THE INHIBITION OF FIBROBLAST GROWTH-FACTOR-II EXPORT BY CARDENOLIDES IMPLIES A NOVEL FUNCTION FOR THE CATALYTIC SUBUNIT OF NA-ATPASE(,K+)

Basic fibroblast growth factor (FGF-S) is one of a select group of pro teins that can exit cells through an alternate, endoplasmic reticulum/ Golgi apparatus independent exocytic pathway, This alternate pathway h as been termed protein export, In an attempt to better understand this process, we have identified a family of related compounds, ''cardenol ides,'' that inhibit FGF-2 export, The cardenolides inhibit FGF-2 expo rt in a time and concentration dependent fashion, Inhibition of FGF-S export is specific in that the cardenolides have no effect on conventi onal protein secretion as measured by their inability to block release of the secreted protein human chorionic gonadotropin-alpha. Because c ardenolides are known to inhibit ion transport activity mediated by Na +,K+-ATPase, we investigated whether there are functional interactions between FGF-S and their only known molecular target: the alpha-subuni t of Na+,K+-ATPase. Export of FGF-2 from COS-l cells is selectively in hibited when co-transfected with expression vectors encoding the alpha -subunit and FGF-2. Moreover, antibodies to the alpha-subunit specific ally co-immunoprecipitate FGF-2 along with the alpha-subunit while con versely, antibodies to FGF-2 specifically co-immunoprecipitate the alp ha-subunit along with FGF-2. Finally, the ion transporting activities of the Na+,K+-ATPase can be uncoupled from protein export, Varying the external concentration of K+ has little effect on export of FGF-2. Ta ken together, these data: 1) identify a novel activity for cardenolide s; 2) suggest a previously unknown role for the alpha-subunit of Na+, K+-ATPase in FGF-2 export; and 3) raise the possibility that the alpha -subunit itself may be an integral component of this alternate exocyti c pathway mediating translocation of cytosolic FGF-2 to the cell surfa ce.