Protective effect of transforming growth factor-β1 on β-amyloid neurotoxicity in rat hippocampal neurons

Neurodegeneration associated with Alzheimer's disease is believed to involve toxicity to β-amyloid (Aβ) and related peptides. Treatment of cultured rat hippocampal neurons with Aβ 1-40 (1 μM) or the active fragment Aβ 25-35 (1 μM) for 5 days led to a ~40-50% decrease in neuronal viability. The hydrophilic anti-oxidant ascorbic acid (300 μM) and the lipophilic antioxidant 2-mercaptoethanol (10 μM) both protected significantly against Aβ neurotoxicity. Despite the protective effects of these anti-oxidants, both acute and chronic treatments with Aβ 25-35 did not increase production of superoxide anions, as monitored with the fluorescent probe hydroethidine. Similarly, overexpression of Cu/Zn-superoxide dismutase using adenovirus-mediated gene transfer did not protect against Aβ neurotoxicity. Aβ neurotoxicity, however, was prevented in cultures infected with a recombinant, replication-defective adenovirus overexpressing the Ca²⁺ binding protein calbindin D(28k). Transforming growth factor-β1 (TGF- β1) has been shown to protect neurons against both Ca²⁺- and free radical- mediated neuronal degeneration. We found that Aβ neurotoxicity was significantly attenuated by single treatments with TGF-β1 (0.1-10 ng/ml) and prevented by repetitive treatments (10 ng/ml/day). The protective effects of TGF-β1 were associated with a preservation of mitochondrial potential and function, as determined with rhodamine-123-based microfluorimetry. Because both increased oxidative stress and pathophysiological Ca²⁺ fluxes can impair mitochondrial function, preservation of mitochondrial potential by TGF-β1 could be directly associated with its protection against Aβ neurotoxicity. The ability of TGF-β1 to increase the expression of the anti- apoptotic proteins Bcl-2 and Bcl-x(L) is discussed in this context.