RIP1 negatively regulates basal autophagic flux through TFEB to control sensitivity to apoptosis.

In a synthetic lethality/viability screen, we identified the serine-threonine kinase RIP1 (RIPK1) as a gene whose knockdown is highly selected against during growth in normal media, in which autophagy is not critical, but selected for in conditions that increase reliance on basal autophagy. RIP1 represses basal autophagy in part due to its ability to regulate the TFEB transcription factor, which controls the expression of autophagy-related and lysosomal genes. RIP1 activates ERK, which negatively regulates TFEB though phosphorylation of serine 142. Thus, in addition to other pro-death functions, RIP1 regulates cellular sensitivity to pro-death stimuli by modulating basal autophagy.

Hedgehog signaling alters reliance on EGF receptor signaling and mediates anti-EGFR therapeutic resistance in head and neck cancer.

The EGF receptor (EGFR)-directed monoclonal antibody cetuximab is the only targeted therapy approved for the treatment of squamous cell carcinoma of the head and neck (HNSCC) but is only effective in a minority of patients. Epithelial-to-mesenchymal transition (EMT) has been implicated as a drug resistance mechanism in multiple cancers, and the EGFR and Hedgehog pathways (HhP) are relevant to this process, but the interplay between the two pathways has not been defined in HNSCC. Here, we show that HNSCC cells that were naturally sensitive to EGFR inhibition over time developed increased expression of the HhP transcription factor GLI1 as they became resistant after long-term EGFR inhibitor exposure. This robustly correlated with an increase in vimentin expression. Conversely, the HhP negatively regulated an EGFR-dependent, EMT-like state in HNSCC cells, and pharmacologic or genetic inhibition of HhP signaling pushed cells further into an EGFR-dependent phenotype, increasing expression of ZEB1 and VIM. In vivo treatment with cetuximab resulted in tumor shrinkage in four of six HNSCC patient-derived xenografts; however, they eventually regrew. Cetuximab in combination with the HhP inhibitor IPI-926 eliminated tumors in two cases and significantly delayed regrowth in the other two cases. Expression of EMT genes TWIST and ZEB2 was increased in sensitive xenografts, suggesting a possible resistant mesenchymal population. In summary, we report that EGFR-dependent HNSCC cells can undergo both EGFR-dependent and -independent EMT and HhP signaling is a regulator in both processes. Cetuximab plus IPI-926 forces tumor cells into an EGFR-dependent state, delaying or completely blocking tumor recurrence.

Caspase 3-mediated stimulation of tumor cell repopulation during cancer radiotherapy.

In cancer treatment, apoptosis is a well-recognized cell death mechanism through which cytotoxic agents kill tumor cells. Here we report that dying tumor cells use the apoptotic process to generate potent growth-stimulating signals to stimulate the repopulation of tumors undergoing radiotherapy. Furthermore, activated caspase 3, a key executioner in apoptosis, is involved in the growth stimulation. One downstream effector that caspase 3 regulates is prostaglandin E(2) (PGE(2)), which can potently stimulate growth of surviving tumor cells. Deficiency of caspase 3 either in tumor cells or in tumor stroma caused substantial tumor sensitivity to radiotherapy in xenograft or mouse tumors. In human subjects with cancer, higher amounts of activated caspase 3 in tumor tissues are correlated with markedly increased rate of recurrence and death. We propose the existence of a cell death-induced tumor repopulation pathway in which caspase 3 has a major role.

Regulation of the Pro-apoptotic scaffolding protein POSH by Akt.

POSH (Plenty of SH3 domains) binds to activated Rac and promotes apoptosis by acting as a scaffold to assemble a signal transduction pathway leading from Rac to JNK activation. Overexpression of POSH induces apoptosis in a variety of cell types, but apoptosis can be prevented by co-expressing the pro-survival protein kinase Akt. We report here that POSH is a direct substrate for phosphorylation by Akt in vivo and in vitro, and we identify a major site of Akt phosphorylation as serine 304 of POSH, which lies within the Rac-binding domain. We further show that phosphorylation of POSH results in a decreased ability to bind activated Rac, as does phosphomimetic S304D and S304E mutation of POSH. S304D mutant POSH also shows a strongly reduced ability to induce apoptosis. These findings identify a novel mechanism by which Akt promotes cell survival.

Activation of caspase pathways during iron chelator-mediated apoptosis.

Iron chelators have traditionally been used in the treatment of iron overload. Recently, chelators have also been explored for their ability to limit oxidant damage in cardiovascular, neurologic, and inflammatory disease as well as to serve as anti-cancer agents. To determine the mechanism of cell death induced by iron chelators, we assessed the time course and pathways of caspase activation during apoptosis induced by iron chelators. We report that the chelator tachpyridine sequentially activates caspases 9, 3, and 8. These caspases were also activated by the structurally unrelated chelators dipyridyl and desferrioxamine. The critical role of caspase activation in cell death was supported by microinjection experiments demonstrating that p35, a broad spectrum caspase inhibitor, protected HeLa cells from chelator-induced cell death. Apoptosis mediated by tachpyridine was not prevented by blocking the CD95 death receptor pathway with a Fas-associated death domain protein (FADD) dominant-negative mutant. In contrast, chelator-mediated cell death was blocked in cells microinjected with Bcl-XL and completely inhibited in cells microinjected with a dominant-negative caspase 9 expression vector. Caspase activation was not observed in cells treated with N-methyl tachpyridine, an N-alkylated derivative of tachpyridine which lacks an ability to react with iron. These results suggest that activation of a mitochondrial caspase pathway is an important mechanism by which iron chelators induce cell death.