Inhibition of lipid biosynthesis

The Surprising Effect of Phenformin on Cutaneous Darkening and Characterization of Its Underlying Mechanism by a Forward Chemical Genetics Approach

Melanin in the epidermis is known to ultimately regulate human skin pigmentation. Recently, we exploited a phenotypic-based screening system composed of ex vivo human skin cultures to search for effective materials to regulate skin pigmentation. Since a previous study reported the potent inhibitory effect of metformin on melanogenesis, we evaluated several biguanide compounds. The unexpected effect of phenformin, once used as an oral anti-diabetic drug, on cutaneous darkening motivated us to investigate its underlying mechanism utilizing a chemical genetics approach, and especially to identify alternatives to phenformin because of its risk of severe lactic acidosis. Chemical pull-down assays with phenformin-immobilized beads were performed on lysates of human epidermal keratinocytes, and subsequent mass spectrometry identified 7-dehydrocholesterol reductase (DHCR7). Consistent with this, AY9944, an inhibitor of DHCR7, was found to decrease autophagic melanosome degradation in keratinocytes and to intensely darken skin in ex vivo cultures, suggesting the involvement of cholesterol biosynthesis in the metabolism of melanosomes. Thus, our results validated the combined utilization of the phenotypic screening system and chemical genetics as a new approach to develop promising materials for brightening/lightening and/or tanning technologies.

Sterol carrier and lipid transfer proteins

The discovery of the sterol carrier and lipid transfer proteins was largely a result of the findings that cells contained cytosolic factors which were required either for the microsomal synthesis of cholesterol or which could accelerate the transfer or exchange of phospholipids between membrane preparations. There are two sterol carrier proteins present in rat liver cytosol. Sterol carrier protein 1 (SCP1) (Mr 47 000) participates in the microsomal conversion of squalene to lanosterol, and sterol carrier protein 2 (SCP2) (Mr 13 500) participates in the microsomal conversion of lanosterol to cholesterol. In addition SCP2 also markedly stimulates the esterification of cholesterol by rat liver microsomes, as well as the conversion of cholesterol to 7 alpha-hydroxycholesterol - the major regulatory step in bile acid formation. Also, SCP2 is required for the intracellular transfer of cholesterol from adrenal cytoplasmic lipid inclusion droplets to mitochondria for steroid hormone production, as well as cholesterol transfer from the outer to the inner mitochondrial membrane. SCP2 is identical to the non-specific phospholipid exchange protein. While SCP2 is capable of phospholipid exchange between artificial donors/acceptors, e.g. liposomes and microsomes, it does not enhance the release of lipids other than unesterified cholesterol from natural donors/acceptors, e.g. adrenal lipid inclusion droplets, and will not enhance exchange of labeled phosphatidylcholine between lipid droplets and mitochondria. Careful comparison of SCP2 and fatty acid binding protein (FABP) using six different assay procedures demonstrates separate and distinct physiological functions for each protein, with SCP2 participating in reactions involving sterols and FABP participating in reactions involving fatty acid binding and/or transport. Furthermore, there is no overlap in substrate specificities, i.e. FABP does not possess sterol carrier protein activity and SCP2 does not specifically bind or transport fatty acid. The results described in the present review support the concept that intracellular lipid transfer is a highly specific process, far more substrate-specific than suggested by the earlier studies conducted using liposomal techniques.

Regulation of Lipid Metabolism by a Lipid-Carrying Protein

The overall conclusion to be made from the information presented here is that for many reasons SCP is a highly unusual protein. Some of these reasons are, first, SCP serves as cofactor for a number of different membrane-bound enzymes catalyzing specific steps in lipid metabolism. Second, SCP is involved in intracellular transport or movement of both cholesterol and fatty acids. Third, SCP is remarkably abundant and ubiquitous; its structure is conserved throughout nature. Fourth, SCP is exported to the blood stream from its site of synthesis by some, perhaps unique, mechanism and then rapidly taken up by specific tissues, e.g., the adrenal. Fifth, SCP is free in the cytosol and can also move to the inner mitochondrial membrane, where it is tightly bound. Sixth, SCP undergoes a dramatic diurnal variation in amount, reflecting changes in synthetic rate. Its half-life is less than an hour. Seventh, the diurnal variation in amount is triggered by feeding and influenced by several hormones. The diurnal variation is lost but a high level of SCP is maintained in the face of debilitating conditions, i.e., starvation, diabetes. Eighth, malignant cells exhibit defects in the uptake, synthesis, or turnover of SCP. Ninth, the synthesis of SCP is regulated by the efficiency of translation of its ever abundant mRNA. Tenth, there is much more to be learned about the functions and regulation of SCP.

