Estrogen partially down-regulates PTEN to prevent apoptosis in VSC4.1 motoneurons following exposure to IFN-γ

PTEN is a tumor suppressor gene that is either mutated or deleted in a number of human cancers. PTEN acts as a negative regulator of the PI3K/Akt survival pathway and thus plays an important role in cell fate, proliferation, growth, and migration. Recent evidence suggests that PTEN may also be involved in the pathophysiology of neurodegenerative disorders such as spinal cord injury (SCI). Overexpression of PTEN appears to cause inactivation/dephosphorylation of Akt in neurons, resulting in increased cell death. Given this newly discovered role for PTEN, it has been identified as a potential molecular target for the development of novel therapeutic strategies against neurodegeneration. Motoneuron degeneration following SCI may occur due to up-regulation of pro-inflammatory and cytotoxic cytokines including IFN-γ. Exposure of VSC4.1 motoneurons to IFN-γ (10 ng/ml) for 24 h resulted in significant overexpression of PTEN and decreased levels of activated Akt. Up-regulation of PTEN following IFN-γ exposure was associated with decreased overall cell viability due to increased apoptosis, as assessed by Wright staining and analysis of cell death markers including Bax, Bcl-2, calpain activity, and caspase-3 activity, indicating a prominent role for PTEN in suppression of the PI3K/Akt survival pathway to promote motoneuron death. Addition of estrogen (100 nM) to VSC4.1 cells for 1 h prior to IFN-γ exposure partially decreased PTEN expression, allowing adequate activation or phosphorylation of Akt (p-Akt) to prevent apoptotic cell death. Thus, it appears that estrogen may mediate neuroprotection through decrease in PTEN expression. In conclusion, our studies suggest that PTEN inactivation may be used as an important parameter for evaluation of the efficacy of estrogen in prevention of neuronal loss in neurodegenerative disorders.