Identification and development of novel cyclic peptide inhibitors of IDOL mediated LDLR degradation

Abstract

Cholesterol is an essential component of the plasma membrane however if present in excess, free cholesterol is toxic to cells and can lead to complications such as atherosclerosis, and ultimately cardiovascular disease. The low density lipoprotein receptor binds and internalises circulating plasma cholesterol, present in low density lipoprotein molecules. The low density lipoprotein receptor has two main transcriptional regulators: liver X receptors and sterol regulatory element binding proteins and post-transcriptional regulators, namely the E3-ubiquitin ligase IDOL, which binds and ubiquitinates the low density lipoprotein receptor intracellular tail, targeting it for lysosomal degradation. Several protein-protein interactions are required for the ubiquitination activity of IDOL, serving as potential therapeutic modulation sites for the treatment of hypercholesterolemia via an upregulation of low density lipoprotein receptor protein levels.

Bacterial reverse-two hybrid systems were designed for the IDOL-LDLR heterodimerisation and the IDOL homodimerisation interactions. The latter system was used to screen a library of 3.2 million cyclic peptides, identifying a series of peptides capable of inhibiting the IDOL homodimerisation event. These peptides were synthesised and their activity assessed using a selection of in vitro assays, providing a lead candidate.

The efficacy of the lead candidate peptide was improved through the development of a small library of non-natural derivatives, improving binding activity by 7-fold. The activity of the new generation peptide was assessed both in vitro for ability to inhibit autoubiquitination and in hepatic cells, elucidating biological implications of the inhibition of IDOL mediated LDLR degradation.

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Identifiers

ORCID for Ali Tavassoli: orcid.org/0000-0002-7420-5063

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