A Study On ILKAP Induced Tumor Apoptosis

ILKAP (integrin-linked kinase-associated phosphatase) is a novel member of PP2C (protein phosphatase 2C) family. Although it has not been understanded completely, the primary results have revealed that ILKAP is an apoptosis-associated phosphatase in almost all types of tissues and cells in the body. ILKAP is ubiquitously expressed in human tissues, especially in skeletal muscle, liver and kidney ones. ILKAP has a major physiological role in the regulation on cellular apoptosis, and involved in the suppression of the tumorigenesis and tumor development. ILKAP played its role mainly based on the integrin and JNK/MAPK (c-Jun N-terminal kinase/mitogen-activated protein kinase) signaling pathways. On the other hand, the loss of heterozygosity of ILKAP gene has been confirmed in many tumor tissues. Moreover, we noted that a truncated isoform of ILKAP by Splice-Site Mutations of ILKAP gene also existed in many tumor tissues, which failed to suppress the tumorigenesis and tumor development.Though ILKAP negatively regulates integrin-linked kinase (ILK) activity to promote apoptosis, the mechanism by which ILKAP inhibits ILK signaling has remained unclear. Here we proposed a regulatory mechanism of ILKAP signaling governing ILK signaling. In a variety of tumor cells, the intact ILKAP was expressed at a lower level, whereas two ILKAP variants missing PP2C catalytic domain were predicted only in HeLa cells and gastric cancer tissue, respectively. More importantly, most, if not all, the intact ILKAP were predominantly localized in the nucleus of these cells by virtue of its N-terminal domain (1-107aa), while in the presence of the exocellular damaging stresses such as ethanol, cold, heat and UV, its cytoplasmic level was increased and perhaps its nuclear level was decreased by its nuclear export. The cytoplasmic ILKAP co-localized with ILK as well as integrinβ-1 subunit, leading to the selectively inhibition of ILK-GSK-3βsignaling axis and consequential apoptosis. These findings suggest that the subcellular localization of ILKAP also plays an important role in its functional regulation in addition to its activity. Our work provides further details of the apoptosis-promoting mechanism of ILKAP signaling.ILKAP is a negative regulator for ILK-integrinβ1 signaling by associating with ILK-integrinβ1 complex. Palladin was a novel binding partner of ILKAP in eukaryotic cells. Palladin's C-terminal fragment including only its last three Ig domains (residues 710–1106) and the PP2C domain of ILKAP (residues 108–392) were necessary and sufficient for their interaction.The biological significance of the interaction between palladin and ILKAP was that palladin recruited the cytoplasmic ILKAP to initiate ILKAP-induced apoptosis.