TRIM24 (TIF1α), TRIM28 (TIF1β), and TRIM33 (TIF1γ) are three related cofactors belonging to the tripartite motif superfamily that interact with distinct transcription factors. TRIM24 interacts with the liganded retinoic acid (RA) receptor to repress its transcriptional activity. Germ line inactivation of TRIM24 in mice deregulates RA-signaling in hepatocytes leading to the development of hepatocellular carcinoma (HCC). Here we show that TRIM24 can be purified as at least two macromolecular complexes comprising either TRIM33 or TRIM33 and TRIM28. Somatic hepatocyte-specific inactivation of TRIM24, TRIM28, or TRIM33 all promote HCC in a cell-autonomous manner in mice. Moreover, HCC formation upon TRIM24 inactivation is strongly potentiated by further loss of TRIM33. These results demonstrate that the TIF1-related subfamily of TRIM proteins interact both physically and functionally to modulate HCC formation in mice.

Hepatocellular carcinoma (HCC) is a major cause of death worldwide. Here, we provide evidence that the ligand-dependent nuclear receptor co-regulator Trim24 (also known as Tif1alpha) functions in mice as a liver-specific tumor suppressor. In Trim24-null mice, hepatocytes fail to execute proper cell cycle withdrawal during the neonatal-to-adult transition and continue to cycle in adult livers, becoming prone to a continuum of cellular alterations that progress toward metastatic HCC. Using pharmacological approaches, we show that inhibition of retinoic acid signaling markedly reduces hepatocyte proliferation in Trim24-/- mice. We further show that deletion of a single retinoic acid receptor alpha (Rara) allele in a Trim24-null background suppresses HCC development and restores wild-type expression of retinoic acid-responsive genes in the liver, thus demonstrating that in this genetic background Rara expresses an oncogenic activity correlating with a dysregulation of the retinoic acid signaling pathway. Our results not only provide genetic evidence that Trim24 and Rara co-regulate hepatocarcinogenesis in an antagonistic manner but also suggest that aberrant activation of Rara is deleterious to liver homeostasis.

While nuclear lamina abnormalities are hallmarks of human diseases, their interplay with epigenetic regulators and precise epigenetic landscape remain poorly understood. Here, we show that loss of the lysine acetyltransferase MOF or its associated NSL-complex members KANSL2 or KANSL3 leads to a stochastic accumulation of nuclear abnormalities with genomic instability patterns including chromothripsis. SILAC-based MOF and KANSL2 acetylomes identified lamin A/C as an acetylation target of MOF. HDAC inhibition or acetylation-mimicking lamin A derivatives rescue nuclear abnormalities observed in MOF-deficient cells. Mechanistically, loss of lamin A/C acetylation resulted in its increased solubility, defective phosphorylation dynamics and impaired nuclear mechanostability. We found that nuclear abnormalities include EZH2-dependent histone H3 Lys 27 trimethylation and loss of nascent transcription. We term this altered epigenetic landscape "heterochromatin enrichment in nuclear abnormalities" (HENA). Collectively, the NSL-complex-dependent lamin A/C acetylation provides a mechanism that maintains nuclear architecture and genome integrity.

Trim24 (Tif1α) and Trim33 (Tif1γ) interact to form a co-repressor complex that suppresses murine hepatocellular carcinoma. Here we show that Trim24 and Trim33 cooperatively repress retinoic acid receptor-dependent activity of VL30-class endogenous retroviruses (ERVs) in liver. In Trim24-knockout hepatocytes, VL30 derepression leads to accumulation of reverse-transcribed VL30 cDNA in the cytoplasm that correlates with activation of the viral-defense interferon responses mimicking the preneoplastic inflammatory state seen in human liver following exogenous viral infection. Furthermore, upon derepression, VL30 long terminal repeats (LTRs) act as promoter and enhancer elements deregulating expression of neighboring genes and generating enhancer RNAs that are required for LTR enhancer activity in hepatocytes in vivo. These data reinforce the role of the TRIM family of proteins in retroviral restriction and antiviral defense and provide an example of an ERV-derived oncogenic regulatory network.

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