Norepinephrine and epinephrine are catecholamines CA release
Norepinephrine and epinephrine are catecholamines (CA) released from synaptic nerves and the adrenal gland that mediate systemic responses to nutritional stressors (Cannon and De la Paz, 1911). CA mobilize plasma glucose by suppressing insulin secretion from the pancreatic β-cell (Katada and Ui, 1981, Laychock and Bilgin, 1987) Nine isoforms of adrenergic receptors (ADR) mediate the actions of CA (Roth et al., 1991). Predominantly the β isoforms (ADRβ1, ADRβ2, ADRβ3) couple to stimulatory G proteins (Gs). While, α isoforms (ADRα1A, ADRα1B, ADRα1C) couple to Gq and (ADRα2A, ADRα2B, ADRα2C) couple to inhibitory G proteins (Gi), however other G proteins, such as Gz and Go have recently been associated with ADR (Straub and Sharp, 2012). Islets from all species examined express several isoforms of ADR but α2-ADR isoforms are responsible for CA inhibition of insulin secretion from β-cells (Kelly et al., 2014, Kelly et al., 2017, Lacey et al., 1996, Urano et al., 2004, Kelly et al., 2017). Several mechanisms for α2-ADR inhibition of GSIS are described: 1) activation of ATP-sensitive K+ channels (KATP) and repolarization of the β-cells, 2) inhibition of L-type Ca2+ channels, 3) decreased activity of adenylyl cyclase (AC) and 4) inhibition of distal exocytosis through interactions with members of the SNARE protein family (Zhao et al., 2010b, Zhao et al., 2010c, Keahey et al., 1989).
Material and methods
Discussion We show that epinephrine exposure suppresses oxidative metabolism in β-cells and islets under optimal (nutrient rich) and glucose-only substrate conditions through α2-ADR, which expands previously defined mechanisms for ADR inhibition of insulin secretion as well as designates novel downstream effects of the α2-ADRs. Epinephrine stimulated Gi-coupled α2-ADRs to decrease β-cell oxidative metabolism through glycolytic and mitochondrial respiration. We establish that the decrease in oxidative metabolism occurred in the presence of pharmacologically stimulated AC and occurred independent from insulin secretion. Thus we proposed novel metabolic targets downstream of Gi-coupled ADR signaling that we evaluated with proteomics on Min6 cells exposed to epinephrine for 4 h. The proteomic analysis for epinephrine exposure implicated metabolic rearrangements of proteins involved in the ETC and the citric 5003 cycle. These differentially expressed proteins were confirmed with immunoblots and correlated with metabolic assays that show decreased cellular reducing agents, lower intracellular ATP concentrations, and higher MMP. These results indicate that epinephrine causes an interruption in mitochondrial respiratory chain reactions and ATP production by β-cells. These data reaffirm that stimulation through α2-ADR inhibits insulin secretion but also indicate a previously undefined mechanism that involves the oxidative metabolism of glucose (Aarnio et al., 2001, Katada and Ui, 1981). The significance of α2-ADR regulation of β-cell function has been demonstrated through studies manipulating expression of the predominant isoform Adrα2a. Mice that lack Adrα2a are hyperinsulinemic and hypoglycemic, indicating loss of inhibitory control in β-cells (Savontaus et al., 2008). Overexpression of Adrα2a decreased β-cell function and contributed to the development of diabetes in adulthood (Rosengren et al., 2010, Liggett, 2009, Ahren, 2009). The ADR expression profile of Min6 in concert with physiological findings using α2-ADR antagonist yohimbine demonstrate α2-ADR are responsible for decreased oxidative phosphorylation of islets and Min6, which expands the mechanisms underlying adrenergic signaling in β-cells. In rat islets glucose utilization rates were inhibited with epinephrine and the α2-ADR agonist clonidine (Laychock and Bilgin, 1987). However, others have demonstrated epinephrine suppresses insulin secretion but does not affect glucose utilization rates (Hermann and Deckert, 1977). We showed epinephrine through α2-ADR decreases glucose oxidation rate and oxygen consumption rates in complete nutrient (amino acid and glucose supplemented) as well as nutrient-deprived (glucose-only) situations in isolated rat islets and Min6 cells. Furthermore, pharmacologic activation of AC to elevate intracellular cAMP increased OCR for Min6 cells but this enhancement was suppressed with epinephrine. Therefore, mechanisms independent of cAMP may be partially responsible for epinephrine inhibition of OCR. Lower AC activity to reduce cAMP is a prominent mechanism underlying Gi-coupled inhibition of insulin secretion (Howell and Montague, 1973, Liggett, 2009). Thus we hypothesized that α2-ADR activation would suppress OCR through lowering cAMP production. Here, we found epinephrine suppressed forskolin-stimulated OCR but not to the magnitude seen in control media. However, these results are reflected by studies that show epinephrine decreased insulin secretion when cAMP concentrations were maintained and studies that demonstrate α2-ADR has cAMP independent mechanisms to suppress insulin secretion (Jones et al., 1987, Bradley et al., 2005, Gloerich and Bos, 2010, Straub and Sharp, 2012). Further studies are required to determine the exact role of cAMP in coordinating oxidative metabolism and α2-ADR activation.