This paper demonstrates that Y79 human retinoblastoma cells contain im
munoreactive insulin (TRI) and release it in the medium. Cells culture
d either in suspension or in monolayer showed a similar content of IRI
. Moreover, in both conditions, IRI concentration was higher in cells
cultured in serum-supplemented medium rather than in serum-free medium
. Retinoblastoma cells are capable of synthesizing insulin. This was d
emonstrated by incubating Y79 cells with [H-3]leucine. The synthesized
radioactive insulin was separated and assayed by means of a HPLC proc
edure described in this paper. Both cell growth and [3H]thymidine and
[H-3]uridine incorporation into acid-insoluble fraction was reduced (-
75%) in Y79 cells cultured without serum with respect to those cultur
ed in the presence of serum. The addition of insulin to the serum-free
medium stimulated both cell division and DNA and RNA labeling, with v
alues approaching those obtained with serum supplemented cultures. Ins
ulin-like growth factor I exerted similar effects, but at a much lower
concentration than insulin. We suggest that both insulin and IGF-I ma
y represent mitogenic signals for these cells which might be mediated
through insulin-like growth factor I receptors.