Does nonsense-mediated mRNA decay explain the ovarian cancer cluster region of the BRCA2 gene?

BRCA2 (BReast CAncer susceptibility gene 2) germline mutation carriers are at increased risk for breast and ovarian cancers. Mutations occurring in the ovarian cancer cluster region (OCCR) are linked to higher ovarian cancer and/or lower breast cancer risk(s) than mutations occurring elsewhere in BRCA2. Most BRCA2 germline mutations introduce premature termination codons (PTCs), making their mRNAs likely targets of nonsense-mediated mRNA decay (NMD), a mechanism that eliminates PTC-bearing transcripts to prevent expression of truncated proteins. Contradictory evidence exists regarding whether NMD can be triggered by PTCs located far upstream of the nearest exon-exon junction (EEJ). Since the OCCR comprises a major portion of the 4.9 kb exon 11 of BRCA2, we investigated if transcripts bearing PTCs in this large exon are unable to trigger NMD, and if this might contribute to the phenotypic difference associated with the OCCR. We examined cDNA from 18 carriers of PTC-introducing germline mutations located throughout BRCA2, and found that PTC-bearing transcripts were 1.4-3.3-fold less prevalent than their nonmutated counterparts irregardless of PTC position. We conclude that NMD can recognize PTCs up to 4.5 kb upstream of the nearest EEJ, demonstrating that a general inability of NMD to recognize PTCs in exon 11 is unlikely to explain the genotype-phenotype correlation associated with the OCCR.

SHIP's C-terminus is essential for its hydrolysis of PIP3 and inhibition of mast cell degranulation

The SH2-containing inositol-5'-phosphatase, SHIP, restrains bone marrow-derived mast cell (BMMC) degranulation, at least in part, by hydrolyzing phosphatidylinositol (PI)-3-kinase generated PI-3,4,5-P(3) (PIP3) to PI-3,4-P(2). To determine which domains within SHIP influence its ability to hydrolyze PIP3, bone marrow from SHIP(-/-) mice was retrovirally infected with various SHIP constructs. Introduction of wild-type SHIP into SHIP(-/-) BMMCs reverted the Steel factor (SF)-induced increases in PIP3, calcium entry, and degranulation to those observed in SHIP(+/+) BMMCs. A 5'-phosphatase dead SHIP, however, could not revert the SHIP(-/-) response, whereas a SHIP mutant in which the 2 NPXY motifs were converted to NPXFs (2NPXF) could partially revert the SHIP(-/-) response. SF stimulation of BMMCs expressing the 2NPXF, which could not bind Shc, led to the same level of mitogen-activated protein kinase (MAPK) phosphorylation as that seen in BMMCs expressing the other constructs. Surprisingly, C-terminally truncated forms of SHIP, lacking different amounts of the proline rich C-terminus, could not revert the SHIP(-/-) response at all. These results suggest that the C-terminus plays a critical role in enabling SHIP to hydrolyze PIP(3) and inhibit BMMC degranulation.

SHIPs ahoy

In 1996 three groups independently cloned a hemopoietic specific, src homology 2-containing inositol 5'-phosphatase which, based on its structure, was called SHIP. More recently, a second more widely expressed SHIP-like protein has been cloned and called SHIP2. Both specifically hydrolyze phosphatidylinositol-3,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in vitro. Moreover, SHIP has been shown in vivo to be the primary enzyme responsible for breaking down phosphatidylinositol-3,4,5-trisphosphate to phosphatidylinositol-3,4-bisphosphate in normal mast cells and, as a result, limits normal and prevents inappropriate mast cell degranulation. Because of their ability to break down phosphatidylinositol-3,4,5-trisphosphate, the SHIPs have the potential to regulate many, if not all, phosphatidylinositol-3-kinase induced events including, proliferation, differentiation, apoptosis, end cell activation, cell movement and adhesion and will thus likely be the subject of intensive research over the next few years.

