Drug induced phospholipidosis: an acquired lysosomal storage disorder

Abstract

There is a strong association between lysosome enzyme deficiencies and monogenic disorders resulting in lysosomal storage disease. Of the more than 75 characterized lysosomal proteins, two thirds are directly linked to inherited diseases of metabolism. Only one lysosomal storage disease, Niemann-Pick disease, is associated with impaired phospholipid metabolism. However, other phospholipases are found in the lysosome but remain poorly characterized. A recent exception is lysosomal phospholipase A2 (group XV phospholipase A2). Although no inherited disorder of lysosomal phospholipid metabolism has yet been associated with a loss of function of this lipase, this enzyme may be a target for an acquired form of lysosomal storage, drug induced phospholipidosis. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Figure 1

Common properties of lysosomal hydrolases. (1) There is subcompartmentalization leading to separation from the limiting membrane. The hydrolases are active against intralysosomal vesicles that contain the substrates for these enzymes. (2) A binding interaction occurs between the enzyme and the intralysosomal membrane at acidic pH. (3) The hydrolases catalyze reactions reversibly. (4) The anionic phospholipids within the membranes such as BMP and phosphatidylinositol are slowly metabolized.

Figure 2

The synthesis of BMP. The pathway as proposed by Waite consists of four steps. These include the hydrolysis of the sn-2 fatty acyl group from phosphatidylglycerol by an acidic phospholipase A2, the transacylation of the sn-1' hydroxyl, the reorientation of the phosphoryl ester from the sn-3 to sn-1 position, and the transacylation of the original glycerol backbone.

Figure 3

Dual activities of lysosomal phospholipase A2 (GXVPLA2). In the presence of N-acetylsphingosine, the enzyme is both an acidic phospholipase A2 and transacylase of the 1-hydroxyl group. When only water is present as an acceptor, only phospholipase A2 activity is observed.

Figure 4

The amino acid sequence of lysosomal phospholipase A2. The signal peptide sequence is highlighted in yellow. The catalytic domain that is shared with lecithin cholesterol acyltransferase is underlined in purple. The catalytic serine and its associated consensus sequence are highlighted in green as are other amino acids comprising the catalytic triad. Finally, glycosylation sites are highlighted in blue.

Figure 5

Inhibition of lysosomal phospholipase activity by amiodarone and PDMP. LPLA2 activity was measured as the formation of 1-O-acylceramide in the presence of varying concentrations of amiodarone, tetracycline, and PDMP. The cationic amphiphiles amiodarone and PDMP inhibited the transacylation reaction but tetracycline did not. This figure is reprinted from reference [80].

Figure 6

Role of anionic lipids in the transacylation of N-acetylsphingosine by lysosomal phospholipase A2. The reaction mixture contained 48 mM sodium citrate (pH 4.5), 10 μg/ml BSA, liposomes (127 μM phospholipid), and 14.5 ng/ml of recombinant mouse LPLA2 in 500 μl of total volume. A: Liposomes consisting of DOPC-NAS (3:1, molar ratio) or DOPC-sulfatide-NAS (3:0.3:1, molar ratio) were incubated with the enzyme for 5, 10, 15, 30, and 45 min at 37°C. The reaction products were extracted and separated by an HPTLC plate using a solvent system consisting of chloroform-acetic acid (9:1, v/v). The reaction product, 1-O-oleoyl-NAS, quantified by scanning the plate, was plotted against time (B). C: Liposomes consisting of DOPC-galactosylceramide-NAS or DOPC-sulfatide-NAS with a different molar ratio were incubated with the recombinant enzyme at 37°C. The reaction products were extracted and separated as described in A. Error bars indicate SD. (n = 3). This figure is reprinted with permission from reference [81].

Figure 7

Proposed model for the inhibition of lysosomal phospholipase A2 and induction of phospholipidosis by amiodarone. Under normal conditions, lysosomal phospholipase A2 is bound through electrostatic charge interactions to the inner membranes of the lysosome. Amiodarone, a cationic amphiphile, accumulates in the lysosome and inserts into the inner membrane decreasing or eliminating the anionic binding sites for the lipase. The lipase is no longer able to bind to its substrates and is potentially susceptible to degradation by lysosomal proteases.