Substrate-based inhibitors of lanosterol 14 alpha-methyl demethylase: II. Time-dependent enzyme inactivation by selected oxylanosterol analogs

Synthesis and Biological Activity of Sterol 14α-Demethylase and Sterol C24-Methyltransferase Inhibitors

Sterol 14α-demethylase (SDM) is essential for sterol biosynthesis and is the primary molecular target for clinical and agricultural antifungals. SDM has been demonstrated to be a valid drug target for antiprotozoal therapies, and much research has been focused on using SDM inhibitors to treat neglected tropical diseases such as human African trypanosomiasis (HAT), Chagas disease, and leishmaniasis. Sterol C24-methyltransferase (24-SMT) introduces the C24-methyl group of ergosterol and is an enzyme found in pathogenic fungi and protozoa but is absent from animals. This difference in sterol metabolism has the potential to be exploited in the development of selective drugs that specifically target 24-SMT of invasive fungi or protozoa without adversely affecting the human or animal host. The synthesis and biological activity of SDM and 24-SMT inhibitors are reviewed herein.

Effects of FoxO4 overexpression on cholesterol biosynthesis, triacylglycerol accumulation, and glucose uptake

The Forkhead transcription factors FoxO1, FoxO3a, and FoxO4 play a prominent role in regulating cell survival and cell cycle. Whereas FOXO1 was shown to mediate insulin sensitivity and adipocyte differentiation, the role of the transcription factor FoxO4 in metabolism remains ill defined. To uncover the effects of FoxO4, we generated a cellular model of stable FoxO4 overexpression and subjected it to microarray-based gene expression profiling. While pathway analysis revealed a disruption of cholesterol biosynthesis gene expression, biochemical studies revealed an inhibition of cholesterol biosynthesis, which was coupled with decreased mRNA levels of lanosterol 14alpha demethylase (CYP51). FoxO4-mediated repression of CYP51 led to the accumulation of 24,25 dihydrolano-sterol (DHL), which independently and unlike lanosterol inhibited cholesterol biosynthesis. Furthermore, FoxO4-overexpressing cells accumulated lipid droplets and triacylglycerols and had an increase in basal glucose uptake. Recapitulation of these effects was obtained following treatment with CYP51 inhibitors, which also induce DHL buildup. Moreover, DHL but not lanosterol strongly stimulated liver X receptor alpha (LXRalpha) activity, suggesting that DHL and LXRalpha mediate the downstream effects initiated by FoxO4. Together, these studies suggest that FoxO4 acts on CYP51 to regulate the late steps of cholesterol biosynthesis.

Sterol 14alpha-demethylase cytochrome P450 (CYP51), a P450 in all biological kingdoms

The CYP51 family is an intriguing subject for fundamental P450 structure/function studies and is also an important clinical drug target. This review updates information on the variety of the CYP51 family members, including their physiological roles, natural substrates and substrate preferences, and catalytic properties in vitro. We present experimental support for the notion that specific conserved regions in the P450 sequences represent a CYP51 signature. Two possible roles of CYP51 in P450 evolution are discussed and the major approaches for CYP51 inhibition are summarized.

CYP51 from Trypanosoma cruzi: a phyla-specific residue in the B' helix defines substrate preferences of sterol 14alpha-demethylase

A potential drug target for treatment of Chagas disease, sterol 14alpha-demethylase from Trypanosoma cruzi (TCCYP51), was found to be catalytically closely related to animal/fungi-like CYP51. Contrary to the ortholog from Trypanosoma brucei (TB), which like plant CYP51 requires C4-monomethylated sterol substrates, TCCYP51 prefers C4-dimethylsterols. Sixty-six CYP51 sequences are known from bacteria to human, their sequence homology ranging from approximately 25% between phyla to approximately 80% within a phylum. TC versus TB is the first example of two organisms from the same phylum, in which CYP51s (83% amino acid identity) have such profound differences in substrate specificity. Substitution of animal/fungi-like Ile105 in the B' helix to Phe, the residue found in this position in all plant and the other six CYP51 sequences from Trypanosomatidae, dramatically alters substrate preferences of TCCYP51, converting it into a more plant-like enzyme. The rates of 14alpha-demethylation of obtusifoliol and its 24-demethyl analog 4alpha-,4alpha-dimethylcholesta-8,24-dien-3beta-ol(norlanosterol) increase 60- and 150-fold, respectively. Turnover of the three 4,4-dimethylated sterol substrates is reduced approximately 3.5-fold. These catalytic properties correlate with the sterol binding parameters, suggesting that Phe in this position provides necessary interactions with C4-monomethylated substrates, which Ile cannot. The CYP51 substrate preferences imply differences in the post-squalene portion of sterol biosynthesis in TC and TB. The phyla-specific residue can be used to predict preferred substrates of new CYP51 sequences and subsequently for the development of new artificial substrate analogs, which might serve as highly specific inhibitors able to kill human parasites.

Effect of 26-oxygenosterols from Ganoderma lucidum and their activity as cholesterol synthesis inhibitors

Ganoderma lucidum is a medicinal fungus belonging to the Polyporaceae family which has long been known in Japan as Reishi and has been used extensively in traditional Chinese medicine. We report the isolation and identification of the 26-oxygenosterols ganoderol A, ganoderol B, ganoderal A, and ganoderic acid Y and their biological effects on cholesterol synthesis in a human hepatic cell line in vitro. We also investigated the site of inhibition in the cholesterol synthesis pathway. We found that these oxygenated sterols from G. lucidum inhibited cholesterol biosynthesis via conversion of acetate or mevalonate as a precursor of cholesterol. By incorporation of 24,25-dihydro-[24,25-3H2]lanosterol and [3-3H]lathosterol in the presence of ganoderol A, we determined that the point of inhibition of cholesterol synthesis is between lanosterol and lathosterol. These results demonstrate that the lanosterol 14alpha-demethylase, which converts 24,25-dihydrolanosterol to cholesterol, can be inhibited by the 26-oxygenosterols from G. lucidum. These 26-oxygenosterols could lead to novel therapeutic agents that lower blood cholesterol.

