Differential phosphorylation of mitogen-activated protein kinase families by epidermal growth factor and ultraviolet B irradiation in SV40-transformed human keratinocytes

Molecular signaling cascades involved in nonmelanoma skin carcinogenesis

Nonmelanoma skin cancer (NMSC) is the most common cancer worldwide and the incidence continues to rise, in part due to increasing numbers in high-risk groups such as organ transplant recipients and those taking photosensitizing medications. The most significant risk factor for NMSC is ultraviolet radiation (UVR) from sunlight, specifically UVB, which is the leading cause of DNA damage, photoaging, and malignant transformation in the skin. Activation of apoptosis following UVR exposure allows the elimination of irreversibly damaged cells that may harbor oncogenic mutations. However, UVR also activates signaling cascades that promote the survival of these potentially cancerous cells, resulting in tumor initiation. Thus, the UVR-induced stress response in the skin is multifaceted and requires coordinated activation of numerous pathways controlling DNA damage repair, inflammation, and kinase-mediated signal transduction that lead to either cell survival or cell death. This review focuses on the central signaling mechanisms that respond to UVR and the subsequent cellular changes. Given the prevalence of NMSC and the resulting health care burden, many of these pathways provide promising targets for continued study aimed at both chemoprevention and chemotherapy.

Skin mild hypoxia enhances killing of UVB-damaged keratinocytes through reactive oxygen species-mediated apoptosis requiring Noxa and Bim

The naturally occurring skin hypoxia has emerged as a crucial host factor of the epidermal microenvironment. We wanted to systematically investigate how reduced oxygen availability of the epidermis modulates the response of keratinocytes and melanocytes to noxious ultraviolet B radiation (UVB). We report that the exposure of normal human keratinocytes (NHKs) or melanocytes (NHEMs) to mild hypoxia drastically impacts cell death responses following UVB irradiation. The hypoxic microenvironment favors survival and reduces apoptosis of UVB-irradiated NHEMs and their malignant counterparts (melanoma cells). In contrast, NHKs, but not the transformed keratinocytes, under hypoxic conditions display increased levels of reactive oxygen species (ROS) and are significantly sensitized to UVB-mediated apoptosis as compared to NHKs treated under normoxic conditions. Prolonged exposure of UVB-treated NHKs to hypoxia triggers a sustained and reactive oxygen species-dependent activation of the stress kinases p38(MAPK) and JNKs, which in turn, engage the activation of Noxa and Bim proapoptotic proteins. Combined silencing of Noxa and Bim significantly inhibits UVB-mediated apoptosis under hypoxic conditions, demonstrating that hypoxia results in an amplification of the intrinsic apoptotic pathway. Physiologically occurring skin hypoxia, by facilitating the specific removal of UVB-damaged keratinocytes, may represent a decisive host factor impeding important steps of the photocarcinogenesis process.

Bcl-2 family members: essential players in skin cancer

Skin cancer has reached epidemic proportions and is considered to be a direct consequence of ultraviolet (UV) radiation exposure. Excessive exposure of epidermal cells to UV results in apoptosis of irreparably damaged cells to avoid malignant transformation. The Bcl-2 family of proteins is emerging as a crucial regulator of epidermal homeostasis and cell's fate in the stressed skin. Not surprisingly, deregulation of Bcl-2 family members is also chiefly involved in skin carcinogenesis and response to cancer therapy. Here we discuss the physiopathological role of epidermal Bcl-2 family members, their implications in skin carcinogenesis and as potential targets in cancer therapy.

Protein kinases and transcription factors activation in response to UV-radiation of skin: implications for carcinogenesis

Solar ultraviolet (UV) radiation is an important environmental factor that leads to immune suppression, inflammation, photoaging, and skin carcinogenesis. Here, we reviewed the specific signal transduction pathways and transcription factors involved in the cellular response to UV-irradiation. Increasing experimental data supporting a role for p38, MAPK, JNK, ERK1/2, and ATM kinases in the response network to UV exposure is discussed. We also reviewed the participation of NF-κB, AP-1, and NRF2 transcription factors in the control of gene expression after UV-irradiation. In addition, we discussed the promising chemotherapeutic intervention of transcription factors signaling by natural compounds. Finally, we focused on the review of data emerging from the use of DNA microarray technology to determine changes in global gene expression in keratinocytes and melanocytes in response to UV treatment. Efforts to obtain a comprehensive portrait of the transcriptional events regulating photodamage of intact human epidermis after UV exposure reveals the existence of novel factors participating in UV-induced cell death. Progress in understanding the multitude of mechanisms induced by UV-irradiation could lead to the potential use of protein kinases and novel proteins as specific targets for the prevention and control of skin cancer.

