alpha-chymotrypsin and clasto-lactacystin-beta-lactone

Lithium affects several enzymatic activities, however, the molecular mechanisms of lithium actions are not fully understood. We previously showed that LiCl interacts synergistically with all-trans-retinoic acid to promote terminal differentiation of WEHI-3B D(+) cells, a phenomenon accompanied by the recovery of the retinoid-induced loss of retinoic acid receptor alpha protein pools. Here, we demonstrate the effects of LiCl on proteasome-dependent degradation of retinoic acid receptor alpha proteins. LiCl alone, or in combination with all-trans-retinoic acid, increased cellular levels of ubiquitinated retinoic acid receptor alpha and markedly reduced chymotryptic-like activity of WEHI-3B D(+) 20 S and 26 S proteasome enzymes. Neither KCl nor all-trans-retinoic acid affected enzyme activity, whereas NaCl produced a modest reduction at relatively high concentrations. In addition, LiCl inhibited 20 S proteasome chymotryptic-like activity from rabbits but had no effect on tryptic-like activity of the 26 S proteasome. This effect has significant consequences in stabilizing the retinoic acid receptor alpha protein levels that are necessary to promote continued differentiation of leukemia cells in response to all-trans-retinoic acid. In support of this concept, combination of proteasome inhibitors beta-clastolactacystin or benzyloxycarbonyl-Leu-Leu-Phe with all-trans-retinoic acid increased differentiation of WEHI-3B D(+) cells in a manner that was analogous to the combination of LiCl and all-trans-retinoic acid.

Topics: Adjuvants, Immunologic; Animals; Blotting, Western; Cell Differentiation; Cell Line; Chymotrypsin; Cysteine Endopeptidases; Humans; Kinetics; Lactones; Leukemia; Lithium Chloride; Multienzyme Complexes; Oligopeptides; Peptide Hydrolases; Precipitin Tests; Proteasome Endopeptidase Complex; Protein Binding; Protein Biosynthesis; Rabbits; Receptors, Retinoic Acid; Retinoic Acid Receptor alpha; Time Factors; Tretinoin; Trypsin; Tumor Cells, Cultured; Ubiquitin