Rab4b Is a Small GTPase Involved in the Control of the Glucose Transporter GLUT4 Localization in Adipocyte

Endosomal small GTPases of the Rab family, among them Rab4a, play an essential role in the control of the glucose transporter GLUT4 trafficking, which is essential for insulin-mediated glucose uptake. We found that adipocytes also expressed Rab4b and we observed a consistent decrease in the expression of Rab4b mRNA in human and mice adipose tissue in obese diabetic states. These results led us to study this poorly characterized Rab member and its potential role in glucose transport.We used 3T3-L1 adipocytes to study by imaging approaches the localization of Rab4b and to determine the consequence of its down regulation on glucose uptake and endogenous GLUT4 location. We found that Rab4b was localized in endosomal structures in preadipocytes whereas in adipocytes it was localized in GLUT4 and in VAMP2-positive compartments, and also in endosomal compartments containing the transferrin receptor (TfR). When Rab4b expression was decreased with specific siRNAs by two fold, an extent similar to its decrease in obese diabetic subjects, we observed a small increase (25%) in basal deoxyglucose uptake and a more sustained increase (40%) in presence of submaximal and maximal insulin concentrations. This increase occurred without any change in GLUT4 and GLUT1 expression levels and in the insulin signaling pathways. Concomitantly, GLUT4 but not TfR amounts were increased at the plasma membrane of basal and insulin-stimulated adipocytes. GLUT4 seemed to be targeted towards its non-endosomal sequestration compartment.Taken our results together, we conclude that Rab4b is a new important player in the control of GLUT4 trafficking in adipocytes and speculate that difference in its expression in obese diabetic states could act as a compensatory effect to minimize the glucose transport defect in their adipocytes

HMGA2-NFIB fusion in a pediatric intramuscular lipoma: a novel case of NFIB alteration in a large deep-seated adipocytic tumor.

International audienceLipomas are frequently characterized by aberrations of the 12q13 approximately q15 chromosomal region and often by rearrangements of the HMGA2 gene. These rearrangements include the formation of chimeric genes that fuse the 5' region of HMGA2 with a variety of partners, such as LPP (3q28) or NFIB (9p22). We describe here the fourth reported case of lipoma showing a HMGA2-NFIB fusion, and the first one in a child. We found a translocation t(9;12)(p22;q14) in a deep-seated intramuscular lipoma occurring in the buttock of a 5-year-old boy. By fluorescence in situ hybridization and reverse-transcription polymerase chain reaction, we have shown that the translocation t(9;12) resulted in an in-frame fusion of the first four exons of HMGA2 with the last exon of NFIB. Intramuscular lipomas are very rare in childhood. Our results confirm that lipomas containing NFIB rearrangements may be related to peculiar clinicohistologic features, including large size, deep situation, infiltration of surrounding muscles, or precocious occurrence. Both the truncation of HMGA2 and the nature of its fusion partner gene might be relevant in the adipose tissue tumorigenesis

Clues to CD2-associated Protein Involvement in Cytokinesis

Cytokinesis requires membrane trafficking coupled to actin remodeling and involves a number of trafficking molecules. CD2-associated protein (CD2AP) has been implicated in dynamic actin remodeling and membrane trafficking that occurs during endocytosis leading to the degradative pathway. In this study, we present several arguments for its implication in cytokinesis. First, endogenous CD2AP was found concentrated in the narrow region of the midzone microtubules during anaphase and in the midbody during late telophase. Moreover, we found that CD2AP is a membrane- and not a microtubule-associated protein. Second, the overexpression of the first two Src homology 3 domains of CD2AP, which are responsible for this localization, led to a significant increase in the rate of cell multinucleation. Third, the CD2AP small interfering RNA interfered with the cell separation, indicating that CD2AP is required for HeLa cells cytokinesis. Fourth, using the yeast two-hybrid system, we found that CD2AP interacted with anillin, a specific cleavage furrow component, and the two proteins colocalized at the midbody. Both CD2AP and anillin were found phosphorylated early in mitosis and also CD2AP phosphorylation was coupled to its delocalization from membrane to cytosol. All these observations led us to propose CD2AP as a new player in cytokinesis

Let-7 MicroRNA and HMGA2 levels of expression are not inversely linked in adipocytic tumors: Analysis of 56 lipomas and liposarcomas with molecular cytogenetic data.

