In addition to its production by oxidative phosphorylation, ATP is synthesized by cytosolic glycolysis. The mitochondrial membrane potential (ΔΨ m) is utilized by the mitochondria for numerous processes, including the powering of mitochondrial ATP synthase.
Atp production series#
Electron transfer occurs along a series of mitochondrial polypeptide complexes-complex I (NADH dehydrogenase), complex III (cytochrome c reductase), and complex IV (cytochrome c oxidase) -and is electrochemically coupled to the translocation of protons across the inner mitochondrial membrane, generating a proton-motive force composed of an electrical gradient (ΔΨ m) and an H + gradient (ΔpH). In many cells, mitochondrial ATP synthesis provides the bulk of cellular ATP through oxidative phosphorylation, a process in which electrons flow from electron donors (NADH and FADH 2) generated by mitochondrial metabolic processes to the terminal electron acceptor, oxygen. In addition to its central role in cholesterol transport and metabolism in steroidogenic cells, the mitochondria are best known for their role in the synthesis of ATP. Androstenedione is finally metabolized in the ER by 17β-hydroxysteroid dehydrogenase (17βHSD) to yield the androgen testosterone. Subsequently, P5 is metabolized by the 3β-hydroxysteroid dehydrogenase/isomerase (3βHSD) enzyme in the endoplasmic reticulum (ER) to progesterone (P4), which is converted to androstenedione by the ER cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17). Upon transfer to the mitochondrial matrix, cholesterol is metabolized to P5 by CYP11A1. Several proteins, including the steroidogenic acute regulatory (STAR) protein and translocator protein (TSPO), are critical for this mitochondrial cholesterol transfer, operating as components of a larger “cholesterol transfer” complex recently named the transduceosome. This is the primary point of postreceptor control of steroidogenesis, because cholesterol does not freely diffuse across the mitochondrial intermembrane space. LH, through cAMP, promotes the transfer of cholesterol to the inner mitochondrial membrane, where it is metabolized into pregnenolone (P5) via the P450 cholesterol side-chain cleavage enzyme (CYP11A1). Acute synthesis of testosterone is stimulated by the binding of circulating luteinizing hormone (LH) to high-affinity receptors on the Leydig cell plasma membrane, which results in the formation of 3′,5′-cAMP. Synthesis of testosterone in mammalian males is performed almost exclusively by testicular Leydig cells. This work underscores the importance of mitochondrial ATP for hormone-stimulated steroid production in both MA-10 and primary Leydig cells while indicating that caution must be exercised in extrapolating data from tumor cells to primary tissue. Inhibitor studies also suggested that the MA-10 ETC is impaired. In contrast, primary cells appear to be almost completely dependent on mitochondrial respiration for their energy provision. Further studies revealed that a significant proportion of cellular ATP in MA-10 cells derives from glycolysis. However, in striking contrast to primary cells, perturbation of ΔΨ m in MA-10 cells did not substantially decrease cellular ATP content, a perplexing finding because ΔΨ m powers the mitochondrial ATP synthase. We show that mitochondrial ATP synthesis is critical for steroidogenesis in both primary and tumor Leydig cells.
Thus, to further understand the similarities and differences between the two systems as well as the impact of ATP disruption on steroidogenesis, we performed comparative studies of MA-10 and primary Leydig cells under similar conditions of mitochondrial disruption. These results suggest significant differences between the two systems and call into question the extent to which results from tumor Leydig cells relate to primary cells. In contrast, studies of primary Leydig cells indicated that the ETC, ΔΨ m, and ATP synthesis cooperatively affected steroidogenesis. Previous studies in MA-10 tumor Leydig cells demonstrated that disruption of the mitochondrial electron-transport chain (ETC), membrane potential (ΔΨ m), or ATP synthesis independently inhibited steroidogenesis.
0 kommentar(er)
