To address the possibility that the expression of the myc gene might be regulated at a post-transcriptional level, we have investigated the half-life of myc mRNA in various cells. Our survey included normal human embryonic fibroblasts as well as transformed human cells of various origins: cervix carcinoma (HeLa), breast carcinoma (MCF7), Burkitt lymphoma (Daudi), and promyelocytic leukemia (HL60). All these cells revealed an extreme instability of myc mRNA (half-life, approximately equal to 10 min), suggesting that the control of myc mRNA degradation might be a general means (although not necessarily exclusive) of regulating both the level and the timing of myc gene expression. Inhibition of protein synthesis resulted in a dramatic stabilization of myc mRNA in HeLa, MCF7, and HL60 cells, suggesting that the controlling element might itself be, at least in these cells, a protein of rapid turnover. This finding opens the way to studying the mechanism of myc mRNA inactivation in these different cell types. However, protein synthesis inhibition had no effect on myc mRNA instability in other transformed (Daudi) cell lines as well as normal embryonic human fibroblasts. These different types of behavior suggest that the post-transcriptional control of myc gene expression might involve multiple factors that would be differently affected in various cell types.

Here, we report the isolation of a human multipotent adipose-derived stem (hMADS) cell population from adipose tissue of young donors. hMADS cells display normal karyotype; have active telomerase; proliferate >200 population doublings; and differentiate into adipocytes, osteoblasts, and myoblasts. Flow cytometry analysis indicates that hMADS cells are CD44⁺, CD49b⁺, CD105⁺, CD90⁺, CD13⁺, Stro-1⁻, CD34⁻, CD15⁻, CD117⁻, Flk-1⁻, gly-A⁻, CD133⁻, HLA-DR⁻, and HLA-I^low. Transplantation of hMADS cells into the mdx mouse, an animal model of Duchenne muscular dystrophy, results in substantial expression of human dystrophin in the injected tibialis anterior and the adjacent gastrocnemius muscle. Long-term engraftment of hMADS cells takes place in nonimmunocompromised animals. Based on the small amounts of an easily available tissue source, their strong capacity for expansion ex vivo, their multipotent differentiation, and their immune-privileged behavior, our results suggest that hMADS cells will be an important tool for muscle cell–mediated therapy.

The gene product of the recently cloned mouse obese gene (ob) is important in regulating adipose tissue mass. ob RNA is expressed specifically by mouse adipocytes in vivo in each of several different fat cell depots, including brown fat. ob RNA is also expressed in cultured 3T3-442A preadipocyte cells that have been induced to differentiate. Mice with lesions of the hypothalamus, as well as mice mutant at the db locus, express a 20-fold higher level of ob RNA in adipose tissue. These data suggest that both the db gene and the hypothalamus are downstream of the ob gene in the pathway that regulates adipose tissue mass and are consistent with previous experiments suggesting that the db locus encodes the ob receptor. In db/db and lesioned mice, quantitative differences in expression level of ob RNA correlated with adipocyte lipid content. The molecules that regulate expression level of the ob gene in adipocytes probably are important in determining body weight, as are the molecules that mediate the effects of ob at its site of action.

Hyperplasia of adipose tissue is critical for the development of obesity, but molecular mechanisms governing normal or pathological recruitment of new adipocytes remain unclear. The extracellular signal–regulated kinase (ERK) pathway plays a pivotal role in many essential cellular functions, such as proliferation and differentiation. Using ERK1^−/− mice, we investigated the role of this isoform in adipose tissue development. Mice lacking ERK1 have decreased adiposity and fewer adipocytes than wild-type animals. Furthermore, ERK1^−/− mice challenged with high-fat diet are resistant to obesity, are protected from insulin resistance, and have a higher postprandial metabolic rate. To get insights into cellular mechanisms implicated in reduced adiposity in ERK1^−/− animals, we analyzed adipocyte differentiation in ERK1^−/− cells. Compared with wild-type control cells, mouse embryo fibroblasts and cultures of adult preadipocytes isolated from ERK1^−/− adult animals exhibit impaired adipogenesis. An inhibitor of the ERK pathway does not affect the residual adipogenesis of the ERK1^−/− cells, suggesting that ERK2 is not implicated in adipocyte differentiation. Our results clearly link ERK1 to the regulation of adipocyte differentiation, adiposity, and high-fat diet–induced obesity. This suggests that a therapeutic approach of obesity targeting specifically the ERK1 isoform and not ERK2 would be of particular interest.