Anke Klippel

The caspase-3 (CPP32, apopain, YAMA) family of cysteinyl proteases has been implicated as key mediators of apoptosis in mammalian cells. Gelsolin was identified as a substrate for caspase-3 by screening the translation products of small complementary DNA pools for sensitivity to cleavage by caspase-3. Gelsolin was cleaved in vivo in a caspase-dependent manner in cells stimulated by Fas. Caspase-cleaved gelsolin severed actin filaments in vitro in a Ca2+-independent manner. Expression of the gelsolin cleavage product in multiple cell types caused the cells to round up, detach from the plate, and undergo nuclear fragmentation. Neutrophils isolated from mice lacking gelsolin had delayed onset of both blebbing and DNA fragmentation, following apoptosis induction, compared with wild-type neutrophils. Thus, cleaved gelsolin may be one physiological effector of morphologic change during apoptosis.

We have found that insulin-like growth factor I (IGF-I) can protect fibroblasts from apoptosis induced by UV-B light. Antiapoptotic signalling by the IGF-I receptor depended on receptor kinase activity, as cells overexpressing kinase-defective receptor mutants could not be protected by IGF-I. Overexpression of a kinase-defective receptor which contained a mutation in the ATP binding loop functioned as a dominant negative and sensitized cells to apoptosis. The antiapoptotic capacity of the IGF-I receptor was not shared by other growth factors tested, including epidermal growth factor (EGF) and thrombin, although the cells expressed functional receptors for all the agonists. However, EGF was antiapoptotic for cells overexpressing the EGF receptor, and expression of activated pp60v-src also was protective. There was no correlation between protection from apoptosis and activation of mitogen-activated protein kinase, p38/HOG1, or p70S6 kinase. On the other hand, protection by any of the tyrosine kinases against UV-induced apoptosis was blocked by wortmannin, implying a role for phosphatidylinositol 3-kinase (PI3 kinase). To test this, we transiently expressed constitutively active or kinase-dead PI3 kinase and found that overexpression of activated phosphatidylinositol 3-kinase (PI3 kinase) was sufficient to provide protection against apoptosis. Because Akt/PKB is believed to be a downstream effector for PI3 kinase, we also examined the role of this serine/threonine protein kinase in antiapoptotic signalling. We found that membrane-targeted Akt was sufficient to protect against apoptosis but that kinase-dead Akt was not. We conclude that the endogenous IGF-I receptor has a specific antiapoptotic signalling capacity, that overexpression of other tyrosine kinases can allow them also to be antiapoptotic, and that activation of PI3 kinase and Akt is sufficient for antiapoptotic signalling.

Cereblon (CRBN), the molecular target of lenalidomide and pomalidomide, is a substrate receptor of the cullin ring E3 ubiquitin ligase complex, CRL4(CRBN) . T cell co-stimulation by lenalidomide or pomalidomide is cereblon dependent: however, the CRL4(CRBN) substrates responsible for T cell co-stimulation have yet to be identified. Here we demonstrate that interaction of the transcription factors Ikaros (IKZF1, encoded by the IKZF1 gene) and Aiolos (IKZF3, encoded by the IKZF3 gene) with CRL4(CRBN) is induced by lenalidomide or pomalidomide. Each agent promotes Aiolos and Ikaros binding to CRL4(CRBN) with enhanced ubiquitination leading to cereblon-dependent proteosomal degradation in T lymphocytes. We confirm that Aiolos and Ikaros are transcriptional repressors of interleukin-2 expression. The findings link lenalidomide- or pomalidomide-induced degradation of these transcriptional suppressors to well documented T cell activation. Importantly, Aiolos could serve as a proximal pharmacodynamic marker for lenalidomide and pomalidomide, as healthy human subjects administered lenalidomide demonstrated Aiolos degradation in their peripheral T cells. In conclusion, we present a molecular model in which drug binding to cereblon results in the interaction of Ikaros and Aiolos to CRL4(CRBN) , leading to their ubiquitination, subsequent proteasomal degradation and T cell activation.

Phosphatidylinositol (Pl)-3 kinase is one of many enzymes stimulated by growth factors. A constitutively activated mutant, p110, that functions independently of growth factor stimulation was constructed to determine the specific responses regulated by Pl-3 kinase. The p110 protein exhibited high specific activity as a Pl-3 kinase and as a protein kinase. Expression of p110 in NIH 3T3 cells induced transcription from the fos promoter. Co-expression of dominant negative Ras blocked this response. When expressed in Xenopus laevis oocytes, p110 increased the amount of guanosine 5'-triphosphate-bound Ras, caused activation of the Ras effector Raf-1, and induced Ras-dependent oocyte maturation. These findings show that Pl-3 kinase can stimulate diverse Ras-dependent cellular processes, including oocyte maturation and fos transcription.

Phosphatidylinositol (PI) 3-kinase is a cytoplasmic signaling molecule that is recruited to activated growth factor receptors after growth factor stimulation of cells. Activation of PI 3-kinase results in increased intracellular levels of 3' phosphorylated inositol phospholipids and the induction of signaling responses, including the activation of the protein kinase Akt, which is also known as RAC-PK or PKB. We tested the possibility that the phospholipid products of PI 3-kinase directly mediate the activation of Akt. We have previously described a constitutively active PI 3-kinase, p110, which can stimulate Akt activity. We used purified p110 protein to generate a series of 3' phosphorylated inositol phospholipids and tested whether any of these lipids could activate Akt in vitro. Phospholipid vesicles containing PI3,4 bisphosphate (P2) specifically activated Akt in vitro. By contrast, the presence of phospholipid vesicles containing PI3P or PI3,4,5P3 failed to increase the kinase activity of Akt. Akt could also be activated by synthetic dipalmitoylated PI3,4P2 or after enzymatic conversion of PI3,4,5P3 into PI3,4P2 with the signaling inositol polyphosphate 5' phosphatase SIP. We show that PI3,4P2-mediated activation is dependent on a functional pleckstrin homology domain in Akt, since a point mutation in the pleckstrin homology domain abrogated the response to PI3,4P2. Our findings show that a phospholipid product of PI 3-kinase can directly stimulate an enzyme known to be an important mediator of PI 3-kinase signaling.