The fuss over lipo“fuss”cin: not all autofluorescence is the same

Abstract

Since the first description of cellular auto-fluorescence over a century ago, we have now come to appreciate that autofluorescence should not be discarded as a biological artifact but embraced as a biological phenomenon with potentially important cellular relevance. Indeed, cellular and tissue autofluorescence has been attributed to a spectrum of unrelated molecules such as porphyrins, vitamins (vita-min A, riboflavin, thiamine), structural pro-teins, lipofuscin and ceroid pigments. We have recently shown that freshly isolated epithelial cancer stem cells (CSCs) bear autofluorescent vesicles in the cytoplasm. Our studies defini-tively prove that riboflavin and not lipofuscin is the source of autofluorescence in CSCs as the inhibition of ATP and not autophagy eliminates CSC autofluorescence, that the ATP-dependent transporter ABCG2, for which riboflavin is a substrate, is overexpressed in autofluorescent CSCs and co-localizes with the membrane of intracellular autofluorescent vesicles, the ABCG2-specific inhibitor Fumitremorgin C reversibly eliminates CSC autofluorescence, riboflavin is a substrate for ABCG2, and only the addition of riboflavin to vitamin-deprived CSC cultures is capable of restoring autofluorescence. Thus, the sum of these data unequivocally supports the conclu-sion that the source of CSC autofluorescence is the vitamin riboflavin.