Synthesis of insulin and its effects in Y79 human retinoblastoma cells

This paper demonstrates that Y79 human retinoblastoma cells contain immunoreactive insulin (IRI) and release it in the medium. Cells cultured 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 demonstrated by incubating Y79 cells with [3H]leucine. The synthesized radioactive insulin was separated and assayed by means of a HPLC procedure described in this paper. Both cell growth and [3H]thymidine and [3H]uridine incorporation into acid-insoluble fraction was reduced (-75%) in Y79 cells cultured without serum with respect to those cultured in the presence of serum. The addition of insulin to the serum-free medium stimulated both cell division and DNA and RNA labeling, with values approaching those obtained with serum supplemented cultures. Insulin-like growth factor I exerted similar effects, but at a much lower concentration than insulin. We suggest that both insulin and IGF-I may represent mitogenic signals for these cells which might be mediated through insulin-like growth factor I receptors.