Summary
Factor VIII (FVIII) processing within mammalian cells is demonstrated to be much less
efficient than proteins of similar size. The deletion of the B-domain from FVIII improves
the level of production, due partly to the increase in mRNA synthesis. We aimed to
characterise the cellular fate and the intracellular processing of the FVIII molecule
devoid of B-domain. A B-domain deleted factor VIII (BDD-FVIII) possessing a furin
consensus cleavage site in the connecting segment between the heavy and the light
chain, was produced in CHO cell line. In such cells, FVIII was retained as two single
chain products from which a majority was aggregated. The two species were located
in Triton X-100 soluble (for 60–80%) and insoluble fractions (for 20–40%). The incubation
of the expressing cells with tunicamycin (5 μg/ml) and the treatment of the intracellular
species with a mixture of Neuraminidase and N-glycosidase-F revealed that both intracellular
species were N-glycosylated. Furin over-expression neither diminished the intracellular
FVIII contents nor improved its extracellular production. Intracellular FVIII was
degraded through both lysosomal and proteasomal pathways as evidenced by inhibitor
treatments (e.g. NH4Cl, leupeptin, clasto-Lactacystin β-lactone and MG-132), pulse-chase analysis and confocal observations.
This study demonstrates that a BDD-FVIII expressed in CHO cells is inefficiently processed
consecutively to intracellular aggregation, proteasomal degradation, and routage to
lysosomes.
Keywords
Degradation - factor VIII - furin - lysosome