Loss of the p16/MTS1 tumor suppressor gene causes E2F-mediated deregulation of essential enzymes of the DNA precursor metabolism

TYMS promotes genomic instability and tumor progression in Ink4a/Arf null background

We previously showed that elevated TYMS exhibits oncogenic properties and promotes tumorigenesis after a long latency, suggesting cooperation with sequential somatic mutations. Here we report the cooperation of ectopic expression of human TYMS with loss of Ink4a/Arf, one of the most commonly mutated somatic events in human cancer. Using an hTS/Ink4a/Arf -/- genetically engineered mouse model we showed that deregulated TYMS expression in Ink4a/Arf null background accelerates tumorigenesis and metastasis. In addition, tumors from TYMS-expressing mice were associated with a phenotype of genomic instability including enhanced double strand DNA damage, aneuploidy and loss of G1/S checkpoint. Downregulation of TYMS in vitro decreased cell proliferation and sensitized tumor cells to antimetabolite chemotherapy. In addition, depletion of TYMS in vivo by TYMS shRNA reduced tumor incidence, delayed tumor progression and prolonged survival in hTS/Ink4a/Arf -/- mice. Our data shows that activation of TYMS in Ink4a/Arf null background enhances uncontrolled cell proliferation and tumor growth, supporting the development of new agents and strategies targeting TYMS to delay tumorigenesis and prolong survival.

Pharmacodynamic Modeling of CDK4/6 Inhibition-Related Biomarkers and the Characterization of the Relationship Between Biomarker Response and Progression-Free Survival in Patients With Advanced Breast Cancer

Identification of a pharmacodynamic (PD) biomarker, which is predictive of the efficacy outcome, is of ultimate interest in drug development. The objectives of the current analyses are to develop the pharmacokinetic (PK)/PD model for biomarkers (thymidine kinase 1 [TK1] in serum and phosphor-retinoblastoma protein [pRb] and Ki67 in skin tissues) related to cyclin-dependent kinase (CDK) 4/6 inhibition by palbociclib and to explore the relationship of the biomarker response with the efficacy end point (progression-free survival). The data used for analysis consisted of extensive sampling of palbociclib PK and longitudinal rich sampling for the PD biomarkers TK1, pRb, and Ki67 in 26 patients. A 2-compartment model was used to describe the PK of palbociclib. A precursor-dependent indirect response PD model was developed to describe the pRb time course, whereas a similar PD model with an additional transit compartment to model the delayed effect on Ki67 and TK1 response was used to describe the Ki67 and TK1 time course. Palbociclib effect on biomarkers was modeled as a maximum inhibition model. A Cox proportional hazard model was used to assess the relationship of progression-free survival with the biomarker response. The PK/PD models adequately described the observed PK of palbociclib and the longitudinal change of pRb, Ki67, and TK1. Palbociclib exposure significantly correlated with the reduction of all 3 biomarkers, and the estimated concentration to achieve 50% inhibition of the synthesis rate values were 45.2, 42.4, 50.2 ng/mL, respectively, for pRb, Ki67, and TK1. The exploratory biomarker-response analyses showed that a longer PFS was associated with lower baseline TK1 and simulated minimum TK1. Such results may warrant further confirmation from future large-scale study. Clinical Trial Registration: NCT02499146.

Opposing roles for p16Ink4a and p19Arf in senescence and ageing caused by BubR1 insufficiency

Expression of p16(Ink4a) and p19(Arf) increases with age in both rodent and human tissues. However, whether these tumour suppressors are effectors of ageing remains unclear, mainly because knockout mice lacking p16(Ink4a) or p19(Arf) die early of tumours. Here, we show that skeletal muscle and fat, two tissues that develop early ageing-associated phenotypes in response to BubR1 insufficiency, have high levels of p16(Ink4a) and p19(Arf). Inactivation of p16(Ink4a) in BubR1-insufficient mice attenuates both cellular senescence and premature ageing in these tissues. Conversely, p19(Arf) inactivation exacerbates senescence and ageing in BubR1 mutant mice. Thus, we identify BubR1 insufficiency as a trigger for activation of the Cdkn2a locus in certain mouse tissues, and demonstrate that p16(Ink4a) is an effector and p19(Arf) an attenuator of senescence and ageing in these tissues.

Basis of FLT as a cell proliferation marker: comparative uptake studies with [3H]thymidine and [3H]arabinothymidine, and cell-analysis in 22 asynchronously growing tumor cell lines

The usefulness of radiolabeled 3'-fluoro-3'-deoxythymidine (FLT), a thymidine derivative with affinity to cytoplasmic thymidine kinase 1 (TK1), as a tumor proliferation marker was evaluated using [3H]FLT and 22 cultured tumor cell lines. Asynchronously growing tumor cells were used for studies to mimic in vivo status of tumors. FLT uptake in each cell line was compared with [3H]thymidine ([3H]Thd) uptake and %S-phase fraction, both known as acceptable markers of proliferation. Uptake of the mitochondrial TK2 specific substrate [3H]arabinothymidine ([3H]AraT) was studied as a reference. Metabolic fate of FLT in tumor cells was also analyzed to elucidate the retention mechanism of FLT. [3H]FLT uptake was mildly correlated with the %S-phase fraction (r=0.76, p<0.0001) and correlated better with [3H]Thd uptake (r=0.88, p<0.0001). In contrast, the TK(2) specific substrate, [3H]AraT, was not significantly correlated with the %S-phase fraction (r=0.19, p=0.39), although it showed some correlation with the [3H]Thd uptake (r=0.47, p<0.05). Over 90% of radioactivity of [3H]Thd was found in the DNA fraction after 60 minutes incubation. In contrast, most of the radioactivity of [3H]FLT was found in the acid-soluble fraction (95%). [3H]FLT incorporation into the DNA fraction was negligible (0.2%). The [3H]AraT was mainly distributed in the acid-soluble fraction (70%) and the DNA fraction (20%). From our results, we concluded that FLT uptake in tumor cells reflects tumor cell proliferation. However, much more convincing validation is needed to clarify the difference between FLT and true substrates for DNA synthesis, like thymidine.

