Differential expression of mu-opioid receptor gene in CXBK and B6 mice by Sp1.

It is well known that there are individual differences in the sensitivity to analgesics. The CXBK mice are characterized by reduced sensitivity to morphine and by partial deficiency in mu-opioid receptor (MOR) expression. The sequences of MOR genes in CXBK and B6 mice are identical in their coding regions but differ at 5'-untranslated region (UTR) nucleotide -202 (C nucleotide in CXBK, but A nucleotide in B6). In this report, we identified an Sp1 element (-211 to -204) immediately before the polymorphic nucleotide. In electrophoretic mobility shift assay, nuclear protein binding to the B6-Sp1 sequence was more efficient than to the CXBK-Sp1 sequence, and anti-Sp1 but not anti-CREB antibody interfered with the formation of the DNA-protein complex. In MOR-expressing cell lines SH-SY5Y, P19, and PC12, B6 MOR promoter possessed high transcription activity than the CXBK promoter, and Sp1 inhibitor PDTC reduced the promoter activities. In SL2 cells that lack endogenous Sp1 expression, B6 and CXBK MOR promoters demonstrated equal activity, whereas overexpression of Sp1 in SL2 cells enhanced B6 MOR promoter activity better than the CXBK promoter. Together, the A-to-C change at MOR 5'-UTR decreases Sp1 binding and MOR gene transcription, which could underlie the reduced morphine expression in CXBK mice.