STATs as activators of apoptosis

Studies directed at understanding how interferons (IFNs) mediate rapid induction of genes led to the identification of the JAK-STAT pathwayl. In the case of type II IFNs (e.g. IFN-y), ligand binding promotes the activation of two receptor-associated tyrosine kinases Jakl and Jak2 (see Fig. 1). These JAKs mediate the phosphorylation of Statl, a member of the STAT (signal transducer and activator of transcription) family of transcription factors, on tyrosine 701. Activated Stat1 is released from the receptor and homodimerizes through the reciprocal interaction between phosphotyrosine 701 of one Stat1 and the Src-homology 2 (SHZ) domain of its partner. While it is widely believed that this entails a pair of phosphotyrosyl-SH2 interactions, actually only one is required to stabilize dimerizatior?. Stat1 dimers translocate to the nucleus, whereupon they bind to members of the GAS family enhancers. It should be noted that Statlp, a naturally occurring Stat1 isoform, lacks the C-terminal transcription-activation domain, rendering it inert in the IFN-7 signalling pathway. To date, seven STATS have been identified in mammalsl. Both genetic and biochemical studies have determined that these STATS play a crucial role in the transduction of signals for all members of the haematopoietin family of receptors. Additionally, several STATS transduce signals for ligands binding to receptors that function directly as tyrosine kinases. While biochemical studies have implicated the same STATS in multiple cytokine pathways, genetic studies (e.g. targeted deletions of genes encoding STATS) indicate a much higher degree of specificity. For example, Stat1 is stimulated by a large number of ligands, yet the Stat1 knockout mouse only exhibits significant defects in its ability to respond to IFNs’,~,~.This includes a loss of the potent antiproliferative activity of IFN-y. Some of these antiproliferative effects could include apoptosis as some studies indicate that Stat1 can induce genes involved in programmed cell death5n6.A recent study from George Stark’s laboratory7 suggests that Stat1 regulates apoptosis by a novel signalling pathway independent of STAT activation.

STATSas activators of apoptosis STATS are a family of transcription factors that are activated in a l&and-dependent manner and promote the rapid induction of genes. They achieve this by directly transducing signals from the receptor into the nucleus. Consistent with their co-evolution with the cytokine family of ligands, studies have determined that STATS play a pivotal role in mediating the biological response for this family of ligands. However, STATS appear to have a dark side as well and contribute to programmed cell death. Intriguingly, a recent study suggests that this might occur in a manner that is distinct from the standard STAT signalling paradigm.

Moreover, reintroduction of Stat1 into the U3 cells rescued the defect, demonstrating that the Stat1 gene product was responsible for the death phenotype. Taking advantage of their ability to reconstitute apoptosis in U3 cells, the authors investigated the ability of various Stat1 mutants to rescue programmed cell death. Surprisingly, Stat1 mutants inactive in IFN-7 signalling (e.g. Tyr701 or the SH2 domain mutants) were able to rescue programmed cell death. In addition, Stat1 mutated at Ser727, which has been implicated in potentiating transcriptional activation, had an intermediate defect, although C-terminally truncated Statlp, which is missing Ser7.27, was able to rescue the mutant phenotype. The authors then evaluated the expression pattern of two potential

Do STATS regulate apoptosis?

That a vital component of the IFN signalling cascade has been implicated in apoptosis is perhaps not surprising. Viruses and their target cells engage in an elaborate dance of death. While one important host defence to viral infection is cell death, several viruses have developed elaborate mechanisms to avoid this fate, often manipulating genes activated to promote death [e.g. IRF-1 (Ref. 7)]. In an effort to explore directly the potential role for IFN signalling components in apoptosis, Stark’s group examined a panel of mutant fibroblast cell lines that they had originally developed to characterize IFN signalling. This panel included mutants in virtually every component of the type II IFN signalling pathways (see Fig. 1) and the partially overlapping type I IFN signalling pathway (not shown). To carry out their studies, each of the mutant lines was stimulated with a potent inducer of apoptosis. Only cells defective in Stat1 expression (U3) were insensitive to this treatment. trends in CELL BIOLOGY

(Vol.

8) March

1998

Christian Schindler is in the Depts of Microbiology

FIGURE

1

and Medicine,

Upon stimulation associated

with interferon

tyrosine

This activated phosphorylates

complex

Stat1 on tyrosine

a reciprocal

Stat1 Src-homology translocate

two receptor-

kinases, Jakl and Jak2, become

receptor

released from the receptor through

y (IFN-r), subsequently

701. Once activated,

complex

interaction

activated.

recruits and

between

2 (SH2) domain.

Stat1 is

and homodimerizes tyrosine

701 and the

The homodimers

to the nucleus, where they bind to members of the

GAS family of enhancers and activate transcription.

Copyright 0 1998 Elsevier Science Ltd. All rights reserved. 0962-8924/98/$19.00 PII:SO962-8924(98)01233-l

Columbia University, 1208 Hammer Health Sciences Center, 701 West 168th St, NY 10032,

USA.

