Abstract
In human intestinal epithelial crypt (HIEC) cells, the PI3-K/Akt-1 pathway is crucial for the promotion of cell survival and suppression of anoikis. Class I PI3-K consists of a complex formed by a catalytic (C) and regulatory (R) subunit. Three R (p85α, β, and p55γ) and four C (p110α, β, γ and δ) isoforms are known. Herein, we analyzed the expression of PI3-K isoforms in HIEC cells and determined their roles in cell survival, as well as in the β1 integrin/Fak/Src-mediated suppression of anoikis. We report that: (1) the predominant PI3-K complexes expressed by HIEC cells are p110α/p85β and p110α/p55γ; (2) the inhibition and/or siRNA-mediated expression silencing of p110α, but not that of p110β, γ or δ, results in Akt-1 down-activation and consequent apoptosis; (3) the expression silencing of p85β or p55γ, but not that of p85α, likewise induces Akt-1 down-activation and apoptosis; however, the impact of a loss of p55γ on both Akt-1 activation and cell survival is significantly greater than that from the loss of p85β; and (4) both the p110α/p85β and p110α/p55γ complexes are engaged by β1 integrin/Fak/Src signaling; however, the engagement of p110α/p85β is primarily Src-dependent, whereas that of p110α/p55γ is primarily Fak-dependent (but Src-independent). Hence, HIEC cells selectively express PI3-K isoform complexes, translating into distinct roles in Akt-1 activation and cell survival, as well as in a selective engagement by Fak and/or Src within the context of β1 integrin/Fak/Src-mediated suppression of anoikis.
Highlights
Caspase-dependent apoptosis constitutes a complex and finely tuned process which performs crucial functions in development, tissue homeostasis and repair, as well as in the pathogenesis of several diseases [1,2,3,4,5]
We report that: (1) the predominant phosphatidylinositol-3 kinase (PI3-K) complexes expressed by human intestinal epithelial crypt (HIEC) cells are p110a/p85b and p110a/p55c; (2) the inhibition and/or Small interference RNA (siRNA)-mediated expression silencing of p110a, but not that of p110b, c or d, results in Akt-1 downactivation and consequent apoptosis; (3) the expression silencing of p85b or p55c, but not that of p85a, likewise induces Akt-1 down-activation and apoptosis; the impact of a loss of p55c on both Akt-1 activation and cell survival is significantly greater than that from the loss of p85b; and (4) both the p110a/p85b and p110a/p55c complexes are engaged by b1 integrin/focal adhesion kinase (Fak)/Src signaling; the engagement of p110a/p85b is primarily Srcdependent, whereas that of p110a/p55c is primarily Fakdependent
We have shown that the PI3-K/Akt-1 pathway is critical for the survival of HIEC cells [28,29,30,31,32], but is engaged by integrin b1/Fak/Src-mediated signaling for anoikis suppression in a Fak- and Src-dependent manner [32, 33]
Summary
Caspase-dependent apoptosis constitutes a complex and finely tuned process which performs crucial functions in development, tissue homeostasis and repair, as well as in the pathogenesis of several diseases [1,2,3,4,5]. It is well understood that normal cells are intrinsically wired by default to undergo apoptosis and, require the input of signals in order to maintain apoptosis in a suppressed mode when not needed, or warranted [2,3,4,5,6]. Such critical cell survival signals are provided by various extracellular cues, including cell adhesion. Depending on the cell and tissue context, signaling originating from b1 integrins to promote cell survival and anoikis suppression will often implicate focal adhesion kinase (Fak; p125Fak), Src (p60Src) and the phosphatidylinositol-3 kinase (PI3-K)/Akt-1 (PKB; p57Akt-1) pathway [6,7,8, 10,11,12,13,14,15,16]
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