The role of Syk kinase in ultraviolet-mediated skin damage

BACKGROUND

Ultraviolet (UV) irradiation is the main cause of skin photodamage; the resulting modulation of matrix metalloproteinases (MMPs) leads to collagen degradation. There is no easily accessible molecular indicator of early skin UV damage.

OBJECTIVES

In this study, we investigated the effects of Syk kinase on MMP expression and evaluated the sensitivity and usefulness of Syk as an early indicator of skin UV damage.

METHODS

Human dermal fibroblasts (HDFs) were transfected with Syk cDNA to overexpress Syk. MMP-1 expression and Syk activity were determined by Western blot after UV exposure. The effect of Syk on MMP-1 expression in HDFs was further explored by either Syk siRNA or a selective Syk inhibitor. Possible downstream molecules of Syk were also evaluated in HDFs upon UV exposure. The relationship between Syk and collagenase was further explored in vivo (MMP-13, hairless mice).

RESULTS

Our studies in HDFs demonstrated that both a Syk inhibitor and Syk siRNA were able to inhibit MMP-1 expression in HDFs exposed to UV and that overexpression of Syk increased MMP-1 expression and the activity of JNK kinase, but not p38 or Erk1/2 MAP kinase. UV exposure enhanced both expression and activity of Syk in HDFs. Experiments with hairless mice suggested that Syk expression is an earlier indicator of UV exposure than MMP-13 expression.

CONCLUSIONS

Our results demonstrate that Syk expression correlates well with increase of MMPs (MMP-1 in humans and MMP-13 in mice) in response to UV exposure. The findings suggest that Syk may be a novel target for the prevention and treatment of skin photodamage by modulating MMPs.

Galectin-3 protects keratinocytes from UVB-induced apoptosis by enhancing AKT activation and suppressing ERK activation

Keratinocytes undergo apoptosis in a variety of physiological and pathological conditions. Galectin-3 is a member of a family of beta-galactoside-binding animal lectins expressed abundantly in keratinocytes and other epithelial cells. Here, we have studied the regulatory role of galectin-3 in keratinocyte apoptosis by using cells from gene-targeted galectin-3 null (gal3(-/-)) mice. We showed that galectin-3 mRNA was transiently upregulated in ultraviolet-B (UVB)-irradiated wild-type keratinocytes. We found that gal3(-/-) keratinocytes were significantly more sensitive to apoptosis induced by UVB as well as various other stimuli, both in vitro and in vivo, than wild-type cells. Moreover, we demonstrated that increased apoptosis in gal3(-/-) keratinocytes was attributable to higher extracellular signal-regulated kinase (ERK) activation and lower AKT activation after UVB irradiation. We conclude that endogenous galectin-3 is an anti-apoptotic molecule in keratinocytes functioning by suppressing ERK activation and enhancing AKT activation and may play a role in the development of apoptosis-related skin diseases.

UVB-induced activation of NF-kappaB is regulated by the IGF-1R and dependent on p38 MAPK

To manage the frequent exposure to carcinogenic UVB wavelengths found in sunlight, keratinocytes have extensive protective measures to handle UVB-induced DNA damage. Recent in vitro evidence and epidemiological data suggest that one possible protective mechanism is dependent on the functional status of the IGF-1R signaling network. A second important signaling pathway regulating the response of keratinocytes to UVB involves the activation of the NF-kappaB transcription factor. Although it is clear that proper functioning of both the IGF-1R and NF-kappaB signaling networks are critical for the appropriate response of keratinocytes to UVB irradiation, it is currently uncertain if these two pathways interact. We now demonstrate that the activation of the NF-kappaB transcription factor by UVB is altered by the functional status of the IGF-1R. In the absence of ligand-activated IGF-1R, UVB-induced NF-kappaB consisted primarily of p50:p50 homodimers. Furthermore, the p38 kinase MAPK directs the subunit composition of NF-kappaB following UVB irradiation, most likely in an IGF-1R-dependent manner. We hypothesize that UVB irradiation leads to an activated p38 MAPK that is regulated in an IGF-1R-dependent manner, leading to NF-kappaB p50:RelA/p65 activation and a survival phenotype. In the absence of ligand-activated IGF-1R, UVB irradiation leads to the induction of NF-kappaB p50:p50 homodimers and a p38-dependent increased susceptibility to apoptosis.

