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Drug transporters, a special family of membrane proteins, are recognized as an important determinant for drug absorption, distribution and elimination. Understanding the molecular mechanism regulating the expression of drug transporters is crucial in pharmacokinetics research. Sphingomyelin synthase (SMS), the last enzyme involved in the biosynthesis of sphingomyelin (SM), plays a critical role in the constitution of cell membrane and has impact on the expression of membrane proteins. SMS2, which is predominantly located in the plasma membrane, is particularly the dominant SMS of its two isoforms (SMS1 and SMS2) in the brain. Therefore, it is conceivable that SMS2 deficiency may have impact on expression of some membrane proteins, such as membrane-bound drug transporters. Using SMS2 gene deficient mouse brain tissues, we studied the gene and protein expression profiles of drug transporters, ERM proteins (ezrin/radixin/moesin) and the cytoskeleton protein,β-actin, in mouse brain by RT-PCR, western blot and immunohistochemistry analysis. We found that the mRNA expression of Mdrl rather than the other drug transporters was significantly decreased in the SMS2 deficient brain. Accordingly, the expression and function of P-glycoprotein (Pgp), encoded by Mdrl, was significantly downregulated. In addition, the substantially downregulated expression of the ezrin and theβ-actin proteins was also observed in the SMS2 deficient brain. The immunohistochemistry analysis further revealed the suppressed expression of Pgp, ezrin and P-actin in both cortex and paraventricular areas of SMS2 knockout mice. Furthermore, both Pgp andβ-actin were found to be co-immunoprecipitated with ezrin from the total brain proteins, suggesting the possible association between Pgp, ezrin andβ-actin in the brain. These results indicate that SMS2 participates in the expression regulation of drug transporters, particularly Pgp, and suggest that SMS2 may be a potential target for enhancing drug access to the brain. In addition, we studied the gene expression profiles of drug transporters in SMS2-null mouse liver and intestine by RT-PCR analysis, and observed the downregulation of Octl in liver, as well as upregulation of some Mrp family members in intestine. These results suggest SMS2 may also play an important role in the drug disposition in mouse liver and intestine.
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