Abstract
Homeodomain interacting protein kinase-2 (HIPK2) is an evolutionary conserved kinase that modulates several key molecular pathways to restrain tumor growth and induce p53-depending apoptotic cell-death in response to anticancer therapies. HIPK2 silencing in cancer cells leads to chemoresistance and cancer progression, in part due to p53 inhibition. Recently, hyperglycemia has been shown to reduce p53 phosphorylation at serine 46 (Ser46), the target residue of HIPK2, thus impairing p53 apoptotic function. Here we asked whether hyperglycemia could, upstream of p53, target HIPK2. We focused on the effect of high glucose (HG) on HIPK2 protein stability and the underlying mechanisms. We found that HG reduced HIPK2 protein levels, therefore impairing HIPK2-induced p53 apoptotic activity. HG-triggered HIPK2 protein downregulation was rescued by both proteasome inhibitor MG132 and by protein phosphatase inhibitors Calyculin A (CL-A) and Okadaic Acid (OA). Looking for the phosphatase involved, we found that protein phosphatase 2A (PP2A) induced HIPK2 degradation, as evidenced by directly activating PP2A with FTY720 or by silencing PP2A with siRNA in HG condition. The effect of PP2A on HIPK2 protein degradation could be in part due to hypoxia-inducible factor-1 (HIF-1) activity which has been previously shown to induce HIPK2 proteasomal degradation through several ubiquitin ligases. Validation analysed performed with HIF-1α dominant negative or with silencing of Siah2 ubiquitin ligase clearly showed rescue of HG-induced HIPK2 degradation. These findings demonstrate how hyperglycemia, through a complex protein cascade, induced HIPK2 downregulation and consequently impaired p53 apoptotic activity, revealing a novel link between diabetes/obesity and tumor resistance to therapies.
Highlights
Homeodomain interacting protein kinase 2 (HIPK2) is a nuclear serine/threonine kinase that is considered a central switch in driving cancer cells toward apoptotic cell death upon genotoxic stress [1]
We previously showed that high glucose (HG) reduces p53 phosphorylation at serine 46 (Ser46) that can be rescued by the use of Calyculin A (CL-A) [23], a cell-permeable phosphatase inhibitor which has been shown to inhibit protein phosphatase A2 (PP2A) and enhance ionizing radiation-induced p53Ser46 phosphorylation [24]
To assess whether HG could affect Homeodomain interacting protein kinase-2 (HIPK2) cellular localization, HIPK2-GFP protein was overexpressed in human embryo kidney (HEK)-293 cells in dose and time conditions that did not modify cell viability and, twentyfour hours after transfection, cells were transferred in low glucose (LG) and HG conditions
Summary
Homeodomain interacting protein kinase 2 (HIPK2) is a nuclear serine/threonine kinase that is considered a central switch in driving cancer cells toward apoptotic cell death upon genotoxic stress [1]. HIPK2 is degraded by the RING family ligase seven in absentia homolog-1 (Siah1) while, in response to DNA damage, the HIPK2/ Siah complex is disrupted by the ATM/ATR-dependent phosphorylation of Siah resulting in HIPK2 stabilization and activation [12, 13]. An intriguing regulatory circuitry between MDM2 and HIPK2/p53 axis revealed that sublethal DNA damage induces HIPK2 inhibition by a protein degradation mechanism involving p53-induced MDM2 activity [14]. Other E3 ubiquitin ligases involved in HIPK2 degradation are induced by hypoxia, a hallmark of tumor progression and failure of tumor therapies [16]. Under hypoxia, the RING family ligase Siah is activated to increase HIPK2/Siah interaction, by a still unknown mechanism, that induces HIPK2 degradation [22]
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