Polyamine regulation of Na+/glucose symporter expression in LLC‐PK1 cells

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Abstract
Addition of polyamines or their analogs to newly confluent LLC‐PK1 cells resulted in down‐regulation of Na+‐glucose transport (symport) activity. Polyamines prevented the induction of this symporter by the differentiation inducer hexamethylene bisacetamide (HMBA) but did not influence induction by the phosphodiesterase inhibitor 3‐isobutyl‐1‐methylxanthine (IBMX). Partial depletion of endogenous polyamines after addition of α ‐difluoromethylornithine (DFMO) resulted in a 4 to 5‐fold increase in symporter expression. Symporter induction by either HMBA or DFMO was inhibited by the protein kinase inhibitor H‐7 but H‐7 did not affect symporter induction by IBMX. Changes in symporter activity were accompanied by changes in levels of the 75 kD symporter subunit detected by Western blot. Cultures exposed to HMBA exhibited reduced levels of ornithine decarboxylase activity. Our results suggest that induction of symporter expression by HMBA may be mediated in part by its effects on polymine metabolism, and point to parallel roles of polyamines and cyclic AMP in regulating the expression of this physiologically important renal transport system.