Parabens (alkyl esters of 4-hydroxybenzoic acid) are widely used as preservatives in drugs, cosmetic products, and foodstuffs. Safety concerns have recently increased due to the potential health risks associated to exposure to large amounts of these substances. Biotransformation of parabens mainly includes hydrolysis of the ester bond and glucuronidation reactions. The hydrolysis and glucuronidation of a series of six parabens differing by the nature of the alkyl group were investigated in human liver microsomes and plasma, and the major human UDP-glucuronosyltransferase (UGT) isoforms involved in the reaction were identified. Methyl- and ethylparaben were stable in human plasma, with 95% of the initial concentration remaining after 24 h. On the other hand, propyl-, butyl- and benzylparaben concentrations decreased by 50% under similar conditions. In contrast, rapid hydrolysis was measured with human liver microsomes depending on the alkyl chain length, with t(1/2) varying from 22 min for methylparaben to 87 min for butylparaben. All parabens were actively glucuronidated by liver microsomes, in comparison to 4-hydroxybenzoic acid. They were mainly substrates of human recombinant UGT1A1, UGT1A8, UGT1A9, UGT2B7, UGT2B15 and UGT2B17. In conclusion, the parabens were readily metabolized in human liver through esterase hydrolysis and glucuronidation by several UGT isoforms. These results suggest that these parabens do not accumulate in human tissue.

The cellular and molecular mechanisms underlying senile osteoporosis remain poorly understood. In this study, transgenic mCol1α1-Pitx1 mice overexpressing paired-like homeodomain 1 (PITX1), a homeobox transcription factor, rapidly develop a severe type-II osteoporotic phenotype with significant reduction in bone mass and biomechanical strength similar to that seen in humans and reminiscent of the phenotype previously observed in Sca-1 ( Ly6a )-null mice. PITX1 plays a critical role in hind limb formation during fetal development, while loss of expression is associated with primary knee/hip osteoarthritis in aging humans. Through in vivo and in vitro analyses, we demonstrate that Pitx1 directly regulates the self-renewal of mesenchymal progenitors and indirectly regulates osteoclast differentiation through the upregulation of Wnt signaling inhibitors DKK1 , SOST , and GSK3-β . This is confirmed by elevated levels of plasma DKK1 and the accumulation of phospho-β-catenin in transgenic mice osteoblasts. Furthermore, overexpressed Pitx1 in mice osteoblasts results in severe repression of Sca-1 ( Ly6a ) that was previously associated with senile osteoporosis. Our study is the first to demonstrate the novel roles of PITX1 in senile osteoporosis where PITX1 regulates the self-renewal of mesenchymal stem cells or progenitor cells through Sca-1 (Ly6a) repression and, in addition, inhibits the Wnt signaling pathway.

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