The transepithelial absorption of food-type proteins has been shown to proceed by endocytosis along two functional pathways: a minor direct pathway allowing transport of intact protein and J major lysosomal degradative pathway. The human colon carcinoma cell line CaCo-2 grown on Millipore filters was used here further to characterize these pathways by measuring HRP transport across the cell monolayer in Ussing chambers. In the apical-basal direction, this transport mainly occurred along the degradative pathway and was inhibited at 4°C (7.41 i 1.26 pnioles/h-cm' vs. 27.40 -+ 8.91 at 37°C). T h e amount conveyed via the direct pathway was very small (0.89 * 0.35 prnoles/h.cm') and did not diminish at 4°C (1.43 * 0.59 pmoles/h-cm'). In the basal-apical direction, HRP transport along the degradative pathway at 37°C was similar to the apical-basal value and wa5 inhibited at 4°C (16.40 2 4.05 vs. 2.72 5 2.52 prnoles/h.cm'), but along the direct pathway, it was eight times the apical-basal value (8.36 * 3.1 1 pmoIrDs/ h.cm2) and was inhibited at 4°C (2.43 * 0.78 pmoles/h.cm2). Intact HRP fluxes
METHODS
Cell culture
The human cell line CaCo-2 (Fogh et al., 1977) was obtained from Dr A. Zweibaum, INSERM U. 178, Paris, at passage 64. Cells were routinely grown as previously described (Pinto et al., 1983). Briefly, they were seeded at 1.2 lo4 cellskm' in Dulbecco's modified Eagle's minimum essential medium (Eurobio, Paris, France) and grown at 37°C in a 10% C02-90% air atmosphere. The medium was supplemented with 20%' fetal bovine serum (Boehringer, Mannheim, FRG), 1%