Induction of heat shock proteins and the proteasome system by casein-N epsilon-(carboxymethyl)lysine and N epsilon-(carboxymethyl)lysine in Caco-2 cells.

Repeated mild heat shock treatment has been shown to have anti-aging effects on cellular mechanisms in vitro. Among these, the age-associated accumulation of advanced glycation end products (AGEs), such as N(epsilon)-(carboxymethyl)lysine (CML), has been demonstrated to be effectively prevented in glyoxal-exposed human skin fibroblasts following mild heat shock treatment. The biochemical mechanism responsible for this inhibition is not yet known. However, the involvement of heat shock proteins (HSPs) and the misfolded proteins degrading the ubiquitin-proteasome system have been hypothesized. As AGE-modified proteins are likely to be conformationally modified, we investigated whether treatment of human intestinal cells with casein-linked CML or nonprotein-linked CML affects the expression of HSPs and the ubiquitin-proteasome system by using matrix-assisted laser desorption/ionization-time-of-flight tandem mass spectroscopy (after protein separation by two-dimensional gel electrophoresis) and by Western blotting. Compared to nontreated control cells, expression of HSP90, HSP60, HSP70 chaperones, and the proteasome S26 ATPase subunit 2 were significantly upregulated in casein-CML and in CML-treated cells. Exposure of Caco-2 cells to beta-amyloid, a nonglycation product, revealed similar results. In conclusion, the results indicate that CML and casein-linked CML activate the expression of HSPs as well as the proteasome system, which are involved in the degradation of misfolded and possibly glycated proteins. Whether this mechanism is based on binding to cell surface receptors, such as the receptor for AGE, has to be clarified in future studies.