After an initial burst of cell proliferation, the type 1 insulin-like growth factor receptor (IGF-IR) induces granulocytic differentiation of 32D IGF-IR cells, an interleukin-3-dependent murine hemopoietic cell line devoid of insulin receptor substrate-1 (IRS-1). The combined expression of the IGF-IR and IRS-1 (32D IGF-IR/IRS-1 cells) inhibits IGF-I-mediated differentiation, and causes malignant transformation of 32D cells. Because of the role of IRS-1 in changing the fate of 32D IGF-IR cells from differentiation (and subsequent cell death) to malignant transformation, we have looked for differences in IGF-IR signaling between 32D IGF-IR and 32D IGF-IR/IRS-1 cells. In this report, we have focused on p70^S6K, which is activated by the IRS-1 pathway. We find that the ectopic expression of IRS-1 and the inhibition of differentiation correlated with a sustained activation of p70^S6K and an increase in cell size. Phosphorylationin vivo of threonine 389 and, to a lesser extent, of threonine 421/serine 424 of p70^S6K seemed to be a requirement for inhibition of differentiation. A role of IRS-1 and p70^S6K in the alternative between transformation or differentiation of 32D IGF-IR cells was confirmed by findings that inhibition of p70^S6K activation or IRS-1 signaling, by rapamycin or okadaic acid, induced differentiation of 32D IGF-IR/IRS-1 cells. We have also found that the expression of myeloperoxidase mRNA (a marker of differentiation, which sharply increases in 32D IGF-IR cells), does not increase in 32D IGF-IR/IRS-1 cells, suggesting that the expression of IRS-1 in 32D IGF-IR cells causes the extinction of the differentiation program initiated by the IGF-IR, while leaving intact its proliferation program.

3T3 cells null for the type 1 insulin-like growth factor receptor are refractory to stimulation by a variety of purified growth factors that are known to be required for the stimulation of other 3T3 cells. However, these cells, known as R⁻cells, grow in serum-supplemented medium and also in media conditioned by certain cell lines. We report here the purification of a growth factor that stimulates DNA synthesis (and growth) of R⁻cells. The growth factor, purified to homogeneity by SDS-polyacrylamide gel electrophoresis, was identified as the granulin/epithelin precursor by an accurate determination of the masses of endoproteinase Lys-C peptides using matrix-assisted laser desorption ionization mass spectrometry, followed by a data base search. The granulin/epithelin precursor is a little known growth factor, secreted by a variety of epithelial and hemopoietic cells. It is at present the only purified growth factor that can stimulate the growth of mouse embryo fibroblasts null for the type 1 insulin-like growth factor receptor.

Loss of zfp36 expression in colorectal cancer correlates to wnt/ ß-catenin activity and enhances epithelial-to-mesenchymal transition through upregulation of zeb1, sox9 and macc1