Research Progress on the Antifungal Mechanism of Natural Products Against Candida Species (Part 2)

This article will continue to introduce the antifungal mechanism of natural products achieved through influencing the changes in cellular structures such as the cell wall, cell membrane, and mitochondria of Candida species; inducing Candida cell apoptosis; inhibiting the expression of drug target enzymes and efflux pumps in Candida; and enhancing immune regulation.

The previous article introduced the antifungal mechanisms of natural products, including inhibiting Candida adhesion and invasion, yeast-to-hyphae transition, biofilm formation, and the synthesis and secretion of hydrolytic enzymes. This article will continue to elaborate on the antifungal mechanisms of natural products achieved through influencing changes in cellular structures such as the cell wall, cell membrane, and mitochondria of Candida species; inducing Candida cell apoptosis; inhibiting the expression of drug target enzymes and efflux pumps; and enhancing immune regulation.

Related Reading: "Research Progress on the Antifungal Mechanism of Natural Products Against Candida Species (Part 1)"

5.The Effect of Natural Products on Candida Cell Wall

The fungal cell wall, primarily composed of β-glucan, chitin, and mannoproteins, plays a crucial role in fungal growth. The unique structure of the fungal cell wall forms a significant difference from mammalian cells, making it an effective target for antifungal treatment. Echinocandin derivatives are the first clinical drugs targeting fungal cell walls by inhibiting the synthesis of β-1,3-glucan. SCY-078 (ibrexafungerp), a triterpenoid compound acting on the same target, has a natural compound enfumafungin as its precursor. Unlike other echinocandin drugs, SCY-078 is a newly isolated antifungal drug that can be administered orally, and it has been approved by the FDA for the treatment of vulvovaginal candidiasis. Acetic acid extracted from plants can specifically bind to β-1,3-glucan without inhibiting its synthesis, significantly damaging the integrity of the cell wall and inhibiting the growth of various fungi, including Candida albicans. Nikkomycin Z, a pyrimidine nucleoside isolated from Streptomyces tendae, competitively inhibits chitin synthase, causing osmotic stress on fungal cells and inhibiting the synthesis of chitin, a basic component of the fungal cell wall. Plagiochin E, a compound isolated from Marchantia polymorpha, can reduce the activities of three chitinases, Chs1, Chs2, and Chs3, in a concentration-dependent manner, resulting in the failure of chitinase synthesis and cell wall structural defects in Candida albicans.

6.The Effect of Natural Products on Candida Cell Membrane

The fungal cell membrane is primarily composed of sterols, glycerophospholipids, and sphingomyelins. It provides a material basis for various functional proteins and maintains the structural integrity of the cell. Ergosterol, a rich sterol, is related to the permeability and fluidity of the cell membrane. Most antifungal drugs interfere with ergosterol function by disrupting different steps of ergosterol biosynthesis (such as allylamines and azoles) or directly binding to ergosterol (such as polyenes), leading to cell death. Glycerophospholipids and sphingomyelins regulate cell function and signal transduction. Several compounds have been reported to inhibit the biosynthesis of fungal glycerophospholipids and sphingomyelins, exerting antifungal effects by inhibiting various enzymes in this pathway. Therefore, targeting the fungal cell membrane is a common approach to explore and develop antifungal drugs. At the same time, a large number of natural products exhibit significant antibacterial effects by disrupting the cell membrane.

Recent studies have shown that the monoterpenoid perillyl alcohol disrupts the calcineurin pathway in Candida albicans, leading to a significant decrease in ergosterol content and thereby damaging the integrity of the cell membrane. Similarly, geraniol, a common component of various essential oils, significantly reduces ergosterol content at sub-MIC concentrations. RsAFP2, a plant antifungal peptide isolated from radish, exhibits strong antifungal activity against Candida species at micromolar concentrations. Studies have shown that RsAFP2 interacts with fungal glucosylceramide, inducing the production of endogenous reactive oxygen species (ROS) in Candida albicans, leading to cell apoptosis. Several sesquiterpenoid compounds have been discovered from bryophytes, which exhibit antifungal activity against Candida albicans with efflux pump defects. For example, entisoalantolactone exerts anti-Candida albicans effects by targeting Erg6 and Erg11 to interfere with sterol synthesis. Natural products can also disrupt the structure of cell membranes by affecting the physical and chemical properties of the membranes. For instance, Giochidioboside from elderberry exerts antibacterial activity by inducing membrane depolarization and permeability.

7.The Effect of Natural Products on Candida Mitochondria

The classical respiratory chain of mitochondria is the center of energy production through oxidative phosphorylation, and it is also an organelle that produces metabolic intermediates. Energy supply and metabolites are essential for the survival and growth of Candida albicans. In recent years, it has been discovered that some natural products can exert antifungal effects against Candida albicans by affecting mitochondrial function, mainly through mechanisms involving reactive oxygen species (ROS) pathways, Caspase signaling pathways, and calcium pump pathways.

Mitochondria produce adenosine triphosphate (ATP), which is essential for all cells. This process is accompanied by the generation of a large amount of ROS. The accumulation of ROS can inactivate mitochondrial enzymes, lead to loss of membrane potential, resulting in mitochondrial dysfunction, and ultimately induce early apoptosis of Candida species. Magnolol, isolated from the bark of Magnolia officinalis, disrupts mitochondrial membrane potential by generating excessive ROS. Other compounds such as plant antifungal resveratrol, berberine, an important component of traditional Chinese herbal medicine, and sophorolipids, a secondary metabolite of Candida species, also exert antifungal effects through similar mechanisms. Dill seed essential oil not only inhibits mitochondrial dehydrogenase by disrupting the citric acid cycle and inhibits ATP synthesis, but also exerts antifungal effects by causing ROS accumulation in Candida albicans cells.