Effect of EGF on [3H]-thymidine incorporation and cell cycle regulatory proteins in primary cultured chicken hepatocytes: Involvement of Ca2+/PKC and MAPKs

The reported studies on the metabolism in chicken hepatocytes in comparison with those of mammals are quite different. Therefore, this study examined the effect of EGF on DNA synthesis along with its related signal cascades in primary cultured chicken hepatocytes. EGF stimulated DNA synthesis in a dose (> or =10 ng/ml)-dependent manner, which correlated with the increase in CDK-2 and CDK-4 expression. The EGF-induced increase in [3H]-thymidine incorporation was blocked by AG 1478 (an EGF receptor tyrosine kinase antagonist), genistein, and herbimycin A (tyrosine kinase inhibitors), suggesting a role in the activation and tyrosine phosphorylation of the EGF receptor. In addition, the EGF-induced stimulation of [3H]-thymidine incorporation was prevented by staurosporine, H-7, or bisindolylmaleimide I (protein kinase C inhibitors), suggesting a role of PKC. In addition, PD 98059 (a MEK inhibitor), SB 203580 (a p38 MAPK inhibitor), and SP 600125 (a JNK inhibitor) blocked the EGF-induced stimulation of [3H]-thymidine incorporation and CDK-2/4 expression. Indeed, EGF increased the translocation of PKC from the cytosol to the membrane fraction, and increased the activation of p44/42 MAPK, p38 MAPK, and JNK. Moreover, EGF increased the CDK-2, CDK-4, cyclin D1, and cyclin E expression levels but decreased the p21 and p27 expression levels. These EGF-induced increases were blocked by an EGF receptor antagonist, tyrosine kinase inhibitors, PKC inhibitors, and MAPKs inhibitors. In conclusion, EGF stimulates DNA synthesis of primary cultured chicken hepatocytes via Ca2+/PKC and the MAPKs signaling pathways.

Apoptotic and necrotic mechanisms of stress-induced human lens epithelial cell death

Exposure to ultraviolet radiation (UVR) and reactive oxygen species (ROS) can damage the human lens and contribute to cataract formation. Recent evidence suggests that apoptosis in lens epithelial cells (LEC) is an initiating event in noncongenital cataract formation in humans and animals. The present study examines the cellular and molecular mechanisms by which environmental (ultraviolet B [UVB]) and chemical (hydrogen peroxide [H(2)O(2)], t-butyl hydroperoxide [TBHP]) stress induces cell death in an SV-40 immortalized human lens epithelial (HLE) cell line. Treatment of HLE cells with UVB, H(2)O(2), and TBHP significantly decreased cell density with LD50 values of 350 J/m(2), 500 muM, and 200 muM, respectively. Cellular morphology, DNA fragmentation, and annexin/propidium iodide staining consistent with apoptosis was observed only in UVB-treated cells, whereas lactate dehydrogenase (LDH) release was significantly higher in H(2)0(2)- and TBHP-treated cells. In addition, activation of apoptotic stress-signaling proteins, including c-Jun NH2-terminal kinase (JNK), caspase-3, and DNA fragmentation factor 45 (DFF45) was observed only in UVB-treated cells. Inhibition of JNK activity increased UVB-induced cell death, suggesting that this pathway may serve a prosurvival role in HLE cells. These findings suggest UVB predominantly induces apoptosis in HLE cells, whereas H(2)O(2) and TBHP induce necrosis.

Cyclic AMP differentially regulates cell proliferation of normal human keratinocytes through ERK activation depending on the expression pattern of B-Raf

Intracellular cyclic AMP (cAMP) increased by extracellular stimuli induces various biological effects, such as cell proliferation, differentiation, and migration. Previous reports regarding the effect of cAMP on keratinocyte proliferation are contradictory and indicate that the effect apparently depends on cellular density. Recent studies have revealed that cAMP signaling regulates cell proliferation by modulating mitogen-activated protein kinase (MAPK) activity. The precise mechanism by which cAMP affects keratinocyte proliferation and/or the crosstalk between the cAMP and MAPK signaling pathways, however, remain to be determined. Using normal human keratinocytes (NHK), we investigated the effect of cAMP on keratinocyte proliferation and its molecular mechanism in terms of cellular density. In confluent NHK, cyclic AMP decreased extracellular regulated kinase (ERK) phosphorylation and cell proliferation in a Ras-independent and Rap1-dependent manner. The decreased cell proliferation by cAMP was blocked by the MEK-1 inhibitor, PD98059. In contrast, in subconfluent NHK, cAMP increased ERK phosphorylation and cell proliferation. Western blot analysis revealed that NHK expressed B-Raf and Rap-1. Although both 95 kDa and 62 kDa B-Raf isoforms were expressed in subconfluent NHK, only 62 kDa B-Raf was detected in confluent NHK. Transfection of 95 kDa B-Raf into confluent NHK resulted in a cAMP-dependent increase in ERK phosphorylation and cell proliferation. These findings indicate that differential expression of B-Raf isoforms is critical for cAMP-dependent regulation of NHK proliferation that depends on phosphorylation of ERK.

