The protein at the center of the new work is the retinoic acid receptor alpha, or RARα, which is highly active in colorectal tumors and drives both proliferation and metastatic invasion. Traditional RARα modulators, built on retinoic acid scaffolds, suffer from chemical instability, excessive lipophilicity, and poor oral bioavailability, and they can provoke toxicity in the liver, skin, and bones. To escape these limitations, the researchers turned to proteolysis-targeting chimera, or PROTAC, technology. PROTACs are bifunctional molecules that simultaneously grip a target protein and an E3 ubiquitin ligase, forming a ternary complex that tags the target for destruction by the cell’s own ubiquitin–proteasome system. Because each PROTAC molecule dissociates after every degradation event and can engage targets repeatedly, this catalytic mechanism holds the promise of overcoming the drug resistance that plagues conventional inhibitors.

The team began with AR7, an atypical RARα antagonist, and selected its optimized derivative CA77.1 as the target-binding ligand. Molecular docking against the RARα structure (PDB:5K13) revealed that CA77.1 binds with a calculated energy of −7.379 kcal/mol, anchored by a key hydrogen bond to Arg276 and an extensive hydrophobic network. Crucially, a terminal methyl group on CA77.1 protrudes into the solvent-exposed region outside the ligand-binding pocket, providing an ideal attachment point for a linker without disturbing the core binding interactions. By covalently joining CA77.1 to thalidomide, a ligand for the E3 ubiquitin ligase CRBN, the researchers created a degrader that remains active in both oxygen-rich and hypoxic tumor regions, overcoming the hypoxia-dependent activation that limited the parent compounds.

Through systematic synthesis of six candidate molecules with varying carbon-chain and piperazine linkers, the team identified Z1 as the standout. Z1 degraded RARα with a DC50 of 6.02 ± 1.05 μM and a maximum degradation of 85.4 percent, an activity roughly 43.6 times greater than that of CA77.1 itself. Western blotting confirmed that Z1 left the related receptors RARβ and RARγ untouched, demonstrating high subtype selectivity. Mechanistic experiments sealed the case: cycloheximide chase assays showed accelerated decay of pre-existing RARα protein, while pre-treatment with the competitive ligand ATRA, the E1 enzyme inhibitor MLN4924, the proteasome inhibitor MG132, or excess thalidomide all blocked degradation, confirming the classical PROTAC pathway of direct binding and ubiquitin-proteasome-mediated clearance.

Molecular dynamics simulations running 100 nanoseconds provided atomistic support for the design. Z1 formed stable hydrogen bonds with both RARα and CRBN, with calculated binding free energies of −139.394 kJ/mol and −111.138 kJ/mol respectively, and per-residue energy decomposition pinpointed Pro407, Val395, and Ser225 as the thermodynamic core of RARα recognition. Functionally, Z1-induced degradation suppressed phosphorylation of AKT at Ser473 in a concentration-dependent manner while leaving total AKT unchanged, and it markedly reduced expression of MMP2, an enzyme that tumors use to invade surrounding tissue. This AKT–MMP2 axis, the authors note, had not been systematically characterized in earlier RARα degradation studies and offers a fresh mechanistic window into how acute protein elimination restrains cancer progression.

In vitro assays in HCT116 colorectal cancer cells painted a striking picture of Z1’s antitumor potency. Flow cytometry showed concentration-dependent arrest of cells at the G2/M phase, with the arrested population rising 3.7-fold at 100 μM. At 50 μM, Z1 completely abolished colony formation, whereas the same concentration of CA77.1 allowed more than 300 colonies to grow. Annexin V staining revealed that Z1 drove 34.6 to 55.2 percent of cells into late apoptosis, well above the 24.6 percent seen with CA77.1. Scratch assays demonstrated up to 90.4 percent inhibition of cell migration, and Transwell experiments showed that 50 μM Z1 cut the invasion rate to 10.8 percent, an 81.2 percent reduction relative to the parent compound.

Yet like all PROTACs, Z1 carries the field’s chronic handicaps: high molecular weight, poor water solubility, and weak membrane permeability. The team’s solution came from an unexpected source, the roots of Polygonatum sibiricum, a plant long valued in traditional medicine. Using a juicing, filtration, and ultracentrifugation workflow, the researchers isolated exosome-like nanoparticles, or PsELNs, that measure roughly 116 nanometers across, carry a mildly negative zeta potential of −5.8 mV, and display the characteristic disk-like morphology of plant vesicles. Their cargo of triglycerides forms a hydrophobic core that stabilizes hydrophobic drugs, while their natural tropism for colonic tissue and their ability to accumulate in tumors through the enhanced permeability and retention effect make them unusually well suited to colorectal cancer applications.

