Cyclin L2, a Novel RNA Polymerase II-associated Cyclin, Is Involved in Pre-mRNA Splicing and Induces Apoptosis of Human Hepatocellular Carcinoma Cells

Lianjun Yang,Nan Li,Chunmei Wang,Yizhi Yu,Minghui Zhang,Xuetao Cao

doi:10.1074/jbc.m312895200

Lianjun Yang, Nan Li + Show 4 more

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https://doi.org/10.1074/jbc.m312895200

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Abstract

We report the cloning and functional characterization of human cyclin L2, a novel member of the cyclin family. Human cyclin L2 shares significant homology to cyclin L1, K, T1, T2, and C, which are involved in transcriptional regulation via phosphorylation of the C-terminal domain of RNA polymerase II. The cyclin L2 protein contains an N-terminal "cyclin box" and C-terminal dipeptide repeats of alternating arginines and serines, a hallmark of the SR family of splicing factors. A new isoform and the mouse homologue of human cyclin L2 have also been cloned in this study. Human cyclin L2 is expressed ubiquitously in normal human tissues and tumor cells. We show here that cyclin L2 co-localizes with splicing factors SC-35 and 9G8 within nuclear speckles and that it associates with hyperphosphorylated, but not hypophosphorylated, RNA polymerase II and CDK p110 PITSLRE kinase via its N-terminal cyclin domains. It can also associate with the SC-35 and 9G8 through its RS repeat region. Recombinant cyclin L2 protein can stimulate in vitro mRNA splicing. Overexpression of human cyclin L2 suppresses the growth of human hepatocellular carcinoma SMMC 7721 cells both in vitro and in vivo, inducing cellular apoptosis. This process involves up-regulation of p53 and Bax and decreased expression of Bcl-2. The data suggest that cyclin L2 represents a new member of the cyclin family, which might regulate the transcription and RNA processing of certain apoptosis-related factors, resulting in tumor cell growth inhibition and apoptosis.

Highlights

Cyclins are key regulatory proteins that complex with and activate cyclin-dependent kinase (CDK)1 subunits [1,2,3,4], playing pivotal roles in the regulation of cell cycle progression [5]
Based on the sequence similarity with known cyclins, especially cyclin L1, the conservation of cyclin features, and its capacity of associating with RNAPol II and splicing factors, the novel protein was designated as human cyclin L2 (CCNL2), according to the cyclin family nomenclature system recommended by HUGO
To explore the possibility that human cyclin L2 may associate with RNA polymerase II or other CDKs and splicing factors and to delineate the domain(s) of cyclin L2 required for these interactions, several domain-negative constructs tagged by myc-6His were overexpressed in SMMC 7721 cells and immunoprecipitated with anti-Myc antibody crosslinked with protein G-Sepharose beads

Summary

Introduction

Cyclins are key regulatory proteins that complex with and activate cyclin-dependent kinase (CDK)1 subunits [1,2,3,4], playing pivotal roles in the regulation of cell cycle progression [5]. The data suggest that cyclin L2 represents a new member of the cyclin family, which might regulate the transcription and RNA processing of certain apoptosis-related factors, resulting in tumor cell growth inhibition and apoptosis.

Results

Conclusion