Insulin-inducible changes in the relative ratio of PTP1B splice variants

Cell and molecular biology of the novel protein tyrosine-phosphatase-interacting protein 51

This chapter examines the current state of knowledge about the expression profile, as well as biochemical properties and biological functions of the evolutionarily conserved protein PTPIP51. PTPIP51 is apparently expressed in splice variants and shows a particularly high expression in epithelia, skeletal muscle, placenta, and germ cells, as well as during mammalian development and in cancer. PTPIP51 is an in vitro substrate of Src- and protein kinase A, the PTP1B/TCPTP protein tyrosine phosphatases and interacts with 14-3-3 proteins, the Nuf2 kinetochore protein, the ninein-interacting CGI-99 protein, diacylglycerol kinase alpha, and also with itself forming dimers and trimers. Although the precise cellular function remains to be elucidated, the current data implicate PTPIP51 in signaling cascades mediating proliferation, differentiation, apoptosis, and motility.

Tyrosine Phosphorylation, Thiol Status, and Protein Tyrosine Phosphatase in Rat Epididymal Spermatozoa

Sperm thiol oxidation and the ability to undergo protein tyrosine phosphorylation are associated with the acquisition of sperm motility and fertilizing ability during passage of spermatozoa through the epididymis. Phosphotyrosine levels in various cells are controlled by tyrosine kinase versus phosphatase, with the latter known to be inhibited by oxidation. In the present paper we examine whether changes in thiol status during sperm maturation affect rat sperm protein phosphotyrosine levels and protein phosphotyrosine phosphatase (PTP) activity. Tyrosine phosphorylation, as demonstrated by immunoblotting (IB), was significantly increased in several sperm tail proteins during maturation in the epididymis. Sperm thiol oxidation with diamide enhanced tail protein phosphorylation; reduction of disulfides with dithiothreitol diminished phosphorylation. In the sperm head, a moderate increase in tyrosine phosphorylation was accompanied by altered localization of phosphotyrosine proteins during maturation. Blocking of thiols and PTP activity with N-ethylmaleimide led to increased tyrosine phosphorylation of protamine in caput sperm heads. Several PTP bands were identified by IB. In the caput spermatozoa, a prominent level of the 50 kDa band was present, whereas in the cauda spermatozoa a very low level of the 50 kDa band was found. PTP activity, measured by using p-nitrophenyl phosphate as a substrate, was significantly higher in the caput spermatozoa (high thiol content) than in the cauda spermatozoa (low thiol content). Our results show that PTP activity is correlated with sperm thiol status and suggest that tyrosine phosphorylation of sperm proteins during sperm maturation is promoted by thiol oxidation and diminished PTP.

PTP1B: from the sidelines to the front lines!

Although initially viewed as housekeeping enzymes, research over the last 15 years has revealed that the protein tyrosine phosphatases (PTPs) are critical regulators of tyrosine phosphorylation-dependent signaling events and may represent novel targets for therapeutic intervention in a variety of human diseases. In this review I will describe some of the key advances in the characterization of the structure, regulation and function of the prototypic PTP, PTP1B, and illustrate how our understanding of the properties of this enzyme has revealed principles that apply to the PTP family as a whole.

Protein tyrosine phosphatase 1B: a new target for the treatment of obesity and associated co-morbidities

Impaired insulin action is important in the pathophysiology of multiple metabolic abnormalities such as obesity and type 2 diabetes. Protein tyrosine phosphatase 1B (PTP1B) is considered a negative regulator of insulin signalling. This is best evidenced by studies on knockout mice showing that lack of PTP1B is associated with increased insulin sensitivity as well as resistance to obesity and in vitro studies whilst studies in animals and humans have given contradictory results. However, several studies support the notion that insulin signalling can be enhanced by the inhibition of PTP1B providing an attractive target for therapy against type 2 diabetes and obesity. In addition, recent genetic studies support the association between PTP1B with insulin resistance. The development of PTP1B inhibitors has already begun although it has become clear that is not easy to find both a selective, safe and effective PTP1B inhibitor. The objective of this paper is to review the current evidence of PTP1B in the pathophysiology of obesity, type 2 diabetes and cancer as well as in the treatment of these disorders.

