Multiple forms of p55PIK, a regulatory subunit of phosphoinositide 3-kinase, are generated by alternative initiation of translation. Biochem J. 1999;341:831-837. [PMID: 10417350 DOI: 10.1042/0264-6021:3410831]

Overexpression of the N-terminal 24-amino acid domain of p55γ regulatory subunit of PI3K inhibits the migration of human gastric carcinoma MGC803 cells

AIM: To investigate the effects of overexpression of the N-terminal 24-amino acid domain of p55γ regulatory subunit (N24p55γ) of phosphoinositide 3-kinase (PI3K) on the migration of human gastric carcinoma MGC803 cells.

METHODS: MGC803/GFP-N24 and MGC803/GFP cell lines stably expressing N24p55γ-GFP fusion protein (GFP-N24) and GFP protein alone were generated by transfection of MGC803 cells with pEGFPN24 plasmid and control plasmid pEGFPC1, respectively. The impact of GFP-N24 overexpression on cell motility and migration was detected by wound healing assay and Transwell migration assay. The expression and secretion of tumor metastasis-related protein matrix metalloproteinase-9 (MMP9) were determined by gelatin zymography. The effect of GFP-N24 overexpression on the activity of the PI3K-Akt signaling pathway was analyzed by Western blot.

RESULTS: The MGC803/GFP-N24 cell line steadily expressing the GFP-N24 fusion protein and the MCG803/GFP cell line steadily expressing GFP were successfully established. The results of wound healing assay and Transwell migration assay showed that the in vitro migration of MGC803/GFP-N24 cells was restricted (t = 0.003, P < 0.01). GFP-N24 overexpression inhibited the activity of PI3K-Akt signaling pathway by decreasing the expression of phospho-Akt in MGC803 cells. However, GFP-N24 overexpression did not alter the expression and secretion of MMP9 protein.

CONCLUSION: Ectopic expression of N24p55γ inhibits cell migration in vitro by suppressing the activity of the PI3K-Akt signaling pathway in MGC803 cells. The N24p55γ peptide can potentially be used to treat gastric carcinoma.

Signalling by PI3K isoforms: insights from gene-targeted mice

Phosphoinositide 3-kinases (PI3Ks) generate lipids that control a wide variety of intracellular signalling pathways. Part of this diversity in PI3K actions stems from the broad range of protein effectors of the PI3K lipids. A further layer of complexity is added by the existence of multiple isoforms of PI3K. Gene-targeting studies in the mouse have recently uncovered key roles for specific PI3K isoforms in immunity, metabolism and cardiac function. Remarkably, some of these actions do not require PI3K catalytic activity. In addition, loss-of-expression of certain PI3K genes leads to increased PI3K signalling following insulin stimulation. PI3K gene targeting has, in many cases, led to altered expression of the non-targeted PI3K subunits, making it difficult to exclude that some of the reported phenotypes result from 'knock-on' effects of PI3K gene deletion. Targeting strategies that take into account the complex interplay between members of the PI3K family will be crucial to gain a full understanding of the physiological roles of the isoforms of PI3K.

Analysis of the expression pattern of Ebp1, an ErbB-3-binding protein

Ebp1, a member of the PA2G4 family, was isolated as an ErbB-3-binding protein in our laboratory using yeast two hybrid analysis. Although Ebp1 mRNA is ubiquitously expressed, little is known about either the expression of Ebp1 protein in vivo or its translation initiation site. Western blotting analysis of a wide range of cell lines and primary tissue indicated that in the majority of cases Ebp1 is expressed as a single protein which migrates at 48 kDa in SDS-polyacrylamide gels. We show using epitope-tagged expression constructs that the second, not the first, in-frame ATG is used for the initiation of translation of the endogenous protein, encoding a protein predicted to be 41.5 kDa. The molecular mass of endogenous Ebp1 protein derived from mouse liver and brain was determined by mass spectrometry and the data confirm that translation of endogenous Ebp1 in tissues is initiated from the second in-frame ATG.

Signaling by distinct classes of phosphoinositide 3-kinases

Many signaling pathways converge on and regulate phosphoinositide 3-kinase (PI3K) enzymes whose inositol lipid products are key mediators of intracellular signaling. Different PI3K isoforms generate specific lipids that bind to FYVE and pleckstrin homology (PH) domains in a variety of proteins, affecting their localization, conformation, and activities. Here we review the activation mechanisms of the different types of PI3Ks and their downstream actions, with focus on the PI3Ks that are acutely triggered by extracellular stimulation.