Encapsulation of Z1 within PsELNs achieved a drug-loading efficiency of 43.1 percent, exceeding values reported for goji berry and ginger vesicles, and the loaded particles grew only modestly to 143 nm while retaining an intact bilayer. Stability testing was impressive: at acidic pH 5.5 mimicking the tumor microenvironment, 77.38 percent of Z1 remained intact after 24 hours versus just 40.65 percent for the free drug, and the formulation resisted thermal stress at 44 °C, stayed stable in serum for 72 hours, and showed minimal leakage over 60 days of storage. Confocal microscopy and flow cytometry confirmed that Z1/PsELNs entered HCT116 cells far more efficiently than free Z1, and the encapsulated drug degraded RARα with a DC50 of 2.95 μM, a 2.2-fold improvement that translated into stronger apoptosis induction and greater cytotoxicity in vitro.

The decisive test came in living animals. Near-infrared imaging of tumor-bearing nude mice showed that DiR-labeled PsELNs accumulated in tumors at 2.13 times the level of the free dye, peaking at 24 hours after injection, while hemolysis rates below 5 percent confirmed blood compatibility. Over 16 days of tail-vein treatment, the Z1/PsELNs group showed significantly stronger tumor growth inhibition than mice receiving free Z1, with in vivo antitumor efficacy increasing 1.8-fold. Histopathology revealed extensive tumor cell damage, Ki67 staining showed the fewest proliferating cells, and both immunohistochemistry and Western blotting confirmed the lowest RARα levels in the nanoparticle-treated tumors, all without body-weight loss or organ damage. The authors propose that the intestinal stability and colon-targeting behavior of PsELNs could eventually support an oral PROTAC formulation, a goal that would address both gastrointestinal degradation and site-specific accumulation. By fusing event-driven protein degradation chemistry with a biocompatible, naturally derived delivery vehicle, the study charts a credible translational path for PROTAC therapeutics in colorectal cancer and illustrates how plant nanotechnology may help the next generation of degrader drugs reach their targets intact.

Subject of Research: A RARα-targeting PROTAC delivered by Polygonatum sibiricum exosome-like nanoparticles for colorectal cancer therapy

Article Title: Synthesis of RARα–PROTAC and its delivery via Polygonatum sibiricum exosome-like nanoparticles for the treatment of colorectal cancer

Article References: Zhu, G., Zhao, Y., Chen, M., Lu, S., Xu, L., Chen, G., Zeng, L., & Chen, J. (2026). Synthesis of RARα–PROTAC and its delivery via Polygonatum sibiricum exosome-like nanoparticles for the treatment of colorectal cancer. Materials Today Bio, 40, Article 103651. https://doi.org/10.1016/j.mtbio.2026.103651

Image Credits: AI Generated

DOI: 10.1016/j.mtbio.2026.103651

Keywords: colorectal cancer, PROTAC, RARα, protein degradation, Polygonatum sibiricum, exosome-like nanoparticles, drug delivery, nanomedicine, ubiquitin-proteasome system, AKT signaling, MMP2, targeted therapy

Enter PROTAC (Proteolysis Targeting Chimera) technology, a transformative approach that diverges fundamentally from traditional inhibition by harnessing the cell’s own ubiquitin-proteasome system to selectively degrade the LSD1 enzyme. PROTAC molecules operate catalytically, tethering the target protein to an E3 ubiquitin ligase which flags it for destruction. This elegant mechanism ensures profound and durable depletion of LSD1 at lower compound concentrations compared to occupancy-driven inhibitors. Such catalytic degradation promises not only heightened efficacy but also reduced emergence of drug resistance and diminished off-target toxicities, positioning PROTACs as a next-generation therapeutic platform in oncology.

In this groundbreaking study, the research team engineered a series of PROTAC compounds by fusing an LSD1-binding moiety named LI-1 with the cereblon (CRBN)-recruiting ligand thalidomide via linkers of varying lengths. Through meticulous structure-activity relationship (SAR) investigations, a candidate designated LD-110 surfaced as the most potent and selective degrader of LSD1. Biochemical assays revealed that LD-110 substantially diminished LSD1 protein levels in breast and lung cancer cell lines in both a time-dependent and dose-dependent fashion. Notably, the half-maximal degradation concentrations (DC₅₀) of LD-110 were impressively low in MDA-MB-231 and MDA-MB-453 breast cancer cells, registering at 9.54 and 7.08 nanomolar, respectively. Although lung cancer H520 cells exhibited a higher DC₅₀ of 446 nanomolar, this still represents significant efficacy given the challenge of targeting non-hematological tumors.

