219 MECHANISM OF SP-A-MEDIA TED SURFACTANT ENDOCYTOSIS BY TYPE II CELLS

Abstract

AIM: Surfactant protein A (SP-A) enhances surfactant lipid uptake by type II pneumocytes. In the presence of SP-A internalized surfactant lipids are reported to bypass the degradative pathway and are recycled towards lamellar bodies. We wanted to further clarify the role of SP-A in surfactant lipid endocytosis. METHODS: A previously described antibody (2H5) against a type II cell membrane protein which stimulates surfactant lipid uptake by type II cells (Pediatr. Res. 1994, 35: 278) was used in parallel with SP-A to study uptake and intracellular fate of liposomes with surfactant-like composition in rat type II cells. RESULTS: In the presence of 2H5 or SP-A significantly more labeled lipid is internalized in a time- and concentration-dependent fashion by type II cells than in their absence (2H5 2-fold, SP-A 3-fold above control). In cells in solution no difference in the distribution of label in phospholipid classes between control cells and cells incubated with either 2H5 on SP-A was found. In adherent 24 hour-old cells after one hour of incubation 82% of the internalized 3H-label is still associated with PC in control cells vs. 87% in the presence of 2H5 and 94% with SP-A. Surprisingly, inhibition of coated pit formation (uptake pathway for SP-A) by K+-depletion enhanced lipid uptake by type II cells significantly. Also, inhibition of protein kinase C (PKC) (staurosporine 100 nM) enhanced lipid uptak^ by type II cells in the presence of SP-A. CONCLUSIONS: SP-A and 2H5 do quantitatively enhance lipid uptake in type II cells. The subsequent intracellular fate of the PC molecule may depend on the type of assay used. Uptake via coated pits and PKC activation are involved in these processes.