In vitro model of attachment of Giardia intestinalis trophozoites to IEC-6 cells, an intestinal cell line.

Attachment of giardias to intestinal cells has been difficult to study because of a lack of a convenient in vitro model. We developed an assay for attachment of radiolabeled trophozoites to IEC-6 cells that can be done in microtiter trays. Attachment was confirmed by scanning and transmission electron microscopy. Trophozoites remained attached to the IEC-6 cells for 24 h with little evidence of damage to the IEC-6 cells. Preincubation of trophozoites with cytochalasins A, B, and D reduced attachment to approximately 20% of that of controls, whereas colchicine had no effect. Chelation of divalent cations with EDTA and EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] reduced attachment to 24 and 26% of control values, respectively, and incubation at 4 degrees C reduced attachment to 7% of the value for controls incubated at 37 degrees C. Glutaraldehyde fixation of trophozoites or IEC-6 cells resulted in significantly diminished attachment to the live substrate (17 and 40% of control values, respectively). Coincubation of IEC-6 cells and trophozoites on a rotary shaker resulted in detachment of 40% of trophozoites, but EDTA, EGTA, glutaraldehyde fixation of trophozoites, and low temperature diminished attachment markedly and significantly. Similar results were obtained in selected experiments with three strains of giardia.

Modulation of epidermal growth factor-induced cell proliferation and receptor binding by insulin in cultured intestinal epithelial cells.

The effect of insulin on EGF-induced cell proliferation and on modulation of EGF receptors was examined in IEC-6 cells in culture. Insulin-induced cell proliferation was not seen until concentration of the hormone reached the microgram (10 micrograms/ml) level, producing an increase in EGF receptor binding affinity (Ka 8.2 +/- .05 x 10(9)M-1 to 1.25 +/- .05 10(10) M-1). In the presence of supraphysiologic concentrations of insulin (10 micrograms/ml), EGF-induced cell proliferation could only be detected at a concentration of 50 ng/ml as opposed to a concentration of 2.0 ng/ml in the absence of insulin. This study suggests that insulin stimulates intestinal crypt cell proliferation only in supraphysiologic concentrations in a manner more characteristic of IGF-I than insulin, while producing enhanced binding of EGF to its receptor as well as an elevated threshold to EGF induced cell proliferation.