Fibrogenic stresses activate different mitogen-activated protein kinase pathways in renal epithelial, endothelial or fibroblast cell populations

Fibrogenic stresses promote progression of renal tubulointerstitial fibrosis, disparately affecting survival, proliferation and trans-differentiation of intrinsic renal cell populations through ill-defined biomolecular pathways. We investigated the effect of fibrogenic stresses on the activation of cell-specific mitogen-activated protein kinase (MAPK) in renal fibroblast, epithelial and endothelial cell populations. The relative outcomes (cell death, proliferation, trans-differentiation) associated with activation or inhibition of extracellular-regulated protein kinase (ERK) or stress activated/c-Jun N terminal kinase (JNK) were analysed in each renal cell population after challenge with oxidative stress (1 mmol/L H2O2), transforming growth factor-beta1 (TGF-beta1, 10 ng/mL) or tumour necrosis factor-alpha (TNF-alpha, 50 ng/mL) over 0-20 h. Apoptosis increased significantly in all cell types after oxidative stress (P < 0.05). In fibroblasts, oxidative stress caused the activation of ERK (pERK) but not JNK (pJNK). Inhibition of ERK by PD98059 supported its role in a fibroblast death pathway. In epithelial and endothelial cells, oxidative stress-induced apoptosis was preceded by early induction of pERK, but its inhibition did not support a pro-apoptotic role. Early ERK activity may be conducive to their survival or promote the trans-differentiation of epithelial cells. In epithelial and endothelial cells, oxidative stress induced pJNK acutely. Pretreatment with SP600125 (JNK inhibitor) verified its pro-apoptotic activity only in epithelial cells. Transforming growth factor-beta1 did not significantly alter mitosis or apoptosis in any of the cell types, nor did it alter MAPK activity. Tumor necrosis factor-alpha caused increased apoptosis with no associated change in MAPK activity. Our results demonstrate renal cell-specific differences in the activation of ERK and JNK following fibrotic insult, which may be useful for targeting excessive fibroblast proliferation in chronic fibrosis.

The role of H2O2 as a mediator of UVB-induced apoptosis in keratinocytes

Apoptosis is an active form of cell death that is initiated by a variety of stimuli, including reactive oxygen species (ROS) and ultraviolet (UV) radiation. Previously, it has been reported that UVB-irradiation of keratinocytes leads to intracellular generation of hydrogen peroxide (H2O2) and that antioxidants can inhibit ROS-induced apoptosis. Although both UVB-irradiation and H2O2-incubation led to increased intracellular H2O2 levels, the antioxidants catalase and glutathione monoester (GME), inhibited apoptosis only when induced by H2O2, not by UVB. Furthermore, extracellular signal-regulated kinase (ERK), a prominent member of the mitogen-activated protein kinase (MAPK) family, was found to be activated by treatment with both UVB and H2O2. Inhibition of ERK phosphorylation by pre-treatment with PD98059 resulted in enhanced apoptosis after H2O2-exposure. However,no significant difference of apoptosis was observed between cells with and without inhibitor pre-treatment upon UVB-irradiation. DNA damage in the form of cyclobutane pyrimidine dimers was observed after exposure to UVB, but no photoproducts were found in H2O2-treated cells. These results suggest a ROS-independent pathway of UVB-induced apoptosis. Although UVB-irradiation causes moderate increase in H2O2, the generation of H2O2 does not contribute to the induction of apoptosis. Moreover, activation of ERK only blocks H2O2-dependent apoptosis but has no impact on UVB-induced apoptosis.

