Multihormonal induction of α 2u ‐globulin in an established rat hepatoma cell line

A subclone of the FU5‐5 rat hepatoma cell line has been isolated which is inducible more than several hundred fold for the 20,000 dalton form of the major rat urinary protein α 2u ‐globulin. The basal relative synthetic rate (RSR) in growth medium containing 10% fetal calf serum was less than 2 × 10 –6 of total protein synthesis. Both dexamethasone and insulin were necessary for induction, and yielded a maximum induced RSR of 4–8 × 10 –3 . Triiodothyronine (T3), dihydrotestosterone (DHT), rat growth hormone (GH), and estrogen, all of which have been shown to influence the induction of α 2u –globulin in the intact rat, were without effect on the cell line. A factor present in fetal calf serum was also necessary for maximum induction, since dexamethasone plus insulin in serum‐free medium raised the RSR to only 3 × 10 –5 ; exogenous T3, GH, and DHT could not substitute for this serum factor. The kinetics of induction by dexamethasone were slow, with a lag of approximately 48 hr followed by a period of increasing RSR for 6–20 days. Removal of dexamethasone from induced cells led to an exponential decline in the RSR (t½ 1/2 15 hr). The concentrations of dexamethasone and insulin that could yield half maximum induction were 5 × 10 –8 M and 3 × 10 –11 M, respectively. Higher concentrations of insulin, although still in physiological range (10 –9 M), inhibited induction. At yet higher insulin levels, beyond the physiological range, α 2u ‐globulin synthesis returned to maximum values. The lack of DHT, T3, and GH requirement for α 2u ‐globulin induction in this cell line may mean that a regulatory aberrancy has occurred in this transformed cell line, or, alternatively, that these hormones act indirectly in the intact animal. This cell line should prove useful for the study of the molecular events associated with α 2u ‐globulin induction and for genetic approaches to the problem of multihormonal regulation of gene expression.