The role of SHIP in growth factor induced signalling

The recently cloned, hemopoietic-specific, src homology 2 (SH2)-containing inositol phosphatase, SHIP, is rapidly gaining prominence as a potential regulator of all phosphatidylinositol (PI)-3 kinase mediated events since it has been shown both in vitro and in vivo to hydrolyze the 5' phosphate from phosphatidylinositol-3,4,5-trisphosphate (PI-3,4,5-P3). Thus SHIP, and its more widely expressed counterpart, SHIP2, could play a central role in determining PI-3,4,5-P3 and PI-3,4-P2 levels in many cell types. To explore the in vivo function of SHIP further we recently generated a SHIP knock out mouse and in this review we discuss experiments carried out with bone marrow derived mast cells (BMMCs) from these animals.

Multiple forms of the SH2-containing inositol phosphatase, SHIP, are generated by C-terminal truncation

The SH2-containing inositol phosphatase, SHIP, often appears as multiple bands in anti-SHIP immunoblots. To characterize these bands, antisera were generated against the N-terminal (anti-N), mid-region (anti-M), and C-terminal (anti-C) portions of SHIP. Immunoprecipitation and immunoblotting studies showed that 145-, 135-, 125-, and 110-kD bands were detected in lysates from the murine hematopoietic cell line, DA-ER, with either anti-N or anti-M antisera, whereas only the 145- and 135-kD bands were recognized by the anti-C antiserum. This finding suggested that the smaller proteins might be C-terminal truncations of the full-length SHIP. To confirm this and determine if these proteins arose through alternate splicing or posttranslational cleavage, a 5'-hemagglutin (HA)-tagged full-length SHIP cDNA was expressed in these cells. We observed, via Western analysis with anti-HA antibodies, the same 4 bands with either anti-N or anti-M and only the 145- and 135-kD bands with anti-C immunoprecipitation. After interleukin-3 stimulation of HA-SHIP-expressing DA-ER cells, only the 145-kD form coprecipitated with Shc, raising the possibility that different forms of SHIP may have distinct intracellular sites. This was confirmed by subcellular fractionation, which showed that only the 110-kD form is present in the cytoskeleton of DA-ER cells. This 110-kD form possesses the same PIP3 5-ptase activity as the 145-kD form and can be generated from the latter in vitro by digestion with calpain. It is therefore possible that the different forms of SHIP are generated in vivo by calpain-mediated C-terminal truncations and perform distinct functions within hematopoietic cells.

Interleukin-3 induces the association of the inositol 5-phosphatase SHIP with SHP2

We recently purified and cloned a 145-kDa protein that becomes tyrosine phosphorylated and associated with Shc in response to multiple cytokines. Based on its predicated amino acid sequence and its enzymatic activity, we have called this protein SHIP, for Src homology 2-containing inositol phosphatase. To gain further insight into the intracellular pathways that this putative signal transduction intermediate might regulate we have investigated whether SHIP binds to intracellular proteins other than Shc. The results presented herein demonstrate that following interleukin-3 stimulation, SHIP binds to the tyrosine phosphatase, SHP2 (also called Syp, PTP1D, SHPTP2, and PTP2C) and that Shc is not present in these SHIP-SHP2 complexes. Time course studies reveal that SHIP's association with SHP2 is transient and is maximal at 10 min of stimulation with interleukin-3. We further show that the association of SHIP with SHP2 occurs through the direct interaction of the SH2 domain of SHIP with a pYXN(I/V) sequence within SHP2.

Cloning and characterization of human SHIP, the 145-kD inositol 5-phosphatase that associates with SHC after cytokine stimulation

We recently cloned and sequenced a cDNA encoding a 145-kD protein from the murine hematopoietic cell line B6SUtA, that becomes tyrosine phosphorylated and associated with Shc after cytokine stimulation. Based on its domains and enzymatic activity, we named this protein SHIP for SH2-containing inositol phosphatase (Damen et al, Proc Natl Acad Sci USA 93:1689, 1996). We describe here the cloning of the human homologue of murine SHIP (mSHIP) from a human megakaryocytic cell line (MO7e) lambda gt11 cDNA library using two nonoverlapping mSHIP cDNA fragments as probes. Northern blot analysis suggests that human SHIP (hSHIP) is expressed as a 5.3-kb mRNA in human bone marrow and a wide variety of other tissues. Sequence analysis of this cDNA predicts a protein of 1188 amino acids exhibiting 87.2% overall sequence identity with mSHIP. Contained within the defined open reading frame is an N-terminal, group l src homology 2 (SH2) domain; three NXXY motifs that, if phosphorylated, could be bound by phosphotyrosine binding (PTB) domains; a C-terminal proline-rich region; and two centrally located inositol polyphosphate 5-phosphatase motifs. Fluorescence in situ hybridization, using the full-length hSHIP cDNA as a probe, mapped hSHIP to the long arm of chromosome 2 at the border between 2q36 and 2q37.

