No abstract

Mdm2 has been shown to promote its own ubiquitination and the ubiquitination of the p53 tumour suppressor by virtue of its E3 ubiquitin ligase activity. This modi®cation targets Mdm2 and p53 for degradation by the proteasome. The p14ARF tumour suppressor has been shown to inhibit degradation of p53 mediated by Mdm2. Several models have been proposed to explain this eect of p14ARF. Here we have compared the eects of p14ARF overexpression on the in vivo ubiquitination of p53 and Mdm2. We report that the inhibition of the Mdm2-mediated degradation of p53 by p14ARF is associated with a decrease in the proportion of ubiquitinated p53. The levels of polyubiquitinated p53 decreased preferentially compared to monoubiquitinated species. p14ARF overexpression increased the levels of Mdm2 but it did not reduce the overall levels of ubiquitinated Mdm2 in vivo. This is unexpected because p14ARF has been reported to inhibit the ubiquitination of Mdm2 in vitro. In addition we show that like p14ARF, the proteasome inhibitor MG132 can promote the accumulation of Mdm2 in the nucleolus and that this can occur in the absence of p14ARF expression. We also show that the mutation of the nucleolar localization signal of Mdm2 does not impair the overall ubiquitination of Mdm2 but is necessary for the eective polyubiquitination of p53. These studies reveal important dierences in the regulation of the stability of p53 and of Mdm2. Oncogene (2001) 20, 4972 ± 4983.

Phosphoinositide 3-kinases (PI3Ks) are critical regulators of pancreatic β cell mass and survival, whereas their involvement in insulin secretion is more controversial. Furthermore, of the different PI3Ks, the class II isoforms were detected in β cells, although their role is still not well understood. Here we show that down-regulation of the class II PI3K isoform PI3K-C2α specifically impairs insulin granule exocytosis in rat insulinoma cells without affecting insulin content, the number of insulin granules at the plasma membrane, or the expression levels of key proteins involved in insulin secretion. Proteolysis of synaptosomal-associated protein of 25 kDa, a process involved in insulin granule exocytosis, is impaired in cells lacking PI3K-C2α. Finally, our data suggest that the mRNA for PI3K-C2α may be down-regulated in islets of Langerhans from type 2 diabetic compared with non-diabetic individuals. Our results reveal a critical role for PI3K-C2α in β cells and suggest that down-regulation of PI3K-C2α may be a feature of type 2 diabetes.

The life expectancy for pancreatic cancer patients has seen no substantial changes in the last 40 years as very few and mostly just palliative treatments are available. As the five years survival rate remains around 5%, the identification of novel pharmacological targets and development of new therapeutic strategies are urgently needed. Here we demonstrate that inhibition of the G protein-coupled receptor GPR55, using genetic and pharmacological approaches, reduces pancreatic cancer cell growth in vitro and in vivo and we propose that this may represent a novel strategy to inhibit pancreatic ductal adenocarcinoma (PDAC) progression. Specifically, we show that genetic ablation of Gpr55 in the KRAS/TP53/Pdx1-Cre (KPC) mouse model of PDAC significantly prolonged survival. Importantly, KPC mice treated with a combination of the GPR55 antagonist Cannabidiol (CBD) and gemcitabine (GEM, one of the most used drugs to treat PDAC), survived nearly three times longer compared to mice treated with vehicle or GEM alone. Mechanistically, knockdown or pharmacologic inhibition of GPR55 reduced anchorage-dependent and independent growth, cell cycle progression, activation of mitogen-activated protein kinase (MAPK) signalling and protein levels of ribonucleotide reductases in PDAC cells. Consistent with this, genetic ablation of Gpr55 reduced proliferation of tumour cells, MAPK signalling and ribonucleotide reductase M1 levels in KPC mice. Combination of CBD and GEM inhibited tumour cell proliferation in KPC mice and it opposed mechanisms involved in development of resistance to GEM in vitro and in vivo. Finally, we demonstrate that the tumour suppressor p53 regulates GPR55 protein expression through modulation of the microRNA miR34b-3p. Our results demonstrate the important role played by GPR55 downstream of p53 in PDAC progression. Moreover our data indicate that combination of CBD and GEM, both currently approved for medical use, might be tested in clinical trials as a novel promising treatment to improve PDAC patients' outcome.

Purpose: Phosphoinositide 3-kinase (PI3K) signaling is well established as important in cancer. To date most studies have been focused on the PI3K/p110α isoform, which has been found to be mutated in several different cancers. The aim of our study was to determine which specific PI3K isoforms are involved in pancreatic ductal adenocarcinoma (PDAC) and investigate the effects of these isoforms on proliferation, survival, and induction of Akt activation in pancreatic cancer cells.Experimental Design: The expression of all PI3K isoforms and downstream targets was analyzed by immunohistochemistry in human pancreatic cancer tissue and normal counterparts. Isoform selective inhibitors and short interfering RNA (siRNA) were employed to investigate the effects of the different PI3Ks on proliferation, survival, and intracellular signaling in PDAC cell lines.Results: Immunohistochemical screening revealed high specific expression of the PI3K/p110γ isoform. Scoring indicated that 72% of the PDAC tissue stained positive for PI3K/p110γ, whereas no stain was detected in normal pancreatic ducts. Proliferation analyses after selective inhibition and siRNA downregulation of PI3K/p110γ showed that PI3K/p110γ, but not other PI3K isoforms, was required for cell proliferation. Overexpression of PI3K/p110γ indeed increased cell numbers and mediated activation of Akt in PDAC cell lines. Moreover, PI3K/p110γ was required for Akt activation via lysophosphatidic acid receptors.Conclusions: These data represent the first identification of a tumor-specific accumulation of the PI3K isoform p110γ in human cancer. Further, our results signify a critical role for PI3K/p110γ in pancreatic cancer, and we hypothesize that PI3K/p110γ overexpression is a key event in the disease progression. Clin Cancer Res; 16(20); 4928-37. ©2010 AACR.