Micheliolide (MCL) is a guaiane-type sesquiterpene lactone derived from natural plants, which is distributed in the root bark of Magnoliaceae plants such as Michelia champaca and Michelia formosana. Its chemical structure is shown in the figure below. MCL can also be obtained through chemical reactions using parthenolide (PTL) as a raw material. Its activity is comparable to PTL, but it has higher stability, lower toxicity, and lower production costs, making it more promising as a clinical drug. Modern pharmacological research has shown that MCL possesses multiple pharmacological effects such as anti-inflammatory, anti-tumor, immunomodulatory, and neuroprotective effects. However, MCL has poor water solubility, resulting in too low plasma concentration after ingestion, which affects its pharmacological effects in vivo. In recent years, to improve the biological activity and plasma concentration of MCL, a large number of structural modifications and transformations have been conducted, leading to the discovery of a series of MCL derivatives with superior activity to MCL and more promising drug-forming research potential.

1. Anti-inflammatory Effects

Micheliolide possesses potential anti-inflammatory biological activities, which can be used to treat various inflammatory diseases such as enteritis and rheumatoid arthritis. Research has shown that Micheliolide can reduce the expression levels of inflammatory cytokines by regulating multiple related signaling pathways, including nuclear factor-κB (NF-κB) and phosphoinositide 3-kinase/protein kinase B (PI3K/Akt).

Micheliolide is able to inhibit the levels of tumor necrosis factor-α (TNF-α), IL-1β, IL-6, IL-18, and interferon-γ (IFN-γ) in the serum of ankylosing spondylitis (AS) model mice, reduce the protein expression levels of caspase-1 p10, IL-1β p17, nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3), and caspase-3, while inhibiting the expression of phosphorylated p65 (p-p65) and increasing the expression of phosphorylated IκBα (p-IκBα). This indicates that Micheliolide can alleviate AS by inhibiting the activation of NLRP3 inflammasomes, regulating the NF-κB signaling pathway, and maintaining the balance of Th1/Th2. Micheliolide significantly inhibits the activation of the NF-κB signaling pathway induced by lipopolysaccharide, reduces the expression levels of NLRP3 and caspase-1, and decreases the secretion of IL-1β and IL-18. At the same time, Micheliolide can also inhibit the activation of NLRP3 inflammasomes induced by mitochondrial ROS (mROS) agonists such as rotenone, indicating that Micheliolide can inhibit LPS-induced inflammatory responses in rat renal tubular epithelial cells by inhibiting the mROS/NF-κB/NLRP3 pathway.

Research has found that Micheliolide can improve the survival rate of mice with lethal septic shock. Its mechanism of action is that Micheliolide inhibits the production of inflammatory cytokines IL-6, TNF-α, MCP-1, IFN-β, and anti-inflammatory cytokine IL-10 in LPS-induced RAW264.7, primary peritoneal macrophages, and bone marrow dendritic cells. It inhibits LPS-induced inflammatory responses through the NF-κB and PI3K/AKT pathways, protecting mice from LPS attack. In the study of the pathogenesis of diabetic nephropathy (DN), it was found that Micheliolide can inhibit the expression of monocyte chemoattractant protein-1 (MCP-1) induced by advanced oxidation protein products (AOPPs), thereby inhibiting the phosphorylation of NF-κB complex P65 and the degradation of IκB, achieving an anti-inflammatory process.

2. Antitumor Effects

Modern pharmacological studies have shown that Micheliolide exerts antitumor effects by inhibiting tumor cell proliferation, regulating tumor cell apoptosis, inducing tumor cell autophagy, and inhibiting tumor cell invasion and metastasis. Micheliolide can inhibit tumor cells by activating pyruvate kinase M2 (PKM2). PKM2 is a low-activity isoform of pyruvate kinase that plays a crucial role in tumorigenesis. Micheliolide selectively activates PKM2 through covalent binding at residue cysteine 424 (C424), which is not present in PKM1. This interaction promotes the formation of more PKM2 tetramers, inhibits the acetylation of lysine 433 (K433), and affects the translocation of PKM2 to the nucleus, thus inhibiting the proliferation of leukemia cells.