Validation of the underlying degradation mechanism was achieved through rigorous control experiments. The authors demonstrated that the proteasome inhibitor MG132 and the neddylation inhibitor MLN4924 effectively abrogated LD-110-induced LSD1 degradation, confirming the critical involvement of the ubiquitin-proteasome system and the CRBN E3 ligase pathway. Furthermore, competitive inhibition with the LI-1 warhead and thalidomide ligand prevented degradation, and a methylated analog of LD-110 (LD-110Me), incapable of binding CRBN, failed to induce either LSD1 degradation or downstream substrate accumulation. These findings collectively solidify that LD-110 functions as a bona fide PROTAC, exploiting CRBN recruitment to catalyze targeted proteostasis.

Beyond biochemical validation, the anti-proliferative effects of LD-110 were striking. The compound exhibited potent growth inhibition across diverse cancer cell lines, yielding half-maximal inhibitory concentrations (IC₅₀) ranging broadly but often in the sub-micromolar range. Of equal importance, pharmacokinetic profiling demonstrated that LD-110 possesses favorable in vivo characteristics, including bioavailability and metabolic stability, which translated into marked tumor growth suppression in both breast and lung cancer xenograft models. Remarkably, this potent anti-tumor activity was achieved without detectable systemic toxicity, suggesting an attractive therapeutic window for further development.

Delving into the mechanistic underpinnings of LD-110’s cytotoxicity revealed a sophisticated orchestration of cellular stress pathways. LD-110 was found to induce apoptotic cell death primarily via triggering endoplasmic reticulum (ER) stress, converging on activation of the ATF4-CHOP axis—a central regulator of stress-induced apoptosis. On one hand, transcriptional modulation stems from LSD1 degradation leading to increased H3K4 dimethylation (H3K4me2), which facilitates ATF4 gene expression. On the other hand, LD-110 also stimulates reactive oxygen species (ROS) production resulting in DNA damage, which activates the GCN2-eIF2α pathway to augment translational synthesis of ATF4 protein. This dual mechanism synergistically amplifies ATF4 levels, engaging downstream apoptotic effectors.

The ATF4-CHOP pathway modulates critical determinants of cell fate by rebalancing members of the BCL-2 protein family. Specifically, LD-110 elevates the expression of NOXA, a potent pro-apoptotic factor, while concomitantly reducing MCL1, an anti-apoptotic protein that often confers resistance to cell death. This shift in protein equilibrium decisively steers cancer cells toward programmed apoptosis, underpinning the robust anticancer effects observed.

Nonetheless, the use of CRBN as the E3 ligase recruitment element inherently carries limitations related to off-target degradation. CRBN naturally targets substrates such as GSPT1 and the IKZF family, which are not the intended therapeutic targets. Consistent with this, LD-110 also promoted degradation of GSPT1 alongside LSD1. Intriguingly, the researchers discovered that GSPT1 competes with LSD1 for LD-110 binding, thereby diminishing the degrader’s efficiency toward LSD1. Through siRNA-mediated knockdown of GSPT1, the inhibitory effect on LSD1 degradation was alleviated, resulting in enhanced LD-110 potency and more pronounced growth inhibition of cancer cells.

This observation suggests a compelling therapeutic strategy: combining LD-110 with a selective GSPT1 degrader could yield synergistic anti-tumor activity through dual pathway engagement. Such a combination may allow for dose reduction, potentially minimizing toxicity while maximizing efficacy—an elegant example of precision polypharmacology.

In summation, this investigation heralds LD-110 as a pioneering PROTAC molecule that effectively depletes LSD1, exhibiting significant anticancer activity in vitro and in vivo. The dual mechanism of inducing ER stress and modulating epigenetic marks represents a novel therapeutic angle to combat cancers characterized by LSD1 overexpression. With favorable pharmacokinetic and safety profiles, LD-110 stands poised as a promising candidate to advance into clinical development, potentially transforming the therapeutic landscape for patients afflicted with breast, lung, and possibly other cancers.

As the field of targeted protein degradation continues to revolutionize drug discovery, studies such as this provide compelling proof-of-concept that PROTACs can surpass the limitations of traditional inhibitors. By capitalizing on polyfunctional molecular design, researchers are opening avenues toward durable, selective, and potent cancer therapies that exploit intrinsic cellular machinery to disarm oncogenic drivers.

Future research will undoubtedly explore further optimization of linker chemistry, E3 ligase selection, and combination regimens with other targeted agents. Moreover, deciphering and mitigating off-target effects inherent to PROTAC technology remains a priority to maximize clinical benefit. The promising results reported here pave the way for a new era in epigenetic cancer therapy, leveraging protein degradation machinery to deliver precise, potent, and lasting tumor suppression.

Subject of Research: LSD1 Protein Degradation Using PROTAC Technology for Cancer Therapy

Article Title: Discovery of LD-110 as a Potent PROTAC Degrader of LSD1 with Therapeutic Efficacy in Breast and Lung Cancer Models

Web References:
10.1016/j.scib.2025.10.024

Keywords:
Life sciences, Health and medicine, Biochemistry, Cancer treatments