E-mail: cws4@ columbia.edu

97

mediators of apoptosis, IRF-1 (see above) and FAS and found them unaltered in the U3 cells, excluding them as causative agents. By contrast, the expression level of three caspases (proteases implicated in programmed cell death) was diminished in apoptosisdeficient U3 cells. The authors concluded that constitutive expression of these caspases, which is necessary for the induction of apoptosis in their cells, is dependent on Statl. Additionally, the ability of both the Stat1 Tyr701 and SH2 mutants to rescue the death phenotype suggests that this basal level of caspase expression does not require the formation of active Stat1 homodimers. They propose that ‘inactive’ Stat1 functions through a novel pathway to promote the constitutive activation of crucial genes. Moreover, since Cpp3Zknockout mice have a significantly more severe phenotype than the Statlknockout mice, Kumar et al. suggest that Stat1 plays a greater role in regulating apoptosis in response to stress than it does during development. Where will this lead?

These intriguing observations raise several exciting possibilities about how cells regulate apoptosis and viral infections. Additional studies will be required to extend these observations to more-physiological stimuli and cell types. Also, these data will need to be reconciled with studies implicating ‘normally’ activated Stat1 (i.e. as shown in Fig. 1) in the activation of caspases6. Are both pathways active simultaneously, or does the pathway employed vary between cell types? While few data are available about the mechanism by which ‘inactive’ Stat1 might regulate programmed cell death, Kumar et al. suggest

r-

Darnell, 1. E. (1997) Science277, 1630-I 635 Gupta, S. et al. (1996) EM80 /. 15, 1075-I 084 Meraz, M. A. et al. (1996) Cell 84, 431442 Durbin, J. E. et al. (1996) Cell 84, 443450 Deiss, L. P. et al. (1995) GenesDev. 9, 15-30 Chin, Y. E. et al. (1997) Mol. Cell. Biol. 17, 5328-5337 Kumar, A. et al. (1997) Science278, 1630-I 632 Schindler, C. et al. (1992) Science257, 809-813

Satoh and colleagues used transgenic

SATOH, A. K., TOKUNACA, F., KAWAMURA, and OZAKI, K. (1997) protein

References

ited the function

In the eye

In situ inhibition

that Stat1 associates with other transcription factors to mediate these effects. Several observations support this speculation. First, the transcriptionally inactive form of Stat1 has been shown to be fully competent in signalling for type I IFNs through its capacity to associate with other ‘active’ transcription factorsl. Moreover, only one STAT needs to be activated to promote dimer formatior?. These observations provide a plausible mechanism by which an ‘inactive’ STAT could participate in signal transduction. Consistent with this notion, numerous groups have noted that there is a constitutive level of nuclear Stat1 in ‘unstimulated’ cells8. This might represent a population of ‘inactive’ Stat1 molecules associated with other factors and promoting the expression of genes either constitutively or in response to an unknown stimulus. The recent observation that N-terminally truncated Stat1 does not exhibit this basal level of nuclear translocation (I. Strehlow and C. Schindler, unpublished) might provide a useful reagent for exploring these novel pathways. Finally, these observations raise the possibility that other STATSpromote the expression of essential genes through pathways that have not yet been elucidated.

of vesicle transport

ing a dominant-negative

mutant

S.

They used the well-studied

polarized

the dominant-negative

and

of the endoplasmic

processing in the dominant negative Rabl mutant of Drosophila

protein

and all their

their first grip on Yptl p in yeast, later called Rabl in mammalian cells. Since then, numerous groups have obtained a great deal of information about Rab localization, molecular headlines were contributed by Heike Pahl, Frank Peter, Debbie Sweet and Fiona Townsley.

98

characteristics,

molecular mechanisms

ated molecules. However, most of this information

and even their associ-

In the surviving

using uni-

membrane

native cells and tissues. To gain a glimpse

in this direction,

fly, six out

the

Rabl(N1241)

mu-

which

have a high turnover require

materials,

of microvillar

a continuous

supply

of

were most affected.

Approaches such as this bring ‘single-cell knowledge’ to whole organisms. In the future, the plethora of molcertainly

of

however,

and therefore

ecular genetic

in the context

reports using

in the whole transgenic

strain,

cell in particular,

opment,

and expression

of

tation caused severe degeneration of the photoreceptor cells. Upon closer examination, the functional parts of the

cellular organisms or cultured cells, leaving open questions regarding the functional requirements of Rabl in cell develdifferentiation

(ER) as well as disassembly

of seven independent transgenic strains were lethal before eclosion, which the authors put down to leaky expression.

newly synthesized was obtained

cell as

the Golgi complex. Moreover, the maturation of opsin was also inhibited, the opsin remaining presumably in the ER.

members seem to be involved in intracellular transport events. About a decade ago, Callwitz and coworkers got

This month’s

photoreceptor

This is all well in line with several previous

family,

overexpressRabl (N1241).

Rabl mutant, they observed swelling reticulum

single cells. Interestingly, are a well-studied

protein,

a model and followed the maturation of the apoprotein opsin to its mature form, rhodopsin. Upon expression of

1. Cell Sci. 110, 2943-2953 Rab proteins

Drosophila and inhib-

of Rabl in situ by inducibly

techniques,

enhance

particularly

our knowledge

in Drosophila,

will

of Rabs and other trans-

port molecules.

trends in CELL BIOLOGY (Vol. 8) March 1998