Activation of metallothioneins and α-crystallin/sHSPs in human lens epithelial cells by specific metals and the metal content of aging clear human lenses

PURPOSE. To identify those metallothionein and α-crystallin/small heat-shock genes induced by toxic metals in human lens cells and to evaluate the levels of these metals between young and aged human lenses. METHODS. Human SRA01/04 and primary human lens epithelial cells were cultured and exposed to Cd²⁺, Cu²⁺, and Zn²⁺. The levels of lens metallothioneins (Ig, If, Ih, Ie, and IIa) and α-crystallin/small heat-shock (αA-crystallin, βB-crystallin, and HSP27) genes were analyzed by semiquantitative and quantitative competitive RT-PCR. The content of aluminum, cadmium, calcium, chromium, copper, iron, lead, magnesium, manganese, nickel, potassium, sodium, and zinc in young (mean, 32.8 years), middle-aged (mean, 52.3 years), and old (mean, 70.5 years) human lenses was analyzed by inductively coupled plasma-emission spectroscopy. RESULTS. Lens metallothioneins (Ig, If, Ih, Ie, and IIa) and α-crystallin/small heat-shock genes (αA-crystallin, βB-crystallin, and HSP27) were differentially induced by specific metals in SRA01/04 human lens epithelial cells. Cd²⁺ and Zn²⁺, but not Cu²⁺, induced the metallothioneins, whereas Cd²⁺ and Cu²⁺, but not Zn²⁺, induced αB-crystallin and HSP27. αA-crystallin was induced by Cu²⁺ only. Similar responses of the metallothionein IIa gene were detected in identically treated primary human lens epithelial cells. Cd²⁺ and Zn²⁺ induced metallothionein IIa to five times higher levels than metallothionein Ig. Of 13 different metals, only iron was altered, exhibiting an 81% decrease in old versus young lenses. CONCLUSIONS. Induction of metallothioneins and α-crystallin/small heat shock proteins by different metals indicates the presence of metal-specific lens regulatory pathways that are likely to be involved in protection against metal-associated stresses.