**1. Introduction**

Cholesterol is a major source of bioactive sterols. Cholesterol modulates the signaling pathways of neoplastic transformation and tumor progression by covalently modifying hedgehog and smoothened family proteins [1,2] and it is also involved in atherosclerosis and metabolic syndrome progression [3]. Tumor growth is accompanied by a significant increase in the expression level of cholesterol biosynthetic enzymes, including lanosterol 14-alpha demethylase (cytochrome P450(51), CYP51A1) [4]. CYP51A1 belongs to the evolutionarily conserved family of cytochrome P450 and catalyzes the oxidative removal of the alpha-methyl group at the C14-position of the steroid substrate in three steps [5]. According to the COSMIC (http://cancer.sanger.ac.uk/cosmic accessed on 29 June 2020)

**Citation:** Kaluzhskiy, L.; Ershov, P.; Yablokov, E.; Shkel, T.; Grabovec, I.; Mezentsev, Y.; Gnedenko, O.; Usanov, S.; Shabunya, P.; Fatykhava, S.; et al. Human Lanosterol 14-Alpha Demethylase (CYP51A1) Is a Putative Target for Natural Flavonoid Luteolin 7,3- -Disulfate. *Molecules* **2021**, *26*, 2237. https://doi.org/10.3390/ molecules26082237

Academic Editor: Celestino Santos-Buelga

Received: 10 March 2021 Accepted: 12 April 2021 Published: 13 April 2021

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resource, the CYP51A1 gene has a fairly low somatic mutation frequency (<0.1%) in various cancers. It has been shown that CYP51A1 gene expression correlates with the estrogen and progesterone receptor status of breast cancer [6] and could be one of the factors in assessing the survival rate of patients with gastric adenocarcinoma [7]. CYP51A1 catalyzes the production of 4,4-dimethyl-5α-cholesta-8,14,24-triene-3β-ol (follicular fluid meiosisactivating sterol, FF-MAS), one of the modulators of meiosis [8]. The ploidy disturbance characteristic for cancer cells is caused by processes similar to meiosis [9]. Thus, FF-MAS might be linked to the ploidy balance of tumor cells. CYP51A1 gene knockout blocked de novo cholesterol synthesis [10], while CYP51A1 inhibition led to the induction of apoptosis in cancer cells [11], indicating the clinical significance of this protein.

We analyzed the potential of natural flavonoids (Figure 1) as modulators of CYP51A1 function using purified human protein. Baicalein and luteolin are flavonoids that were originally isolated from plants of the *Scutellaria* and *Reseda* genus. The inhibitory activity of baicalein and luteolin was demonstrated for some cytochrome P450 isozymes [12–14]. Baicalein exhibited broad antifungal activity [15] and demonstrated strong synergy with fluconazole [16], a known inhibitor of fungal CYP51A1. Luteolin possesses an antibacterial effect in vivo, increasing membrane permeability [17], but does not directly perturb the model membranes in vitro [18,19]. Both baicalein and luteolin possess antitumor activity [20,21] and, as well as their derivatives, have been used in preclinical studies and in experimental oncology [22–27]. Epidemiological studies showed that foods rich in polyphenolic compounds (flavonoids, phenolic acids, lignans and stilbenes) included in the diet reduced the total risk of cancer by up to 50% [28]. Overall, 14 clinical trials were initiated to study baicalein and luteolin as dietary supplements (https://clinicaltrials.gov/, accessed on 1 March 2021). The G.B. Elyakov Pacific Institute of Bioorganic Chemistry has a broad collection of natural compounds from unique Far-Eastern plants and marine species with a wide range of biological activities. Luteolin 7,3- -disulfate, a water-soluble luteolin derivative originally obtained from the seagrass *Zostera marina* [29], also exhibits antitumor activity [30,31]. It was shown that sulfation at the 7-position of the luteolin molecule decreases cytotoxicity [32]. Moreover, the activity of luteolin 7,3- -disulfate in some cases is stronger than that of luteolin [30,33,34], possibly due to bypassing the stage of conjugation by intestinal and liver cells. Cholesterol is a crucial component of membranes, maintaining their permeability and fluidity. We hypothesized that flavonoids might target its synthesis via CYP51A1 and selected baicalein, luteolin and luteolin 7,3- -disulfate for testing. Using surface plasmon resonance (SPR), we showed that only luteolin 7,3- -disulfate interacted with CYP51A1 with high affinity. However, in the spectral binding experiments luteolin 7,3- -disulfate does not induce spectral changes. In contrast, baicalein and luteolin induce a reverse type I response in the difference absorption spectra of CYP51A1, indicating changes around heme iron. In the reconstituted enzymatic assay, among the three tested flavonoids, only luteolin 7,3- -disulfate inhibited the lanosterol 14α-demethylase activity of human CYP51A1 with significant potency. The binding mode distant from the heme was predicted for luteolin 7,3- -disulfate by the performed molecular docking, showing the binding not in the hydrophobic active site but rather in the access channel. The inhibitory effect of the most hydrophilic form of tested flavonoids—luteolin 7,3- -disulfate—is quite unusual. We suggest that, besides the predicted binding, luteolin 7,3- -disulfate could also bind to the proximal surface of CYP51A1, interfering with the interaction to the redox partner. The obtained data open up a new valuable source of flavonoid modulators of CYP51A1 activity as an alternative to the classic inhibition by azole compounds.

**Figure 1.** Flavonoids used in this work.
