Histone deacetylase (HDAC)-inhibitors (HDACis) are well characterized anti-cancer agents with promising results in clinical trials. However, mechanistically little is known regarding their selectivity in killing malignant cells while sparing normal cells. Gene expression-based chemical genomics identified HDACis as being particularly potent against Down syndrome associated myeloid leukemia (DS-AMKL) blasts. Investigating the anti-leukemic function of HDACis revealed their transcriptional and posttranslational regulation of key autophagic proteins, including ATG7. This leads to suppression of autophagy, a lysosomal degradation process that can protect cells against damaged or unnecessary organelles and protein aggregates. DS-AMKL cells exhibit low baseline autophagy due to mTOR activation. Consequently, HDAC inhibition repressed autophagy below a critical threshold, which resulted in accumulation of mitochondria, production of reactive oxygen species, DNA-damage and apoptosis. Those HDACi-mediated effects could be reverted upon autophagy activation or aggravated upon further pharmacological or genetic inhibition. Our findings were further extended to other major acute myeloid leukemia subgroups with low basal level autophagy. The constitutive suppression of autophagy due to mTOR activation represents an inherent difference between cancer and normal cells. Thus, via autophagy suppression, HDACis deprive cells of an essential pro-survival mechanism, which translates into an attractive strategy to specifically target cancer cells.

Receptor-interacting protein kinase 1 (RIPK1) represents an essential signaling node in cell death and inflammation. Ablation of Ripk1 in liver parenchymal cells (LPC) did not cause a spontaneous phenotype, but led to tumor necrosis factor (TNF)-dependent hepatocyte apoptosis and liver injury without affecting inducible nuclear factor κB (NF-κB) activation. Loss of Ripk1 induced the TNF-dependent proteasomal degradation of the E3-ligase, TNF receptor-associated factor 2 (TRAF2), in a kinase-independent manner, thereby activating caspase-8. Moreover, loss of both Ripk1 and Traf2 in LPC not only resulted in caspase-8 hyperactivation but also impaired NF-κB activation, promoting the spontaneous development of hepatocellular carcinoma. In line, low RIPK1 and TRAF2 expression in human HCCs was associated with an unfavorable prognosis, suggesting that RIPK1 collaborates with TRAF2 to inhibit murine and human hepatocarcinogenesis.

FADD, caspase-8, and cFLIP regulate the outcome of cell death signaling. Mice that constitutively lack these molecules die at an early embryonic age, whereas tissue-specific constitutive deletion of FADD or caspase-8 results in inflammatory skin disease caused by increased necroptosis. The function of cFLIP in the skin in vivo is unknown. In contrast to tissue-specific caspase-8 knockout, we show that mice constitutively lacking cFLIP in the epidermis die around embryonic days 10 and 11. When cFLIP expression was abrogated in adult skin of cFLIPfl/fl-K14CreERtam mice, severe inflammation of the skin with concomitant caspase activation and apoptotic, but not necroptotic, cell death developed. Apoptosis was dependent of autocrine tumor necrosis factor production triggered by loss of cFLIP. In addition, epidermal cFLIP protein was lost in patients with severe drug reactions associated with epidermal apoptosis. Our data demonstrate the importance of cFLIP for the integrity of the epidermis and for silencing of spontaneous skin inflammation.