No abstract

Numerous studies have indicated that inactivation of p53 is one of the essential requirements for the unrestrained growth of tumoral cells. When the status of the p53 gene was examined in various types of lymphoid malignancies, mutations in p53 have been predominantly detected in Burkitt's lymphoma (BL) cells, therefore suggesting that alteration of p53 could speci®cally contribute to the malignant phenotype of these tumoral cells. In addition to mutations, functional inactivation of p53 can also occur through interaction of the wild-type gene product with various viral or cellular proteins. The cellular MDM2 protein, for example, is able to inhibit p53 tumor suppressor function by concealing its transactivation domain. Mdm2 gene ampli®cation has been described in several types of sarcomas, resulting in overexpression of the MDM2 protein. In this study, we have examined the status of MDM2 and p53 in 20 BL cell lines. Four were found to contain wild-type p53 and to overexpress MDM2 protein. Within these BL cells, both molecules are physically associated since they can be co-precipitated and p53 is inactivated as cells neither arrest in G1 nor enter apoptosis following g-radiation. We also report that the high level of the MDM2 protein in BL cells is neither associated with an ampli®cation of the mdm2 gene nor with an elevated level of RNA or an increased protein stability, but is rather due to an enhanced translation ability of the mdm2 RNA. These results indicate that in certain BL cells, overexpression of MDM2 protein regulated at the posttranscriptional level, induces an escape from p53-controlled cell growth.

Enlarged fat cells exhibit modified metabolic capacities, which could be involved in the metabolic complications of obesity at the whole body level. We show here that sterol regulatory element-binding protein 2 (SREBP-2) and its target genes are induced in the adipose tissue of several models of rodent obesity, suggesting cholesterol imbalance in enlarged adipocytes. Within a particular fat pad, larger adipocytes have reduced membrane cholesterol concentrations compared with smaller fat cells, demonstrating that altered cholesterol distribution is characteristic of adipocyte hypertrophy per se. We show that treatment with methyl-␤-cyclodextrin, which mimics the membrane cholesterol reduction of hypertrophied adipocytes, induces insulin resistance. We also produced cholesterol depletion by mevastatin treatment, which activates SREBP-2 and its target genes. The analysis of 40 adipocyte genes showed that the response to cholesterol depletion implicated genes involved in cholesterol traffic (caveolin 2, scavenger receptor BI, and ATP binding cassette 1 genes) but also adipocyte-derived secretion products (tumor necrosis factor ␣, angiotensinogen, and interleukin-6) and proteins involved in energy metabolism (fatty acid synthase, GLUT 4, and UCP3). These data demonstrate that altering cholesterol balance profoundly modifies adipocyte metabolism in a way resembling that seen in hypertrophied fat cells from obese rodents or humans. This is the first evidence that intracellular cholesterol might serve as a link between fat cell size and adipocyte metabolic activity.Depending upon physiological or pathological conditions, lipid storage within the adipocyte may vary dramatically. This is particularly obvious in the obese state in which long term disequilibria in energy balance produces massive fat cell hypertrophy. Enlarged fat cells isolated from obese animals or humans exhibit modified metabolic properties and do not respond to hormones as do adipocytes from lean controls. For example, they develop insulin resistance (1-4) and produce increased quantities of secreted products such as leptin (1) or TNF-␣ 1 (5). These characteristics could be secondary to the hormonal and metabolic changes linked to obesity but could also be due to cell hypertrophy per se and thus be part of an adaptative mechanism in relationship with the status of energy stores in adipose tissue.Results from previous studies summarized below lead us to propose a role for cholesterol as a signaling molecule for fat cell hypertrophy. Cholesterol accumulates in large quantities in a non-esterified form within the adipocyte lipid droplet (for review see Ref. 6) proportionally to the triglyceride content (7,8). Several lines of evidence also suggest that concomitantly with the enlargement of the triglyceride stores, adipocyte cholesterol is redistributed from the plasma membrane to the lipid droplet. First, a decrease in plasma membrane cholesterol, associated with increased fluidity, has been reported in hypertrophied adipocytes (9). Second, plasma membran...