Hormone-sensitive Lipase Is a Cholesterol Esterase of the Intestinal Mucosa

Abstract

The identity of the enzymes responsible for lipase and cholesterol esterase activities in the small intestinal mucosa is not known. Because hormone-sensitive lipase (HSL) catalyzes the hydrolysis of acylglycerols and cholesteryl esters, we sought to determine whether HSL could be involved. HSL mRNA and protein were detected in all segments of the small intestine by Northern and Western blot analyses, respectively. Immunocytochemistry experiments revealed that HSL was expressed in the differentiated enterocytes of the villi and was absent in the undifferentiated cells of the crypt. Diacylglycerol lipase and cholesterol esterase activities were found in the different segments. Analysis of gut from HSL-null mice showed that diacylglycerol lipase activity was unchanged in the duodenum and reduced in jejunum. Neutral cholesterol esterase activity was totally abolished in duodenum, jejunum, and ileum of HSL-null mice. Analysis of HSL mRNA structure showed two types of transcripts expressed in equal amounts with alternative 5-ends transcribed from two exons. This work demonstrates that HSL is expressed in the mucosa of the small intestine. The results also reveal that the enzyme participates in acylglycerol hydrolysis in jejunal enterocytes and cholesteryl ester hydrolysis throughout the small intestine. Hormone-sensitive lipase (HSL)1 is a multifunctional enzyme with broad substrate specificity (1). It hydrolyzes tri-, di-, and monoacylglycerols, cholesteryl esters, and retinyl esters. The activity against diacylglycerol is higher than the activity toward tri-and monoacylglycerols. The enzyme also exhibits cholesterol esterase activity, which is almost twice the activity toward triacylglycerols. Much has been learned in the recent years about the domain structure of HSL. Sequence comparisons revealed that HSL belongs to a family of esterases which is mainly represented by prokaryotic enzymes (2, 3). From a structural point of view, HSL is the most complex protein of the family. Sequence alignments together with biochemical experiments suggest that adipocyte HSL is composed of two structural domains (4, 5). The first 315 amino acids make up the N-terminal domain, which shows very little sequence similarities to other known proteins. The region responsible for the interaction with adipocyte lipid-binding protein (ALBP) was mapped to this domain (6, 7). In adipose tissue, ALBP could increase the hydrolytic activity of HSL through its ability to bind and sequester fatty acids and through specific proteinprotein interactions. The C-terminal domain is divided in two functional parts, a catalytic core and a regulatory module. The latter is composed of 150 amino acids, including all of the known phosphorylation sites of HSL. Unlike other known mammalian triacylglycerol lipases, the activity of HSL is regulated by phosphorylation. The phosphorylation sites of protein kinase A, extracellular signal-regulated kinase, and AMP-dependent protein kinase have been mapped (8 -10). The catalytic core is the region that shows homology w...

A Novel Hormone-sensitive Lipase Isoform Expressed in Pancreatic β-Cells

“…The isoform presented here is encoded by exon A, in addition to exons 1-9. Exon A is located ϳ13 kb upstream of exon 1 and has previously been shown to be incorporated in HSL transcripts in adipocytes from both hu- man and mouse (19,20) and enterocytes in mouse (11). In addition to the stated exons, there are several other exons in the region upstream of exon 1 in the HSL gene.…”

“…HSL has a broad substrate specificity, being able to hydrolyze, among others, mono-, di-, and triglycerides and cholesteryl esters. Besides ␤-cells and adipocytes, HSL is also present in testis, ovary, adrenal gland, heart, skeletal muscle (9), macrophages (10), and gastrointestinal tract mucosa (11). The HSL gene is located on chromosome 19 (human) (12) and spans 27 kb (Fig.…”

“…The HSL gene is located on chromosome 19 (human) (12) and spans 27 kb (Fig. 1A) (11,(13)(14)(15). All functional HSL proteins are encoded by, at least, exons 1-9.…”

The Role of Neutral Cholesterol Ester Hydrolysis in Macrophage Foam Cells

“…LIPE is an intracellular neutral lipase that catalyzes the hydrolysis of triacylglycerol, diacylglycerol, monoacylglycerol, and CE as well as other lipids. Although LIPE was initially identified as an adipose-specific lipase 12) , it has since been shown that LIPE is expressed in a wide variety of organs and cells, including the heart, skeletal muscle, adrenal glands, testes, ovaries, pancreatic -cells, liver, intestine and macrophages [13][14][15][16] . However, phenotypic analyses of Lipe-deficient mice generated by several laboratories, including ours, have suggested the redundancy of lipases in macrophages as well as in adipose tissue.…”

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“…[6] HSL gene and protein structure are varying size from 88 to 130 kDa, generated by the same gene with different promoters in various tissue. [7][8][9][10] HSL in adipocyte is made up of catalytic domain in the position of N-terminal and C-terminal which is similar to all isoform of HSL. Amino acid residue such as Ser, Asp, and His present in catalytic domain and also it is phosphorylated site with its regulation.…”