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 responsivenes s, a cell line that secretes IGF-I was tested. SV40-IGF-I cells also regist ered concentration-dependent increases in ECAR; however, unlike the parenta l cell line, signal attenuation upon repeat challenges was not evident. Fur thermore, SV40-IGF-I cells did not proliferate in response to IGF-I. We inv estigated if lack of proliferation was due to differences in the protocols of the assays ([H-3]thymidine incorporation and micro physiometry). We iden tified three key differences in the protocols: surface substrate, cell dens ity, and fluid residence time. We found no increase in [H-3]thymidine incor poration for cells on either tissue-culture plastic or polycarbonate transw ells. Control levels of [H-3]thymidine incorporation were cell-density-depe ndent, but IGF-I did not increase proliferation at any density studied. Fin ally, we investigated IGF-I stimulation for cells under microphysiometer fl ow conditions and found no proliferative response to IGF-I. We found that t he cells do respond to IGF-I with increased amino acid uptake. These data s uggest that IGF-I signaling is operational in the SV40-IGF-I cells, but the transduction pathway for IGF-I-induced proliferation is compromised, despi te the fact that these cells respond to fetal bovine serum with increased g rowth. Ongoing studies are focused on identifying which elements in the sig naling cascade are altered by autocrine secretion of IGF-I.