Renal medullary cells are exposed to high levels of NaCl and urea during antidiuresis. An important mechanism to adapt to these adverse conditions is the expression of various heat shock proteins (HSPs), which act as molecular chaperones. aB-Crystallin is an osmoprotective small HSP, highly abundant in the renal medulla, that is regulated by osmotic stress. In contrast to other osmosensitive genes, aB-Crystallin is not transcriptionally regulated by TonEBP/NFAT5, but its regulatory region contains a Peroxisome Proliferator-Activated Receptor (PPAR) response element. We hypothesized that tonicity-induced expression of aB-Crystallin in the renal medulla is mediated by enhanced concentrations of COX-2 derived cyclopentenone prostaglandins, which are known activators of the intracellular PPARg. MDCK cells were used as model for renal medullary cells, in which both COX-2 and aB-Crystallin expression is induced by osmotic stress. Cells were incubated with various cyclopentenone prostaglandines and aB-Crystallin expression was analyzed. 15-deoxy-D^12-14-PGJ₂ and D¹²-PGJ₂ both increased aB-Crystallin expression in MDCK cells, while PGA₁ and Prostacyclin (PGI₂) had no significant effects. Accordingly, the synthesis of 15-deoxy-D^12-14-PGJ₂ in MDCK cells was significantly increased during osmotic stress. Furthermore, the pharmacological inhibition of COX-2 decreased osmotic stress-induced aB-Crystallin expression. The role of COX-2 derived prostanoids in aB-Crystallin expression was also assessed in COX-2 knockout mice. After dehydration for 24–48 hours, significant higher levels of aB-Crystallin were detected in the inner medulla of wild-type mice compared to the COX-2 knockout animals, suggesting that the situation in cultured cells is comparable to the situation in vivo. Taken together, these results indicate that osmotic stress induces COX-2 expression. This is accompanied by an increase in synthesis of prostaglandins, namely 15-deoxy-D^12-14-PGJ₂ and D¹²-PGJ₂, which stimulate aB-Crystallin expression, probably by activation of the intracellular PPARg with subsequent binding of the receptor to the PPAR response element in the regulatory region of the aB-Crystallin gene.