Riboflavin-mediated delivery of a macromolecule into cultured human cells

Cell surface receptors for the vitamins folic acid and biotin have been previously reported to mediate the endocytosis of vitamin-conjugated macromolecules into cultured cells. To evaluate whether a similar uptake pathway for riboflavin-conjugated macromolecules might exist, riboflavin was covalently linked to bovine serum albumin (BSA) via the vitamin's ribityl side chain, and uptake of the protein by cultured human cells was examined. Whereas unconjugated BSA was not internalized by KB, A549, SK-LU-1 or SK-OV cells, riboflavin-conjugated BSA was readily internalized (>106 molecules/cell). Analysis of the uptake pathway revealed that the riboflavin-BSA conjugate likely docks on cells at a carrier/transport protein that is distinct from the uptake pathway for free riboflavin and then enters via normal membrane cycling. Evidence for this contention is: (i) the internalized conjugate accumulates in endosomal compartments, (ii) uptake into cells is halted at temperatures near 0 degreesC where membrane trafficking is abrogated, (iii) cell association is inhibited by unlabeled riboflavin-BSA, but not by free riboflavin, and (iv) cellular uptake of [3H]riboflavin is only partially inhibited by riboflavin-BSA. Regardless of the pathway of internalization, these data demonstrate that riboflavin conjugation can facilitate protein entry into human cells in culture.