An inducible autoregulatory loop between HIPK2 and Siah2 at the apex of the hypoxic response

Abstract

Oxygen deprivation (hypoxia) results in reprogrammed gene expression patterns that induce multifaceted cellular responses. Here we identify a regulated interaction between the serine/threonine kinase HIPK2 and the ubiquitin E3 ligase Siah2 as a mechanism controlling the hypoxic response. Under normoxic conditions, several mechanisms ensure HIPK2 stability: only a fraction of HIPK2 is found in association with Siah2, whereas HIPK2-mediated phosphorylation of this E3 ligase at positions 26, 28 and 68 weakens mutual binding and destabilizes its phosphorylated interaction partner. Hypoxic conditions allow a markedly increased HIPK2/Siah2 interaction and result in efficient polyubiquitylation and proteasomal degradation of the kinase. Accordingly, hypoxia-induced HIPK2 elimination is markedly reduced in Siah2-deficient cells. As HIPK2 has an important role as a negative regulator of gene expression, its elimination from promoter-associated repressor complexes allows the induction of a substantial fraction of hypoxia-induced genes.

MYCN Sensitizes Human Neuroblastoma to Apoptosis by HIPK2 Activation through a DNA Damage Response

“…In particular, we showed that this proapoptotic phenotype of MYCN-overexpressing cells also required p53 phosphorylation at serine 46 and were associated to a more pronounced activation of proapoptotic target genes compared with those involved in cell cycle arrest, such as the p21 WAF1 , in keeping with the current understanding of p53 activity (32,33). Among a few kinases able to phosphorylate p53 S46 , we focused our attention on HIPK2, a protein required for apoptosis induced by neurotrophin deprivation in developing sympathetic neurons (45), whose role in p53 S46 phosphorylation and p53-dependent apoptosis has clearly been established (26,36 (37)(38)(39)(40). DNA damage was shown to promote HIPK2 accumulation through an ATM/ATRdependent inhibition of its degradation by Siah1 and/or WSB1 ubiquitin ligases (38,40).…”

“…3D), suggesting that HIPK2 induction by MYCN might occur at the level of protein stability. HIPK2 expression is largely controlled via proteasomal degradation by several E3 ubiquitin ligases (37)(38)(39)(40). Importantly, in response to DNA damage, the ATM and ATR kinases were shown to promote HIPK2 accumulation by inhibiting its ubiquitin ligases Siah1 and WSB1 (38,40).…”

Regulation of p53 activity by HIPK2: molecular mechanisms and therapeutical implications in human cancer cells

“…Siah-2 and HIPK2 show a mutual regulation including HIPK2-mediated Siah-2 phosphorylation at positions 26, 28 and 68, which weakens mutual binding between both the proteins and destabilizes its phosphorylated interaction partner. Hypoxic condition triggers increased HIPK2/ Siah-2 interaction, which occurs by a still unknown mechanism but that induces HIPK2 polyubiquitination (Calzado et al, 2009a). Accordingly, hypoxia-induced HIPK2 elimination is markedly reduced in Siah-2-deficient cells.…”

“…HIPK2 is downregulated by RING family ligase seven in absentia homolog-2 (Siah2) during hypoxia (Calzado et al, 2009a). Siah-2 and HIPK2 show a mutual regulation including HIPK2-mediated Siah-2 phosphorylation at positions 26, 28 and 68, which weakens mutual binding between both the proteins and destabilizes its phosphorylated interaction partner.…”

“…HIPK2 downregulation during hypoxia strongly reduces ADR-induced p53Ser46 phosphorylation, moreover, it allows the HIPK2 elimination from promoter-associated repressor complexes with induction of a substantial fraction of hypoxia-induced genes such as PDK1, TGFB3, CYP1B1 (cytochrome P450) (Calzado et al, 2009a) and hypoxia-induced factor (HIF)-1a (Nardinocchi et al, 2009). Other ubiquitin ligases involved in HIPK2 downregulation may take place during hypoxia including WD40-repeat/SOCS box protein WSB-1 (Choi et al, 2008), which is a novel predicted HIF1 target gene, as identified by an integrative genomic approach, upregulated upon hypoxia (Benita et al, 2009); and the RING family member Siah-1, originally identified along with Siah-2 as HIPK2-interacting protein (Winter et al, 2009;Calzado et al, 2009a). As for WSB-1 and Siah-2, Siah-1 depletion by siRNA partially inhibits hypoxia-induced HIPK2 degradation (Moehlenbrink et al, 2010).…”