Polarized membrane expression of brush-border hydrolases in primary cultures of kidney proximal tubular cells depends on cell differentiation and is induced by dexamethasone

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Abstract
To analyze the influence of cell differentiation and the effects of hormones on the subcellular distribution of apical antigens in polarized epithelial cells, we have compared the localization of three brush border (BB) hydrolases [neutral endopeptidase (ENDO), aminopeptidase N (APN), and dipeptidylpeptidase IV (DPPIV)] in primary cultures of renal proximal tubule cells grown in various culture media. The degree of cell differentiation modulated by medium composition was estimated by measuring proximal functions, including glucose transport, specific enzymatic activities, and PTH responsiveness. In the dedifferentiated state observed in cells grown in 1% fetal calf serum (FCS)‐supplemented medium, the three hydrolases are abnormally concentrated in a cytoplasmic vesicle compartment with weak expression on both membrane domains. By contrast, in serum‐free hormonally defined medium (DM: insulin, 5 μg/ml; dexamethasone, 5 × 10−8 M), which markedly enhances morphological and functional cell differentiation, the distribution of hydrolases parallels that observed in the normal tubule. When added to the DM devoid of hormones, insulin has little polarizing effect, whereas dexamethasone dramatically increases the apical expression of the hydrolases, which then almost disappear from the basolateral membrane and cytoplasmic vesicular compartments. This glucocorticoid hormone augments the amount of immunoreactive antigen detectable on the apical domain in paraformaldehyde‐fixed cells but does not change the total enzymatic activity. This suggests the presence in tubular cells of a dexamethasone‐dependent polarizing machinery that requires de novo RNA and protein synthesis, and probably acts mainly by targeting a storage cytoplasmic pool of enzyme to the apical domain.

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