Date Published: April 24, 2019
Publisher: Public Library of Science
Author(s): Sanshiro Tateya, Norma Rizzo-De Leon, Andrew M. Cheng, Brian P. Dick, Woo Je Lee, Madeleine L. Kim, Kevin O’Brien, Gregory J. Morton, Michael W. Schwartz, Francis Kim, Nobuyuki Takahashi.
During periods in which glucose absorption from the gastrointestinal (GI) tract is insufficient to meet body requirements, hepatic gluconeogenesis plays a key role to maintain normal blood glucose levels. The current studies investigated the role in this process played by vasodilatory-associated phosphoprotein (VASP), a protein that is phosphorylated in hepatocytes by cAMP/protein kinase A (PKA), a key mediator of the action of glucagon. We report that following stimulation of hepatocytes with 8Br-cAMP, phosphorylation of VASP preceded induction of genes encoding key gluconeogenic enzymes, glucose-6-phosphatase (G6p) and phosphoenolpyruvate carboxykinase (Pck1), and that VASP overexpression enhanced this gene induction. Conversely, hepatocytes from mice lacking VASP (Vasp-/-) displayed blunted induction of gluconeogenic enzymes in response to cAMP, and Vasp-/- mice exhibited both greater fasting hypoglycemia and blunted hepatic gluconeogenic enzyme gene expression in response to fasting in vivo. These effects of VASP deficiency were associated with reduced phosphorylation of both CREB (a key transcription factor for gluconeogenesis that lies downstream of PKA) and histone deacetylase 4 (HDAC4), a combination of effects that inhibit transcription of gluconeogenic genes. These data support a model in which VASP functions as a molecular bridge linking the two key signal transduction pathways governing hepatic gluconeogenic gene expression.
In the fasting state, increased hepatic gluconeogenesis is essential to avert hypoglycemia and maintain normal plasma glucose levels. Glucagon and epinephrine are key hormonal drivers of gluconeogenesis by virtue of increased intracellular cAMP levels induced by binding of their respective receptors on the hepatocyte plasma membrane. This response in turn activates cAMP-dependent protein kinase A (PKA), which ultimately promotes the interaction of cAMP-responsive binding protein (CREB) with CREB-regulated transcription co-activators, resulting in increased expression of genes encoding two key enzymes involved in gluconeogenesis, glucose-6-phosphatase (G6p) and phosphoenolpyruvate kinase (Pck1) [1, 2].
Anti-VASP(#3132), anti-phosphorylated VASP (Ser157)(#3111), anti-phosphorylated serum response factor (serine103) (SRF)(#4261) and total SRF(#5147) antibodies were purchased from Cell Signaling (Denvers, MA). Anti-GAPDH (sc-25778) rabbit polyclonal antibody and anti CREB binding protein (CBP) (sc369) were obtained from Santa Cruz Biotechnology (Santa Cruz, CA).
Glucose homeostasis is achieved through a highly coordinated regulatory system that governs both glucose production (occuring primarily in liver) and utilization and, in normal individuals, maintains blood glucose within narrow physiological limits. Control of gluconeogenesis is a key element of this regulatory system, and dysregulation of this process contributes to hyperglycemia in patients with diabetes . In the fed state, gluconeogenesis is inhibited both by reduced secretion of glucagon  and through the suppressive effects of insulin. Conversely, increased hepatic glucose production during a fast is crucial for maintenance of euglycemia, a process that over time becomes increasingly dependent on gluconeogenesis . Our new findings support a novel role for VASP, acting via two distinct hepatocyte signal transduction pathways, in this process.
In summary, we provide evidence that in the liver, VASP phosphorylation contributes to activation of the CREB transcriptional complex required for stimulation of Pck1, G6p, and provides a molecular bridge linking CREB to SIRT1/HDAC4 signaling in the control of this process.