Wnt Down, Tumors Wind Up?

In mouse intestinal tumors induced by the inhibition of APC, the restoration of APC function causes complete tumor regression with normal differentiation and return of stem cell function irrespective of whether tumors also carried mutations in Kras and p53. These findings by Dow et al. validate the Wnt pathway as an exquisite target for intervention

Pre-TCR Signaling and Inactivation of p53 Induces Crucial Cell Survival Pathways in Pre-T Cells

AbstractSignaling through the pre-TCR is essential for early T cell development and is severely impaired in mice lacking the CD3γ chain of the pre-TCR. We here address the molecular mechanisms underlying this defect. Impaired pre-TCR signaling is shown to be associated with a profound increase in the number of apoptotic CD4−CD8− (DN) thymocytes. Introduction of p53 deficiency into CD3γ-deficient mice completely reverses the cell survival defect in CD3γ-deficient DN thymocytes and rescues the block in pre-T cell differentiation. In addition, the CD4+CD8+ (DP) compartment is expanded to its normal size. These findings suggest that the pre-TCR regulates progression through the DNA-damage checkpoint of the DN to DP transition by inactivating p53

The Loss of PTEN Allows TCR αβ Lineage Thymocytes to Bypass IL-7 and Pre-TCR–mediated Signaling

The phosphatase and tensin homologue deleted on chromosome 10 (PTEN) negatively regulates cell survival and proliferation mediated by phosphoinositol 3 kinases. We have explored the role of the phosphoinositol(3,4,5)P3-phosphatase PTEN in T cell development by analyzing mice with a T cell–specific deletion of PTEN. Ptenflox/floxLck-Cre mice developed thymic lymphomas, but before the onset of tumors, they showed normal thymic cellularity. To reveal a regulatory role of PTEN in proliferation of developing T cells we have crossed PTEN-deficient mice with mice deficient for interleukin (IL)-7 receptor and pre–T cell receptor (TCR) signaling. Analysis of mice deficient for Pten and CD3γ; Pten and γc; or Pten, γc, and Rag2 revealed that deletion of PTEN can substitute for both IL-7 and pre-TCR signals. These double- and triple-deficient mice all develop normal levels of CD4CD8 double negative and double positive thymocytes. These data indicate that PTEN is an important regulator of proliferation of developing T cells in the thymus

Contributions of the T Cell Receptor–associated CD3γ–ITAM to Thymocyte Selection

The immunoreceptor tyrosine-based activation motifs (ITAMs) in the CD3 chains associated with the T cell receptor (TCR) are crucial for TCR signaling. To probe the role of the CD3γ–ITAM in T cell development, we created knock-in mice in which the CD3γ chain of the TCR complex is replaced by a mutant signaling-deficient CD3γ chain, lacking the CD3γ–ITAM. This mutation results in considerable impairment in positive selection in the polyclonal TCR repertoire. When CD3γ–ΔITAM mice are crossed to mice expressing transgenic F5 TCRs, their thymocytes are completely unable to perform positive selection in vivo in response to intrathymic ligands. Also, the in vitro positive selection response of double-positive (DP) thymocytes with F5–CD3γ–ΔITAM mutant receptors to their agonist ligand and many of its variants is severely impaired or abrogated. Yet, the binding and dissociation constants of agonist ligands for the F5 receptor are not affected by the CD3γ–ΔITAM mutation. Furthermore, DP thymocytes with mutant receptors can respond to agonist ligand with normal antigen sensitivity and to normal levels, as shown by their ability to induce CD69 up-regulation, TCR down-regulation, negative selection, and ZAP70 and c-Jun NH2-terminal kinase activation. In sharp contrast, induction of extracellular signal-regulated kinase (ERK) activation and linker for activation of T cells (LAT) phosphorylation are severely impaired in these cells. Together, these findings underscore that intrinsic properties of the TCR–CD3 complex regulate selection at the DP checkpoint. More importantly, this analysis provides the first direct genetic evidence for a role of the CD3γ–ITAM in TCR-driven thymocyte selection

Identification of an alternative triglyceride biosynthesis pathway

Triacylglycerols (TAGs) are the main source of stored energy in the body, providing an important substrate pool for mitochondrial beta-oxidation. Imbalances in the amount of TAGs are associated with obesity, cardiac disease and various other pathologies 1,2. In humans, TAGs are synthesized from excess, coenzyme A-conjugated fatty acids by diacylglycerol O-acyltransferases (DGAT1 and DGAT2) 3. In other organisms, this activity is complemented by additional enzymes 4, but whether such alternative pathways exist in humans remains unknown. Here we disrupt the DGAT pathway in haploid human cells and use iterative genetics to reveal an unrelated TAG-synthesizing system composed of a protein we called DIESL (also known as TMEM68, an acyltransferase of previously unknown function) and its regulator TMX1. Mechanistically, TMX1 binds to and controls DIESL at the endoplasmic reticulum, and loss of TMX1 leads to the unconstrained formation of DIESL-dependent lipid droplets. DIESL is an autonomous TAG synthase, and expression of human DIESL in Escherichia coli endows this organism with the ability to synthesize TAG. Although both DIESL and the DGATs function as diacylglycerol acyltransferases, they contribute to the cellular TAG pool under specific conditions. Functionally, DIESL synthesizes TAG at the expense of membrane phospholipids and maintains mitochondrial function during periods of extracellular lipid starvation. In mice, DIESL deficiency impedes rapid postnatal growth and affects energy homeostasis during changes in nutrient availability. We have therefore identified an alternative TAG biosynthetic pathway driven by DIESL under potent control by TMX1. </p

A Context-Specific Role for Retinoblastoma Protein-Dependent Negative Growth Control in Suppressing Mammary Tumorigenesis

The ability to respond to anti-growth signals is critical to maintain tissue homeostasis and loss of this negative growth control safeguard is considered a hallmark of cancer. Negative growth regulation generally occurs during the G0/G1 phase of the cell cycle, yet the redundancy and complexity among components of this regulatory network has made it difficult to discern how negative growth cues protect cells from aberrant proliferation.The retinoblastoma protein (pRB) acts as the final barrier to prevent cells from entering into the cell cycle. By introducing subtle changes in the endogenous mouse Rb1 gene (Rb1(ΔL)), we have previously shown that interactions at the LXCXE binding cleft are necessary for the proper response to anti-growth signals such as DNA damage and TGF-β, with minimal effects on overall development. This disrupts the balance of pro- and anti-growth signals in mammary epithelium of Rb1(ΔL/ΔL) mice. Here we show that Rb1(ΔL/ΔL) mice are more prone to mammary tumors in the Wap-p53(R172H) transgenic background indicating that negative growth regulation is important for tumor suppression in these mice. In contrast, the same defect in anti-growth control has no impact on Neu-induced mammary tumorigenesis.Our work demonstrates that negative growth control by pRB acts as a crucial barrier against oncogenic transformation. Strikingly, our data also reveals that this tumor suppressive effect is context-dependent