GOING TO WAR ON COVID19: MOBILIZING AN ACADEMIC NEPHROLOGY GROUP PRACTICE

10.1111/nep.13753Nephrolog

SUMO modification of Stra13 is required for repression of cyclin D1 expression and cellular growth arrest.

Stra13, a basic helix-loop-helix (bHLH) transcription factor is involved in myriad biological functions including cellular growth arrest, differentiation and senescence. However, the mechanisms by which its transcriptional activity and function are regulated remain unclear. In this study, we provide evidence that post-translational modification of Stra13 by Small Ubiquitin-like Modifier (SUMO) dramatically potentiates its ability to transcriptionally repress cyclin D1 and mediate G(1) cell cycle arrest in fibroblast cells. Mutation of SUMO acceptor lysines 159 and 279 located in the C-terminal repression domain has no impact on nuclear localization; however, it abrogates association with the co-repressor histone deacetylase 1 (HDAC1), attenuates repression of cyclin D1, and prevents Stra13-mediated growth suppression. HDAC1, which promotes cellular proliferation and cell cycle progression, antagonizes Stra13 sumoylation-dependent growth arrest. Our results uncover an unidentified regulatory axis between Stra13 and HDAC1 in progression through the G(1)/S phase of the cell cycle, and provide new mechanistic insights into regulation of Stra13-mediated transcriptional repression by sumoylation

Sumoylation is essential for Stra13-dependent growth inhibition.

<p>(A) Lysates of NIH3T3 cells transfected with Myc-Stra13, Stra13 2KR and SENP1 were immunoblotted with anti-Myc antibody. (B–C) After selection, colony assays were performed and colonies were stained with crystal violet. Representative plates are shown (B). The mean relative absorbance after extraction of crystal violet stain from plates in shown in C. Error bars indicate mean ±SD.</p

Stra13 is sumoylated.

<p>(A) Schematic representation of the Stra13 domain structure (upper panel). The basic and HLH domains are shown along with three α-helices in the C-terminal repression domain. Potential sumoylation acceptor lysines at 159 and 279 (K159 and K279) are indicated. Numbers indicate amino acid residues in the mouse Stra13 cDNA. Alignment of Stra13 cDNA from various species revealed a highly conserved SUMO consensus motif IKQE, and a somewhat less conserved motif AKHE that are highlighted. K159 and K279 are indicated by arrowheads (lower panel). (B) Cells were co-transfected with Myc-Stra13, SUMO1 and SENP1 as indicated. Lysates were immunoprecipitated with Myc-agarose beads followed by immunoblotting with anti-SUMO1 antibody. Input shows expression of Stra13 using anti-Myc antibody. β-actin served as a loading control. (C) Cells were co-transfected with Myc-Stra13, or point mutants (Stra13 K279R, Stra13 K159R, Stra13 2KR) together with SUMO1. Cell lysates were immunoprecipitated with Myc-agarose beads and the immunoprecipitates were subjected to western blotting with anti-SUMO1 antibody. (D) Myc-Stra13 and SUMO1 were expressed along with Flag-PIAS1, PIAS3, PIASxα, or PIASy as indicated. Lysates were immunoprecipitated with Myc- agarose beads followed by western blotting with anti-SUMO1 antibody. Lysates (input) were probed for Stra13 and PIAS.</p

HDAC1 regulates Stra13 sumoylation.

<p>(A) Cells were co-transfected with plasmids expressing Flag-HDAC1 and Myc-Stra13 or Stra13 2KR. 48 hr after transfection, lysates were immunoprecipitated with Myc-agarose beads and analyzed for interaction by western blotting with anti-Flag antibody. (B) Cells were co-transfected with constructs encoding Myc-Stra13, Flag-HDAC1 and SUMO1. TSA was added at a concentration of 300 nM. Cell lysates were immunoprecipitated with Myc-agarose beads followed by western blotting with anti-SUMO1 antibody. (C–D) NIH3T3 cells were left untreated (−) or treated (+) with TSA. Endogenous Stra13 was immunoprecipitated and analyzed for sumoylation (C), as well as for association with HDAC1 (D). (E–F) Down-regulation of endogenous HDAC1 expression in siHDAC1 cells compared to control cells was examined by western blotting (E). Control and siHDAC1 cells were transfected with Stra13 and SUMO1. Lysates were immunoprecipitated as indicated and analyzed with anti-SUMO1 antibody (F). (G–I) Cells were co-transfected with Flag-HDAC1 and Myc-Stra13 or Stra13 2KR. Lysates were subject to western blotting with anti-Myc and anti-Flag antibodies to detect expression of Stra13 and HDAC1 (G). Colony assays were performed, and representative plates stained with crystal violet are shown (H). Colony assays were quantified by measuring the absorbance of extracted crystal violet dye at 570 nm (I). (J) Cells were transfected with the pD1luc reporter (100 ng) with Myc-Stra13 (25 ng) and SUMO1 (25 ng) in the presence of increasing amounts of HDAC1 (25, 50 and 100 ng). 48 hr later, luciferase activity was assayed. (K) siHDAC1 cells and controls were transfected with pD1luc in the absence and presence of Stra13. Luciferase activity was measured 48 hr later. Error bars indicate mean ±SD.</p

Mutation of sumoylation sites abrogates Stra13-mediated growth suppression.

<p>(A) NIH3T3 cells were co-transfected with Stra13 or Stra13 2KR together with a puromycin resistance plasmid. Empty vector (pCS2) was transfected in control cells (Vector). Stra13 expression was determined by western blotting using anti-Myc antibody. (B–C) Colony forming assays were performed with control, Stra13 and Stra13 2KR cells. Colonies were stained with crystal violet 14 days later. Data are representative of three independent experiments (B). Crystal violet dye was extracted and the absorbance measured at a wavelength of 570 nm. The error bars indicate standard deviations for triplicate wells in each experiment (C). (D) Growth of NIH3T3 cells expressing vector alone, Stra13 and Stra13 2KR was evaluated over a five-day period. Cell numbers at each time are represented as mean ±SD. (E) Stra13^−/− MEFs were transfected at passage 5 with equivalent amounts of Stra13 and Stra13 2KR. Cell viability was measured three days later by MTT assays. (F) Cell cycle profile of control (Vector), Stra13 and Stra132KR cells was determined by PI staining and FACS analysis. Representative histograms of cell cycle profiles in cells expressing vector alone, Stra13 and Stra13 2KR. The result shown is representative of three independent experiments.</p

Sumoylation regulates Stra13 transcriptional activity but not its subcellular localization.

<p>(A) mRNA levels of cyclin D1, p21^Cip/WAF, cyclin B1, and cyclin E1 were analyzed by Q-PCR in vector, Stra13 and Stra13 2KR cells. (B) Cells were transfected with the cyclin D1 promoter reporter pD1luc (100 ng) together with Stra13 (25 ng), Stra13 2KR (25 ng), SUMO1 (25 ng) or SENP1 (25 ng), as indicated. Cells were harvested 48 hr after transfection, and assayed for luciferase activity. (C) COS-7 cells were transfected with Stra13 and Stra13 2KR alone or together with SUMO1. Cells were stained with anti-Myc antibody. Nuclei were stained with DAPI. Error bars indicate mean ±SD. (D) NIH3T3 cells were left untreated or treated with TSA. ChIP assays were done to determine Stra13 occupancy on the cyclin D1 promoter.</p