Cyclin C was cloned as a growth-promoting G1 cyclin, and was also shown to regulate gene transcription. Here we report that in vivo cyclin C acts as a haploinsufficient tumor suppressor, by controlling Notch1 oncogene levels. Cyclin C activates an “orphan” CDK19 kinase, as well as CDK8 and CDK3. These cyclin C-CDK complexes phosphorylate Notch1 intracellular domain (ICN1) and promote ICN1 degradation. Genetic ablation of cyclin C blocks ICN1 phosphorylation in vivo, thereby elevating ICN1 levels in cyclin C-knockout mice. Cyclin C ablation or heterozygosity collaborate with other oncogenic lesions and accelerate development of T-cell-acute lymphoblastic leukemia (T-ALL). Furthermore, the cyclin C gene is heterozygously deleted in a significant fraction of human T-ALL, and these tumors express reduced cyclin C levels. We also describe point mutations in human T-ALL that render cyclin C-CDK unable to phosphorylate ICN1. Hence, tumor cells may develop different strategies to evade cyclin C inhibitory function.

SummaryWe investigated the outcome for children and young people with Early T-precursor acute lymphoblastic leukaemia (ETP-ALL), a recently described poor prognosis subgroup of T-ALL, treated on a contemporary protocol, UKALL 2003. After a median follow-up of 4 years and 10 months, the ETP sub-group, representing 16% of T-ALL patients, had non-significantly inferior 5-year event-free survival (76Á7% vs. 84Á6%, P = 0Á2) and overall survival (82Á4% vs. 90Á9%, P = 0Á1), and a higher relapse rate (18Á6% vs. 9Á6%, P = 0Á1) compared to typical T-ALL. ETP-ALL has an intermediate risk outcome, which does not warrant experimental treatment or first remission allogeneic transplant for the group universally.

Acquiring a mechanistic understanding of the processes underlying the renal clearance of drug molecules in man has been hampered by a lack of robust in vitro models of human proximal tubules. Several human renal epithelial cell lines derived from the renal cortex are available, but few have been characterised in detail in terms of transporter expression. This includes the HK-2 proximal tubule cell line, which has been used extensively as a model of nephrotoxicity. The aim of this study was to investigate the expression and function of drug transporters in HK-2 cells and their suitability as an in vitro model of the human proximal tubule. qPCR showed no mRNA expression of the SLC22 transporter family (OAT1, OAT3, OCT2) in HK-2 cells compared to renal cortex samples. In contrast, SLC16A1 (MCT1), which is important in the uptake of monocarboxylates, and SLCO4C1 (OATP4C1) were expressed in HK-2 cells. The functional expression of these transporters was confirmed by uptake studies using radiolabelled prototypic substrates DL-lactate and digoxin, respectively. The mRNA expression of apical membrane efflux transporters ABCB1 (MDR1) and several members of the ABCC family (multidrug resistance proteins, MRPs) was shown by qPCR. ABCG1 (BCRP) was not detected. The efflux of Hoechst 33342, a substrate for MDR1, was blocked by MDR1 inhibitor cyclosporin A, suggesting the functional expression of this transporter. Similarly, the efflux of the MRP-specific fluorescent dye glutathione methylfluorescein was inhibited by the MRP inhibitor MK571. Taken together, the results of this study suggest that HK-2 cells are of limited value as an in vitro model of drug transporter expression in the human proximal tubule.

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