¹Department of Oral Biology and Pathology, School of Dental Medicine, State University of New York at Stony Brook, Stony Brook, New York, USA

Correspondence: Dr Marcia Simon, Department of Oral Biology and Pathology, School of Dental Medicine, State University of New York at Stony Brook, Stony Brook, New York, USA. E-mail: marsimon@notes.cc.sunysb.edu

Received 20 December 2005; Revised 8 February 2006; Accepted 9 March 2006; Published online 1 June 2006.

Abstract

Retinoic acid at nanomolar concentrations modulates epidermal functions by serving as a transcription factor ligand. Under conditions of retinol sufficiency, it is imperative to limit retinoic acid biosynthesis from serum-derived retinol. In the epidermis, this is accomplished by esterifying retinol with long-chain fatty acids. Retinylester (RE) pools serve as a source of retinol for retinoic acid production under retinol deficiency and when required for proper differentiation. We have recently reported that GS2 lipase is expressed in keratinocytes and has the enzymatic properties of keratinocyte RE hydrolase. As GS2 lipase has a robust activity that can affect the intracellular retinol levels, we postulated that its activity must be regulated. Therefore, we screened keratinocyte cDNA expression libraries for the putative inhibitor. Herein, we report the identity of an inhibitor, TIP47, which prevents RE hydrolysis catalyzed by GS2 lipase and hormone-sensitive lipase. This protein was known to transport mannose-6-phosphate receptors from endosome to trans-Golgi and to be distributed between the cytoplasm and lipid droplets. Using a series of deletion mutants, we found two regions involved in the inhibitory activity. Residues within the carboxyl 3–4 helices are essential in the context of the full-length protein. Residues within the amino-terminal also contribute depending on the context.