Differential effects of membrane sphingomyelin and cholesterol on agonist-induced bitter taste receptor T2R14 signaling

A cholesterol-centric outlook on steroidogenesis

Cholesterol, an essential and versatile lipid, is the precursor substrate for the biosynthesis of steroid hormones, and a key structural and functional component of organelle membranes in eukaryotic cells. Consequently, the framework of steroidogenesis across main steroidogenic cell types is built around cholesterol, including its cellular uptake, mobilization from intracellular storage, and finally, its transport to the mitochondria where steroidogenesis begins. This setup, which is controlled by different trophic hormones in their respective target tissues, allows steroidogenic cells to meet their steroidogenic need of cholesterol effectively without impinging on the basic need for organelle membranes and their functions. However, our understanding of the basal steroidogenesis (i.e., independent of trophic hormone stimulation), which is a cell-intrinsic trait, remains poor. Particularly, the role that cholesterol itself plays in the regulation of steroidogenic factors and events in steroid hormone-producing cells remains largely unexplored. This is likely because of challenges in selectively targeting the steroidogenic intracellular cholesterol pool in studies. New evidence suggests that cholesterol plays a role in steroidogenesis. These new findings have created new opportunities to advance our understanding in this field. In this book chapter, we will provide a cholesterol-centric view on steroidogenesis and emphasize the importance of the interplay between cholesterol and the mitochondria in steroidogenic cells. Moreover, we will discuss a novel mitochondrial player, prohibitin-1, in this context. The overall goal is to provide a stimulating perspective on cholesterol as an important regulator of steroidogenesis (i.e., more than just a substrate for steroid hormones) and present the mitochondria as a potential cell-intrinsic factor in regulating steroidogenic cholesterol homeostasis.

Insights into the Role of Membrane Lipids in the Structure, Function and Regulation of Integral Membrane Proteins

Membrane proteins exist within the highly hydrophobic membranes surrounding cells and organelles, playing key roles in cellular function. It is becoming increasingly clear that the membrane does not just act as an appropriate environment for these proteins, but that the lipids that make up these membranes are essential for membrane protein structure and function. Recent technological advances in cryogenic electron microscopy and in advanced mass spectrometry methods, as well as the development of alternative membrane mimetic systems, have allowed experimental study of membrane protein–lipid complexes. These have been complemented by computational approaches, exploiting the ability of Molecular Dynamics simulations to allow exploration of membrane protein conformational changes in membranes with a defined lipid content. These studies have revealed the importance of lipids in stabilising the oligomeric forms of membrane proteins, mediating protein–protein interactions, maintaining a specific conformational state of a membrane protein and activity. Here we review some of the key recent advances in the field of membrane protein–lipid studies, with major emphasis on respiratory complexes, transporters, channels and G-protein coupled receptors.

Taurine attenuates gossypol-induced apoptosis of C2C12 mouse myoblasts via the GPR87-AMPK/AKT signaling

Gossypol, a toxic polyphenol extracted from cotton seeds, is hazardous to human and animal health. Taurine is considered as an essential or semi-essential amino acid and has diverse cytoprotective effects. This study was aimed to investigate the protective effect and molecular mechanism of taurine against apoptosis of C2C12 mouse myoblasts induced by gossypol. C2C12 mouse myoblasts were exposed to gossypol (0, 1 nM, 10 nM, 100 nM, 1 μM, and 10 μM). Cell numbers were rapidly decreased with increasing concentrations of gossypol. Gossypol significantly induced apoptosis, decreased Bcl2 expression, and increased the protein levels of Bax and the cleaved caspase 3. Taurine (0.24 mM) treatment largely rescued the cell number decreased by gossypol, attenuated gossypol-induced cell apoptosis. GPR87 knockdown abolished the inhibition by taurine of cell apoptosis. Furthermore, GPR87 overexpression attenuated cell apoptosis induced by gossypol. Both taurine treatment and GPR87 overexpression stimulated AKT phosphorylation but inhibited AMPK phosphorylation, whereas gossypol had the opposite effects. Taurine treatment promoted GPR87 expression and subcellular localization and partially rescued the inhibition of gossypol on this expression. In summary, these data reveal that taurine attenuates gossypol-induced apoptosis of C2C12 mouse myoblasts via the GPR87-AMPK/AKT signaling.