De novo ceramide synthesis participates in the ultraviolet B irradiation-induced apoptosis in undifferentiated cultured human keratinocytes

Ultraviolet irradiation is a major environmental cause of skin cancers, whereas ultraviolet-induced DNA repair and apoptosis are defense mechanisms that rescue and/or protect keratinocytes from this risk. Multiple pathways are involved in ultraviolet-induced keratinocyte apoptosis, including activation of p38-mitogen-activated protein kinase, protein kinase C, and CD95, each of which are associated with caspase activation. Alternatively, ceramides could serve as ultraviolet-induced, second messenger lipids, because they induce cell cycle arrest and apoptosis in a variety of cell types, including keratinocytes. We investigated the role of ceramide versus caspase, and the responsible pathway for ceramide generation in ultraviolet B-induced apoptosis of cultured normal human keratinocytes maintained in low calcium (0.07 mm) medium. Ultraviolet B (40 mJ per cm2) significantly inhibited cultured normal human keratinocyte proliferation, assessed as [3H-methyl]thymidine-thymidine incorporation into DNA, 2 h after irradiation. Terminal nick deoxynucleotide end-labeling-positive apoptotic cells (14.8% at 24 h and 34.4% at 48 h) and trypan blue-positive apoptotic cells (8.4% at 24 h and 28.6% at 48 h) became evident in a time-dependent manner after ultraviolet B irradiation, in parallel with activation of caspase-3. The ceramide content of irradiated cultured normal human keratinocytes increased significantly by 8 h, whereas glucosylceramide only modestly increased, and sphingomyelin content remained unaltered. Metabolic studies with radiolabeled serine, palmitic acid, and phosphorylcholine revealed that the ultraviolet B-induced increase in ceramide results primarily from increased de novo synthesis rather than accelerated sphingomyelin hydrolysis. Increased ceramide synthesis, in turn, could be attributed to increased activity of ceramide synthase (i.e., 1.7-fold increase 8 h after ultraviolet B irradiation), whereas serine palmitoyltransferase activity did not change. Both fumonisin B1, an inhibitor of ceramide synthase, and ISP-1, myriocin an inhibitor of serine palmitoyltransferase, significantly attenuated the ultraviolet B-induced apoptosis in a caspase-3-independent fashion, whereas co-incubation with a caspase-3 inhibitor (Ac-DEVD-chloromethyl-ketone) further attenuated the ultraviolet B-induced apoptosis. Thus, increased de novo ceramide synthesis signals ultraviolet B-induced apoptosis, by a pathway independent of, but in concert with, caspase-3 activation.

Mitogen-activated protein kinase activation in UV-induced signal transduction

Experimental evidence supported by epidemiological findings suggests that solar ultraviolet (UV) irradiation is the most important environmental carcinogen leading to the development of skin cancers. Because the ozone layer blocks UVC (wavelength, 180 to 280 nm) exposure, UVA (UVA I, 340 to 400 nm; UVA II, 320 to 340 nm) and UVB (280 to 320 nm) are probably the chief carcinogenic components of sunlight with relevance for human skin cancer. Substantial contributions to the elucidation of the specific signal transduction pathways involved in UV-induced skin carcinogenesis have been made over the past few years, and most evidence suggests that the cellular signaling response is UV wavelength-dependent. The mitogen-activated protein kinase (MAPK) signaling cascades are targets for UV and are important in the regulation of the multitude of UV-induced cellular responses. Experimental studies have used a range of UVA, UVB, UVC, and various combinations in multiple doses, and the observed effects on activation and phosphorylation of MAPKs are varied. This review focuses on the mechanistic data supporting a role for MAPKs in UV-induced skin carcinogenesis. Progress in understanding the mechanisms of UV-induced signal transduction could lead to the use of these protein kinases as specific targets for the prevention and control of skin cancer.

Keratinocyte survival, differentiation, and death: many roads lead to mitogen-activated protein kinase

The epidermis is a dynamic and continually renewing surface that provides and maintains a life-sustaining interface with the environment. The epidermal keratinocyte, the major cell type of the epidermis, undergoes a complex and carefully choreographed program of differentiation. This process requires a balance between keratinocyte proliferation, differentiation, and apoptosis. This overview will concentrate on cascades that regulate the balance between keratinocyte cell proliferation and survival, and apoptosis and cell differentiation, with a particular emphasis on the role of the mitogen-activated protein kinase cascades. A summary of the literature suggests that extracellular regulated kinases function to promote keratinocyte proliferation and survival, whereas p38 mitogen-activated protein kinase functions to promote differentiation and apoptosis.

