CYCLIC-AMP-DEPENDENT UP-REGULATION OF THE TAURINE TRANSPORTER IN A HUMAN RETINAL-PIGMENT EPITHELIAL-CELL LINE

This investigation was undertaken to study the role of cAMP in the reg ulation of the taurine transporter expressed in a human retinal pigmen t epithelial (HRPE) cell line. Treatment of the HRPE cells with choler a toxin for 24 h was found to stimulate the taurine transporter activi ty, as measured by taurine transport into the cells in the presence of NaCl, to a significant extent. The stimulation was 50-60% at 100 ng/m l cholera toxin. This stimulation was specific to the taurine transpor ter since the transport of two other amino acids (leucine and alanine) , which are not substrates for the taurine transporter, was not affect ed by cholera toxin under similar conditions. Exposure of the cells to cholera toxin for a time period >4 h was needed to elicit the stimula tory effect. The cholera toxin-induced stimulation of the taurine tran sporter activity was associated with an increase in the maximal veloci ty of the transport system. The affinity of the transporter for taurin e was not altered by the treatment. The stimulatory effect was markedl y blunted when the treatment of the cells with cholera toxin was done in the presence of actinomycin D, an inhibitor of transcription, or cy cloheximide, an inhibitor of translation. The increase in the taurine transporter activity induced by cholera toxin was associated with a 2. 6-fold increase in the steady state levels of the transporter mRNA. Me asurement of cyclic nucleotides in control and cholera toxin-treated c ells revealed that the toxin caused a 20-fold increase in the cellular levels of cAMP, the levels of cGMP remaining unaffected. Treatment of the cells with membrane permeable cAMP analogs (dibutyryl cAMP and X- bromo cAMP) or with agents which are known to increase intracellular c AMP levels (forskolin and isobutylmethylxanthine) also stimulated the taurine transporter activity. It is concluded that the taurine transpo rter expressed in the HRPE cells is subject to cAMP-dependent up-regul ation and that the process involves de novo synthesis of the transport er protein.