Megakaryocyte and hepatocyte origins of human fibrinogen biosynthesis
exhibit hepatocyte-specific expression of gamma chain-variant polypeptides
PJ Haidaris, CW Francis, LA Sporn, DS Arvan, FA Collichio and VJ Marder
Department of Medicine, University of Rochester School of Medicine and
Dentistry, NY.
The gamma chain of human fibrinogen is heterogeneous in length at the C-
terminus due to differential RNA processing of the gamma chain-gene primary
transcript. We have produced two specific monoclonal antibodies (MoAbs)
against the gamma-chain epitopes generated by this alternative processing
event: anti-gamma 57.5(408-416) (L2B), which reacts with gamma 57.5 and
gamma 55 chains, and anti-gamma 50(337-411) (H9B7), which reacts
preferentially with gamma 50 chains. Using these MoAbs we have studied the
expression of gamma-chain polypeptides by immunofluorescence microscopy in
the tissues of fibrinogen biosynthesis and have determined that gamma 57.5
polypeptide is expressed in hepatocytes but is absent or present in
significantly reduced amounts in megakaryocytes. Therefore the gamma 50
chain is found in plasma, platelet, and megakaryocyte fibrinogens, but the
gamma 57.5 chain is found only in plasma fibrinogen. The C-terminal amino
acid sequence of gamma 55 includes the L2B epitope 57.5(408-416). Using
MoAb L2B we have determined that gamma 55, which is a post-translationally
modified gamma 57.5 chain, is found only in plasma fibrinogen and is absent
or present in markedly reduced amounts in platelet or megakaryocyte
fibrinogen. In addition, the conformation of the L2B epitope is preserved
in gamma 55, as determined by Western blot analysis. The
hepatocyte-specific expression of the gamma 57.5-chain polypeptide and the
post-translational modification to gamma 55 result in a
compartmentalization of gamma-chain polypeptide expression. This is
suggestive of different mechanisms regulating human fibrinogen gamma- chain
gene expression in hepatocytes v megakaryocytes that may operate in a
tissue-specific manner at the level of 3' RNA processing events.
Volume 74,
Issue 2,
pp. 743-750,
08/01/1989
Copyright © 1989 by The American Society of Hematology