Coagulation and fibrinolysis in estrogen-treated surgically menopausal women

The short-term effects of different types and doses of estrogen therapy on coagulation and fibrinolysis were studied in 35 surgically menopausal women. Dynamic tests of the coagulation cascade, tests indicative of ongoing, intravascular coagulation, and assessments of coagulation inhibition and fibrinolysis were performed. No clinically abnormal responses were found with the tested regimens--1 and 2 mg of 17 beta-estradiol and 0.625 and 1.25 mg of conjugated equine estrogens. Increased plasminogen antigen and activity were found with the conjugated estrogens but not with the 17 beta-estradiol preparations. The age of the woman had no effect on either the direction or magnitude of response to treatment. Estrogen therapy at the reported doses does not appear to adversely affect the coagulation-fibrinolysis systems of surgically menopausal women. Based on their ability to enhance plasminogen activity, conjugated estrogens may be preferred over the 17 beta-estradiol preparations for this clinical population.

Lipids and lipoproteins in women after oophorectomy and the response to oestrogen therapy

The short-term effects of different types and doses of oestrogen on serum lipids and lipoproteins were studied in 35 oophorectomized women. After 3 months treatment, serum cholesterol levels were unaffected by 1 and 2 mg of micronized 17 beta-oestradiol or 0.625 and 1.25 mg of conjugated equine oestrogens. Triglyceride levels were significantly elevated after treatment with 1.25 mg of conjugated oestrogens. A trend towards a higher relative proportion of high-density lipoproteins and a lower relative proportion of low-density lipoproteins was observed for all four oestrogen regimens, however, statistical significance was not achieved. The proportion of very-low-density lipoprotein was unaffected by oestrogen treatment. The age of the oophorectomized women was found to have no effect on either the direction or magnitude of the lipid or lipoprotein responses to oestrogen. Using FSH depression as an index, 1.25 mg of conjugated oestrogens was found to be the most potent of the four oestrogen regimens tested. Therefore, with respect to lipid balance, little additional clinical benefit is achieved by using a more potent regimen and the risk of adverse side effects may be increased.

Oestrogen--progestin therapy and the lipid balance of post-menopausal women

The lipid and lipoprotein profiles of 20 post-menopausal women treated with cyclic conjugated oestrogens (0.625 or 1.25 mg) and medroxyprogesterone acetate (10 mg for 7 days) were compared to those of 18 untreated women of similar age and menopausal status. No statistically significant between-group differences were observed during the 18-mth period for cholesterol, triglycerides or lipoprotein distribution. After 12 mth, a significant shift in lipoprotein distribution manifested in the treated and untreated groups. The proportion of high-density lipoproteins significantly increased and that of the low-density lipoproteins significantly decreased. Although the shift was more pronounced in the treated group, there was no significant difference between the treated and untreated groups. These results indicated that such relatively nonandrogenic progestins as medroxyprogesterone acetate, have no adverse effects on the lipid milieu of post-menopausal women when used with long-term oestrogen therapy.

Benzodiazepines decrease grooming in response to novelty but not ACTH or beta-endorphin

Excessive grooming in response to intracerebroventricular (ICV) ACTH1-24 was assayed following various doses of diazepam, chlordiazepoxide and flurazepam. Grooming scores were only affected by doses of the benzodiazepines higher than those that depressed locomotor activity. Similarly, diazepam did not affect excessive grooming induced by ICV beta-endorphin, nor did chronic chlordiazepoxide affect ACTH-induced grooming. By contrast similar doses of the benzodiazepines decreased the increased grooming score observed when mice were observed in a novel as opposed to the home cage. This result is consistent with the hypothesis that novel cage-induced grooming is caused by an increase in the ventricular content of ACTH or beta-endorphin, and that the benzodiazepines decrease or prevent this increase. It is not consistent with hypotheses of a functional antagonism between ACTH and benzodiazepines, at least insofar as the mechanisms involved in the production of grooming are concerned.