BACKGROUND: HDAC inhibitors (HDACi) belong to a new group of chemotherapeutics that are increasingly used in the treatment of lymphocyte-derived malignancies, but their mechanisms of action remain poorly understood. Here we aimed to identify novel protein targets of HDACi in B- and T-lymphoma cell lines and to verify selected candidates across several mammalian cell lines.
METHODS: Jurkat T- and SUDHL5 B-lymphocytes were treated with the HDACi SAHA (vorinostat) prior to SILAC-based quantitative proteome analysis. Selected differentially expressed proteins were verified by targeted mass spectrometry, RT-PCR and western analysis in multiple mammalian cell lines. Genomic uracil was quantified by LC-MS/MS, cell cycle distribution analyzed by flow cytometry and class switch recombination monitored by FACS in murine CH12F3 cells.
RESULTS: SAHA treatment resulted in differential expression of 125 and 89 proteins in Jurkat and SUDHL5, respectively, of which 19 were commonly affected. Among these were several oncoproteins and tumor suppressors previously not reported to be affected by HDACi. Several key enzymes determining the cellular dUTP/dTTP ratio were downregulated and in both cell lines we found robust depletion of UNG2, the major glycosylase in genomic uracil sanitation. UNG2 depletion was accompanied by hyperacetylation and mediated by increased proteasomal degradation independent of cell cycle stage. UNG2 degradation appeared to be ubiquitous and was observed across several mammalian cell lines of different origin and with several HDACis. Loss of UNG2 was accompanied by 30-40% increase in genomic uracil in freely cycling HEK cells and reduced immunoglobulin class-switch recombination in murine CH12F3 cells.
CONCLUSION: We describe several oncoproteins and tumor suppressors previously not reported to be affected by HDACi in previous transcriptome analyses, underscoring the importance of proteome analysis to identify cellular effectors of HDACi treatment. The apparently ubiquitous depletion of UNG2 and PCLAF establishes DNA base excision repair and translesion synthesis as novel pathways affected by HDACi treatment. Dysregulated genomic uracil homeostasis may aid interpretation of HDACi effects in cancer cells and further advance studies on this class of inhibitors in the treatment of APOBEC-expressing tumors, autoimmune disease and HIV-1.
UNG2 degradation appeared to be ubiquitous and was observed across several mammalian cell lines of different origin and with several HDACis.
orthogonal validation across cultured cell systems supports recurrence, but cell-line sampling and incompletely quantified blots limit generalisation
UNG2 depletion was accompanied by hyperacetylation and mediated by increased proteasomal degradation independent of cell cycle stage.
inhibitor rescue and cell-cycle sorting support proteasome involvement, while gel overlap provides indirect acetylation evidence
Loss of UNG2 was accompanied by 30-40% increase in genomic uracil in freely cycling HEK cells
drug perturbation does not isolate UNG2's contribution, with only one nominally significant comparison and three biological replicates
MS-275 mediated a dose-dependent decrease in CSR and a > fivefold reduction at the 0.5 µM concentration.
representative cytograms lack stated replication and quantitative uncertainty, and broad drug effects prevent UNG-specific attribution
SAHA treatment resulted in differential expression of 125 and 89 proteins in Jurkat and SUDHL5, respectively, of which 19 were commonly affected.
unspecified discovery replication and unadjusted differential-expression testing limit confidence in the selected protein lists
Derived from the full evaluation — not a separate score.
Strengths
The B- and T-cell proteomic comparison is followed by evidence of UNG2 depletion in cells of different origins and with multiple inhibitors, extending the finding beyond a single discovery system.
↳ Results, first two subsections; Figs. 1 and 3
Targeted mass spectrometry, western analysis, and microscopy converge on UNG2 loss. Transcript measurements, cell-cycle sorting, and rescue with three proteasome inhibitors help distinguish protein depletion from transcriptional or cell-cycle explanations.
↳ Figs. 3–5; Methods, Targeted PRM analysis
The article acknowledges that AID and UNG contributions to CSR cannot be separated and discusses backup uracil glycosylases. It also identifies freshly prepared lymphocytes as a necessary next setting for investigation.
↳ Results, CH12F3 and genomic uracil subsections; Discussion
Limitations
The SILAC analysis does not specify the number or independence of the replicates collapsed before testing. Central western-blot and CSR presentations also lack the replication and quantitative uncertainty needed to assess their precision.
↳ Methods, Bioinformatic analysis of SILAC MS data; Figs. 3, 5, and 6b
Only one of eight genomic-uracil comparisons reaches nominal significance, and no UNG2-specific rescue or knockdown separates its contribution from other HDACi effects. The 48 h uracil measurement also coincides with reported UNG2 re-expression after SAHA.
↳ Results, UNG2 time course and genomic uracil subsection; Fig. 6
The Abstract's pathway-establishment claim extends beyond the non-significant PCLAF discovery result and protein-level observations. Proteasome rescue and overlapping gel signals do not directly establish polyubiquitin independence or identify the acetylation mechanism.
↳ Abstract, Conclusion; Results, PCLAF passage and Fig. 5; Discussion, paragraph 3
The central UNG2-depletion result benefits from complementary protein assays, multiple cell systems, and experiments addressing transcription and cell-cycle explanations. The proteasome-inhibitor rescue supports a degradation mechanism, but acetylation and polyubiquitin independence are less directly identified. Unspecified SILAC replication and limited quantitative reporting constrain methodological confidence despite the breadth of assays. Genomic uracil and CSR remain functional correlates rather than isolated consequences of UNG2 loss, which also limits the proposed therapeutic pathway.
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Confidence mediumThe study extends prior Jurkat proteomics with a B-cell comparison and multi-method evidence for HDACi-associated UNG2 depletion. The advance is narrower than a demonstrated UNG2-specific explanation of genomic uracil, CSR, or therapeutic effects.
Orthogonal protein measurements, proteasome-inhibitor rescue, and cell-cycle experiments support the central observation. Unspecified discovery replication, limited quantification of central blots and CSR, and unaddressed differential-expression multiplicity constrain the broader analysis.
The discovery-to-validation sequence is followable and the Results explicitly delimit the uracil and CSR findings. Central statements about pathway establishment and polyubiquitin independence imply more than the reported assays directly identify.
The discussion engages prior proteomics, AID-related explanations, backup uracil glycosylases, and the need for freshly prepared lymphocytes. Statistical uncertainty and the distance from cell-line findings to disease applications are not traced as fully.
Caveats4 of 4 checks
The printed proteomics accession has an atypical digit count, and the Fig. 3c legend names SAHA twice where the Results describe two inhibitors. Additional experiments are reported as data not shown.
Data availability: declared
105 references in manuscript 105 of 105 checkable references found in an index
References not verified automatically are resolved manually within 24 hours.
No retraction or expression of concern found in Crossref (includes Retraction Watch).
Correction published 04 Jun 2021 · listed in Crossref
Sources: Crossref (incl. Retraction Watch) ✓
Where this paper’s evidence sits on the path from initial observation to real-world use.
Mechanistic and functional experiments remain confined to cultured cells. No disease-specific intervention, primary-cell confirmation, or in vivo benefit is demonstrated.
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