Obese gene expression at in vivo levels by fat pads derived from s.c. implanted 3T3-F442A preadipocytes

3T3-F442A preadipocytes implanted s.c. into athymic mice develop into fat pads that are indistinguishable from normal adipose tissue. Implanted preadipocytes harboring a β-galactosidase transgene gave rise to fat pads in which almost all adipocytes expressed β-galactosidase. This finding proved that the implanted 3T3-F442A preadipocytes, rather than endogenous preadipose cells, gave rise to the newly developed “adipose tissue.” 3T3-F442A preadipocytes, when differentiated into adipocytes in cell culture, express the obese gene at an unexpectedly low level, i.e., ≤1% the level in adipose tissue. However, adipose tissue derived from s.c. implanted 3T3-F442A preadipocytes expressed leptin mRNA at a level comparable to that in epididymal adipose tissue. These findings indicate that a factor(s) or condition, present in the tissue context and necessary for maximal obese gene expression, is lacking in cell culture. Furthermore, adipocytes derived from the implanted cells were hormonally responsive in that leptin mRNA levels were up-regulated 3- to 8-fold by glucocorticoid injection into the host animal. Thus, these findings indicate that adipose-specific promoter–reporter constructs, transfected into 3T3-F442A preadipocytes, can be tested in an in vivo context during and after development of these cells into adipose tissue. Furthermore, the effect of transgenes on the adipogenic development of the implanted preadipocytes can be assessed. Thus, this approach offers a faster and less costly alternative to the transgenic mouse method for assessing adipose gene function.