Estrogen stimulation of creatine kinase B specific activity in 3T3L1 adipocytes after their differentiation in culture: dependence on estrogen receptor.

We have demonstrated previously that rat adipose tissue showed sex and depot-specific responses to gonadal steroids. The epididymal fat pad in males responded exclusively to androgens by increased specific activity of the brain type isozyme of creatine kinase (CK). In females, the parametrial adipose tissue responded exclusively to estrogens. The present study was undertaken to follow the responsiveness to steroid hormones, and the presence of estrogen receptors (ER), in 3T3L1 cells during their differentiation from pre-adipocytes to adipocytes. In pre-adipocytes in which the basal CK specific activity is low, there was no CK response to 17beta estradiol (E2) or dihydrotestosterone (DHT). Differentiation of the cells into adipocytes was accompanied by increased basal CK activity which was stimulated by E2, but not by DHT. Responsiveness to E2 began 5 days after switching pre-adipocytes to differentiation medium. Upon differentiation, ER became demonstrable in the cell nuclei by staining with FITC labeled anti-idiotypic antibody (clone 1D5) directed against the steroid binding domain of ER. The response to E2 was time-dependent and blocked completely by cycloheximide or actinomycin D. 1D5 itself, which has an estrogen mimetic effect, stimulated CK activity in the cells similarly to E2. The antiestrogen tamoxifen which also stimulated CK activity in the adipocytes, completely blocked E2 action. The 'pure' antagonist of E2, ICI 164,384 and the tissue-selective antiestrogens, raloxifene or tamoxifen methiodide were also complete antagonists with no agonistic effects. The response of the 3T3L1 adipocytes to E2 was upregulated by 1,25(OH)2D3. Moreover, IGF1 was also a potent stimulator of CK in these cells, and therefore may mediate partially the stimulation by E2. Transient transfection of the pre-adipocytes with ER permitted E2 induction of CK. Thus, the appearance of ER and concomitant responsiveness to E2 is another hormone-related change occurring in 3T3L1 cells during differentiation, in addition to changes such as development of insulin responsiveness. The interactions in this system provide a useful in vitro model for investigating the development of responsiveness to E2.