An Alternative Splice Variant of HIPK2 with Intron Retention Contributes to Cytokinesis.

Veronica Gatti,Silvia Matteoni,Maria Grazia Diodoro,Ilaria Virdia,Cinzia Rinaldo,Giuliana Di Rocco,Manuela Ferrara,Silvia Soddu,Simona Di Martino,Laura Monteonofrio

doi:10.3390/cells9020484

Abstract

HIPK2 is a DYRK-like kinase involved in cellular stress response pathways, development, and cell division. Two alternative splice variants of HIPK2, HIPK2-FL and HIPK2-Δe8, have been previously identified as having different protein stability but similar functional activity in the stress response. Here, we describe one additional HIPK2 splice variant with a distinct subcellular distribution and functional activity in cytokinesis. This novel splice variant lacks the last two exons and retains intron13 with a stop codon after 89 bp of the intron, generating a short isoform, HIPK2-S, that is detectable by 2D Western blots. RT-PCR analyses of tissue arrays and tumor samples show that HIPK2-FL and HIPK2-S are expressed in normal human tissues in a tissue-dependent manner and differentially expressed in human colorectal and pancreatic cancers. Gain- and loss-of-function experiments showed that in contrast to HIPK2-FL, HIPK2-S has a diffuse, non-speckled distribution and is not involved in the DNA damage response. Rather, we found that HIPK2-S, but not HIPK2-FL, localizes at the intercellular bridge, where it phosphorylates histone H2B and spastin, both required for faithful cell division. Altogether, these data show that distinct human HIPK2 splice variants are involved in distinct HIPK2-regulated functions like stress response and cytokinesis.

Highlights

HIPK2 (Homeodomain-Interacting Protein Kinase 2) is a Y-regulated S/T kinase [1,2] originally identified for its capacity to interact with homeodomain transcription factors [3]
Increased HIPK2 expression with or without gene amplification has been observed in pilocytic astrocytoma [34,35], cervical [36], and renal carcinomas [37], while HIPK2-mediated protection against genotoxic insults has been found in NRF2-overexpressing tumor cells [38]
Together with the already described ∆e8 variant (NM_001113239), we obtained various RNA transcripts spreading into intron 13, while no other HIPK2 exon shows ESTs protruding from exon boundaries (Figure S1a)

Summary

Introduction

HIPK2 (Homeodomain-Interacting Protein Kinase 2) is a Y-regulated S/T kinase [1,2] originally identified for its capacity to interact with homeodomain transcription factors [3]. HIPK2’s function as a tumor suppressor has been supported by several pieces of evidence, including the HIPK2 pro-apoptotic and growth-arresting activities through p53-dependent and -independent mechanisms [19,20,21,22,23,24,25,26], the observation that the Hipk gene works as a haploinsufficient tumor suppressor in mice [27], and the identification of a few mechanisms of HIPK2 inactivation in human cancers, such as reduced expression and loss of heterozygosity [28,29,30,31]. Strong proproliferative activity of HIPK2 has been related to kidney and skin fibrosis, in which excessive fibroblast proliferation is coupled with the deposition of extracellular matrix and epithelial-to-mesenchymal transition [39,40]

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