We found that the low-dose rapamycin treatment that increases life span in aged mice [5
] caused glucose intolerance by reducing plasma insulin levels before and after glucose stimulation. In lean C57B/L6 mice, a higher dose of rapamycin treatment not only reduced plasma insulin and plasma C-peptide levels but also caused insulin resistance accompanied with compromised hepatic mTOR and insulin signaling. We further characterized the impairment in insulin secretion of pancreatic islets from rapamycin-treated mice, showing that it likely arose from a reduction in beta cell mass and insulin content.
Previous studies report that chronic rapamycin treatment raises plasma insulin of DIO mice [19
] and of rats displaying impaired hepatic insulin clearance [18
]. In our study, we show that chronic rapamycin treatment of young as well as old mice causes glucose intolerance in a dose-dependent and reversible manner (Fig. ). Under standard rapamycin regimen (0.5 mg/kg), it took about 2 weeks to fully develop glucose intolerance, and glucose tolerance was restored 2 weeks after stopping the rapamycin regimen (Fig. ). Given that the beta cell proliferation rate is very slow, we suspect that a reduction of insulin content upon rapamycin treatment may contribute significantly to a faster onset and offset of glucose intolerance (Fig. , ). Previous studies have shown that insulin turnover rate is about 48–72 h in rodents [31
]. Since both the insulin secretory machinery and the excitation–secretion coupling are not affected, rapamycin may slow down the de novo synthesis of insulin without affecting preexisting insulin in the secretory granules. This way, it may take a few days, if not weeks, to deplete the preexisting insulin pool even if rapamycin halts the synthesis of insulin. Also, we suspect that during the first few days after stopping rapamycin treatment, the residual rapamycin in the system might still be effective in suppressing insulin production and inhibiting beta cell proliferation so that the recovery is delayed until rapamycin level is reduced below its pharmacological effective concentration. Indeed, in vivo pharmacokinetic analysis has revealed that rapamycin has a relatively long half-life (5–12 h), larger volume of distribution, and extensive tissue binding [32
Rapamycin treatment of lean C57B/L6 mice reduces the glucose-stimulated insulin secretion in both in vivo and ex vivo conditions (Fig. ). Our electrophysiological recordings revealed no alterations in glucose stimulus–excitation coupling and the exocytotic machinery of beta cells from rapamycin-treated mice (Fig. ). Therefore, the impaired insulin secretion is likely due to a reduction of the total insulin content by 70% (Fig. ) and of the average beta cell mass by 50% in rapamycin-treated mice (Fig. ). Since both the insulin-positive cell area (Fig. ) and the membrane capacitance of beta cells were normal in islets from rapamycin-treated mice, these mice most likely have fewer beta cells. Indeed, we found a reduction in dividing cells as well as insulin-containing cells in islets from rapamycin-treated mice (Fig. ), consistent with previous studies [17
]. Nir et al. [33
] have demonstrated the regenerative capacity of mouse pancreatic beta cells after diabetogenic injury, and they have also shown that immunosuppressants such as rapamycin and tarcolimus used in the Edmonton protocol, a pancreatic islet transplantation procedure for treating type I diabetes in patients, prevents beta cell proliferation.
Our study has important clinical implications, given that long-term rapamycin (sirolimus) exposure is required to prolong life span or treat illnesses such as Alzheimer’s disease, organ transplant, and cancer [3
] and given the consideration of rapamycin in human clinical trials [6
]. Our findings raise the concern that chronic low-dose rapamycin regimen has the potential to disrupt glucose homeostasis, thereby elevating the risk for hyperglycemia, which could lead to diabetes. A reduction in circulating insulin causing glucose intolerance could be a double-edged sword: lowering the insulin level impairs glucose homeostasis, extending life span, but also likely compromising the quality of life.
How can the conflicting reports of rapamycin effects be reconciled? On the one hand, overactivation of the mTOR-S6K pathway may be diabetogenic in mammals. For example, in a short-term in vivo study on human subjects, rapamycin ameliorates glucose intolerance caused by nutrient abundance of amino acids [34
]. Additionally, deleting S6K1 alleviates insulin resistance and improves beta cell survival under conditions of chronic hyperglycemia [15
], and rapamycin injection prevents the onset of insulin dependent diabetes in non-obese diabetic mice [35
]. Conversely, other studies have suggested that rapamycin is diabetogenic: a retrospective study has shown that kidney transplant patients taking rapamycin have a significantly higher risk of developing diabetes [16
]; rapamycin also induces diabetes in DIO sand rats [17
] and mice [19
]. Our study has further uncovered the diabetogenic effects of rapamycin in young and old lean inbred C57B/L6 mice. One way to reconcile these conflicting findings is to consider that the effects of rapamycin are different depending on the duration of the treatment: short-term rapamycin treatment seems to alleviate the metabolic symptoms [34
], whereas chronic rapamycin treatment reduces insulin secretion and decreases insulin sensitivity; hence, chronic rapamycin treatment is diabetogenic and causes deterioration of metabolic symptoms in mice of different ages and strains (Figs. and ).
In summary, our study shows that chronic rapamycin is diabetogenic in mice. Chronic inhibition of mTORC1 with rapamycin impairs insulin secretion and causes insulin resistance. It would be prudent to monitor glucose homeostasis in patients treated with rapamycin or other mTOR inhibitors to minimize the risk of developing metabolic syndromes.