Summary
The ability of cultured human fibroblasts to synthesize and secrete a mitogenic, somatomedin
C-like peptide suggests that human fibroblasts unlike certain other cell lines may
not require exogenous somatomedin for cell proliferation. The aim of this study was
to examine the effects of hypophysectomy and subsequent growth hormone (GH) replacement
on the ability of rat serum to stimulate proliferation and DNA synthesis in cultured
human fetal skin fibroblasts. For comparison we have also examined the effects of
serum from hypophysectomized (hypox) and GH-replaced hypox rats on mouse BALB/c 3T3
fibroblasts, a cell line which has been shown to require exogenous somatomedin (SM)
for DNA synthesis and cell proliferation.
Human fibroblast proliferation and DNA synthesis (measured as thymidine incorporation)
were significantly enhanced (P < 0.025) when these cells were cultured in low concentrations
of hypox rat serum compared to normal rat serum. When thymidine incorporation into
DNA was expressed in terms of a normal rat serum pool (arbitrarily assigned a potency
of 1 unit/ml), serum from hypox rats was significantly more potent than serum from
normal rats (1.52 ± 0.16 (SEM) vs. 0.99 ± 0.10 units/ml, P < 0.0125).
Replacement treatment of hypox rats with human GH, 0.2 IU/100 g body weight/day or
0.4 IU/100 g body weight/day decreased the potency of the rat serum in terms of stimulation
of thymidine incorporation from 1.52 ± 0.16 to 1.30 ± 0.14 and 1.08 ± 0.06 units/ml
respectively. In contrast serum from untreated hypox rats was significantly less potent
than normal rat serum in stimulating BALB/c 3T3 DNA synthesis (0.36 ± 0.06 vs. 1.04
± 0.16 units/ml, P < 0.0025). When hypox and normal rat serum were mixed in various
ratios and tested for potency in stimulating thymidine incorporation into DNA of quiescent
human fibroblasts, and BALB/c 3T3 cells, values intermediate between normal and hypox
rat serum were obtained. These data are consistent with the presence of an inhibitor
of human fibroblast DNA synthesis in serum from both normal and GH-treated hypox rats.
The effect of Multiplication Stimulation Activity (MSA), a SM analog, on thymidine
incorporation into DNA of human fibroblasts in the presence of normal or hypox rat
serum was examined. Significantly more thymidine incorporation occurred at each concentration
of MSA in the presence of hypox rat serum compared to normal rat serum consistent
with enhanced inhibitory activity in normal rat serum.
From these observations we conclude that under the conditions described here human
fibroblast DNA synthesis is not dependent on the serum somatomedin concentration.
In addition inhibitory activity present in normal rat serum may in part be GH-dependent.
Key-Words:
Somatomedins
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Growth Factors
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DNA Synthesis
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Mitogenesis
