IGF-I stimulation of extracellular acidification is not linked to cell proliferation for autocrine cells

Insulin-like growth factor-I (IGF-I) increases extracellular acidification rate (ECAR), a measure correlated with proliferation for nonautocrine cells. To evaluate the effect of autocrine IGF-I secretion on cell responsiveness, a cell line that secretes IGF-I was tested. SV40-lGF-I cells also registered concentration-dependent increases in ECAR; however, unlike the parental cell line, signal attenuation upon repeat challenges was not evident. Furthermore, SV40-IGF-I cells did not proliferate in response to IGF-I. We investigated if lack of proliferation was due to differences in the protocols of the assays ([3H]thymidine incorporation and microphysiometry). We identified three key differences in the protocols: surface substrate, cell density, and fluid residence time. We found no increase in [3H]thymidine incorporation for cells on either tissue-culture plastic or polycarbonate transwells. Control levels of [3H]thymidine incorporation were cell-density-dependent, but IGF-I did not increase proliferation at any density studied. Finally, we investigated IGF-I stimulation for cells under microphysiometer flow conditions and found no proliferative response to IGF-I. We found that the cells do respond to IGF-I with increased amino acid uptake. These data suggest that IGF-I signaling is operational in the SV40-IGF-I cells, but the transduction pathway for IGF-I-induced proliferation is compromised, despite the fact that these cells respond to fetal bovine serum with increased growth. Ongoing studies are focused on identifying which elements in the signaling cascade are altered by autocrine secretion of IGF-I.