Insulin aggregation and asymmetric transport across human bronchial epithelial cell monolayers (Calu-3)

The purpose of this work was to elucidate the transport pathways of zinc insulin across the Calu-3 cell monolayer, an in vitro model of the human airway epithelium. Calu-3 cells grown in liquid-covered conditions formed a confluent monolayer with a high transepithelial electrical resistance value of 1000 +/- 150 Omega small middle dot cm(2). The cell monolayer was characterized by a low mannitol permeability of 4.7 +/- 0.5 10(-7)cm/s. Transport of zinc insulin (donor concentration 1 U/mL) in Dulbecco's modified phosphate buffer saline at 37 degrees C was found to be higher in the basolateral (BL) to apical (AP) (P(app) = 3.0 +/- 0.2 10(-8) cm/s), than in the AP to BL direction (P(app) = 0.41 +/- 0.02 10(-8) cm/s). P-glycoprotein efflux or specific enzymatic degradation did not appear to contribute toward this asymmetric transport. Insulin receptors, though apparently more abundant on the BL side than on the AP side of Calu-3 cells, did not mediate the direction-dependent transport of insulin. However, transport of a monomeric human insulin analog, Asp(B10)des(B28-30), across the Calu-3 cell monolayer was similar in both directions (BL to AP and AP to BL). The corresponding permeability, P(app) = 2.9 +/- 0.2 10(-8) cm/s, was not significantly different from the permeability of zinc insulin in the BL to AP direction. The paracellular pathway seems to play a major role in the insulin transport across the Calu-3 cell monolayers. We hypothesize that the transport of zinc insulin oligomers is restricted at the AP surface by the presence of the tight junctional complexes. From the BL side, oligomers may undergo dissociation in the intercellular space and diffuse readily as monomers to the AP surface of the membrane.