Tafazzin-dependent cardiolipin composition in C6 glioma cells correlates with changes in mitochondrial and cellular functions, and cellular proliferation.

The mitochondrial phospholipid cardiolipin (CL) has been implicated with mitochondrial morphology, function and, more recently, with cellular proliferation. Tafazzin, an acyltransferase with key functions in CL remodeling determining actual CL composition, affects mitochondrial oxidative phosphorylation. Here, we show that the CRISPR-Cas9 mediated knock-out of tafazzin (Taz) is associated with substantial alterations of various mitochondrial and cellular characteristics in C6 glioma cells. The knock-out of tafazzin substantially changed the profile of fatty acids incorporated in CL and the distribution of molecular CL species. Taz knock-out was further associated with decreased capacity of oxidative phosphorylation that mainly originates from impaired complex I associated energy metabolism in C6 glioma cells. The lack of tafazzin switched energy metabolism from oxidative phosphorylation to glycolysis indicated by lower respiration rates, membrane potential and higher levels of mitochondria-derived reactive oxygen species but keeping the cellular ATP content unchanged. The impact of tafazzin on mitochondria was also indicated by altered morphology and arrangement in tafazzin deficient C6 glioma cells. In the cells we observed tafazzin-dependent changes in the distribution of cellular fatty acids as an indication of altered lipid metabolism as well as in stability/morphology. Most impressive is the dramatic reduction in cell proliferation in tafazzin deficient C6 glioma cells that is not mediated by reactive oxygen species. Our data clearly indicate that defects in CL phospholipid remodeling trigger a cascade of events including modifications in CL linked to subsequent alterations in mitochondrial and cellular functions.

Effects of siRNA-dependent knock-down of cardiolipin synthase and tafazzin on mitochondria and proliferation of glioma cells.

The mitochondrial phospholipid cardiolipin (CL) has been implicated with mitochondrial morphology, function, and cell proliferation. Changes in CL are often paralleled by changes in the lipid environment of mitochondria that may contribute to mitochondrial function and proliferation. This study aimed to separate the effects of CL content and CL composition from cellular free fatty acid distribution on bioenergetics and proliferation in C6 glioma cells. To this end, cardiolipin synthase and the CL remodelling enzyme, tafazzin, were knocked-down by siRNA in C6 cells. After 72 h of cultivation, we analysed CL composition by means of LC/MS/MS, distribution of cellular fatty acids by means of gas chromatography, and determined oxygen consumption and proliferation. Knock-down of cardiolipin synthase affected the cellular CL content in the presence of linoleic acid (LA) in the culture medium. Knock-down of tafazzin had no consequence with respect to the pattern of cellular fatty acids but caused a decrease in cell proliferation. It significantly changed the distribution of molecular CL species, increased CL content, decreased oxygen consumption, and decreased cell proliferation when cultured in the presence of linoleic acid (LA). The addition of linoleic acid to the culture medium caused significant changes in the pattern of cellular fatty acids and the composition of molecular CL species. These data suggest that tafazzin is required for efficient bioenergetics and for proliferation of glioma cells. Supplementation of fatty acids can be a powerful tool to direct specific changes in these parameters.

The Angiotensin-(1-7)/Mas Axis Improves Pancreatic ß-Cell Function in Vitro and in Vivo.

The angiotensin-converting enzyme 2/angiotensin (Ang)-(1-7)/Mas axis of the renin-angiotensin system often opposes the detrimental effects of the angiotensin-converting enzyme/Ang II/Ang II type 1 receptor axis and has been associated with beneficial effects on glucose homeostasis, whereas underlying mechanisms are mostly unknown. Here we investigate the effects of Ang-(1-7) and its receptor Mas on ß-cell function. Isolated islets from Mas-deficient and wild-type mice were stimulated with Ang-(1-7) or its antagonists and effects on insulin secretion determined. Islets' cytoplasmic calcium and cAMP concentrations, mRNA amounts of Ins1, Ins2, Pdx1, and Mafa and effects of inhibitors of cAMP downstream signaling were determined. Ang-(1-7) was also applied to mice by osmotic pumps for 14 days and effects on glucose tolerance and insulin secretion were assessed. Ang-(1-7) increased insulin secretion from wild-type islets, whereas antagonists and genetic Mas deficiency led to reduced insulin secretion. The Mas-dependent effects of Ang-(1-7) on insulin secretion did not result from changes in insulin gene expression or changes in the excitation-secretion coupling but from increased intracellular cAMP involving exchange protein activated directly by cAMP. Administration of Ang-(1-7) in vivo had only marginal effects on glucose tolerance in wild-type mice but still resulted in improved insulin secretion from islets isolated of these mice. Interestingly, although less pronounced than in wild types, Ang-(1-7) still affected insulin secretion in Mas-deficient islets. The data indicate a significant function of Ang-(1-7) in the regulation of insulin secretion from mouse islets in vitro and in vivo, mainly, but not exclusively, by Mas-dependent signaling, modulating the accessory pathway of insulin secretion via increase in cAMP.