Abstract
Class I phosphoinositide 3-kinase (PI3K) generates phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) at the plasma membrane in response to growth factors, activating a signalling cascade that regulates many cellular functions including cell growth, proliferation, survival, migration and metabolism. The PI3K pathway is commonly dysregulated in human cancer, and drives tumorigenesis by promoting aberrant cell growth and transformation. PtdIns(3,4,5)P3 facilitates the activation of many pleckstrin homology (PH) domain-containing proteins including the serine/threonine kinase AKT. There are three AKT isoforms that are frequently hyperactivated in cancer through mutation, amplification or dysregulation of upstream regulatory proteins. AKT isoforms have converging and opposing functions in tumorigenesis. PtdIns(3,4,5)P3 signalling is degraded and terminated by phosphoinositide phosphatases such as phosphatase and tensin homologue (PTEN), proline-rich inositol polyphosphate 5-phosphatase (PIPP) (INPP5J) and inositol polyphosphate 4-phosphatase type II (INPP4B). PtdIns(3,4,5)P3 is rapidly hydrolysed by PIPP to generate phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2), which is further hydrolysed by INPP4B to form phosphatidylinositol 3-phosphate (PtdIns3P). PtdIns(3,4)P2 and PtdIns3P are also important signalling molecules; PtdIns(3,4)P2 together with PtdIns(3,4,5)P3 are required for maximal AKT activation and PtdIns3P activates PI3K-dependent serum and glucocorticoid-regulated kinase (SGK3) signalling. Loss of Pten, Pipp or Inpp4b expression or function promotes tumour growth in murine cancer models through enhanced AKT isoform-specific signalling. INPP4B inhibits PtdIns(3,4)P2-mediated AKT activation in breast and prostate cancer; however, INPP4B expression is increased in acute myeloid leukaemia (AML), melanoma and colon cancer where it paradoxically promotes cell proliferation, transformation and/or drug resistance. This review will discuss how PTEN, PIPP and INPP4B distinctly regulate PtdIns(3,4,5)P3 signalling downstream of PI3K and how dysregulation of these phosphatases affects cancer outcomes.
Highlights
Class I phosphoinositide 3-kinase (PI3K) generates phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) at the plasma membrane in response to growth factors, activating a signalling cascade that regulates many cellular functions including cell growth, proliferation, survival, migration and metabolism
AKT is phosphorylated at Threonine-308 (Thr308) within the T-loop region of the catalytic domain by phosphoinositide-dependent kinase 1 (PDK1) and at Serine-473 (Ser473) in the C-terminal hydrophobic motif by mammalian target of rapamycin complex 2, thereby promoting its kinase activity to phosphorylate a diverse spectrum of protein targets [5,6]
The inositol polyphosphate phosphatases are dysregulated in many human cancers, and they were originally predicted to be negative regulators of canonical PI3K/AKT signalling, recent studies suggest they may play more complex roles in tumorigenesis [128, 143, 144, 145, 146]
Summary
Regulation of PI3K effector signalling in cancer by the phosphoinositide phosphatases. Class I phosphoinositide 3-kinase (PI3K) generates phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) at the plasma membrane in response to growth factors, activating a signalling cascade that regulates many cellular functions including cell growth, proliferation, survival, migration and metabolism. Loss of Pten, Pipp or Inpp4b expression or function promotes tumour growth in murine cancer models through enhanced AKT isoform-specific signalling. PtdIns(3,4,5)P3 is rapidly dephosphorylated at the D5-position of the inositol ring by inositol polyphosphate 5-phosphatases producing phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2) Both PtdIns(3,4,5)P3 and PtdIns(3,4)P2 facilitate the plasma membrane recruitment of pleckstrin homology (PH)-domain containing proteins such as the serine/threonine kinase AKT [3,4,5]. PI3K-dependent AKT signalling is inhibited by phosphatase and tensin homologue (PTEN), which hydrolyses PtdIns(3,4,5)P3 at the D3-position phosphate of the inositol ring to form PtdIns(4,5)P2 directly opposing PI3K. II (INPP4B) to generate phosphatidylinositol 3-phosphate (PtdIns3P), which terminates PI3K/AKT signalling [7,8]
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