Thus the presence or absence of pRb directly modulates the levels and activity of Ppar and Runx2 in accordance with the preferential commitment of our OS cell lines to the osteogenic versus the adipogenic lineage. == Determine 3. adipose tissuein vivo. Mutations inRB1(7090% of cases) andTP53(5070% of cases) are strongly associated with human osteosarcoma67. To model osteosarcoma in the mouse, we crossedRbfl/fl(8) andp53fl/fl(9) conditional mutant SC-26196 mice with a transgenic collection,Prx1-Cre10, which expresses Cre recombinase in uncommitted mesenchymal cells that contribute to bone, muscle mass, and both white and brown adipose tissue (Supplementary Determine 1ac). The homozygous deletion ofRband/orp53byPrx1-Creyielded viable neonates with no detectable developmental defects (data not shown), allowing us to determine the impact ofRband/orp53loss on sarcomagenesis (Determine 1a). ThePrx1-Cre;p53fl/flanimals developed osteosarcoma (62%), rhabdomyosarcomas (15%) and/or undifferentiated sarcomas (12%). In contrast, deletion ofRbalone did not yield sarcomas. However,Rbmutation experienced a profound effect on the tumour spectrum ofPrx1-Cre;p53fl/flmice (Determine 1a,b): deletion of oneRballele increased the frequency of osteosarcomas (to 92%), while mutation of bothRballeles shifted the tumour spectrum away from osteosarcoma (now 18%) and towards hibernomas (91%;Supplementary figure 2). This propensity for brown fat, as opposed to white fat, tumors fits with prior studies showing thatRbloss promotes brown fat over white fat differentiation11,12,13. Genotyping confirmed that this tumour cells experienced undergone Cre-mediated recombination ofRband/orp53(Determine 1c, data not shown). Moreover, it showed that thePrx1-Cre;Rb+/fl;p53fl/fltumours consistently retained the wildtypeRballele (Determine 1c, data not shown). ThusRbacts in a dose dependent manner to modulate the spectrum of tumours arising from p53-deficient, uncommitted mesenchymal stem cells: osteosarcomas predominant in the presence ofRb, whileRbloss strongly favours hibernoma formation. == Determine 1.Rbcooperates withp53and modulates mesenchymal tumor fate in a dosage-dependent manner. == a, Mesenchymal tumor distribution (percentage of animals analyzed up SC-26196 to 24 months of age) forPrx1-Cre;Rband/orp53compound mutant animals.b, H+E staining of representative sarcomas (20 magnification).c, PCR genotyping to detectRbwildtype (wt) and recombined conditional mutant (loxp) alleles in controlRbfl/+;p53fl/+tissues (lane 1) or cell lines derived fromPrx1-Cre;Rbfl/fl;p53fl/fl(DKO) orPrx1-Cre;Rbfl/+;p53fl/flosteosarcomas. Cell lines were cultured for 20 passages prior to genotyping to eliminate stromal cell contribution. Given thatRbloss inp53mutant uncommitted mesenchymal cells disfavours osteosarcoma formation, we also investigated the impact ofRbloss in a bone-committed progenitor. For this, we deletedRband/orp53using theOsx-Cretransgenic14which usesOsterixpromoter sequences to express Cre in the pre-oestoblast (Supplementary Determine 1d). In this model15,Osx-Cre;p53fl/flmice specifically develop osteosarcoma (100%) whileOsx-Cre;Rbfl/fl;p53fl/fldevelop osteosarcoma (53%), hibernomas (46%) and sarcomas (2%). We established cell lines from multiple (3) independentOsx-Cre;p53fl/flandOsx-Cre;Rbfl/fl;p53fl/flosteosarcomas and discovered that the two genotypes Mouse monoclonal to VAV1 have distinct differentiation properties (Determine 2, data not shown). TheRb;p53(DKO) osteosarcoma (OS) cell lines expressed mRNAs that are characteristic of bone and SC-26196 fat differentiation (Determine 2a). Indeed, their expression pattern more closely resembled that of mesenchymal stem cells (MSCs) than main osteoblasts (Supplementary Determine 3). Accordingly, culture in the appropriate differentiation media induced theseDKOcells to adopt either the adipogenic or osteoblastic fate (Determine 2a). In contrast, thep53KOOS cell lines closely resembled pre-osteoblasts based on their gene expression patterns, but these cells were unable to differentiate into either bone or fat (Determine 2aandSupplementary Determine 3). Since this differentiation block occurs in the p53-null OS cell lines, but not p53-deficient main osteoblasts16(Determine 4a), it likely reflects their transformed state. We used thesep53KOOS cells to determine whetherRbloss was sufficient to alter the differentiation potential of blocked pre-oestoblasts by introducing control (shLuc) orRb-specific (shRb) shRNAs. pRb was readily detectable inshLuc-p53KO, but notshRb-p53KO, OS cells (Determine 2b). Strikingly, without addition of differentiation media, pRb knockdown downregulated the bone-specific mRNABspand upregulated the fat regulatorPpar (Determine 2b). Accordingly, theseshRb-p53KOOS cells were now able to differentiate into either bone or fatin vitro(Determine 2c). Moreover, when transplanted into nude SC-26196 mice, theshRb-p53KOOS cells formed more aggressive tumors than the parentalp53KOOS cells, and these were of mixed lineage (fat, bone and undifferentiated sarcomas), in stark contrast to the undifferentiated osteoblastic tumours arising from either controlshLuc-p53KOor parentalp53KOOS cell lines (Determine 2d,Supplementary Determine 4, and data not shown). Thus, pRB loss is sufficient to over-ride the differentiation block of these p53-deficient, tumor cell lines and also expand their fate commitment to include the adipogenic state. == Determine 2.Rbregulates osteosarcoma-cell lineage plasticityin vitroandin vivo. == The differentiation potential of 3 differentOsx-Cre;Rbfl/fl:p53fl/fl(DKO) andOsx-Cre:p53fl/fl(p53KO) OS cell lines was assessed 0, 7, 14, or 21 days after addition of differentiation media.a, Representative staining for: (left lane) alkaline phosphatase prior to differentiation induction; (middle lane) Alizarin Reddish to detect bone mineralization 14.