A novel hPepT1 stably transfected cell line: Establishing a correlation between expression and function

Stably transfected MDCK/hPepT1-V5&His clonal cell lines expressing varying levels of epitope-tagged hPepT1 protein were established to quantify the relationship between transgene hPepT1 expression levels and its functional kinetics in facilitating peptide and peptide-like drug uptake and transport in vitro. The hPepT1 sequence was amplified from Caco-2 cell mRNA, inserted into the pcDNA3.1-V5&His TOPO plasmid, and transfected into MDCK cells. Transgene protein levels were quantified by Western Blot analysis utilizing a standard curve generated with a positive control protein containing a V5&His epitope. Three clones expressing different levels of the hPepT1 fusion protein (low, medium, and high) were selected for the functional characterization with [C-14]Gly-Sar and [H-3]carnosine. The MDCK/hPepT1 cells expressed a novel hPepT1/epitope tag protein with an apparent molecular mass of 110 kDa. The [C-14]Gly-Sar uptake in the transfected cells was sodium-independent and pH-dependent, demonstrating enhanced uptake, the rate of which increased significantly from the weakly to strongly expressing hPepT1 MDCK/hPepT1-V5&His clones as compared to the mock cell line at pH 6.0. The uptake and permeability of [C-14]Gly-Sar and [H-3]carnosine demonstrated a direct correlation between the hPepT1 level of expression, uptake, and transport capabilities. Molecular and functional characterization of the MDCK/hPepT1-V5&His cell line confirmed a directly proportional relationship between V-max and P-app versus the molar levels of hPepT1 transgene expression. This stably transfected hPepT1 cell line may serve as a useful in vitro model for screening and quantifying peptide and peptide-like drug transport as a function of hPepT1 expression in drug discovery.