Transfection alters ion transport in MDCK cells

In the course of an investigation into the effect of Tamm-Horsfall protein (THP) on ion transport, we performed stable transfection of THP into MDCK cells using the SV40 or the cytomegalovirus (CMV) promoter. As controls, we transfected MDCK cells with an "empty" plasmid containing SV40 or CMV promoter but without THP cDNA. In another set of controls, we subjected cells to transfection procedures without DNA (mock transfection). K influx was not altered in cells subjected to mock transfection procedures without DNA, but both ouabain sensitive (OS) and ouabain resistant (OR) components of K influx were diminished in cells transfected with THP cDNA using either SV40 or CMV promoter. However, K influx was also reduced in cells transfected with a control plasmid containing either the SV40 promoter alone, or the CMV promoter alone, without the THP cDNA. Thus, the transport alterations were caused by transfection and not by THP. The reduction in ouabain-sensitive K influx was accompanied by a proportional reduction in the abundance of Na-K pump units as assessed by [3H] ouabain binding. [3H] bumetanide binding, a measure of the number of functioning NaK2Cl cotransporter sites, was reduced pari passu with the reduction in bumetanide-sensitive K influx. These results highlight the possibility that alterations in properties of transfected cells may not be solely due to the presence of transfected protein, but the result of some process associated with transfection itself. Without appropriate controls to evaluate this possibility, results of transfection studies are subject to potentially faulty and misleading interpretation.