Knockdown of MTP18, a Novel Phosphatidylinositol 3-Kinase-dependent Protein, Affects Mitochondrial Morphology and Induces Apoptosis

Abstract

We identified a novel human cDNA encoding a mitochondrial protein, MTP18 (mitochondrial protein, 18 kDa) as a transcriptional downstream target of phosphatidylinositol (PI) 3-kinase signaling. We demonstrate that MTP18 mRNA as well as protein expression is dependent on PI 3-kinase activity. Confocal microscopy and biochemical fractionation revealed a mitochondrial localization of MTP18. Loss-of-function analysis employing antisense molecules revealed that MTP18 is essential for cell viability in PC-3 and HaCaT cells. We show that knockdown of MTP18 protein level results in a cytochrome c release from mitochondria and consequently leads to apoptosis. In addition, HaCaT cells with reduced levels of MTP18 become more sensitive to apoptotic stimuli. This effect is accompanied by dramatic subcellular alterations. Reduction of MTP18 impairs mitochondrial morphology resulting in the formation of a highly interconnected mitochondrial reticulum in COS-7 cells. Conversely, overexpression of MTP18 induces a punctuate morphology of mitochondria suggesting also a functional role of MTP18 in maintaining the mitochondrial integrity. Hence, our data indicate an unexpected connection of PI 3-kinase signaling, apoptosis and the morphology of mammalian mitochondria.

Mitochondria and cancer: is there a morphological connection?

“…Modification of lipid composition is likely to be involved, and additional proteins may participate in mitochondrial fission. For example, endophilin B1, a putative fatty acyl transferase, has been shown to act downstream of Drp1 in the control of mitochondrial morphology and MTP18 is a novel mitochondrial protein that is able to trigger mitochondrial fragmentation in a Drp1-dependent fashion (Tondera et al, 2004). GDPAP1, which is expressed in Schwann cells and neurons, has also been reported to mediate mitochondrial fission (Niemann et al, 2005).…”

Regulation of Mitochondrial Dynamics by Proteolytic Processing and Protein Turnover

“…MTP18 protein expression is dependent on Phosphatidylinositol 3-kinase activity, and inhibition of this pathway results in loss of MTP18. After serum withdrawal or inhibition of PI3K by the small molecule inhibitor LY294002 MTP18 protein decreases in expression by 50% in approximately 5 hours and 48 hours respectively [47]. The stability of this protein has not been interrogated and further study is required to determine whether MTP18 expression is regulated like other mitochondrial dynamics proteins.…”

Mitochondrial outer-membrane permeabilization and remodelling in apoptosis

“…In cells lacking this protein, fission of the inner membrane can occur without fission of the MOM. Another protein, Mtp-18, induces Drp-1-dependent mitochondrial fission (Tondera et al, 2004(Tondera et al, , 2005. Finally, over-expression of Gdap-1, a protein of the MOM, induces mitochondrial fission, whereas its loss of function induces mitochondrial elongation (Niemann et al, 2005;Pedrola et al, 2005).…”