The Tumor Suppressor, PTEN/MMAC1, Dephosphorylates the Lipid Second Messenger, Phosphatidylinositol 3,4,5-Trisphosphate

Abstract

Phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P 3 ) is a key molecule involved in cell growth signaling. We demonstrated that overexpression of PTEN, a putative tumor suppressor, reduced insulininduced PtdIns(3,4,5)P 3 production in human 293 cells without effecting insulin-induced phosphoinositide 3-kinase activation. Further, transfection of the catalytically inactive mutant of PTEN (C124S) caused PtdIns(3,4,5)P 3 accumulation in the absence of insulin stimulation. Purified recombinant PTEN catalyzed dephosphorylation of PtdIns(3,4,5)P 3 , specifically at position 3 on the inositol ring. PTEN also exhibited 3-phosphatase activity toward inositol 1,3,4,5-tetrakisphosphate. Our results raise the possibility that PTEN acts in vivo as a phosphoinositide 3-phosphatase by regulating PtdIns(3,4,5)P 3 levels. As expected, the C124S mutant of PTEN was incapable of catalyzing dephosphorylation of PtdIns(3,4,5)P 3 consistent with the mechanism observed in protein-tyrosine phosphatase-catalyzed reactions.A recently identified candidate tumor suppressor gene, PTEN/MMAC1, shares sequence identity with the family of protein-tyrosine phosphatases (PTPases) 1 (1). Deletions and mutations within the PTEN gene have been observed in several cancer cell types and tumor cell lines (2, 3). Additional evidence that PTEN functions as a tumor suppressor was obtained by Furnari et al. (4), who showed that PTEN had a growth suppressor activity in glioma cells. PTEN encodes the active site consensus motif HCXXGXXR(S/T) found in all PTPases. In contrast, the recombinant protein is a poor catalyst toward both phosphoproteins and peptide substrates with the highest activity of PTEN observed toward the highly negatively charged, multiply phosphorylated polymer of (Glu-Tyr) n (5, 6). Based on these observations we thought it possible that PTEN could catalyze the dephosphorylation of acidic nonproteinaceous substrate. Identification of possible in vivo substrates would not only suggest a possible physiological function of PTEN, but they might also provide insight into how PTEN functions as a tumor suppressor.PtdIns(3,4,5)P 3 is an important second messenger involved in cell growth signaling (7). PtdIns(3,4,5)P 3 is specifically produced from PtdIns(4,5)P 2 by PI 3-kinase upon stimulation by a variety of ligands (7). Recent studies have identified that PtdIns(3,4,5)P 3 can directly activate Akt, which in turn activates p70 S6 kinase and inhibits glycogen synthase kinase-3 (8, 9). Although there are several phosphoinositide 5-phosphatases, the mechanism of regulation and particularly the degradation pathway of PtdIns(3,4,5)P 3 in vivo is still unclear (10,11). In the present study we demonstrate that recombinant PTEN has PtdIns(3,4,5)P 3 3-phosphatase activity. In addition, we provide evidence that PTEN may act in vivo as a regulator of PtdIns(3,4,5)P 3 , which produces a substrate that can be recycled by PI 3-kinase. EXPERIMENTAL PROCEDURESDetermination of PtdIns(3,4,5)P 3 in Vivo-The coding sequence of human PTEN and the C124S mutant o...

PTEN expression is associated with prognosis for patients with advanced endometrial carcinoma undergoing postoperative chemotherapy

“…The former activity is dephosphorylation of the 3-position of phosphatidylinositol 3,4,5-triphosphate, a second messenger of phosphatidylinositol 3-kinase (PI3K). 13,14 PTEN antagonizes PI3K activity and negatively regulates the PI3K/Akt pathway, which is associated with cell survival and proliferation. 13,15 However, PTEN dephosphorylates focal adhesion kinase and inhibits cell migration, spreading and focal adhesion formation.…”

Lipid signalling in disease

“…Many known oncogenes, such as mutated ERBB, insulin-like growth factor-1 (IGF1), c-Kit (stem-cell factor receptor), overexpressed chemokine receptors, mutated Ras, BCR-ABL and elevated levels of the proto-oncogene Src, to name but a few, provide constitutive input signals to PI3K 6,30 . The finding that the 3-lipid phosphatase PTEN (phosphatase and tensin homologue deleted on chromosome ten) is frequently mutated in numerous late-stage tumours finally demonstrated that elevated levels of PtdIns(3,4,5)P 3 contribute to oncogenesis 31,32 . PI3K has also been found to be mutated in colon, gastric and breast cancer 33 , and structural studies have recently elucidated how key mutations can constitutively activate the PI3K complex by relieving the inhibitory action of the p85 regulatory subunit on the catalytic subunit p110 .…”

P53 and PTEN expression contribute to the inhibition of EGFR downstream signaling pathway by cetuximab

“…15 PTEN antagonizes the action of PI3K and negatively controls the downstream pathway by inhibiting the phosphorylation of AKT. 16 Consequently, the loss of PTEN expression leads to an elevated activation of AKT. 17 A recent study investigating protein expression demonstrated that PTEN has a fundamental function in predicting the response to cetuximab against EGFR in metastatic colorectal cancer.…”