Enhanced Na+-dependent bile salt uptake by WIF-B cells, a rat hepatoma hybrid cell line, following growth in the presence of a physiological bile salt

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Abstract
Although bile salts are toxic to the liver at high plasma concentrations, the effects of physiological concentrations of bile salts on normal hepatic function are poorly understood. We examined the effect of taurocholate (TC) on the basolateral uptake of [3H]TC in WIF-B cells, a hybrid cell line stably exhibiting in vitro the structural and functional polarity of hepatocytes. Cells were grown in the absence or presence of TC (50 μmol/L) over 12 days, and then incubated with [3H]TC concentrations ranging from 1 to 250 μmol/L. For both control and TC-grown cells, uptake of [3H]TC was linear over 2 minutes. In control cells, the K mfor [3H]TC Na+-dependent uptake over 1 minute was 6 ± 5 μmol/L, and the Vmax was 45 ± 6 pmol TC/mg protein/min (± SEM). TC-grown cells exhibited no significant change in K m but showed a doubling of V max to 87 ± 6 pmol TC/mg protein/min (P< .005). In both control and TC-grown cells, maximal uptake of [3H]TC occurred following 10 to 12 days in culture, with TC-grown cells consistently showing greater rates of [3H]TC uptake from 4 to 14 days in culture. Western blots immunostained for the basolateral Na+-dependent plasma membrane protein, ntcp, revealed the appropriate ≈50-kd band in control and TC-grown cells, and confocal immunofluorescence microscopy demonstrated staining along the basolateral plasma membrane. Northern blots hybridized with a cDNA probe directed against ntcp indicated a modest TC-induced increase in mRNA levels. Reverse-transcriptase polymerase chain reaction (RT-PCR) using RNA isolated from WIF-B cells and oligonucleotide primers specific for rat ntcp or human NTCP transcripts revealed only the presence of the rat ntcp transcript. We conclude that bile salts, at concentrations normally found in mammalian portal blood, may be capable of promoting enhanced hepatocellular bile salt uptake via an increase in basolateral Na+-dependent plasma membrane transport capacity.