Control of Cell Growth and Proliferation by the Tribbles Pseudokinase: Lessons from Drosophila.

The Tribbles (Trib) family of pseudokinase proteins regulate cell growth, proliferation, and differentiation during normal development and in response to environmental stress. Mutations in human Trib isoforms (Trib1, 2, and 3) have been associated with metabolic disease and linked to leukemia and the formation of solid tumors, including melanomas, hepatomas, and lung cancers. Drosophila Tribbles (Trbl) was the first identified member of this sub-family of pseudokinases and shares a conserved structure and similar functions to bind and direct the degradation of key mediators of cell growth and proliferation. Common Trib targets include Akt kinase (also known as protein kinase B), C/EBP (CAAT/enhancer binding protein) transcription factors, and Cdc25 phosphatases, leading to the notion that Trib family members stand athwart multiple pathways modulating their growth-promoting activities. Recent work using the Drosophila model has provided important insights into novel facets of conserved Tribbles functions in stem cell quiescence, tissue regeneration, metabolism connected to insulin signaling, and tumor formation linked to the Hippo signaling pathway. Here we highlight some of these recent studies and discuss their implications for understanding the complex roles Tribs play in cancers and disease pathologies.

A Doubletime Nuclear Localization Signal Mediates an Interaction with Bride of Doubletime to Promote Circadian Function.

Doubletime (DBT) has an essential circadian role in Drosophila melanogaster because it phosphorylates Period (PER). To determine if DBT antagonism can produce distinct effects in the cytosol and nucleus, forms of a dominant negative DBT(K/R) with these 2 alternative localizations were produced. DBT has a putative nuclear localization signal (NLS), and mutation of this signal confers cytosolic localization of DBT in the lateral neurons of Drosophila clock cells in the brain. By contrast, addition of a strong NLS domain (e.g., SV40 NLS) to DBT's C terminus leads to more nuclear localization. Expression of DBT(K/R) with the mutated NLS (DBT(K/R) NLS(-)) using a timGAL4 driver does not alter the circadian period of locomotor activity, and the daily oscillations of PER detected by immunoblot and immunofluorescence persist, like those of wild-type flies. By contrast, expression of DBT(K/R) with the strong NLS (DBT(K/R) stNLS) using the timGAL4 driver lengthens period more strongly than DBT(K/R), with damped oscillations of PER phosphorylation and localization. Both DBT(K/R) and DBT(WT) without the NLS fail to interact with Bride of Doubletime (BDBT) protein, which is related to FK506-binding proteins and shown to interact with DBT to enhance its circadian function. This result suggests that the DBT(K/R) NLS(-) has lost its dominant negative property because it does not form normal clock protein complexes. DBT(WT) proteins with the same changes (NLS(-) and stNLS) also produce equivalent changes in localization that do not produce opposite period phenotypes. Additionally, a DBT(K/R) protein with both the stNLS and NLS(-) mutation does not affect circadian period, although it is nuclear, demonstrating that the lack of a dominant negative for the DBT(K/R) NLS(-) is not due to failure to localize to nuclei. Finally, bdbt RNAi increases the cytosolic localization of DBT(K/R) but not of DBT(WT), suggesting a role for BDBT in DBT kinase-dependent nuclear localization of DBT.

Unfavorable therapeutic index of cisplatin/gemcitabine/vinorelbine in advanced non-small-cell lung cancer.

In view of favorable reports with the 3-drug combination of PGV (cisplatin/gemcitabine/vinorelbine), this multicenter phase II study evaluated the therapeutic index of PGV in patients with advanced non-small-cell lung cancer (NSCLC). Thirty-two patients with stage IV NSCLC and 1 with stage IIIB were studied. There were 23 men and 10 women, with a median age of 63 years (range, 38-80 years). Twelve patients had a performance status (PS) of 0, and 21 patients had a PS of 1. Treatment consisted of cisplatin 50 mg/m2, gemcitabine 1000 mg/m2, and vinorelbine 25 mg/m2 all given intravenously on days 1 and 8, in 21-day cycles. Fifteen patients (45%; 95% confidence interval (CI), 28%-64%) achieved a partial response. Median response duration was 3 months (range, 1-9 months). The median and 1-year survival rates were 9.4 months and 39% (95% CI, 23%-58%), respectively. The median number of cycles was 4. Only 3 patients (9%) completed treatment without regimen modifications. Median dose intensity (% planned) was cisplatin 24 mg/m2/week (72%), gemcitabine 483 mg/m2/week (72%), and vinorelbine 12 mg/m2/week (72%). Toxicities were predominantly hematologic, with grade 3/4 neutropenia (67%), febrile neutropenia (21%), and thrombocytopenia (67%). There were 3 (9%) treatment-related deaths due to neutropenic complications. This study confirms the substantial antineoplastic activity of PGV. We observed a high rate of severe hematologic toxicity, especially febrile neutropenia, despite a lower delivered dose intensity of PGV. Given these results, PGV appears to offer no therapeutic advantage to current doublet regimens.