The SKP2 E3 ligase regulates basal homeostasis and stress-induced regeneration of HSCs

Abstract

Exit from quiescence and reentry into cell cycle is essential for HSC self-renewal and regeneration. Skp2 is the F-box unit of the SCF E3-ligase that targets the CDK inhibitors (CKIs) p21 Cip1 , p27 Kip1 , p57 Kip2 , and p130 for degradation. These CKIs inhibit the G 1 to S-phase transition of the cell cycle, and their deletion results in increased cell proliferation and decreased stem cell self-renewal. Skp2 deletion leads to CKIs stabilization inducing cell-cycle delay or arrest, and conversely, increased Skp2 expression is often found in cancers. Here, we show that SKP2 expression is increased in HSC and progenitors in response to hematopoietic stress from myelosuppression or after transplantation. At steady state, SKP2 deletion decreased the mitotic activity of HSC and progenitors resulting in enhanced HSC quiescence, increased HSC pool size, and maintenance. However, the inability to rapidly enter cell cycle greatly impaired the short-term repopulating potential of SKP2 null HSC and their ability to regenerate after myeloablative stress. Mechanistically, deletion of SKP2 in HSC and progenitors stabilized CKIs in vivo, particularly p27 Kip1 IntroductionThe molecular mechanisms regulating adult hematopoietic stem cell fate decisions during the BM response to stress are not yet well defined. Several molecules have been implicated in the regulation of HSC quiescence and cell-cycle entry in particular cell-cycle regulators such as the cyclin CDK inhibitors (CKIs). 1 CKIs regulate directly the activity of cyclin/CDK complexes, specifically the CIP/KIP family members (p21 Cip1 , p27 Kip1 , p57 Kip2 ), which preferentially inhibit CDK2 activity at the G 1 -S-phase transition. 2,3 Overexpression of each of these CKIs has been shown to induce growth arrest, whereas diminished expression is linked to increased proliferation in progenitors or loss of self-renewal in HSC. [4][5][6] Their regulation occurs predominantly at the posttranslational level and involves proteasome-dependent degradation mediated by SCF complexes. 3,7 The authors of recent reports have indicated that the regulation of proteasomal degradation and protein stability by SCF complexes may contribute to the regulation of HSC self-renewal and differentiation. 8,9 The Skp1-Cullin-F-box (SCF) complex is a multi-unit E3 ubiquitin ligase in which the F-box family of proteins confers substrate specificity. 10 In the SCF Skp2 complex, the F-box SKP2 protein is necessary for ubiquitination and degradation of p27 Kip ,11 p21 Cip1 , 4 and p57 Kip212 and Rb family member p130. 13 SKP2 overexpression causes quiescent cells to enter the cell cycle 14 and is frequently associated with human cancers. 15,16 Conversely, its down-regulation is critical for cell-cycle arrest, 17 and its deletion restricts oncogenicity and induces senescence. 18 SCF SKP2 is undoubtedly the major ubiquitin ligase regulating the abundance of cell-cycle regulatory proteins at the G 1 -S transition.The fact that SKP2 controls the destruction of several CKIs counteracting S-phase entry und...

p57Kip2 and Cancer: Time for a Critical Appraisal

“…Two recent articles (96,97) confirmed the central role played by decreased p57 Kip2 in the exit from quiescence and reentry into the cell cycle, which are essential for HSC self-renewal and regeneration. The first investigation showed that SKP2 expression (and thus p57 Kip2 degradation) is increased in HSC and progenitors in response to hematopoietic stress from myelosuppression or after transplantation (96). The study also suggested a previously unrecognized role for SKP2 in regulating HSC and progenitor expansion and hematopoietic regeneration after stress.…”

“…A subsequent investigation suggested that p57 Kip2 is responsible for the G 0 /G 1 block of HSC and that its decrease is required for S entry (95). Two recent articles (96,97) confirmed the central role played by decreased p57 Kip2 in the exit from quiescence and reentry into the cell cycle, which are essential for HSC self-renewal and regeneration. The first investigation showed that SKP2 expression (and thus p57 Kip2 degradation) is increased in HSC and progenitors in response to hematopoietic stress from myelosuppression or after transplantation (96).…”

Reactive Oxygen Species in Normal and Tumor Stem Cells

“…These defects in ROS high HSCs were associated with an increased activation of p38 MAPK (p38) and mammalian target of rapamycin (mTOR), which could be attenuated by the treatment with an antioxidant, a p38 inhibitor, or rapamycin. Furthermore, incubation of mouse BM HSCs with high concentrations of BSO resulted in a dramatic reduction in HSC clonogenicity (Juntilla et al, 2010; Rodriguez et al, 2011). In addition, increased production of ROS has been found to have a cause and effect relationship with HSC defects in various pathological conditions.…”

Knockdown of SCFSkp2 Function Causes Double-Parked Accumulation in the Nucleus and DNA Re-Replication in Drosophila Plasmatocytes

“…These data suggest that Dup may be the target substrate for the SCF complex being studied, with a secondary target possibly being Cyclin E. Previous research in human cells has shown that SCF Skp2 regulates the degradation of Cdt1 (the homolog of Drosophila Dup) [32]. It has also been shown that the activated SCF Skp2 complex plays a role in murine hematopoiesis, by ubiquitinating proteins necessary for proper cell cycle, such as Cyclin E. There are still many questions to be answered about SCF regulation in blood cells, as some of these results are contradictory [36-38]. …”