International audienceThe aim of our study was first to assess the role of HMGA2 expression in the pathogenesis of adipocytic tumors (AT) and, second, to seek a potential correlation between overexpression of HMGA2 and let-7 expression inhibition by analyzing a series of 56 benign and malignant AT with molecular cytogenetic data. We measured the levels of expression of HMGA2 mRNA and of eight members of the let-7 microRNA family using quantitative RT-PCR and expression of HMGA2 protein using immunohistochemistry. HMGA2 was highly overexpressed in 100% of well-differentiated/dedifferentiated liposarcomas (WDLPS/DDLPS), all with HMGA2 amplification, and 100% of lipomas with HMGA2 rearrangement. Overexpression of HMGA2 mRNA was detected in 76% of lipomas without HMGA2 rearrangement. HMGA2 protein expression was detected in 100% of lipomas with HMGA2 rearrangement and 48% of lipomas without HMGA2 rearrangement. We detected decreased expression levels of some let-7 members in a significant proportion of AT. Notably, let-7b and let-7g were inhibited in 61% of WDLPS/DDLPS. In lipomas, each type of let-7 was inhibited in approximately one-third of the cases. Although overexpression of both HMGA2 mRNA and protein in a majority of ordinary lipomas without HMGA2 structural rearrangement may have suggested a potential role for let-7 microRNAs, we did not observe a significant link with let-7 inhibition in such cases. Our results indicate that inhibition of let-7 microRNA expression may participate in the deregulation of HMGA2 in AT but that this inhibition is neither a prominent stimulator for HMGA2 overexpression nor a surrogate to genomic HMGA2 rearrangements. © 2011 Wiley-Liss, Inc

Clinical and biological significance of CDK4 amplification in well-differentiated and dedifferentiated liposarcomas.

International audiencePURPOSE: The MDM2 and HMGA2 genes are consistently amplified in well-differentiated/dedifferentiated liposarcomas (WDLPS/DDLPS) whereas CDK4 is frequently but not always amplified in these tumors. Our goal was to determine whether the absence of CDK4 amplification was (a) correlated to a specific clinico-histopathologic profile; and (b) compensated by another genomic anomaly involving the CCND1/CDK4/P16INK4a/RB1/E2F pathway. EXPERIMENTAL DESIGN: We compared the clinical characteristics of a series of 143 WDLPS/DDLPS with amplification of both MDM2 and CDK4 (MDM2+/CDK4+) to a series of 45 WDLPS/DDLPS with MDM2 amplification and no CDK4 amplification (MDM2+/CDK4-). We used fluorescence in situ hybridization, real time quantitative reverse transcription PCR, and immunohistochemistry to explore the status of CCND1, P16INK4a, P14ARF, and RB1. RESULTS: We found that MDM2+/CDK4- WDLPS/DDLPS represent a distinct clinical subgroup with favorable prognostic features, including low-grade lipoma-like histology, peripheral location, and lower rate of recurrence. By using fluorescence in situ hybridization, we found that genomic aberrations expected to be alternative mechanisms for compensating the lack of CDK4 amplification, such as RB1 and CDKN2A deletions or CCND1 amplification, were very uncommon. In contrast, by using real time quantitative reverse transcription PCR and immunohistochemistry, we observed that overexpression of P16INK4a (and P14ARF) and CCND1 and reduced expression of RB1 were very frequent, independently of the CDK4 status. Conclusions: Our results underscore the complex coordinated regulation of the RB and p53 growth-control pathways in WDLPS/DDLPS. Because the absence of CDK4 amplification is not specifically counterbalanced by a genomic alteration of the CCND1/CDK4/P16INK4a/RB1/E2F pathway, CDK4 amplification may only represent a "MDM2-HMGA2-helper" in WDLPS/DDLPS tumorigenesis

HMGA2 is the partner of MDM2 in well-differentiated and dedifferentiated liposarcomas whereas CDK4 belongs to a distinct inconsistent amplicon.

