Cytoplasmic transfer of the mitogenic response to platelet-derived growth factor

BALB/c 3T3 mouse cells exposed briefly to platelet-derived growth factor (PDGF) become "competent" to replicate their DNA and divide. When cells are treated with PDGF and then fused to untreated cells, the resulting heterokaryons become competent to replicate their DNA. Cytoplasts derived from PDGF-treated cells are also able to transfer the growth response to untreated cells. After cytoplasmic transfer to another cell, the strength of the PDGF-induced mitogenic signal is attenuated by a factor roughly proportional to the increase in total cytoplasmic volume. When RNA synthesis is blocked during PDGF treatment, cells do not acquire the capacity to transfer the PDGF growth signal to untreated cells. By contrast, exposure to cycloheximide during PDGF treatment has no effect. These observations suggest that cytoplasmic transfer of the growth response to PDGF (competence) is mediated by a PDGF-induced stable RNA rather than by PDGF itself or a PDGF--receptor complex. The onset of DNA synthesis in PDGF--control heterokaryons occurs a minimum of 11 hr after cell fusion. Thus the substance that is transferred in these cell fusions is not directly involved in DNA synthesis; rather, it seems to trigger a sequence of events culminating in DNA synthesis.