Autophagy induced by deficiency of sphingosine-1-phosphate phosphohydrolase 1 is switched to apoptosis by calpain-mediated autophagy-related gene 5 (Atg5) cleavage

Sphingosine 1-phosphate (S1P) and ceramide have been implicated in both autophagy and apoptosis. However, the roles of these sphingolipid metabolites in the links between these two processes are not completely understood. Depletion of S1P phosphohydrolase-1 (SPP1), which degrades intracellular S1P, induces the unfolded protein response and endoplasmic reticulum stress-induced autophagy (Lépine, S., Allegood, J. C., Park, M., Dent, P., Milstien, S., and Spiegel, S. (2011) Cell Death Differ. 18, 350-361). Surprisingly, however, treatment with doxorubicin, which by itself also induced autophagy, markedly reduced the extent of autophagy mediated by depletion of SPP1. Concomitantly, doxorubicin-induced apoptosis was greatly enhanced by down-regulation of SPP1. Autophagy and apoptosis seemed to be sequentially linked because inhibiting autophagy with 3-methyladenine also markedly attenuated apoptosis. Moreover, silencing Atg5 or the three sensors of the unfolded protein response, IRE1α, ATF6, and PKR-like eIF2α kinase (PERK), significantly decreased both autophagy and apoptosis. Doxorubicin stimulated calpain activity and Atg5 cleavage, which were significantly enhanced in SPP1-depleted cells. Inhibition or depletion of calpain not only suppressed Atg5 cleavage, it also markedly decreased the robust apoptosis induced by doxorubicin in SPP1-deficient cells. Importantly, doxorubicin also increased de novo synthesis of the pro-apoptotic sphingolipid metabolite ceramide. Elevation of ceramide in turn stimulated calpain; conversely, inhibiting ceramide formation suppressed Atg5 cleavage and apoptosis. Hence, doxorubicin switches protective autophagy in SPP1-depleted cells to apoptosis by calpain-mediated Atg5 cleavage.     

Disruption of the ribosomal P complex leads to stress-induced autophagy

The human ribosomal P complex, which consists of the acidic ribosomal P proteins RPLP0, RPLP1, and RPLP2 (RPLP proteins), recruits translational factors, facilitating protein synthesis. Recently, we showed that overexpression of RPLP1 immortalizes primary cells and contributes to transformation. Moreover, RPLP proteins are overexpressed in human cancer, with the highest incidence in breast carcinomas. It is thought that disruption of the P complex would directly affect protein synthesis, causing cell growth arrest and eventually apoptosis. Here, we report a distinct mechanism by which cancer cells undergo cell cycle arrest and induced autophagy when RPLP proteins are downregulated. We found that absence of RPLP0, RPLP1, or RPLP2 resulted in reactive oxygen species (ROS) accumulation and MAPK1/ERK2 signaling pathway activation. Moreover, ROS generation led to endoplasmic reticulum (ER) stress that involved the EIF2AK3/PERK-EIF2S1/eIF2α-EIF2S2-EIF2S3-ATF4/ATF-4- and ATF6/ATF-6-dependent arms of the unfolded protein response (UPR). RPLP protein-deficient cells treated with autophagy inhibitors experienced apoptotic cell death as an alternative to autophagy. Strikingly, antioxidant treatment prevented UPR activation and autophagy while restoring the proliferative capacity of these cells. Our results indicate that ROS are a critical signal generated by disruption of the P complex that causes a cellular response that follows a sequential order: first ROS, then ER stress/UPR activation, and finally autophagy. Importantly, inhibition of the first step alone is able to restore the proliferative capacity of the cells, preventing UPR activation and autophagy. Overall, our results support a role for autophagy as a survival mechanism in response to stress due to RPLP protein deficiency.     

Nelfinavir, a new anti-cancer drug with pleiotropic effects and many paths to autophagy

The development of cancer drugs is slow and costly. One approach to accelerate the availability of new drugs is to reposition drugs approved for other indications as anti-cancer agents. HIV protease inhibitors (HIV PIs) are useful in treating HIV infection and cause toxicities in humans that are similar to those observed when the kinase Akt, a target for cancer therapy, is inhibited. To test whether HIV PIs inhibited Akt and cancer cell proliferation, we screened 6 HIV PIs and found that three, ritonavir, saquinavir and nelfinavir, inhibit the growth of over 60 cancer cell lines derived from 9 different tumor types; Nelfinavir is the most potent. Nelfinavir causes caspase-dependent apoptosis and non-apoptotic death, as well as endoplasmic reticulum (ER) stress and autophagy. Nelfinavir blocks growth factor receptor activation and decreases growth factor-induced and endogenous Akt signaling. In vivo, nelfinavir inhibits tumor growth and upregulates markers of ER stress, autophagy and apoptosis. Nelfinavir is currently being tested in cancer patients in Phase I clinical trials where biomarkers will be assessed. Current studies are focused on measuring autophagy in clinical specimens and identifying combination strategies that will exploit the induction of autophagy and increase the effectiveness of nelfinavir.     

