Subtilisin-like proprotein convertases, PACE4 and PC8, as well as furin, are endogenous proalbumin convertases in HepG2 cells

Serum albumin is synthesized as a larger precursor form, proalbumin, which undergoes proteolytic processing at a dibasic site by a hepatic proprotein convertase within the secretory pathway to generate the mature form, Althou gh furin, a member of the subtilisin-like proprotein convertase (SPC) famil y, was thought to be the only candidate hepatic convertase for proalbumin, SPC family members other than furin were recently suggested to also be invo lved in proalbumin processing. This study was designed to identify the endo genous proprotein convertases involved in proalbumin processing. Since huma n hepatoma HepG2 cells are highly differentiated and produce major plasma p roteins, this cell line was used as a model for hepatocytes. Northern blot analysis revealed that PACE4, furin and PC8 of the SPC family were expresse d in HepG2 cells as well as in the liver, Ribonuclease protection assay sho wed that PACE4A-II mRNA is the major transcript in HepG2 cells among the PA CE4 isoforms, The coexpression studies showed that furin, PACE4A-II and PC8 were all able to convert proalbumin to albumin correctly, To elucidate the roles of these endogenous SPC family members in proalbumin processing, the antisense RNA for PACE4, furin and PC8 was stably expressed in HepG2 cells , respectively. The expression of each antisense RNA resulted in approximat ely 30% inhibition of endogenous proalbumin processing. We therefore conclu ded that PACE4 and PC8, as well as furin, are involved in the processing of proalbumin in HepG2 cells, and that these SPC family members are functiona lly redundant in this processing.