Identification of Arabidopsis meiotic cyclins reveals functional diversification among plant cyclin genes.

Meiosis is a modified cell division in which a single S-phase is followed by two rounds of chromosome segregation resulting in the production of haploid gametes. The meiotic mode of chromosome segregation requires extensive remodeling of the basic cell cycle machinery and employment of unique regulatory mechanisms. Cyclin-dependent kinases (CDKs) and cyclins represent an ancient molecular module that drives and regulates cell cycle progression. The cyclin gene family has undergone a massive expansion in angiosperm plants, but only a few cyclins were thoroughly characterized. In this study we performed a systematic immunolocalization screen to identify Arabidopsis thaliana A- and B-type cyclins expressed in meiosis. Many of these cyclins exhibit cell-type-specific expression in vegetative tissues and distinct subcellular localization. We found six A-type cyclins and a single B-type cyclin (CYCB3;1) to be expressed in male meiosis. Mutant analysis revealed that these cyclins contribute to distinct meiosis-related processes. While A2 cyclins are important for chromosome segregation, CYCB3;1 prevents ectopic cell wall formation. We further show that cyclin SDS does not contain a D-box and is constitutively expressed throughout meiosis. Analysis of plants carrying cyclin SDS with an introduced D-box motif determined that, in addition to its function in recombination, SDS acts together with CYCB3;1 in suppressing unscheduled cell wall synthesis. Our phenotypic and expression data provide extensive evidence that multiplication of cyclins is in plants accompanied by functional diversification.

The transcription factor c-Jun protects against sustained hepatic endoplasmic reticulum stress thereby promoting hepatocyte survival.

UNLABELLED: Endoplasmic reticulum (ER) stress due to accumulation of hepatoviral or misfolded proteins is increasingly recognized as an important step in the pathogenesis of inflammatory, toxic, and metabolic liver diseases. ER stress results in the activation of several intracellular signaling pathways including Jun N-terminal kinase (JNK). The AP-1 (activating protein 1) transcription factor c-Jun is a prototypic JNK target and important regulator of hepatocyte survival, proliferation, and liver tumorigenesis. Because the functions of c-Jun during the ER stress response are poorly understood, we addressed this issue in primary hepatocytes and livers of hepatocyte-specific c-Jun knockout mice. ER stress was induced pharmacologically in vitro and in vivo and resulted in a rapid and robust induction of c-Jun protein expression. Interestingly, ER-stressed hepatocytes lacking c-Jun displayed massive cytoplasmic vacuolization due to ER distension. This phenotype correlated with exacerbated and sustained activation of canonical ER stress signaling pathways. Moreover, sustained ER stress in hepatocytes lacking c-Jun resulted in increased cell damage and apoptosis. ER stress is also a strong inducer of macroautophagy, a cell-protective mechanism of self-degradation of cytoplasmic components and organelles. Interestingly, autophagosome numbers in response to ER stress were reduced in hepatocytes lacking c-Jun. To further validate these findings, macroautophagy was inhibited chemically in ER-stressed wildtype hepatocytes, which resulted in cytoplasmic vacuolization and increased cell damage closely resembling the phenotypes observed in c-Jun-deficient cells. CONCLUSION: Our findings indicate that c-Jun protects hepatocytes against excessive activation of the ER stress response and subsequent cell death and provide evidence that c-Jun functionally links ER stress responses and macroautophagy.

A dual role for autophagy in a murine model of lung cancer.

Autophagy is a mechanism by which starving cells can control their energy requirements and metabolic states, thus facilitating the survival of cells in stressful environments, in particular in the pathogenesis of cancer. Here we report that tissue-specific inactivation of Atg5, essential for the formation of autophagosomes, markedly impairs the progression of KRas(G12D)-driven lung cancer, resulting in a significant survival advantage of tumour-bearing mice. Autophagy-defective lung cancers exhibit impaired mitochondrial energy homoeostasis, oxidative stress and a constitutively active DNA damage response. Genetic deletion of the tumour suppressor p53 reinstates cancer progression of autophagy-deficient tumours. Although there is improved survival, the onset of Atg5-mutant KRas(G12D)-driven lung tumours is markedly accelerated. Mechanistically, increased oncogenesis maps to regulatory T cells. These results demonstrate that, in KRas(G12D)-driven lung cancer, Atg5-regulated autophagy accelerates tumour progression; however, autophagy also represses early oncogenesis, suggesting a link between deregulated autophagy and regulatory T cell controlled anticancer immunity.