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Peroxisomes are organelles in eukaryotic cells that are involved in a variety of processes such as the metabolism of lipids and of reactive oxygen species. Their enzyme content can vary depending on the metabolic situation. For a flexible and dynamic adjustment of the peroxisomal metabolism to changing metabolsim situations, the turnover of these organelles has to be precisely controlled. In yeast, two distinct pathways, the macroautophagy and the microautophagy, are involved depending on the degradation stimulus. In higher eukaryotes, a 15-Lipoxygenase-(15-LOX)-mediated destruction of the peroxisomal membrane has been proposed as the initial step in peroxisome turnover. Research on the degradation of peroxisomes in higher eukaryotes, however, has been hampered by the limited number of experimental systems so far.
We have recently identified a mutated version of Pxmp2, the most abundant protein of the peroxisomal membrane that exerts a "toxic" effect on the peroxisomal compartment. A fusion protein of the Pxmp2 with the green fluorescent protein (GFP) was localised to peroxisomes when expressed at low level. At high expression levels, however, peroxisomes could not be detected in those cells. To investigate this phenomenon in more detail, we have established a CHO-cell-line for inducible expression of the Pxmp2-GFP-fusion-protein using the ecdysone-inducible gene expression system. This cell line represents the first experimental system to analyse the degradation of peroxisomes in higher eukaryotes in a synchronised fashion.
The experiments could show that the peroxisomal degradation in these cells require initial steps of autophagy, the degradation process in the CHO-cells was altered in response to known inducers and inhibitors of autophagy. Inhibitors of 15-LOX, however, did not effect the degradation process.
In contrast to the pexophagy-process in yeast we could not show any involvement of lysosomes in the degradation of peroxisomes in the CHO-cells. Since Pxmp2-GFP never colocalized with lysosomal markers, a terminal degradation pathway without or via non-typical lysosomes could function in higher eukaryotes.