufficient to cause torpor.

Hibernation is a natural adaptation that allows certain mammals to survive physiological extremes that are lethal to humans. Near freezing body temperatures, heart rates of 3-10 beats per minute, absence of food consumption, and depressed metabolism are characteristic of hibernation torpor bouts that are periodically interrupted by brief interbout arousals (IBAs). The molecular basis of torpor induction is unknown, however starved mice overexpressing the metabolic hormone fibroblast growth factor 21 (FGF21) promote fat utilization, reduce body temperature, and readily enter torpor-all hallmarks of mammalian hibernation. In this study we cloned FGF21 from the naturally hibernating thirteen-lined ground squirrel (Ictidomys tridecemlineatus) and found that levels of FGF21 mRNA in liver and FGF21 protein in serum are elevated during hibernation torpor bouts and significantly elevated during IBAs compared to summer active animals. The effects of artificially elevating circulating FGF21 concentrations 50 to 100-fold via adenoviral-mediated overexpression were examined at three different times of the year. This is the first time that a transgenic approach has been used in a natural hibernator to examine mechanistic aspects of hibernation. Surgically implanted transmitters measured various metrics of the hibernation phenotype over a 7-day period including changes in motor activity, heart rate and core body temperature. In April fed-state animals, FGF21 overexpression decreased blood insulin and free fatty acid concentrations, effects similar to those seen in obese mice. However, elevated FGF21 concentrations did not cause torpor in these fed-state animals nor did they cause torpor or affect metabolic parameters in fasted-state animals in March/April, August or October. We conclude that FGF21 is strongly regulated during torpor and IBA but that its overexpression is not sufficient to cause torpor in naturally hibernating ground squirrels.

ßKlotho is required for fibroblast growth factor 21 effects on growth and metabolism.

Fibroblast growth factor 21 (FGF21) is a fasting-induced hepatokine that has potent pharmacologic effects in mice, which include improving insulin sensitivity and blunting growth. The single-transmembrane protein ßKlotho functions as a coreceptor for FGF21 in vitro. To determine if ßKlotho is required for FGF21 action in vivo, we generated whole-body and adipose tissue-selective ßKlotho-knockout mice. All of the effects of FGF21 on growth and metabolism were lost in whole-body ßKlotho-knockout mice. Selective elimination of ßKlotho in adipose tissue blocked the acute insulin-sensitizing effects of FGF21. Taken together, these data demonstrate that ßKlotho is essential for FGF21 activity and that ßKlotho in adipose tissue contributes to the beneficial metabolic actions of FGF21.

FGF15/19 regulates hepatic glucose metabolism by inhibiting the CREB-PGC-1α pathway.

Regulation of hepatic carbohydrate homeostasis is crucial for maintaining energy balance in the face of fluctuating nutrient availability. Here, we show that the hormone fibroblast growth factor 15/19 (FGF15/19), which is released postprandially from the small intestine, inhibits hepatic gluconeogenesis, like insulin. However, unlike insulin, which peaks in serum 15 min after feeding, FGF15/19 expression peaks approximately 45 min later, when bile acid concentrations increase in the small intestine. FGF15/19 blocks the expression of genes involved in gluconeogenesis through a mechanism involving the dephosphorylation and inactivation of the transcription factor cAMP regulatory element-binding protein (CREB). This in turn blunts expression of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and other genes involved in hepatic metabolism. Overexpression of PGC-1α blocks the inhibitory effect of FGF15/19 on gluconeogenic gene expression. These results demonstrate that FGF15/19 works subsequent to insulin as a postprandial regulator of hepatic carbohydrate homeostasis.

Long-range transcriptional control of progesterone receptor gene expression.

Estrogen receptor alpha (ERalpha) binds to specific target DNA sequences, estrogen response elements (EREs), to regulate estrogen-responsive gene expression. The progesterone receptor (PR) gene has been used extensively as a marker of estrogen responsiveness. Although we previously identified cis elements within 1 kb of the PR-B transcription start site that are associated with ERalpha and help to confer estrogen responsiveness, the identification of ERalpha binding sites far removed from the transcription start site suggested that long-range regulation of this gene may occur. We now show that eight regions of the PR gene from 311 kb upstream to 4 kb downstream of the PR-B transcription start site interact with ERalpha and that coactivator proteins and acetylated histones are selectively associated with these gene regions. Specific PR gene regions confer estrogen responsiveness to a heterologous reporter plasmid, and mutation of EREs within these regions diminishes estrogen-induced transactivation. Importantly, chromosome conformation capture assays reveal ERalpha- and ligand-dependent interactions between proximal and distal PR gene regions. Taken together, our studies suggest that distal regions of the PR gene participate in the dynamic regulation of this gene and that the coordinated action of proximal and distal PR gene regions allows cells to respond to changes in hormone levels with extraordinary versatility and sensitivity.