Dual-action hypoglycemic and hypocholesterolemic agents that inhibit glycogen phosphorylase and lanosterol demethylase

Diabetic dyslipidemia requires simultaneous treatment with hypoglycemic agents and lipid-modulating drugs. We recently described glycogen phosphorylase inhibitors that reduce glycogenolysis in cells and lower plasma glucose in ob/ob mice (J. Med. Chem., 41: 2934, 1998). In evaluating the series prototype, CP-320626, in dogs, up to 90% reduction in plasma cholesterol was noted after 2 week treatment. Cholesterol reductions were also noted in ob/ob mice and in rats. In HepG2 cells, CP-320626 acutely and dose-dependently inhibited cholesterolgenesis without affecting fatty acid synthesis. Inhibition occurred together with a dose-dependent increase in the cholesterol precursor, lanosterol, suggesting that cholesterolgenesis inhibition was due to lanosterol 14alpha-demethylase (CYP51) inhibition. In ob/ob mice, acute treatment with CP-320626 resulted in a decrease in hepatic cholesterolgenesis with concomitant lanosterol accumulation, further implicating CYP51 inhibition as the mechanism of cholesterol lowering in these animals. CP-320626 and analogs directly inhibited rhCYP51, and this inhibition was highly correlated with HepG2 cell cholesterolgenesis inhibition (R2 = 0.77). These observations indicate that CP-320626 inhibits cholesterolgenesis via direct inhibition of CYP51, and that this is the mechanism whereby CP-320626 lowers plasma cholesterol in experimental animals. Dual-action glycogenolysis and cholesterolgenesis inhibitors therefore have the potential to favorably affect both the hyperglycemia and the dyslipidemia of type 2 diabetes.

Effects of 15-oxa-32-vinyl-lanost-8-ene-3 beta,32 diol on the expression of 3-hydroxy-3-methylglutaryl coenzyme A reductase and low density lipoprotein receptor in rat liver

The mechanisms by which oxylanosterols regulate expression of hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and lower serum cholesterol levels were examined by using a novel nonmetabolizable oxylanosterol mimic, 15-oxa-32-vinyl-lanost-8-ene-3 beta, 32 diol (DMP 565). This compound, unlike other nonmetabolizable oxylanosterols, is not a substrate for lanosterol 14 alpha-methyl demethylase. Feeding rats a diet supplemented with 0.02% DMP 565 markedly decreased HMG-CoA reductase immunoreactive protein and enzyme activity levels without affecting mRNA levels. The rate of reductase protein degradation was unaffected. However, the rate of translation was reduced to less than 20% of control. Thus, DMP 565 appears to regulate hepatic HMG-CoA reductase gene expression primarily at the level of translation. The pronounced inhibition of HMG-CoA reductase by DMP 565 resulted in a compensatory increase in the functioning of the hepatic low density lipoprotein (LDL) receptor, possibly by increased cycling, as evidenced by a marked increase in the rate of degradation of the LDL receptor. The half-life of the receptor was decreased from over 7 h to only 1 h in animals receiving DMP 565. This increase in the rate of degradation occurred without a change in the steady state level of the receptor. Addition of dietary cholesterol attenuated the increased turnover of the LDL receptor. These effects on the hepatic LDL receptor have also been observed with HMG-CoA reductase inhibitors (G. C. Ness et al., 1996, Arch. Biochem, Biophys. 325, 242-248). However, the effect of DMP 565 on the rate of degradation of the hepatic LDL receptor was of a greater magnitude when equal doses of the drugs were used. These regulatory actions of DMP 565 provide, in part, an explanation for the observed hypocholesterolemic action of this compound.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Substrate-based inhibitors of lanosterol 14 alpha-methyl demethylase: I. Assessment of inhibitor structure-activity relationship and cholesterol biosynthesis inhibition properties

A series of 15-, 32-, and 15,32-substituted lanost-8-en-3 beta-ols is described which function as inhibitors of cholesterol biosynthesis. These agents inhibit lanosterol 14 alpha-methyl demethylase activity as well as suppress HMG-CoA reduction activity in cultured cells. Several of these agents are extremely potent as both demethylase inhibitors and reductase suppressors, while others are more selective in their activities. Selected regio double bond isomers show preference for demethylase inhibition with the following order: delta 8 > delta 7 > delta 6 = unsaturated sterols. Comparisons also show that 4,4-dimethyl sterols are always more potent demethylase inhibitors and reductase suppressors than their 4,4-bisnomethyl counterparts. However, evaluation of an extensive oxylanosterol series leads us to conclude that demethylase inhibition and reductase suppression are not parallel in the same molecule. In addition, the oxylanosterols, but not the oxycholesterols, are able to disrupt coordinate regulation of HMG-CoA reductase from the LDL receptor. Thus, oxylanosterol treatment at levels which suppress reductase activity enhances LDL receptor activity. These results demonstrate that compounds can be made which (1) are selective reductase suppressors enabling dissection of the dual inhibitor nature of these compounds and (2) maximize reductase suppression and LDL receptor induction without demethylase inhibition which could lead to novel agents for serum cholesterol lowering.