Disruption of calcium transfer from ER to mitochondria links alterations of mitochondria-associated ER membrane integrity to hepatic insulin resistance

Abstract

Aims/hypothesis Mitochondria-associated endoplasmic reticulum membranes (MAMs) are regions of the endoplasmic reticulum (ER) tethered to mitochondria and controlling calcium (Ca 2+ ) transfer between both organelles through the complex formed between the voltage-dependent anion channel, glucose-regulated protein 75 and inositol 1,4,5-triphosphate receptor (IP3R). We recently identified cyclophilin D (CYPD) as a new partner of this complex and demonstrated a new role for MAMs in the control of insulin's action in the liver. Here, we report on the mechanisms by which disruption of MAM integrity induces hepatic insulin resistance in CypD (also known as Ppif)-knockout (KO) mice. Methods We used either in vitro pharmacological and genetic inhibition of CYPD in HuH7 cells or in vivo loss of CYPD in mice to investigate ER-mitochondria interactions, inter-organelle Ca 2+ exchange, organelle homeostasis and insulin action. Results Pharmacological and genetic inhibition of CYPD concomitantly reduced ER-mitochondria interactions, inhibited inter-organelle Ca 2+ exchange, induced ER stress and altered insulin signalling in HuH7 cells. In addition, histaminestimulated Ca 2+ transfer from ER to mitochondria was blunted in isolated hepatocytes of CypD-KO mice and this was associated with an increase in ER calcium store. Interestingly, disruption of inter-organelle Ca 2+ transfer was associated with ER stress, mitochondrial dysfunction, lipid accumulation, activation of c-Jun N-terminal kinase (JNK) and protein kinase C (PKC)ε and insulin resistance in liver of CypD-KO mice. Finally, CYPD-related alterations of insulin signalling were mediated by activation of PKCε rather than JNK in HuH7 cells. Conclusions/interpretation Disruption of IP3R-mediated Ca 2+ signalling in the liver of CypD-KO mice leads to hepatic insulin resistance through disruption of organelle interaction and function, increase in lipid accumulation and activation of PKCε. Modulation of ER-mitochondria Ca 2+ exchange may thus provide an exciting new avenue for treating hepatic insulin resistance.

Metabolic implications of organelle–mitochondria communication

“…In addition, CypD deficiency is associated with deficient insulin signaling and a reduced number of VDAC1/IP3R1 interactions in human liver cells [27]. The metabolic effects detected under conditions of CypD deficiency in mice and in human liver cells were characterized by UPR ER [176]. In conclusion, these data are coherent with a model in which CYPD plays an important role in the maintenance of mitochondria-ER contact sites, which, upon dysregulation, trigger ER stress and metabolic alterations, namely deficient insulin signaling and insulin resistance ( Fig 4A).…”

“…A shared feature of the absence of MFN2, GRP75, BAP31, or CYPD is the activation of UPR [97,156,166,173,175,176]. ER stress was initially proposed as a mechanism that drives insulin resistance-related diseases [177], and altered reticulum proteostasis alteration has also been in the spotlight as a possible driver of metabolic diseases [178].…”

Reduction of endoplasmic reticulum- mitochondria interactions in beta cells from patients with type 2 diabetes

“…Increased basal and decreased glucose-stimulated Ca 2+ concentrations have been associated with cell dysfunction [8] and alteration of beta cell calcium dynamics is an early event during type 2 diabetes [9]. Whereas recent data suggested a disruption of organelle coupling in insulin resistant liver that may participate in altered glucose homeostasis [10,11], the relevance of MAM in beta cell dysfunction has not been studied.…”

Metformin attenuates ER stress–induced mitochondrial dysfunction

“…30 Pathologic ER stress leads to calcium release, which is directed toward the mitochondria via domains shared by ER and mitochondria called mitochondria-associated membranes (MAM). 31 Therefore, we hypothesized: (1) that metformin will decrease the ER stress-induced MPTP opening measured in isolated heart mitochondria; (2) that metformin will protect the ETC during ER stress; (3) metformin will inhibit ER stress-induced CHOP expression and subsequent cell death.…”