PTPs : redox controlled regulators of cell signaling and transformation

Abstract

Protein tyrosine phosphatases (PTPs) are important redox regulated enzymes that control the tyrosine phosphorylation status in the cell. These studies aimed to increase the understanding on how PTPs are controlled by reversible oxidation in different pathological settings such as hypoxia and restenosis and to which extent oxidized PTPs contribute to certain diseases.

Low levels of oxygen - hypoxia - can occur in tumors due to poor vascularization as well as in myocardial infarction and is associated with increased levels of reactive oxygen species (ROS). In our study we found that hypoxia followed by reoxygenation caused reversible oxidation of PTPs. This created an increased amount of inactivated PTPs and was followed by prolonged PDGF receptor activation. The hypoxia induced increase in phosphorylation lead to elevated ERK signaling as well as increased formation of cytoskeletal re-arrangements which could be inhibited by addition of the antioxidant N-acetylcysteine. Decreased phosphatase activity was also seen when analyzed in heart tissues from an ex-vivo model of rat heart exposed to ischemia-reperfusion.

Atherosclerosis is a disease characterized by artery wall thickening due to a buildup of fatty materials. A common treatment is to open up the vessel with a balloon catheter and thereby increase the blood flow. A problem with this treatment is that restenosis occur after some time. Restenosis is associated with an elevated ROS production and increase in PDGFβ-receptor signaling. Using a rabbit restenosis model we could show that restenosis could be attenuated by administration of antioxidants. Tissue analysis from vessels in combination with cell culture experiments showed that the beneficial effect of antioxidant treatment was prevention of PTP inactivation.

Peroxidized lipids have been found in increased amounts in several diseases with inflammatory components, like atherosclerosis and diabetes. The amounts of peroxidized lipids are tightly regulated by the glutathione peroxidase 4 (GPx4). In an inducible knock out model of GPx4 in cells we found that the rapid accumulation of peroxidized lipids caused PTP oxidation and lead to increased cell signaling. This was the first time peroxidized lipids were shown to oxidize and inactivate PTPs.

PTPs are often regarded as inhibitors of cell signaling and therefore as potential tumor suppressors. SHP-2 is however, an exception and is a bona fida oncogene in which phosphatase-activating mutations have been associated with different forms of leukemia and to a smaller proportion with solid tumors. We found that PDGF-BB dependent growth of subcutaneous tumors was compromised when SHP-2 levels were repressed by shRNA. Cell culture experiments indicated that compromised Src activity and reduced ERK activation underlie the inability of these cells to form tumors.