A novel autoregulatory loop between the Gcn2-Atf4 pathway and (L)-Proline [corrected] metabolism controls stem cell identity.

Increasing evidence indicates that metabolism is implicated in the control of stem cell identity. Here, we demonstrate that embryonic stem cell (ESC) behaviour relies on a feedback loop that involves the non-essential amino acid L-Proline (L-Pro) in the modulation of the Gcn2-Eif2α-Atf4 amino acid starvation response (AAR) pathway that in turn regulates L-Pro biosynthesis. This regulatory loop generates a highly specific intrinsic shortage of L-Pro that restricts proliferation of tightly packed domed-like ESC colonies and safeguards ESC identity. Indeed, alleviation of this nutrient stress condition by exogenously provided L-Pro induces proliferation and modifies the ESC phenotypic and molecular identity towards that of mesenchymal-like, invasive pluripotent stem cells. Either pharmacological inhibition of the prolyl-tRNA synthetase by halofuginone or forced expression of Atf4 antagonises the effects of exogenous L-Pro. Our data provide unprecedented evidence that L-Pro metabolism and the nutrient stress response are functionally integrated to maintain ESC identity.

Substituent and solvent effects on the photosensitized oxygenation of 5,6-dihydro-1,4-oxathiins. Intramolecular oxygen transfer vs normal cleavage of the dioxetane intermediates.

The reaction of singlet oxygen with variously substituted oxathiins 1 affords dicarbonyl compounds 4 and/or ketosulfoxides 7 and 8 depending on the nature of the substituent at C3 and on the reaction conditions. The normal fragmentation of dioxetanes 2 to 4 competes with an intramolecular oxygen transfer to ring sulfur, which leads to 7 and 8, presumably via the labile epoxides 5. This new pathway is promoted by electron-withdrawing groups at C3 and, for unsubstituted and monosubstituted amide derivatives 1h and 1i, respectively, by the solvent basicity. Chemical experiments support the intermediacy of epoxides 5 for 7, whereas they are not conclusive for 8. However, the formation of the latter compounds appears to be favored by polar solvents and cation-stabilizing groups at C2 as phenyl or methyl, and these observations may be well accounted for by the suggested pathway from 5 through charged species as E. Direct oxidation by singlet oxygen to sulfur is unsignificant, except for the amide series and for 1g or when the oxygenation is carried out in methanol.

Unexpected products via singlet oxygen oxygenation of functionalized 5,6-dihydro-1,4-oxathiins

[formula: see text] Single oxygen oxygenation of 5,6-dihydro-1,4-oxathiins substituted at C-3 with an electron-withdrawing group leads stereoselectively to ketosulfoxides 5 and 6, instead of the expected dicarbonyl compounds 3. A mechanism involving an unprecedented intramolecular rearrangement of the corresponding dioxetanes 2 is proposed.