Regulation and gene expression of heme synthesis under heavy metal exposure--review.

Environmental pollution of heavy metals is very abundant nowadays from industry, chemicals, old paints, and pipes or resulting from previous contaminants accumulating in the food chain. Most of the iron demands of the body are needed for heme synthesis and assembly, but iron is also required for Fe-S cluster proteins and other redox enzymes. Heme is an essential, iron-binding molecule used as a prosthetic group of hemoproteins or as a regulator in multiple cellular pathways. In this review, we focused on the effect of exposure to heavy metals, such as Pb, Ga, Cu, Kd, Hg and Al, on heme synthesis as the main iron-sequestering process of the human body. These metals compete with iron on transporters, reduce the cellular iron pool and moreover, bind to proteins, and cause physical and mental disturbances. Heavy metals mainly impair various aspects of the heme synthesis pathway: gene expression, enzyme activity, and iron integration into protoporphyrin IX. Main risk factors are described as well as effects on iron dependent processes in order to increase public awareness to the distribution of heavy metals in our close environment and the harsh consequences of exposure, even in low doses.

Accelerated proteasomal activity induced by Pb2+, Ga3+, or Cu2+ exposure does not induce degradation of alpha-synuclein.

The involvement of environmental heavy metals in Parkinson's disease (PD) has been suggested by epidemiologic studies; however, the mechanism of this effect is unknown. PD is characterized by the aggregation of alpha-synuclein in Lewy bodies. We previously showed that Pb2+ accelerates proteasomal activity. Therefore, we examined the effect of Pb2+, Ga3+, and Cu2+ on alpha-synuclein in human SH-SY5Y cells. The heavy metals induced an increase in heme-oxygenase-1 levels without significant cell death or ROS generation. The metals inhibited ALA-dehydratase, which is the inhibitory subunit of the proteasome, thereby accelerating proteasomal activity and decreasing protein levels of CDK-1 and PBGD. However, alpha-synuclein protein levels increased after exposure to metals, similar to the effect obtained with the proteasome inhibitor, hemin, suggesting that alpha-synuclein is inaccessible to proteasomal degradation. Indeed, electron microscopy revealed the formation of aggresomes in Pb2+- or hemin-treated cells. Thus, although heavy metals enhance proteasomal activity, alpha-synuclein is protected from degradation, and its protein levels and aggregation are increased.