A 'Selective' Ghrelin Blocker Peptide Turns Out to Also Block HIV Entry Into Immune Cells | Research

The takeaway

A peptide assumed to be a specific ghrelin receptor blocker was found to also inhibit the CXCR4 receptor — partially blocking HIV-1 entry into immune cells and calling into question many ghrelin research findings.

Dual receptor blockade
DLS, used in hundreds of studies as a 'selective' ghrelin blocker, also blocks CXCR4 — the receptor HIV uses to enter immune cells — calling into question years of ghrelin research

What the researchers found

D-[Lys3] GHRP-6 (DLS), widely used as a 'selective' ghrelin receptor antagonist, was unexpectedly found to also block the CXCR4 chemokine receptor. DLS blocked CXCL12 binding and signaling through CXCR4 on T cells and PBMCs. Because CXCR4 is a major co-receptor for HIV-1 entry into CD4+ cells, DLS partially blocked HIV-1 entry and replication in activated human PBMCs.

This means that many published studies using DLS as a 'specific' ghrelin receptor tool may have confounded results, as DLS has dual receptor activity. It also suggests that structural analogs of DLS could potentially block HIV infection, CXCR4-dependent cancer cell migration, and inflammatory immune cell trafficking.

Why it matters

This finding has two major implications. First, it calls into question the results of numerous studies that used DLS as a 'selective' ghrelin receptor blocker — any effects attributed to ghrelin receptor blockade could have been partly or wholly due to CXCR4 inhibition. Second, it opens an unexpected therapeutic avenue: a ghrelin-related peptide that can block HIV entry into immune cells, potentially leading to novel anti-HIV or anti-cancer peptide therapeutics based on DLS structural analogs.

The numbers in context

DLS blocks both GHS-R1a and CXCR4 · CXCL12 binding inhibited · HIV-1 entry partially blocked · T cell and PBMC effects confirmed · Dual receptor activity demonstrated

How the study worked

Researchers tested DLS for CXCR4 activity using CXCL12 binding and chemotaxis assays on T cells and PBMCs. Calcium signaling was measured to confirm functional CXCR4 blockade. HIV-1 entry and propagation were assessed in activated human PBMCs in the presence and absence of DLS.

Who was studied

In vitro studies using human T cells, PBMCs, and HIV-1 infection assays

What this study cannot tell us

DLS only partially (not completely) blocked HIV-1 entry, suggesting it is a moderate CXCR4 antagonist. The study does not determine whether DLS acts as a competitive or allosteric CXCR4 antagonist. The binding affinity for CXCR4 versus GHS-R1a was not directly compared. In vivo validation of the anti-HIV or anti-CXCR4 effects was not performed. The HIV blocking was demonstrated in cell culture, not in animal models or patients.

How to read the evidence

This is a preclinical in vitro study demonstrating an unexpected pharmacological property of a widely used research peptide. The CXCR4 blocking and HIV inhibition are well-documented but only shown in cell culture, not in vivo.

When this study was published

Published in 2012, this study raised important concerns about the selectivity of a common ghrelin research tool. The ghrelin research field has since needed to account for possible CXCR4 confounding in DLS-based studies.

The bigger picture

Off-target effects of research tool peptides can both invalidate previous findings and open unexpected therapeutic doors. The CXCR4 receptor is a validated drug target — the CXCR4 antagonist plerixafor is approved for stem cell mobilization, and CXCR4 blockade is being explored for HIV, cancer metastasis, and inflammatory diseases. The finding that a ghrelin-family peptide can access CXCR4 suggests previously unrecognized structural similarities between these receptors and could inspire new dual-target peptide therapeutics.

Questions still open

How many published ghrelin receptor studies using DLS need to be reinterpreted in light of its CXCR4 activity?
Can DLS structural analogs be optimized for potent CXCR4 blockade while minimizing ghrelin receptor effects?
Could dual GHS-R/CXCR4 peptide antagonists have therapeutic value in conditions involving both appetite dysregulation and inflammation or infection?

Common questions

Why does it matter that a research tool peptide has off-target effects?

Scientists use tool compounds to study specific receptors — if a peptide is supposed to block only the ghrelin receptor, any effects observed are attributed to ghrelin blockade. But if the peptide also blocks CXCR4, some effects may actually be due to CXCR4 inhibition. This means conclusions from years of ghrelin research using DLS may be partly wrong, and the true role of ghrelin signaling could be different from what was reported.

Could this peptide really be used against HIV?

DLS only partially blocked HIV entry, so it's not potent enough to be a treatment on its own. However, the finding proves that ghrelin-family peptides can interact with CXCR4 — the HIV co-receptor. Chemists could use DLS as a starting point to design more potent CXCR4-blocking peptides that could contribute to anti-HIV combination therapy, alongside its potential use against CXCR4-driven cancer metastasis.

Read the original research

Identification of ghrelin receptor blocker, D-[Lys3] GHRP-6 as a CXCR4 receptor antagonist.

International journal of biological sciences, 8(1), 108-17

View on PubMed ↗

Citation

Patel, Kalpesh; Dixit, Vishwa Deep; Lee, Jun Ho; Kim, Jie Wan; Schaffer, Eric M; Nguyen, Dzung; Taub, Dennis D. (2012). Identification of ghrelin receptor blocker, D-[Lys3] GHRP-6 as a CXCR4 receptor antagonist.. International journal of biological sciences, 8(1), 108-17.