Jy. Ryoo et al., ENDOCYTOSIS OF SERUM-ALBUMIN IN REGENERATING RAT-LIVER, Proceedings of the Society for Experimental Biology and Medicine, 215(2), 1997, pp. 179-185
Lysosomes, isolated from rat liver after 70% partial hepatectomy (PHX)
, were found, by Western blotting, to contain a considerable amount of
serum albumin. The level of intralysosomal serum albumin after PHX sh
owed biphasic patterns: it increased immediately after PHX, peaked at
30 min, rapidly declined within a few hours, rose again with a peak at
15 hr, and gradually declined thereafter. At 15 hr after PHX, the con
tent of lysosomal proteins in the liver increased to twice the level o
f unoperated control, and the electron-microscopic observation of the
isolated lysosomes revealed numerous large membrane-delimited structur
es with ground substances of variable electron opacities, The increase
in the intralysosomal serum albumin at 30 min and 15 hr was accompani
ed by changes in the buoyant densities of endosomes in Percoll density
gradients, At both time points, the density profiles of endosomes iso
lated from hepatectomized rats shifted to the denser direction, sugges
ting that PHX activates fusion and/or maturation of endosomes. Formald
ehyde-treated bovine serum albumin is known to be taken up by the live
r by receptor-mediated endocytosis. The uptake of the modified heterol
ogous albumin was shown to be activated as early as 30 min after PHX,
Both the uptake of serum albumin into lysosomes and the shift of buoya
nt density profile of endosomes after PHX were inhibited by the admini
stration of adrenergic receptor antagonists, particularly by the alpha
(1)-antagonist prazosin, Further, the concentration of catecholamines
in rat serum, particularly that of norepinephrine, was found to increa
se immediately after PHX, relative to that in serum from sham-operated
rats. These results suggest that the elevation of serum norepinephrin
e levels after PHX activates endocytosis and facilitates delivery of e
ndocytosed serum albumin to lysosomes, where albumin is digested to yi
eld amino acids for possible use in protein synthesis during liver reg
eneration.