Micheliolide can also inhibit the proliferation of breast cancer cells by inducing the expression of the mitochondrial-related protein (Drp) 1. Drp1 is the main molecule responsible for mitochondrial fission. Inducing Drp1 expression can promote mitochondrial fission, increase ROS levels in breast cancer cells, dissipate mitochondrial membrane potential, promote the release of cytochrome C, cleave the apoptotic protein DNA repair enzyme (PARP), and thereby induce apoptosis in breast cancer cells. Therefore, it can inhibit the proliferation of MCF-7 and MDA-MB-231 breast cancer cells in a dose-dependent manner.

Micheliolide can selectively activate PKM2 by covalently binding to cysteine residues, promote the formation of pyruvate kinase tetramers, reduce the lysine acetylation and nuclear translocation of PKM2, and thus significantly inhibit the proliferation and growth of leukemia cells. Micheliolide can inhibit the proliferation of breast cancer TNBC cells and enhance their sensitivity to cisplatin by reducing intracellular glutathione levels. Additionally, Micheliolide significantly inhibits the expression of hypoxia-inducible factor 1α (HIF-1α) in lung cancer H1299 and Calu-1 cells after radiation and hypoxia exposure. It sensitizes p53-deficient lung cancer cells to radiation by inhibiting the HIF-1α pathway, thereby inhibiting abnormal proliferation of lung cancer cells.

Micheliolide can increase the autophagy rate of human colon cancer HCT116 cells and significantly upregulate the phosphorylation level of STAT3, indicating that Micheliolide can induce autophagy in HCT116 cells by regulating the STAT3 pathway. Other studies have found that Micheliolide can selectively eliminate leukemia cells, upregulate the levels of transcription factors such as nuclear factor E2-related factor 2 (Nrf2) and heme oxygenase 1 (HO-1), and inhibit the metastasis of leukemia cells by inhibiting the NF-κB signaling pathway and increasing ROS production.

3. Neuroprotective Effects

Micheliolide can improve neuroinflammation by reducing the accumulation of Aβ and inhibiting the activation of microglia. It exerts therapeutic effects on neurodegenerative diseases by regulating pathways such as IκBα/NF-κB, Akt, JNK, MAPK, ERK1/2, and enhancing the activity of Nrf2. Modern research has shown that Micheliolide can significantly reduce the number and area of Aβ plaques in the dentate gyrus, CA1 region, and cortical region of transgenic Alzheimer's disease (AD) model mice, as well as the number of microglia and inhibit their abnormal activation and aggregation. Additionally, it can also reduce the number of glial fibrillary acidic protein (GFAP)-positive astrocytes, inhibit their activation, and significantly improve cognitive impairment in transgenic AD model mice.

Neuroinflammation mediated by microglia is believed to induce brain damage in various neurodegenerative diseases, and inhibiting the excessive activation of microglia is beneficial for the treatment of neurodegenerative diseases. Other studies have found that Micheliolide can significantly alleviate neuroinflammation induced by LPS-stimulated BV2 microglia in mice, reducing the levels of iNOS, COX-2, TNF-α, IL-6, and NO. This mechanism is related to Micheliolide's ability to inhibit the activation of IκBα/NF-κB, Akt pathways, as well as the activation of c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK), and extracellular signal-regulated kinase 1/2 (ERK1/2). At the same time, Micheliolide can significantly promote the expression of the antioxidant protein HO-1 by enhancing the activity of Nrf2.

