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

Abstract

Hormone-sensitive lipase (HSL) is a key enzyme in fatty acid mobilization in many cell types. Two isoforms of HSL are known to date, namely HSL adi (84 kDa in rat) and HSL tes (130 kDa in rat). These are encoded by the same gene, with exons 1-9 encoding the parts that are common to both and an additional 5 -exon encoding the additional amino acids in HSL tes . HSL of various tissues, among these the islet of Langerhans, is larger than HSL adi , but not as large as HSL tes , indicating that there may be other 5 -coding exons. Here we describe the molecular basis for a novel 89-kDa HSL isoform that is expressed in ␤-cells, adipocytes, adrenal glands, and ovaries in the rat and that is encoded by exons 1-9 and exon A, which is spliced to exon 1 and thereby introducing an upstream start codon. The additional 5 -base pairs encode a 43-amino acid peptide, which is highly positively charged. Conglomerates of HSL molecules are in close association with the secretory granules of the ␤-cell, as determined by immunoelectron microscopy with antibodies targeting two separate regions of HSL. We have also determined that the human genomic sequence upstream of exon A has promoter activity in INS-1 cells as well as glucose sensing capability, mediating an increase in expression at high glucose concentration. The minimal promoter is present within 170 bp from the transcriptional start site and maximal glucose responsiveness is conferred by sequence within 850 bp from the transcriptional start site.Because of the established association between type 2 diabetes and obesity, lipids have received much attention in research on the pathophysiology of type 2 diabetes. Lipids are believed to cause insulin resistance in tissues and to alter ␤-cell function. The effect of lipids on ␤-cells has been studied extensively, but the picture that emerges is complex. Different effects are seen depending on parameters such as duration of exposure, concentration, and fatty acid chain length and saturation. Briefly, the presence of free fatty acids is essential for normal glucosestimulated insulin secretion (1, 2) and also has an amplifying action on glucose-stimulated insulin secretion. On the other hand, a prolonged exposure (24 -48 h) to high concentrations (ϳ1 mM) of free fatty acids is detrimental to ␤-cell function and survival (3, 4).In view of this, knowledge of lipid mobilization, lipid storage, and the regulation of these in the ␤-cell are of great interest. Recently, we showed that hormone-sensitive lipase (HSL) 1 is expressed and active in islets of Langerhans and clonal ␤-cells (5). HSL is a key enzyme in fatty acid mobilization in the adipocytes and presumably also in several non-adipocyte cell types (see Ref. 6 for review). It is unique among known lipases in that its activity is acutely controlled by hormones. Catabolic hormones, such as glucagon and catecholamines, activate the enzyme and anabolic hormones (e.g. insulin) decrease its activity. The activity of HSL is principally determined by the intracellular level of cAMP, which con...

A beta cell-specific knockout of hormone-sensitive lipase in mice results in hyperglycaemia and disruption of exocytosis

“…Had we corrected for total insulin content in the in vitro secretion experiments, thus presenting the data as fractional insulin release, then not only early but also later phases of insulin secretion would appear as impaired. Recent work has shown that HSL is located in close proximity to the secretory granules in the pancreatic beta cell [45]. Our present findings therefore provide a functional correlate of this compartmentation of HSL.…”

“…The association of HSL with the secretory granules provides a structural correlate for these mechanisms [45]. Future work should focus on the identity of the lipid signals generated by HSL and perhaps other lipases in the beta cell.…”

Beta cell compensation for insulin resistance in Zucker fatty rats: increased lipolysis and fatty acid signalling

“…We showed impaired GSIS in only male fasted Hsl −/− mice, and this was completely reversed by the provision of exogenous NEFA [18]. The expression of HSL in beta cells, however, has been shown to be upregulated by a prolonged high glucose concentration [32] and HSL has been shown to be co-localised with insulin granules [33]. There has been particular interest in the role of non-HSL lipases in adipose tissue metabolism [34,35].…”

Transcription of LIPE gene encoding hormone-sensitive lipase/cholesteryl esterase is regulated by SF-1 in human adrenocortical cells: involvement of protein kinase A signal transduction pathway

“…During the search for a stimulatory protein, CREB was excluded since the LIPE promoter does not contain a cAMP-response element (CRE) (Smale 1997, Lindvall et al 2004. The best candidate for such a protein was SF-1, a principal stimulator of expression of almost all genes involved in steroid hormone synthesis (Val et al 2003).…”

“…HSL is most active in adipose tissue. In steroidogenic tissues including the adrenal cortex, the enzyme is about three times less active, and its activity in pancreatic b-cells is even lower, while in the heart and macrophages, it is negligible (Khoo et al 1993, Langfort et al 1999, Lindvall et al 2004.…”