Background/Aims: Hepatic stellate cells (HSC) are located in close proximit
y to hepatocytes in Disse's space. Hepatocyte derived factors have earlier
been implicated in the paracrine regulation of HSC proliferation. The aim o
f the present study was to further characterize this mitogenic activity of
the parenchymal cell conditioned medium (PCcM). Methods: Primary rat HSC we
re cultured for 4 days. DNA synthesis was measured by H-3-thymidine incorpo
ration. TGF beta(1) immunoreactivity was quantified by ELISA. PCcM was obta
ined from hepatocytes cultured in medium without serum or hormones for two
days. Results: Incubation of 4-day-old HSC on plastic surface with PCcM for
2 days increased DNA synthesis, while no effect was seen in HSC cultured o
n Matrigel. Heat-, acid-, and protease-treatment of PCcM abolished its stim
ulatory effect. Size fractionations with spin columns indicated that the st
imulatory effect was contained in the fractions of a molecular size between
30 and 100 kD. The addition of LY 294002, a phosphatidylinositol 3-kinase
(PI3-K) inhibitor, dose-dependently inhibited the PCcM induced increase in
DNA synthesis to about 9% of the control values. The specific MAP kinase (M
APK) inhibitor, PD 98059 only suppressed the PCcM induced DNA synthesis to
35% of control cultures at the highest dose (10 mu M). DNA content in the c
ultures was not affected by either blocker. HSC seemed to produce immunorea
ctive TGF beta(1). However, addition of latency-associated peptide (LAP), a
potent TGF beta(1) blocker, stimulated DNA synthesis to a much less extent
than PCcM. Conclusions: The factor(s) that stimulate DNA synthesis in HSC
from hepatocytes are most likely protein(s) with a molecular size between 3
0-100 kD. These factor(s) rely more on PI3-K than on MAPK for their mitogen
ic effect and are probably not acting via TGF beta(1) inhibition.