Data concerning the fine structure of the 12q13-15 amplicon which contains MDM2 and CDK4 in well-differentiated and dedifferentiated liposarcomas (WDLPS/DDLPS) are scarce. We investigated a series of 38 WDLPS/DDLPS using fluorescence in situ hybridization analysis with 17 probes encompassing the 12q13-15 region. In addition, using quantitative RT-PCR we studied the expression of MDM2, CDK4, DDIT3 (CHOP/GADD153), DYRK2, HMGA2, TSPAN31 and YEATS4 (GAS41) in 11 cases. We showed that CDK4 (12q14.1) belonged to a distinct amplicon than MDM2 (12q15). There was no continuity in the amplified sequences between MDM2 and CDK4. Moreover, while MDM2 was amplified and overexpressed in all cases, CDK4 was not amplified or overexpressed in 13% of cases. The centromeric border of the CDK4 amplicon was located immediately downstream the 5' end of DDIT3, a gene known for being involved in myxoid liposarcoma translocations. DDIT3 was amplified in 3 cases and overexpressed in 9 cases. The overexpression of DDIT3 was correlated to the CDK4 amplification and not to its own amplification status. This suggested that the CDK4 amplicon, as well as the overexpression of DDIT3, might be generated by the disruption of a fragile region in 5' DDIT3. HMGA2 was always amplified and rearranged indicating that it plays a central role in WDLPS/DDLPS. HMGA2 rearrangement frequently resulted in a loss of the 3' end region that is a binding site for let-7. We also found a frequent amplification and overexpression of YEATS4, an oncogene that inactivates P53, suggesting that YEATS4 might play an important role together with MDM2 in WDLPS/DDLPS oncogenesis

Alternative PDGFD rearrangements in dermatofibrosarcomas protuberans without PDGFB fusions

International audienceDermatofibrosarcoma protuberans is underlined by recurrent collagen type I alpha 1 chain-platelet-derived growth factor B chain (COL1A1-PDGFB) fusions but ~ 4% of typical dermatofibrosarcoma protuberans remain negative for this translocation in routine molecular screening. We investigated a series of 21 cases not associated with the pathognomonic COL1A1-PDGFB fusion on routine fluorescence in situ hybridization (FISH) testing. All cases displayed morphological and clinical features consistent with the diagnosis of dermatofibrosarcoma protuberans. RNA-sequencing analysis was successful in 20 cases. The classical COL1A1-PDGFB fusion was present in 40% of cases (n = 8/20), and subsequently confirmed with a COL1A1 break-apart FISH probe in all but one case (n = 7/8). 55% of cases (n = 11/20) displayed novel PDGFD rearrangements; PDGFD being fused either to the 5' part of COL6A3 (2q37.3) (n = 9/11) or EMILIN2 (18p11) (n = 2/11). All rearrangements led to in-frame fusion transcripts and were confirmed at genomic level by FISH and/or array-comparative genomic hybridization. PDGFD-rearranged dermatofibrosarcoma protuberans presented clinical outcomes similar to typical dermatofibrosarcoma protuberans. Notably, the two EMILIN2-PDGFD cases displayed fibrosarcomatous transformation and homozygous deletions of CDKN2A at genomic level. We report the first recurrent molecular variant of dermatofibrosarcoma protuberans involving PDGFD, which functionally mimic bona fide COL1A1-PDGFB fusions, leading presumably to a similar autocrine loop-stimulating PDGFRB. This study also emphasizes that COL1A1-PDGFB fusions can be cytogenetically cryptic on FISH testing in a subset of cases, thereby representing a diagnostic pitfall that pathologists should be aware of