Cytosolic phospholipase A2 regulates lipid homeostasis under osmotic stress through PPARγ.

Physiologically, renal medullary cells are surrounded by a hyperosmolar interstitium. However, different pathological situations can induce abrupt changes in environmental osmolality, causing cell stress. Therefore, renal cells must adapt to survive in this new condition. We previously demonstrated that, among the mechanisms involved in osmoprotection, renal cells upregulate triglyceride biosynthesis (which helps preserve glycerophospholipid synthesis and membrane homeostasis) and cyclooxygenase-2 (which generates prostaglandins from arachidonic acid) to maintain lipid metabolism in renal tissue. Herein, we evaluated whether hyperosmolality modulates phospholipase A2 (PLA2 ) activity, leading to arachidonic acid release from membrane glycerophospholipid, and investigated its possible role in hyperosmolality-induced triglyceride synthesis and accumulation. We found that hyperosmolality induced PLA2 expression and activity in Madin-Darby canine kidney cells. Cytosolic PLA2 (cPLA2) inhibition, but not secreted or calcium-independent PLA2 (sPLA2 or iPLA2 , respectively), prevented triglyceride synthesis and reduced cell survival. Inhibition of prostaglandin synthesis with indomethacin not only failed to prevent hyperosmolality-induced triglyceride synthesis but also exacerbated it. Similar results were observed with the peroxisomal proliferator activated receptor gamma (PPARγ) agonist rosiglitazone. Furthermore, hyperosmolality increased free intracellular arachidonic acid levels, which were even higher when prostaglandin synthesis was inhibited by indomethacin. Blocking PPARγ with GW-9662 prevented the effects of both indomethacin and rosiglitazone on triglyceride synthesis and even reduced hyperosmolality-induced triglyceride synthesis, suggesting that arachidonic acid may stimulate triglyceride synthesis through PPARγ activation. These results highlight the role of cPLA2 in osmoprotection, since it is essential to provide arachidonic acid, which is involved in PPARγ-regulated triglyceride synthesis, thus guaranteeing cell survival.

DMT1 iron uptake in the PNS: bridging the gap between injury and regeneration.

Previous studies by our group demonstrated the key role of iron in Schwann cell maturation through an increase in cAMP, PKA activation and CREB phosphorylation. These studies opened the door to further research on non-transferrin-bound iron uptake, which revealed the presence of DMT1 mRNA all along SC progeny, hinting at a constitutive role of DMT1 in ensuring the provision of iron in the PNS. In light of these previous results, the present work evaluates the participation of DMT1 in the remyelination process following a demyelinating lesion promoted by sciatic nerve crush--a reversible model of Wallerian degeneration. DMT1 was observed to colocalize with a SC marker S100ß at all survival times analyzed. In turn, the assessment of DMT1 mRNA expression exhibited an increase 7 days post-injury, while DMT1 protein levels showed an increase 14 days after crush at the lesion site and distal stump; finally, an increase in iron levels became evident as from 14 days post-injury, in parallel with DMT1 values. To sum up, the present work unveils the role of DMT1 in mediating the neuroregenerative action of iron.