ER stress contributes to renal proximal tubule injury by increasing SREBP-2-mediated lipid accumulation and apoptotic cell death

Renal proximal tubule injury is induced by agents/conditions known to cause endoplasmic reticulum (ER) stress, including cyclosporine A (CsA), an immunosuppressant drug with nephrotoxic effects. However, the underlying mechanism by which ER stress contributes to proximal tubule cell injury is not well understood. In this study, we report lipid accumulation, sterol regulatory element-binding protein-2 (SREBP-2) expression, and ER stress in proximal tubules of kidneys from mice treated with the classic ER stressor tunicamycin (Tm) or in human renal biopsy specimens showing CsA-induced nephrotoxicity. Colocalization of ER stress markers [78-kDa glucose regulated protein (GRP78), CHOP] with SREBP-2 expression and lipid accumulation was prominent within the proximal tubule cells exposed to Tm or CsA. Prolonged ER stress resulted in increased apoptotic cell death of lipid-enriched proximal tubule cells with colocalization of GRP78, SREBP-2, and Ca(2+)-independent phospholipase A(2) (iPLA(2)β), an SREBP-2 inducible gene with proapoptotic characteristics. In cultured HK-2 human proximal tubule cells, CsA- and Tm-induced ER stress caused lipid accumulation and SREBP-2 activation. Furthermore, overexpression of SREBP-2 or activation of endogenous SREBP-2 in HK-2 cells stimulated apoptosis. Inhibition of SREBP-2 activation with the site-1-serine protease inhibitor AEBSF prevented ER stress-induced lipid accumulation and apoptosis. Overexpression of the ER-resident chaperone GRP78 attenuated ER stress and inhibited CsA-induced SREBP-2 expression and lipid accumulation. In summary, our findings suggest that ER stress-induced SREBP-2 activation contributes to renal proximal tubule cell injury by dysregulating lipid homeostasis.     

Nelfinavir,a new anti-cancer drug with pleiotropic effects and many paths to autophagy

The development of cancer drugs is slow and costly. One approach to accelerate the availability of new drugs is to reposition drugs approved for other indications as anti-cancer agents. HIV protease inhibitors (HIV PIs) are useful in treating HIV infection and cause toxicities in humans that are similar to those observed when the kinase Akt, a target for cancer therapy, is inhibited. To test whether HIV PIs inhibited Akt and cancer cell proliferation, we screened 6 HIV PIs and found that three, ritonavir, saquinavir and nelfinavir, inhibit the growth of over 60 cancer cell lines derived from 9 different tumor types; Nelfinavir is the most potent. Nelfinavir causes caspase-dependent apoptosis and non-apoptotic death, as well as endoplasmic reticulum (ER) stress and autophagy. Nelfinavir blocks growth factor receptor activation and decreases growth factor-induced and endogenous Akt signaling. In vivo, nelfinavir inhibits tumor growth and upregulates markers of ER stress, autophagy and apoptosis. Nelfinavir is currently being tested in cancer patients in Phase I clinical trials where biomarkers will be assessed. Current studies are focused on measuring autophagy in clinical specimens and identifying combination strategies that will exploit the induction of autophagy and increase the effectiveness of nelfinavir.     

Site-1 and site-2 proteases: A team of two in regulated proteolysis

The site-1 and site-2 proteases (S1P and S2P) were identified over 20 years ago, and the functions of both have been addressed in numerous studies ever since. Whereas S1P processes a set of substrates independently of S2P, the latter acts in concert with S1P in a mechanism, called regulated intramembrane proteolysis, that controls lipid metabolism and response to unfolded proteins. This review summarizes the molecular roles that S1P and S2P jointly play in these processes. As S1P and S2P deficiencies mainly affect connective tissues, yet with varying phenotypes, we discuss the segregated functions of S1P and S2P in terms of cell homeostasis and maintenance of the connective tissues. In addition, we provide experimental data that point at S2P, but not S1P, as a critical regulator of cell adaptation to proteotoxicity or lipid imbalance. Therefore, we hypothesize that S2P can also function independently of S1P activity.     

Site-2 proteases in prokaryotes: regulated intramembrane proteolysis expands to microbial pathogenesis

Regulated intramembrane proteolysis (RIP) is a widely distributed mechanism of signal transduction in which membrane-bound proteases cleave transmembrane domains of substrate proteins. The site-2 protease (S2P) class of RIP metalloproteases is present in most bacterial genomes but is generally of unknown function except for the well-characterized proteases RseP and SpoIVFB. In this review we will discuss the biochemical functions and physiologic roles of S2P proteases in bacteria and highlight recent data implicating S2P family members in host-pathogen interactions.