Role of the tuberous sclerosis gene-2 product in cell cycle control. Loss of the tuberous sclerosis gene-2 induces quiescent cells to enter S phase

Tuberous sclerosis is an autosomal dominant disorder characterized by the development of benign growths in many tissues and organs. Linkage analysis revealed two disease-determining genes on chromosome 9 and chromosome 16. The TSC2 gene on chromosome 16 encodes a 1784-amino acid tumor suppressor protein, tuberin, that functions as a GTPase-activating protein for Rap1, a member of the superfamily of Ras-related proteins. By immunoblot analyses, we found TSC2 expression to be high in G0 as well as in early small G1 cells. Analyses after different cell synchronization procedures revealed that TSC2 mRNA and protein expression do not fluctuate throughout the cell cycle. Using inducible expression systems we further demonstrated that TSC2 expression is not affected by overexpression of the mitogenic transcription factor E2F-1 or c-Myc. Nevertheless, antisense inhibition of tuberin expression in logarithmically growing cells markedly decreased the percentage of cells in G1. Furthermore, we found that cells exposed to TSC2 antisense oligonucleotides did not undergo G0 arrest after serum withdrawal. Antisense inhibition of TSC2 expression also induced quiescent G0-arrested fibroblasts to reenter the cell cycle. Our data show for the first time that the absence of tuberin can both induce cells to pass through the G1/S transition of the eukaryotic cell cycle and prevent them from entering a quiescent state. These results have clear implications for the tumor suppressor function of TSC2. We further found that reentry into the cell cycle upon loss of TSC2 is dependent on the activity of the G1 cyclin-dependent kinases (CDKs), Cdk2 or Cdk4. Taken together with our finding that antisense inhibition of TSC2 causes up-regulation of cyclin D1 expression, these results provide the first evidence for a connection between tuberin/Rap1 and the G1 CDK-dependent regulation of the transition from G0/G1 to S phase.

Cellular targets for activation by c-Myc include the DNA metabolism enzyme thymidine kinase

Although a remarkable number of genes has been identified that are either activated or repressed via c-Myc, only few of them obviously contribute to Myc's biological effect--the induction of proliferation. We found that in logarithmically growing cells overexpression of Myc specifically induces thymidine kinase (TK) mRNA expression and enzyme activity, whereas loss of one allele of Myc causes downregulation of this enzyme. We show that activation of Myc triggers high levels of this normally strictly S-phase-regulated DNA metabolism enzyme in serum arrested G0 cells and causes high and constant levels of TK expression throughout the entire ongoing cell cycle. Induction of TK by Myc requires an intact transcriptional activation domain. Myc-induced deregulation of this enzyme is paralleled by alterations of protein binding at the E2F-site of the TK promoter. We further show that cell growth arrest by the cyclin-dependent kinase inhibitor p16 is abrogated by overexpression of Myc and that co-overexpression of p16 cannot inhibit the Myc-induced up-regulation of TK expression. Our data demonstrate TK to be a cellular target of Myc independently of the status of cell proliferation and provide evidence that the transcription factor E2F might be involved in this process.

Specific transformation abolishes cyclin D1 fluctuation throughout the cell cycle

We analysed cyclin D1 mRNA and protein expression in several different cell types after separating these cells according to their different cell cycle phases by centrifugal elutriation. In normal human and rat fibroblasts cyclin D1 expression is high in early to mid G1 and decreases about 6-7 fold before onset of replication. It has been demonstrated that specific transforming events, such as loss of functional retinoblastoma protein, overexpression of c-myc, and transfection with the human papillomavirus oncoproteins E6 and E7 cause transcriptional downregulation of cyclin D1 expression in logarithmically growing cells. We found that such transformed cells exhibit loss of the cell cycle-dependent cyclin D1 fluctuation accompanied with reduced upregulation of cyclin D1 in G1 phase. The data presented here provide the experimental support for a recently suggested model involving the function of the retinoblastoma protein in cyclin D1 cell cycle regulation.

Identification of leukaemic cells in bone marrow and blood samples by a new cytofluorometric assay

The expression of thymidine kinase--an enzyme of the DNA precursor pathway--is strictly regulated during the normal cellular cycle, but is much higher and permanently expressed in malignant growing cells. We used this fact to detect neoplastic cells in samples freshly taken from leukaemia patients and kept frozen in liquid nitrogen until analysis. Using a new cytofluorometric assay for thymidine kinase in single cells, we were able to identify leukaemic cells in a surplus of normal ones. Our results demonstrate the benefits of this assay for leukaemia diagnosis.