Characterization of two spliced variants of human phosphatidic acid phosphatase cDNAs that are differentially expressed in normal and tumor cells

Integral membrane lipid phosphatases/phosphotransferases: common structure and diverse functions

Phospholipids and sphingolipids play critical roles in signal transduction, intracellular membrane trafficking, and control of cell growth and survival. We discuss recent progress in the identification and characterization of a family of integral membrane proteins with central roles in bioactive lipid metabolism and signalling. These five groups of homologous proteins, which we collectively term LPTs (lipid phosphatases/phosphotransferases), are characterized by a core domain containing six transmembrane-spanning alpha-helices connected by extramembrane loops, two of which interact to form the catalytic site. LPT family members are localized to all major membrane compartments of the cell. The transmembrane topology of these proteins places their active site facing the lumen of endomembrane compartments or the extracellular face of the plasma membrane. Sequence conservation between the active site of the LPPs (lipid phosphate phosphatases), SPPs (sphingosine phosphate phosphatases) and the recently identified SMSs (sphingomyelin synthases) with vanadium-dependent fungal oxidases provides a framework for understanding their common catalytic mechanism. LPPs hydrolyse LPA (lysophosphatidic acid), S1P (sphingosine 1-phosphate) and structurally-related substrates. Although LPPs can dephosphorylate intracellularly generated substrates to control intracellular lipid metabolism and signalling, their best understood function is to regulate cell surface receptor-mediated signalling by LPA and S1P by inactivating these lipids at the plasma membrane or in the extracellular space. SPPs are intracellularly localized S1P-selective phosphatases, with key roles in the pathways of sphingolipid metabolism linked to control of cell growth and survival. The SMS enzymes catalyse the interconversion of phosphatidylcholine and ceramide with sphingomyelin and diacylglycerol, suggesting a pivotal role in both housekeeping lipid synthesis and regulation of bioactive lipid mediators. The remaining members of the LPT family, the LPR/PRGs (lipid phosphatase-related proteins/plasticity-related genes) and CSS2s (type 2 candidate sphingomyelin synthases), are presently much less well studied. These two groups include proteins that lack critical amino acids within the catalytic site, and could therefore not use the conserved LPT reaction mechanism to catalyse lipid phosphatase or phosphotransferase reactions. In this review, we discuss recent ideas about their possible biological activities and functions, which appear to involve regulation of cellular morphology and, possibly, lipid metabolism and signalling in the nuclear envelope.

Molecular mechanisms regulating the tumor-targeting potential of splice-activated gene expression

Previous studies have suggested that differences in the ability of normal and malignant cells to process certain alternatively spliced pre-mRNA transcripts can be exploited as a potentially powerful means of targeting the expression of therapeutic genes to tumor cells in vivo and in vitro. Specifically, it was shown that efficient processing of minigene constructs containing the alternatively spliced CD44 exons v9 and v10 only occurs in tumor cells that express CD44 isoforms that incorporate these exons (e.g. CD44R1). In the present study, efforts were made to define the molecular mechanisms that underlie the apparent specificity of this process. RT-PCR analysis and DNA sequencing were used to characterize the various splicing events that occur between CD44 exons v8, v9 and v10 following transfection of minigene constructs containing these various exons into CD44R1-positive (PC3) and CD44R1-negative (T24) cell lines. The results obtained confirm that although the v8-v9 intron is efficiently removed in both CD44R1-positive and CD44R1-negative cells, the corresponding v9-v10 intron is accurately spliced and the exons appropriately joined only in lines that express v10-containing CD44 isoforms (e.g. PC3). In CD44R1-negative cell lines (e.g. T24) alternative 5' and 3' splice sites located within the v9-v10 intron are preferentially used, resulting in various portions of the intron being retained within the final processed mRNA product. It is proposed that identification of these functionally important intronic sequence elements will facilitate the development of second generation "splice activated gene expression" vectors that may prove useful in various cancer gene therapy applications.