POSTTRANSCRIPTIONAL REGULATION OF NA+/GLUCOSE COTRANSPORTER (SGTL1) GENE-EXPRESSION IN LLC-PK1 CELLS - INCREASED MESSAGE STABILITY AFTER CYCLIC-AMP ELEVATION OR DIFFERENTIATION INDUCER TREATMENT

We have further investigated the molecular basis of increased differentiation regulated expression of SGTL1, a Na+/glucose cotransporter, in the renal epithelial lial cell line LLC-PK1. Treatment of confluent monolayers either with the differentiation inducer hexamethylene bisacetamide (HMBA) or with cyclic AMP elevating agents promoted increased levels of the SGLT1 mRNA, the immunodetectable 75-kDa cotransporter subunit, and the transport activity. Two molecular species of SGLT1 mRNA (2.2 and 3.9 kilobases (kb)) are transcribed from the same gene in LLC-PK1 cells and differ only in the length of the 3'-untranslated region. The larger transcript is less stable (t(1/2) = 2 h) than the smaller one (t(1/2) = 10 h) in control, confluent monolayers. The 3.9 kb species was stabilized from degradation after either cyclic AMP elevation (t(1/2) = 14 h) or HMBA addition (t(1/2) = 8 h), with negligible effects on the stability of the 2.2-kb species (t(1/2) = 11 h). Inhibition of translation by cycloheximide resulted in a 10-fold increase in the t(1/2) of the 3.9-kb transcript and a 2-fold increase in that of the 2.2-kb species in control monolayers. Our results demonstrate that post-transcriptional regulation of message stability plays a major role in differentiation-dependent SGTL1 expression promoted by either HMBA or cyclic AMP.