Identification of soluble transforming growth factor-β receptor III (sTβIII) in rat milk

Transforming growth factor- (TGF-) is present at high concentrations in maternal milk. In milk TGF-2 is the predominant isoform. For function TGF-2 requires TRIII to facilitate efficient binding to the TGF- receptor types I and II signalling complex. We have shown that TGF- receptor types I (TRI), II (TRII) and III (TRIII) are coexpressed in the suckling rat intestine. Immunostaining for TRIII was also observed in the intestinal lumen prior to weaning. TRIII (or betaglycan) has been reported in serum, cell culture medium and extracellular matrix. To determine whether a soluble form of TRIII is present in milk, the rat milk aqueous phase was analysed by slot-blot and Western blot. Soluble TRIII was detected in milk throughout lactation. Western blot analysis of rat milk revealed a high molecular weight band of glycosylated protein of >200 kDa, with a core protein of approximately 110–120 kDa that comigrated with recombinant TRIII. Immunoabsorption of soluble TRIII (sTRIII) from milk resulted in partial depletion of active TGF- from milk, suggesting that the receptor may interact with ligand in milk. In addition rat pups suckled on mother's milk demonstrated an enhanced labelling of TRIII in the gut, as compared with pups fed on a rat milk substitute (RMS). These findings suggest that milk sTRIII is functional, and may modulate milk-derived TGF- function in the developing intestine.