Research Spotlight: The Acyl Chains of Phosphoinositide PIP3 Alter the Structure and Function of Nuclear Receptor Steroidogenic Factor-1

Posted on June 23, 2021

Sf1 Crystal Structure

Lipids are bound by transcription factors called nuclear receptors. Nuclear receptors are involved in several key processes such as homeostasis, development, metabolism, and signaling. The binding interaction between these nuclear receptors and lipid ligands causes a structural conformation that facilitates interaction with transcriptional coactivators. Nuclear receptor steroidogenic factor-1 (SF-1, NR5A1) specifically binds the signaling phosphoinositides, PI(4,5)P2 and PI(3,4,5)P3, and is responsible for the development and function of steroidogenic tissue. It is required for the proper regulation of gene expression as well as sexual determination via the development of the adrenals, gonads, and hypothalamus in mammals. Research has previously shown how the headgroups of these PIP lipids interact with and regulate SF-1 function. But what role do the acyl chains of these PIPs play in this interaction? That’s what researchers at Vanderbilt University School of Medicine wanted to find out.

The study showed that an increase in acyl chain length and unsaturation alters the binding of PIPs to SF-1. A mobility shift assay that had previously been used was employed to determine the effect of chain length while holding the head group constant. The dioleoyl PIP3 was compared to dipalmitoyl PIP3, and it was determined that the dioleoyl PIP3 had a 3-fold lower binding affinity. The same trend held when comparing dioleoyl and dipalmitoyl PIP2. When using a longer acyl chain with a higher degree of unsaturation (18:0/20:4), an even lower binding affinity was determined. They also used dioleoyl PIP3 to determine if the chain length and degree of unsaturation had any change on the SF-1 crystal structure.

The study was conducted in the same manner as the one that previously solved the structure of dipalmitoyl PIP3 bound to SF-1. The 2.5Å crystal structure of SF-1 was determined bound with the same 14-mer peptide used in previous studies and dioleoyl PIP3. The crystal structure adopts a 12-helical bundle structure which is typical of nuclear receptor ligand-binding domains. The crystal structure also showed that the unsaturated acyl chains of dioleoyl PIP3 took a similar path through the core of SF-1 as do the saturated chains of dipalmitoyl PIP3. However, there were differences in the crystal structures of SF-1 bound to dioleoyl PIP3 versus dipalmitoyl PIP3. The key differences in the crystal structure occur in the loop between helix 2 and helix 3 (2-3 loop) and the coactivator peptide region.

The 2-3 loop is the site of polymorphisms found in patients presenting with adrenal insufficiency, ovarian insufficiency, male infertility, and disorders of sexual development. This is an area of clinical importance and until the current study conformational changes in this loop have never been directly linked to changes in acyl chain length and saturation. Introducing the longer and unsaturated acyl chains present in dioleoyl PIP3 induced disordering in the 2-3 loop. This study shows the importance of structure-activity relationships for acyl chains of varying lengths and degrees of unsaturation. This potentially explains how polymorphisms of SF-1 contribute to sexual development disorders in humans.

It is always amazing to see what Avanti’s products are being used for and we are glad that our PIP lipids are being put to good use at Vanderbilt University School of Medicine!

Click HERE to read the full article, and stay tuned to see what else is being done with our lipids!

Image Credit: Original Research Publication