Cell-matrix interactions modulate transepithelial phosphate transport in Pi-deprived OK cells

Abstract

In opossum kidney (OK) cells as well as in kidney proximal tubules, P(i) depletion increases apical (A) and basolateral (B) Na(+)-dependent P(i) cell influxes. In OK cells' monolayers in contrast to proximal tubules, there is no increase in transepithelial P(i) transport. This limitation may be due to altered cell-matrix interactions. A and B cell (32)P(i) uptakes and transepithelial (32)P(i) and [(14)C]mannitol fluxes were measured in OK cells grown on uncoated or on Matrigel-coated filter inserts. Cells were exposed overnight to solution of either low (0.25 mM) or high (2.5 mM) P(i). When grown on Matrigel, immunofluorescence of apical NaPi4 (an isoform of the sodium-phosphate cotransporter) transporters increased and A and B (32)P(i) uptakes into P(i) depleted cells were five and threefold higher than in P(i) replete cells (P < 0.001). P(i) deprivation resulted in larger increase in A to B (4.6x, P < 0.001) than in B to A (3.5x, P < 0.001) P(i) flux and net P(i) transport from A to B increased 10-fold (P < 0.001). With P(i) depletion increases in B to A (3.4x) and A to B (3.3x) paracellular [(14)C]mannitol fluxes were similar, and its net flux was opposite to that of P(i). In cells grown on uncoated filters, transepithelial and paracellular unidirectional and net P(i) fluxes decreased or did not change with P(i) depletion, despite twofold increases in apical and basolateral P(i) cell influxes. In summary, Matrigel-OK cell interactions, particularly in P(i)-depleted cells, led to enhanced expression of apical NaPi4 transporters resulting in higher P(i) transport rates across cell boundaries; apical P(i) readily entered the transcellular transport pool and paracellular fluxes were smaller fractions of transepithelial P(i) fluxes. These Matrigel-induced changes led to an increase in net transepithelial apical to basolateral P(i) transport.