De novo ceramide regulates the alternative splicing of caspase 9 and Bcl-x in A549 lung adenocarcinoma cells. Dependence on protein phosphatase-1

Previous studies have demonstrated that several splice variants are derived from both the caspase 9 and Bcl-x genes in which the Bcl-x splice variant, Bcl-x(L) and the caspase 9 splice variant, caspase 9b, inhibit apoptosis in contrast to the pro-apoptotic splice variants, Bcl-x(s) and caspase 9. In a recent study, we showed that ceramide induces the dephosphorylation of SR proteins, a family of protein factors that regulate alternative splicing. In this study, the regulation of the alternative processing of pre-mRNA of both caspase 9 and Bcl-x(L) was examined in response to ceramide. Treatment of A549 lung adenocarcinoma cells with cell-permeable ceramide, D-e-C(6) ceramide, down-regulated the levels of Bcl-x(L) and caspase 9b mRNA and immunoreactive protein with a concomitant increase in the mRNA and immunoreactive protein levels of Bcl-x(s) and caspase 9 in a dose- and time-dependent manner. Pretreatment with calyculin A (5 nm), an inhibitor of protein phosphatase-1 (PP1) and protein phosphatase 2A (PP2A) blocked ceramide-induced alternative splicing in contrast to okadaic acid (10 nm), a specific inhibitor of PP2A at this concentrations in cells, demonstrating a PP1-mediated mechanism. A role for endogenous ceramide in regulating the alternative splicing of caspase 9 and Bcl-x was demonstrated using the chemotherapeutic agent, gemcitabine. Treatment of A549 cells with gemcitabine (1 microm) increased ceramide levels 3-fold via the de novo sphingolipid pathway as determined by pulse labeling experiments and inhibition studies with myriocin (50 nm), a specific inhibitor of serine palmitoyltransferase (the first step in de novo synthesis of ceramide). Treatment of A549 cells with gemcitabine down-regulated the levels of Bcl-x(L) and caspase 9b mRNA with a concomitant increase in the mRNA levels of Bcl-x(s) and caspase 9. Again, inhibitors of ceramide synthesis blocked this effect. We also demonstrate that the change in the alternative splicing of caspase 9 and Bcl-x occurred prior to apoptosis following treatment with gemcitabine. Furthermore, doses of D-e-C(6) ceramide that induce the alternative splicing of both caspase 9 and Bcl-x-sensitized A549 cells to daunorubicin. These data demonstrate a role for protein phosphatases 1 (PP1) and endogenous ceramide generated via the de novo pathway in regulating this mechanism. This is the first report on the dynamic regulation of RNA splicing of members of the Bcl-2 and caspase families in response to regulators of apoptosis.

Genomic characterization of the human and mouse protein tyrosine phosphatase-1B genes

PTP-1B is a ubiquitously expressed intracellular protein tyrosine phosphatase (PTP) that has been implicated in the negative regulation of insulin signaling. Mice deficient in PTP-1B were found to have an enhanced insulin sensitivity and a resistance to diet-induced obesity. Interestingly, the human PTP-1B gene maps to chromosome 20q13.1 in a region that has been associated with diabetes and obesity. Although there has been a partial characterization of the 3' end of the human PTP-1B gene, the complete gene organization has not been described. In order to further characterize the PTP-1B gene, we have cloned and determined the genomic organization for both the human and mouse PTP-1B genes including the promoter. The human gene spans >74 kb and features a large first intron of >54 kb; the mouse gene likewise contains a large first intron, although the exact size has not been determined. The organization of the human and mouse PTP-1B genes is identical except for an additional exon at the 3' end of the human that is absent in the mouse. The mouse PTP-1B gene maps to the distal arm of mouse chromosome 2 in the region H2-H3. This region is associated with a mouse obesity quantitative trait locus (QTL) and is syntenic with human chromosome 20. The promoter region of both the human and mouse genes contain no TATA box but multiple GC-rich sequences that contain a number of consensus SP-1 binding sites. The basal activity of the human PTP-1B promoter was characterized in Hep G2 cells using up to 8 kb of 5' flanking sequence. A 432 bp promoter construct immediately upstream of the ATG was able to confer maximal promoter activity. Within this sequence, there are at least three GC-rich sequences and one CCAAT box, and deletion of any of these elements results in decreased promoter activity. In addition, the promoter in a number of mouse strains contains, 3.5 kb upstream of the start codon, an insertion of an intracisternal a particle (IAP) element that possibly could alter the expression of PTP-1B mRNA in these strains.