As someone who is familiar with this literature, it seems like these data could be symptoms of ongoing mitophagy from rapamycin (decreased membrane potential, decreased expression of OXPHOS protein components, decreased OCR).
The Nature authors are drawing a connection between the mitochondrial effects of rapamycin on pancreatic beta cells and the increased blood glucose that I believe many of us have observed with rapamycin. Here is a great quote from the Nature paper:
“Interestingly, the properties of sirolimus observed in β cells are cell-specific and seem to be in contrast with its effects seen in models of neurodegenerative and ischemic disorders, in which the drug has been shown to be overall protective, inducing autophagy and enhancing lysosomal activation in order to remove damaged mitochondria.”
Rapamycin, an inhibitor of the serine/threonine protein kinase mTOR, is an immunosuppressant used to treat renal transplant recipients, but it can cause endothelial and mitochondrial dysfunction. Metformin is used for the treatment of type 2 diabetes and was reported to exert therapeutic effects against rheumatoid arthritis and obesity by improving mitochondrial dysfunction via the activation of fibroblast growth factor 21. We investigated the therapeutic effects of rapamycin–metformin combination therapy in obese mice with collagen-induced arthritis (CIA).
Methods
Mouse embryonic fibroblasts were treated with rapamycin, metformin, or rapamycin–metformin, and their respiratory level and mitochondrial gene expression were assayed. Mice were fed a high-fat diet, immunized with type II collagen, and subsequently treated with rapamycin–metformin daily for 10 weeks.
Results
Rapamycin-treated cells exhibited dysfunction of mitochondrial respiration and decreased mitochondrial gene expression compared with rapamycin–metformin-treated cells. Moreover, rapamycin–metformin reduced the clinical arthritis score and the extent of histological inflammation and improved the metabolic profile in obese mice with CIA. Rapamycin–metformin enhanced the balance between T helper 17 and regulatory T cells in vitro and in vivo.
Conclusions
These results suggest that rapamycin–metformin is a potential therapeutic option for autoimmune arthritis.
Dosing at a pretty high level, daily, for 10 weeks:
Metformin and rapamycin treatment
Metformin and rapamycin were obtained from Sigma-Aldrich and dissolved in saline. Mice were orally administered 50 mg/kg metformin and/or 1 mg/kg rapamycin daily for 10 weeks starting 7 days after the first immunization. Control mice received saline.
Yes, the Nature paper also notes that they treated at 25nm, the dose observed in the blood of transplant patients so presumably our blood concentration is lower.
That paper observed increased superoxide production. Interestingly, rotenone treatment actually decreased ROS that they observed with sirolimus treatment (kind of similar to how in the first paper, metformin rescues the mitochondrial function of sirolimus treated cells, those are both Complex I inhibitors).
What the heck is going on with that? As someone who works with those drugs in the lab (and other ETC inhibitors), that is super interesting to me.
This is also rather strange. The ITP tried giving rapamycin to mice in food in their first study and could not get it to work (the rapamycin is destroyed in the gut and never makes it to the blood stream). So eventually they figured out a micro-encapsulation approach that became known as e-rapa, that did work. But the rapamycin disolved in saline should not, I believe, make it into the blood stream from what I’ve read.