Glucocorticoids induce mitochondrial gene transcription in HepG2 cells: role of the mitochondrial glucocorticoid receptor

Abstract

Glucocorticoids are major regulators of a plethora of cellular functions, acting on target cells through glucocorticoid receptors (GR) and modulation of gene transcription, among other mechanisms. One main site of action of glucocorticoids is the hepatocyte, which responds to the hormonal stimulus with induction of several proteins among them enzymes of oxidative phosphorylation (OXPHOS), both nuclearly and mitochondrially encoded. The induction of OXPHOS is regarded as a result of a nuclear action of the receptor on the respective nuclear genes and on genes encoding mitochondrial transcription factors. The presence of GR in mitochondria and of sequences in the mitochondrial genome similar to glucocorticoid responsive elements, suggested a direct action of GR on mitochondrial transcription. We demonstrate in HepG2 hepatocarcinoma cells specific binding of GR to the regulatory D-loop region of the mitochondrial genome and show that dexamethasone induces the mitochondrial transcription factors A, B1, and B2, the mitochondrial ribosomal RNA, and several mitochondrially encoded OXPHOS genes. Applying α-amanitin, the specific inhibitor of DNA-dependent RNA polymerase II, the dexamethasone-induced transcription of the mitochondrial genes can still proceeds, whereas the DEX effect on transcription of the mitochondrial transcription factors is suppressed. Moreover, HepG2 cells overexpressing mitochondrial targeted GR showed increased RNA synthesis, cytrochrome oxidase subunit I protein expression, and mitochondrial ATP production. We conclude that glucocorticoids can stimulate directly mitochondrial transcription by the mitochondrially localized GR, affecting OXPHOS enzyme biosynthesis. This takes place in addition to their action on mitochondrial genes by way of induction of the nuclearly encoded mitochondrial transcription factors.