The interaction of steroids with phospholipid bilayers and membranes.

Steroids are critical for various physiological processes and used to treat inflammatory conditions. Steroids act by two distinct pathways. The genomic pathway is initiated by the steroid binding to nuclear receptors while the non-genomic pathway involves plasma membrane receptors. It has been proposed that steroids might also act in a more indirect mechanism by altering biophysical properties of membranes. Yet, little is known about the effect of steroids on membranes, and steroid-membrane interactions are complex and challenging to characterise. The focus of this review is to outline what is currently known about the interactions of steroids with phospholipid bilayers and illustrate the complexity of these systems using cortisone and progesterone as the main examples. The combined findings from current work demonstrate that the hydrophobicity and planarity of the steroid core does not provide a consensus for steroid-membrane interactions. Even small differences in the substituents on the steroid core can result in significant changes in steroid-membrane interactions. Furthermore, steroid-induced changes in phospholipid bilayer properties are often dependent on steroid concentration and lipid composition. This complexity means that currently there is insufficient information to establish a reliable structure-activity relationship to describe the effect of steroids on membrane properties. Future work should address the challenge of connecting the findings from studying the effect of steroids on phospholipid bilayers to cell membranes. Insights from steroid-membrane interactions will benefit our understanding of normal physiology and assist drug development.