A novel growth stimulating activity from BRL-3A cell conditioned medium

The R⁻ cell line is a 3T3-like cell line originating from mouse embryos with a homozygous disruption of the type 1 insulin-like growth factor receptor (IGF-IR) genes. Although R⁻ cells cannot grow at all in serum-free medium (SFM) supplemented by several known growth factors, either singly or in combination, they are able to grow in 10% serum, albeit at a reduced rate. These findings suggested that serum contains an unknown, or unidentified, growth factor that can promote cell growth even in cells devoid of IGF-IRs. In an effort to identify such growth factor, we searched, using R⁻ cells, for a growth and DNA synthesis stimulating activity in SFM conditioned by different cell lines. We found that the BRL-3A cell line secreted an activity capable of stimulating DNA synthesis and cell proliferation in R⁻ cells. This activity (which is concentration-dependent) can be collected and concentrated by ultrafiltration, it is heat-labile, proteinase K-sensitive and has a size larger than 10 kDa. Because of the resistance of R⁻ cells to stimulation by known growth factors, we believe that this activity is due to a novel polypeptide secreted by BRL-3A cells. Further characterization of the active component(s) is in progress.