The Role of AMPK and mTOR in Nutrient Sensing in Pancreatic β-Cells

Abstract

The AMP-activated protein kinase (AMPK) is a central regulator of the energy status of the cell, based on its unique ability to respond directly to fluctuations in the ratio of AMP:ATP. Because glucose and amino acids stimulate insulin release from pancreatic ␤-cells by the regulation of metabolic intermediates, AMPK represents an attractive candidate for control of ␤-cell function. Here, we show that inhibition of AMPK in ␤-cells by high glucose inversely correlates with activation of the mammalian Target of Rapamycin (mTOR) pathway, another cellular sensor for nutritional conditions. Forced activation of AMPK by AICAR, phenformin, or oligomycin significantly blocked phosphorylation of p70S6K, a downstream target of mTOR, in response to the combination of glucose and amino acids. Amino acids also suppressed the activity of AMPK, and this at a minimum required the presence of leucine and glutamine. It is unlikely that the ability of AMPK to sense both glucose and amino acids plays a role in regulation of insulin secretion, as inhibition of AMPK by amino acids did not influence insulin secretion. Moreover, activation of AMPK by AICAR or phenformin did not antagonize glucose-stimulated insulin secretion, and insulin secretion was also unaffected in response to suppression of AMPK activity by expression of a dominant negative AMPK construct (K45R). Taken together, these results suggest that the inhibition of AMPK activity by glucose and amino acids might be an important component of the mechanism for nutrient-stimulated mTOR activity but not insulin secretion in the ␤-cell.The ␤-cell is unique compared with other mammalian cell types in that its primary function to synthesize and secrete insulin is tightly coupled to its metabolic rate. Glucose is the most potent nutrient in stimulating insulin release. Upon entry into the ␤-cell, glucose is rapidly metabolized, resulting in the generation of mitochondria-derived metabolic intermediates including ATP. This increase in ATP leads to closure of ATPsensitive K ϩ (K ATP )-channels, depolarization of the plasma membrane and opening of voltage-gated L-type Ca 2ϩ channels. The subsequent increase in intracellular Ca 2ϩ concentration [Ca 2ϩ ] i triggers insulin exocytosis (1). The ␤-cell also utilizes certain key amino acids that, via mitochondrial metabolism, can further generate coupling factors that elicit an insulin secretory response (2, 3). In addition to their role as insulin secretagogues, glucose and other nutrients stimulate protein translation and ␤-cell growth and proliferation (4, 5). While much is known regarding how the ␤-cell couples glucose metabolism to insulin secretion, the mechanisms by which ␤-cells sense metabolism of other fuels, such as amino acids, and augment glucose-stimulated insulin release are less clear. Further, it is unclear how the ␤-cell coordinates nutrient abundance with enhanced protein translation and cell growth. This aspect of ␤-cell function is particularly important under conditions of increased insulin demand, such as obesity a...

Serine-385 phosphorylation of inwardly rectifying K+ channel subunit (Kir6.2) by AMP-dependent protein kinase plays a key role in rosiglitazone-induced closure of the KATP channel and insulin secretion in rats

“…In the present study, we found that short-term treatment with the AMPK activator AICAR stimulated insulin secretion, an effect that was independent of glucose concentration and was mediated by inhibition of K ATP channel activity. Stimulation of insulin secretion in response to AMPK activation by AICAR at physiological glucose concentrations [16][17][18]48] is in agreement with our present findings.…”

The AMP-regulated kinase family: Enigmatic targets for diabetes therapy

“…On the other hand, the effects of metformin on insulin secretion are more controversial; Marchetti and colleagues (Lupi et al, 1997) have provided evidence that insulin secretion is stimulated by metformin in islets from type 2 diabetics, perhaps reflecting respiratory chain inhibition and suppressed reactive oxygen species (ROS) production. By contrast, our own findings using islets from both rodent and human (Leclerc et al, 2004b) It should also be mentioned that two other groups (Wang et al, 2005;Gleason et al, 2007) have provided evidence which argues against a role for AMPK in the control of insulin secretion. In the first of these studies, AICAR was shown to stimulate secretion from isolated islets; it must be stressed, however, that this riboside may have complex effects, including, after triple phosphorylation to generate an ATP analogue (ZTP), closure of ATP-sensitive K + channels, likely to activate insulin release.…”

“…In the first of these studies, AICAR was shown to stimulate secretion from isolated islets; it must be stressed, however, that this riboside may have complex effects, including, after triple phosphorylation to generate an ATP analogue (ZTP), closure of ATP-sensitive K + channels, likely to activate insulin release. On the other hand, Gleason et al (Gleason et al, 2007), using the strongly glucose-responsive INS-1(832/13) cell line, as well as MIN6 cells, demonstrated that expression of an inactivated form of AMPK (2K 45 R) exerted no effect on basal or glucose-stimulated insulin secretion, arguing against a controlling role for AMPK.…”

The AMPK agonist AICAR inhibits the growth of EGFRvIII-expressing glioblastomas by inhibiting lipogenesis

“…AICAR has been postulated to mediate its anti-growth effects primarily through mTOR inhibition (14,28). Thus, the demonstration that its effects were mediated primarily through inhibition of lipogenesis was quite unexpected.…”