FIG. 1.

Mitogenic response to insulin in wild-type and IRS-1-deficient brown adipocytes. (A) Three clones of wild-type (+/+) and IRS-1-deficient (−/−) immortalized brown adipocytes were cultured for 24 h in serum-free medium either in the absence or presence of insulin (1, 10, and 100 nM). DNA synthesis was determined by [³H]thymidine incorporation (0.2 μCi/ml) over the last 4 h of culture. After two washes with ice-cold PBS, cells were lysed and trichloroacetate-precipitable DNA was then counted for incorporated radioactivity. Results are expressed as disintegrations per minute (dpm) per dish and are means ± standard errors from six independent experiments, each one performed in triplicate. (B) Cells were cultured for 24 h either in the absence or presence of various doses of insulin as described for panel A. At the end of the culture time, the percentage of cells in S plus G₂/M phases of the cell cycle was determined as described in Materials and Methods. Results are means ± standard errors from six independent experiments.