4. Liver Protection Effects

Research has shown that Micheliolide can both reduce inflammatory responses and fatty degeneration in liver cells, and induce hepatocyte autophagy to exert its protective effects on the liver. Micheliolide can alleviate hepatic fatty degeneration in diabetic model mice, reduce abnormal elevations in blood lipid levels (triglycerides, total cholesterol), liver enzyme levels (alanine aminotransferase, aspartate aminotransferase), and inflammatory cytokines (TNF-α, IL-1β). It upregulates the content of hepatic peroxisome proliferator-activated receptor (PPAR-γ) and reduces the expression levels of p-IκBα and p-NF-κB/p65, thus inhibiting the NF-κB pathway and reducing the production of TNF-α and IL-1β in hepatocytes. Additionally, Micheliolide can induce autophagy in hepatocytes of diabetic model mice by activating the adenosine monophosphate-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) signaling pathway to upregulate the expression of PPAR-γ.

5. Kidney Protection Effects

Micheliolide exerts protective effects on the kidneys by inhibiting signaling pathways such as MTDH/BMP/MAPK and MTDH/NF-κB, thereby reducing renal fibrosis and inflammation. Current research indicates that Metadherin (MTDH) is a factor involved in renal fibrosis, and Micheliolide can specifically inhibit EMT induced by MTDH overexpression. In two mouse models of renal fibrosis, unilateral ureteral obstruction and ischemia-reperfusion injury, Micheliolide can reduce the expression levels of fibronectin and α-smooth muscle actin (α-SMA) in the model mice and partially restore the expression level of epithelial cadherin. Bone morphogenetic protein (BMP) signaling is another extracellular signal involved in fibrosis, and MTDH overexpression can promote fibrosis by regulating the BMP/MAPK pathway in renal tubular epithelial cells, which Micheliolide can specifically reverse. Additionally, Micheliolide has been found to protect the kidneys of leptin receptor-deficient mice (a diabetic model) from renal failure and inflammation. This protective effect is related to Micheliolide's ability to inhibit abnormal upregulation of MTDH. Furthermore, downregulation of MTDH significantly inhibits the activation of the NF-κB signaling pathway and reduces the expression levels of downstream inflammatory cytokines such as MCP-1, TNF-α, IL-1β, and IL-6. Micheliolide also downregulates MTDH expression by inhibiting MTDH transcription and promoting ubiquitin-mediated degradation.

Structural Modification of Micheliolide

Micheliolide is a sesquiterpene lactone with a tricyclic structure that exhibits various significant pharmacological activities. However, its clinical application is greatly limited due to its low plasma stability in vivo. Additionally, the lack of clarity in understanding the targets of Micheliolide's actions poses challenges for its rational structural modification and druggability studies. Currently, one effective method to improve bioavailability and increase blood drug concentration is to connect non-cyclic lower secondary amines to the C13 position of Micheliolide through a Michael addition reaction to obtain its water-soluble salt adduct. Moreover, introducing ester groups at the C-4 position can maintain the activity of Micheliolide, allowing it to be used as a molecular probe position for exploring the mechanism of action of Micheliolide. Esterification at positions C-2, 14, and 9 leads to enhanced anti-tumor activity. Therefore, the introduction of heteroatoms or modifications that maintain the α-methylene-γ-butyrolactone structure may further enhance the anti-tumor activity of Micheliolide. In-depth research on the mechanisms and targets of Micheliolide and its derivatives will facilitate the rational modification and optimization of Micheliolide.

References

[1] Lin Jing, Mo Junxiao, Zhou Xin, et al. Research Progress on the Pharmacological Effects and Mechanisms of Micheliolide [J]. China Pharmacy, 2022, 33(14): 1787-1792.

[2] Zhao Ru, Zeng Binglin, Pan Xiandao. Research Progress on Structural Modification, Biological Activity, and Structure-Activity Relationships of Micheliolide [J]. Natural Product Research and Development, 2020, 32(03): 532-539+497.

About the Author

Xiaomichong, a pharmaceutical quality researcher, has been committed to pharmaceutical quality research and drug analysis method validation for a long time. Currently employed by a large domestic pharmaceutical research and development company, she is engaged in drug inspection and analysis as well as method validation.