Molecular cloning and characterization of hamster and duck PPAR isoforms and the mechanism of estrogen-induced peroxisome proliferation

Estrogen is involved in the pathological progression of various human diseases, such as breast cancer. It is important to understand the molecular events involved in the actions of estrogen. Estrogen-induced peroxisome proliferation has been observed in hamster kidney and duck uropygial gland, and the phenomena was found to be both species and tissue-specific. To reveal the molecular basis of estrogen-induced peroxisome proliferation, I have cloned the cDNAs of peroxisome proliferator-activated receptors (PPAR) from hamster kidney and duck uropygial gland. The sequence analysis of the isolated {dollar}alpha{dollar} and {dollar}gamma{dollar} isoforms of PPAR showed high homology to the other known PPARs. The involvements of estrogen in peroxisome proliferation in duck was examined at different levels of signal transduction, and the expression of duck PPARs was found not to be induced by treatment with estrogen. The tissue distribution showed that duck PPAR{dollar}gamma1{dollar} was uniquely and highly expressed in the duck uropygial gland but not the goose uropygial gland, where estrogen treatment does not cause peroxisome proliferation. Therefore it is suggested that PPAR{dollar}gamma1{dollar} is involved in estrogen-induced peroxisome proliferation. Transient transfection of NIH 3T3 cells by duck PPAR{dollar}gamma1{dollar} showed that duck PPAR{dollar}gamma1{dollar} did not confer response specific to estrogen. Furthermore, heterodimerization which would involve PPAR and other nuclear receptors such as estrogen receptor was not observed.; Recent finding that prostaglandin J{dollar}sb2{dollar} series are potent activators of PPAR{dollar}gamma{dollar} suggests a possible link of prostaglandins and PPARs in the process of estrogen-induced peroxisome proliferation. We found that estrogen treatment significantly influenced the metabolism of arachidonic acid in the duck uropygial gland, and with the treatment of estrogen, PG D{dollar}sb2{dollar} was specifically and transiently converted into a new metabolite. The rate of this conversion was consistent with the changes of endogenous PG D{dollar}sb2{dollar} level in the duck uropygial gland, which decreased in the first several days and then went back to the original level. Our study showed that this PG D{dollar}sb2{dollar} metabolite was closer in its behavior to PG J{dollar}sb2{dollar} and {dollar}Deltasp{lcub}12{rcub}{dollar}-PG J{dollar}sb2{dollar} in TLC and HPLC. It was also shown that the metabolite had the same retention time as {dollar}Deltasp{lcub}12{rcub}{dollar}-PG J{dollar}sb2{dollar} in GC, and its mass spectrum showed striking similarity to that of standard {dollar}Deltasp{lcub}12{rcub}{dollar}-PG J{dollar}sb2.{dollar} On the other hand, certain minor differences between the new metabolite and {dollar}Deltasp{lcub}12{rcub}{dollar}-PG J{dollar}sb2{dollar} in UV absorbance spectrum and the highest molecular ions in mass spectrum remained. Transient transfection of NIH 3T3 cells with duck PPAR{dollar}gamma1{dollar} showed that this PG D{dollar}sb2{dollar} metabolite was an activator of duck PPAR{dollar}gamma1,{dollar} and was as potent as PG J{dollar}sb2{dollar} series. The fact that estrogen specifically enhanced the transient production of the PG D{dollar}sb2{dollar} metabolite strongly suggests that the PG D{dollar}sb2{dollar} metabolite is responsible for the activation of PPAR{dollar}gamma{dollar} and which in turn induces peroxisome proliferation in duck uropygial gland. These results have great implication for the action of estrogen in other hormone-influenced tissues.