Use of the yeast two-hybrid system for identifying the cascade of protein interactions resulting in apoptotic cell death

A light way for nuclear cell biologists

The nucleus is a very complex organelle present in eukaryotic cells. Having the crucial task to safeguard, organize and manage the genetic information, it must tightly control its molecular constituents, its shape and its internal architecture at any given time. Despite our vast knowledge of nuclear cell biology, much is yet to be unravelled. For instance, only recently we came to appreciate the existence of a dynamic nuclear cytoskeleton made of actin filaments that regulates processes such as gene expression, DNA repair and nuclear expansion. This suggests further exciting discoveries ahead of us. Modern cell biologists embrace a new methodology relying on precise perturbations of cellular processes that require a reversible, highly spatially confinable, rapid, inexpensive and tunEable external stimulus: light. In this review, we discuss how optogenetics, the state-of-the-art technology that uses genetically encoded light-sensitive proteins to steer biological processes, can be adopted to specifically investigate nuclear cell biology.

Apoptosis related protein-1 triggers melanoma cell death via interaction with the juxtamembrane region of p75 neurotrophin receptor

Although chemotherapeutic drugs could theoretically target all metastatic sites, current treatments do not provide complementary therapeutics. Therefore, the development of an alternative approach replacing the traditional therapy is urgently needed. To assess the killing efficiency of the functionally identified apoptosis-related protein (APR)-1 in melanoma cells, we established a system for the regulated expression of APR-1. The induction of APR-1 expression caused apoptosis of melanoma cells via the interaction with the juxtamembrane region of p75 neurotrophin receptor (p75NTR), and possible also via the competition with tumour necrosis factor receptor-associated factor-6 (TRAF6) and the catalytic receptor of neurotrophin (Trk) for the same p75NTR interacting site. The accumulation of APR-1 in melanoma cells may block the physical association of p75NRT with TRAF6 and/or Trk, leading to the disruption of both NF-κB and extracellular signal-regulated kinase (ERK) pathways. Also, accumulation of APR-1 protein enhanced the activity of both c-Jun-N-terminal kinase (JNK) and p38 pathways. However, the analysis of APR-1-modulated pathways demonstrated the involvement of apoptosis-regulating kinase 1-JNK/p38 pathway in the induction of Bax expression leading to both mitochondrial dysregulation [as demonstrated by the loss of mitochondrial membrane potential, the release of both cytochrome c and apoptosis-inducing factor into cytoplasm, and cleavage of caspase-9, caspase-3 and poly (ADP-ribose) polymerase (PARP)] and endoplasmic reticulum stress as demonstrated by the increase of intracellular Ca(2+) release. Thus, besides the analysis of its pro-apoptotic function, our data provide insight into the molecular mechanism of APR-1-induced apoptosis of melanoma cells.

Hyaluronidase induction of a WW domain-containing oxidoreductase that enhances tumor necrosis factor cytotoxicity

To determine how hyaluronidase increases certain cancer cell sensitivity to tumor necrosis factor (TNF) cytotoxicity, we report here the isolation and characterization of a hyaluronidase-induced murine WW domain-containing oxidoreductase (WOX1). WOX1 is composed of two N-terminal WW domains, a nuclear localization sequence, and a C-terminal alcohol dehydrogenase (ADH) domain. WOX1 is mainly located in the mitochondria, and the mitochondrial targeting sequence was mapped within the ADH domain. Induction of mitochondrial permeability transition by TNF, staurosporine, and atractyloside resulted in WOX1 release from mitochondria and subsequent nuclear translocation. TNF-mediated WOX1 nuclear translocation occurred shortly after that of nuclear factor-kappaB nuclear translocation, whereas both were independent events. WOX1 enhanced TNF cytotoxicity in L929 cells via its WW and ADH domains as determined using stable cell transfectants. In parallel with this observation, WOX1 also enhanced TRADD (TNF receptor-associated death domain protein)-mediated cell death in transient expression experiments. Antisense expression of WOX1 raised TNF resistance in L929 cells. Enhancement of TNF cytotoxicity by WOX1 is due, in part, to its significant down-regulation of the apoptosis inhibitors Bcl-2 and Bcl-x(L) (>85%), but up-regulation of pro-apoptotic p53 ( approximately 200%) by the ADH domain. When overexpressed, the ADH domain mediated apoptosis, probably due to modulation of expression of these proteins. The WW domains failed to modulate the expression of these proteins, but sensitized COS-7 cells to TNF killing and mediated apoptosis in various cancer cells independently of caspases. Transient cotransfection of cells with both p53 and WOX1 induced apoptosis in a synergistic manner. WOX1 colocalizes with p53 in the cytosol and binds to the proline-rich region of p53 via its WW domains. Blocking of WOX1 expression by antisense mRNA abolished p53 apoptosis. Thus, WOX1 is a mitochondrial apoptogenic protein and an essential partner of p53 in cell death.

A conditionally expressed third partner stabilizes or prevents the formation of a transcriptional activator in a three-hybrid system

We describe a three-hybrid system that involves three polypeptides that allow or prevent the formation of the transcriptional activator. Beside the two-hybrid fusion proteins, the third partner is under the control of the Met25 promoter, which is positively regulated in medium lacking methionine. We document a situation where such a third partner promotes interaction between two proteins, one fused to a DNA-binding domain and the other fused to an activator domain. This is demonstrated for cdk7-MAT1 interaction stabilized by the presence of cyclin H; these three polypeptides are found either free or associated with the transcription/DNA repair factor TFIIH. We also document the capacity of our system to conditionally inhibit the interaction between two polypeptides that otherwise elicit a positive two-hybrid response. This is demonstrated for Ras-Raf interaction precluded by an excess of Raf. The presence of a methionine-regulated promoter provides an "on" or "off" switch for the formation of the transcriptional activator, thus also providing an excellent control to evaluate the activation or inhibition properties of the third partner.