Importance of apolipoproteins in lipid metabolism

Lipids, which serve as a source of energy and are an important constituent of cell membrane structure, are readily stored in the body. By definition they are insoluble in water. Specific proteins called apolipoproteins interact with lipids to form soluble lipid-protein complexes called lipoproteins. It is in this form that the major lipids--cholesterol, triglyceride and phospholipid--circulate in plasma. Unesterified fatty acids, another major lipid group, are bound to albumin in the circulation. The plasma lipoproteins are complex macromolecules composed of lipids, apolipoproteins and carbohydrates. The relative proportions of these components differ markedly between lipoprotein classes. Hyperlipidemia is a term used for increased concentrations of plasma cholesterol and/or triglycerides. Any one plasma lipid is present in several types of lipoproteins. Thus, hyperlipidemia implies the presence of hyperlipoproteinemia. The latter has important therapeutic implications. Most of the recent attempts at classification have been directed at the lipoprotein level of plasma lipid organization. Decreased concentrations of lipids in plasma can be achieved by altering the rates of metabolism of lipoproteins. Decrease in lipoprotein synthesis, increased catabolism or impaired release from cells into the blood stream may all result in a decrease of plasma lipids. Drugs which affect one or more of these factors are used to treat hyperlipoproteinemia. In order to elucidate the mechanism of action of hypolipidemic drugs it is necessary to understand the lipoprotein defect at the molecular level. This requires a more detailed knowledge of lipoprotein metabolism than is presently available for most of the hyperlipoproteinemias. This paper will review some of the generally accepted properties of the plasma lipoproteins, describe some difficulties which hamper the understanding of lipoprotein metabolism, and identify possible mechanisms by which drugs may affect lipoprotein metabolism.

The effect of phenformin-HCl on patients with diabetes mellitus, studied under strict balance conditions

Under strict balance conditions we studied the effect of phenformin in 5 patients with diabetes mellitus. In all cases phenformin lowered the blood glucose values, and all patients showed a reduction of glycosuria. Contrary to other reports body weight increased during phenformin treatment. This was accompanied by positive nitrogen, phosphorus and calcium balances. The weight gain can be explained by the positive caloric balance, mainly caused by the diminished glycosuria. No change in B.M.R. or R.Q. was seen. During phenformin treatment there was a drop in cholesterol and total lipid levels in 4 patients. No conclusions could be drawn about the effect of phenformin on triglycerides, phospholipids and lipoprotein spectra. Phenformin treatment did not affect the disappearance of glucose, nor the insulin levels after intravenous glucose loading. During oral glucose loading phenformin caused a significant fall in blood glucose levels, accompanied by an increased insulin response in one patient. In the other 4 patients phenformin had no effect on either parameter.

Squalene and sterol carrier protein: structural properties, lipid-binding, and function in cholesterol biosynthesis

Squalene and sterol carrier protein of liver plays a general role as a vehicle for cholesterol and its water-insoluble precursors; the carrier protein is essential for enzymic cholesterol synthesis. Liver microsomal enzymes contain a small amount of endogenous carrier protein, which is readily removed by washing or purification of the enzyme. Enzymic conversion to products of a cholesterol precursor.carrier protein complex is markedly faster than that for initially unbound sterol. The protomer form of the carrier protein has a molecular weight of 16,000; during sodium dodecyl sulfate gel electrophoresis one band is observed. Phospholipid facilitates the aggregation of the protomer to the oligomer form (>150,000 daltons; purified 720-fold) accompanied by the binding of cholesterol precursors to the oligomer. The carrier protein binds fatty acids as well as cholesterol precursors, suggesting that it may more generally be a lipid carrier protein with "squalene and sterol carrier protein" describing the functional aspects of the lipid carrier in cholesterol biosynthesis. Studies with several steroids and related compounds revealed that the binding sites of lipid carrier protein must contain highly specific hydrophobic and polar regions.

Calcium-like action of phenethylbiguanide and related compounds: inhibition of pyruvate kinase

Pyruvate kinase (EC 2.7.1.40) is inhibited by phenethylbiguanide. The kinetics of inhibition are competitive between biguanide and divalent, but not monovalent, metal cation activators of the enzyme; biguanide inhibition thus resembles inhibition by Ca(++). Alteration of either the polar or nonpolar portion of the phenethylbiguanide molecule quantitatively reduces its effectiveness as an inhibitor of pyruvate kinase, but the kinetics of inhibition remain qualitatively unchanged. Measurements of [(3)H]phenethylbiguanide binding to the enzyme indicate the presence of a single class of about 12 binding sites per enzyme molecule; binding characteristics are not significantly different in the presence of either monovalent or divalent metal cations. Studies with (45)Ca(++) and (54)Mn(++) demonstrate about 4 metal binding sites per enzyme molecule; phenethylbiguanide displaces these metal cations from the enzyme. Studies with several enzymes, dependent upon divalent metal cations, of both metal-bridge and substrate-bridge classes fail to show significant inhibition except at much higher phenethylbiguanide concentrations.