Rapamycin is used frequently in both transplantation and oncology. Although historically thought to have little diabetogenic effect, there is growing evidence of β-cell toxicity. This Review draws evidence for rapamycin toxicity from clinical studies of islet and renal transplantation, and of rapamycin as an anticancer agent, as well as from experimental studies. Together, these studies provide evidence that rapamycin has significant detrimental effects on β-cell function and survival and peripheral insulin resistance. The mechanism of action of rapamycin is via inhibition of mammalian target of rapamycin (mTOR). This Review describes the complex mTOR signaling pathways, which control vital cellular functions including mRNA translation, cell proliferation, cell growth, differentiation, angiogenesis, and apoptosis, and examines molecular mechanisms for rapamycin toxicity in β-cells. These mechanisms include reductions in β-cell size, mass, proliferation and insulin secretion alongside increases in apoptosis, autophagy, and peripheral insulin resistance. These data bring into question the use of rapamycin as an immunosuppressant in islet transplantation and as a second-line agent in other transplant recipients developing new-onset diabetes after transplantation with calcineurin inhibitors. It also highlights the importance of close monitoring of blood glucose levels in patients taking rapamycin as an anticancer treatment, particularly those with preexisting glucose intolerance.

Glucagon like peptide-1 (GLP1) is a G s -coupled receptor agonist that exerts multiple effects on pancreatic ␤-cells, including the stimulation of insulin gene expression and secretion. In this report, we show that treatment of the mouse pancreatic ␤-cell line MIN6 with GLP1 leads to the glucose-dependent activation of Erk. These effects are mimicked by forskolin, a direct activator of adenylate cyclase, and blocked by H89, an inhibitor of cAMP-dependent protein kinase. Additionally, we provide evidence that GLP1-stimulated activation of Erk requires an influx of calcium through L-type voltage-gated calcium channels and the activation of calcium/calmodulin-dependent protein kinase II. GLP1-stimulated activation of Erk is blocked by inhibitors of MEK, but GLP1 does not induce the activation of A-Raf, B-Raf, C-Raf, or Ras. Additionally, dominant negative forms of Ras(N17) and Rap1(N17) fail to block GLP1-stimulated activation of Erk. In conclusion, our results indicate that, in the presence of stimulatory concentrations of glucose, GLP1 stimulates the activation of Erk through a mechanism dependent on MEK but independent of both Raf and Ras. This requires 1) the activation of cAMP-dependent protein kinase, 2) an influx of extracellular Ca 2؉ through L-type voltage-gated calcium channels, and 3) the activation of CaM kinase II.

The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), a potent stimulator of Erk, leads to the phosphorylation of 4E-BP1 and its dissociation from eIF4E. In contrast to agonists such as insulin, this occurs independently of PKB activation. In this report, we investigate the mechanism by which TPA regulates 4E-BP1 phosphorylation. and its release from eIF4E. In this case, the phosphorylation of these sites was also blocked by inhibitors of MEK or by rapamycin. In conclusion, the Erk pathway, via mechanisms also requiring mTOR, regulates the phosphorylation of multiple sites in 4E-BP1 in vivo and this is sufficient for the release of 4E-BP1 from eIF4E. mRNA translation is conventionally divided into three stages: initiation, elongation, and termination. Of these, initiation is considered the rate-limiting step and therefore is considered the most important step in the acute regulation of translation. Essential for initiation of cap-dependent protein synthesis is the assembly of the initiation complex, eukaryotic initiation factor (eIF) 1 4F, containing the initiation factors eIF4G, a large scaffolding protein, eIF4E, the protein which binds to the 5Ј cap structure and eIF4A, a bidirectional RNA helicase (for review, see Ref.

Wasting of lean tissue as a consequence of metabolic acidosis is a serious problem in patients with chronic renal failure. A possible contributor is inhibition by low pH of the System A (SNAT2) transporter, which carries the amino acid L-glutamine (L-Gln) into muscle cells. The aim of this study was to determine the effect of selective SNAT2 inhibition on intracellular amino acid profiles and amino acid-dependent signaling through mammalian target of rapamycin in L6 skeletal muscle cells. Inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate, metabolic acidosis (pH 7.1), or silencing SNAT2 expression with small interfering RNA all depleted intracellular L-Gln. SNAT2 inhibition also indirectly depleted other amino acids whose intracellular concentrations are maintained by the L-Gln gradient across the plasma membrane, notably the anabolic amino acid L-leucine. Consequently, SNAT2 inhibition strongly impaired signaling through mammalian target of rapamycin to ribosomal protein S6 kinase, ribosomal protein S6, and 4E-BP1, leading to impairment of protein synthesis comparable with that induced by rapamycin. It is concluded that even though SNAT2 is only one of several L-Gln transporters in muscle, it may determine intracellular anabolic amino acid levels, regulating the amino acid signaling that affects protein mass, nucleotide/nucleic acid metabolism, and cell growth.

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