Plant active ingredients can activate the Caspase-dependent signaling pathway, leading to changes in mitochondrial membrane permeability and inducing apoptosis in Candida cells. Studies have found that 4 μg/mL and 8-16 μg/mL of pterostilbene can induce apoptosis in 14% and 17% of Candida albicans cells respectively. After 3 hours of treatment with pterostilbene, the Caspase enzyme activity in Candida albicans increased, mitochondrial membrane permeability changed, membrane potential decreased, and intracellular ROS accumulation also increased, synergistically inducing apoptosis. In addition, studies have found that 100 μmol/L and 1000 μmol/L of baicalin can induce nuclear pyknosis and fragmentation in Candida albicans cells, and reduce mitochondrial membrane potential, leading to Candida cell apoptosis. Lycopene, a carotenoid mainly found in tomatoes, can cause accumulation of intracellular calcium ions, affect the release of cytochrome C and mitochondrial depolarization, leading to the activation of proteolytic enzymes and the production of ROS, ultimately resulting in mitochondrial dysfunction and apoptosis.

8.Inducing Candida Apoptosis through Endogenous Reactive Oxygen Species

Environmental stress can induce Candida to produce and accumulate a large amount of endogenous reactive oxygen species (ROS), leading to progressive oxidative damage within the cell, damaging components such as DNA, proteins, and lipids. Plant active ingredients can also induce Candida to produce endogenous ROS, damaging the integrity of its organelles and cell membranes, thereby inducing apoptosis in Candida. Studies have found that when treated with 16 μg/mL of magnolol for 2h or 4h, the intracellular ROS content in Candida albicans can significantly increase. After 4h of treatment with 32 μg/mL of magnolol, the intracellular fluorescence intensity of Candida albicans can increase to 741%. Therefore, at low doses, magnolol can induce Candida albicans to rapidly produce a large amount of ROS, promoting its intracellular oxidative stress response, thus inducing apoptosis in Candida albicans. In addition, studies have shown that after 4h of treatment with 5 μg/mL of lycopene, the intracellular rhodamine-123 staining fluorescence intensity and HPF staining fluorescence intensity of Candida albicans increased by 30.13% and 10.01%, respectively. Therefore, lycopene can induce apoptosis in Candida albicans by inducing intracellular ROS accumulation, especially the accumulation of ·OH.

9.Inhibiting the Expression of Drug Target Enzymes and Efflux Pumps in Candida

The overexpression of drug target enzymes and drug active efflux pumps is an important mechanism of Candida resistance. The main genes encoding drug target enzymes are ERG5 and ERG11. Candida resistance-related active efflux pumps include the ATP-binding cassette transporter family (ABCT) and the major facilitator superfamily (MFS), with related encoding genes including CDR1, CDR2, MDR1, MDR2, and FLU1. Studies have shown that when Candida albicans is exposed to fluconazole for a long time, its expression of ERG5, ERG11, CDR1, CDR2, MDR1, and FLU1 genes is upregulated, leading to drug resistance. However, when combined with selaginellin, it can significantly reverse the expression levels of these genes, causing them to be downregulated. Other studies have found that when mangiferin is combined with fluconazole, the downregulation of CDR1 in Candida albicans resistant strains is the most significant, and CDR2 and MDR2 are also significantly downregulated compared to the fluconazole alone group. When berberine is combined with fluconazole, it has good inhibitory effects on fluconazole-resistant Candida albicans and Candida tropicalis. Its mechanism involves promoting the accumulation of intracellular ROS in Candida, inhibiting the biosynthesis of ergosterol in the cell membrane, and inhibiting the expression of efflux pump encoding genes CDR1, CDR2, and MDR1.

10.Enhancement of Immune Regulation by Natural Products

Some natural products can also exert anti-Candida effects by regulating the immune system. The n-butanol extract of Pulsatilla decoction (main components include berberine, pulsatilla saponins, and fraxinellone) not only downregulates the expression of the NLRP3 inflammasome but also reduces the expression of chemokines IL-8 and MIP-2, inhibiting the recruitment of "inefficient" neutrophils to the vaginal cavity, thus playing a role in treating vulvovaginal candidiasis. In vivo experiments have shown that some single components of traditional Chinese medicine can enhance the immune system of immunosuppressed mice by increasing their IgG, IL-4, and IL-12 levels, enabling them to fight against Candida albicans.

References

[1] Yu Yu, She Xiaodong, Liu Weida. Research Progress on Natural Products Against Candida [J]. Chinese Journal of Mycology, 2023, 18(02): 183-187.

[2] Xie Yufei, Zhou Peiru, Hua Hong, et al. Research Progress on the Anti-Candida Mechanisms of Plant Active Ingredients [J]. Chinese Journal of Mycology, 2022, 17(04): 315-318+329.

[3] Xie Lingfeng, He Liya, Wang Xianzhe, et al. Research Progress on the Experimental Study of Chinese Medicine Monomer Components Against Candida albicans [J]. Journal of Diagnosis and Treatment of Dermatovenerology, 2021, 28(06): 524-528.

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.