Life and death signaling pathways contributing to skin cancer

Apoptosis is generally regarded as a critical regulatory event in the development of malignancies in several different organ systems (Thompson, 1995). Initially, oncologists focused on alterations in rates of proliferation and cell cycle kinetics, but more recently an emphasis on apoptosis has dominated the fight against cancer (Evan and Vousden, 2001). As approximately 1,000,000 individuals in the U.S.A. develop skin cancer each year, it is important to elucidate the molecular mechanisms that govern cell survival and cell death in the epidermis (Miller and Weinstock, 1994). Moreover, given that most skin cancers occur on sun-exposed skin, the pro-apoptotic and antiapoptotic response of keratinocytes (KC) to UV light is of particular relevance to the development of skin cancer (Brash et al, 1996). Whereas both squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) arise from epidermal KC, it is becoming increasingly apparent that the natural history of their development, their underlying molecular pathogenesis, and potential involvement of antiapoptotic pathways are significantly different. Nonetheless, as pointed out later in the text, significant progress is being made in our understanding of the pathophysiology of these relatively common epithelial-cell-derived neoplasms. In this review we will explore four topics: first, a review of the life and death signaling pathways operative in normal human skin that prevents premature apoptosis of KC with an emphasis on nuclear factor kappaB (NFkappaB) survival signals; second, the molecular pathways that are engaged and regulate apoptosis after normal KC are exposed to ultraviolet (UV) light; third, the apoptotic resistant mechanisms that premalignant and malignant KC utilize to avoid cell death; fourth, therapeutic strategies that can render malignant cells more susceptible to apoptosis with an emphasis on a death pathway mediated by the death ligand TRAIL.

Extracellular regulated kinase and c-Jun N-terminal kinase are activated in psoriatic involved epidermis

Psoriatic skin shows markedly increased keratinocyte proliferation and altered differentiation with various abnormal signalling pathways. In the present study, we investigated the expression of mitogen-activated protein kinases in psoriatic skin. Immunohistochemical study showed increased extracellular regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) expression in the nuclei of involved epidermis. Western blot analyses confirmed the increased active phospho-ERK and phospho-JNK expression in the involved epidermis. In contrast, expression pattern of p38 was not different between the involved and uninvolved epidermis, which was confirmed by western blot analysis. These results indicate that psoriatic epidermis shows selective activation of ERK and JNK, which might be related to hyperproliferation and abnormal differentiation of psoriatic epidermis.

ILPIP, a novel anti-apoptotic protein that enhances XIAP-mediated activation of JNK1 and protection against apoptosis

We have previously described a new aspect of the Inhibitor of Apoptosis (IAP) family of proteins anti-apoptotic activity that involves the TAK1/JNK1 signal transduction pathway (1,2). Our findings suggest the existence of a novel mechanism that regulates the anti-apoptotic activity of IAPs that is separate from caspase inhibition but instead involves TAK1-mediated activation of JNK1. In a search for proteins involved in the XIAP/TAK1/JNK1 signaling pathway we isolated by yeast two-hybrid screening a novel X chromosome-linked IAP (XIAP)-interacting protein that we called ILPIP (hILP-Interacting Protein). Whereas ILPIP moderately activates JNK family members when expressed alone, it strongly enhances XIAP-mediated activation of JNK1, JNK2, and JNK3. The expression of a catalytically inactive mutant of TAK1 blocked XIAP/ILPIP synergistic activation of JNK1 thereby implicating TAK1 in this signaling pathway. ILPIP moderately protects against interleukin-1beta converting enzyme- or Fas-induced apoptosis and significantly potentiates the anti-apoptotic activity of XIAP. In vivo co-precipitation experiments show that both ILPIP and XIAP interact with TAK1 and tumor necrosis factor receptor-associated factor 6. Finally, expression of ILPIP did not affect the ability of XIAP to inhibit caspase activation, further supporting the idea that XIAP protection against apoptosis is achieved by two separate mechanisms: one requiring JNK1 activation and a second involving caspase inhibition.

IAP Suppression of Apoptosis Involves Distinct Mechanisms: the TAK1/JNK1 Signaling Cascade and Caspase Inhibition

The antiapoptotic properties of the inhibitor of apoptosis (IAP) family of proteins have been linked to caspase inhibition. We have previously described an alternative mechanism of XIAP inhibition of apoptosis that depends on the selective activation of JNK1. Here we report that two other members of the IAP family, NAIP and ML-IAP, both activate JNK1. Expression of catalytically inactive JNK1 blocks NAIP and ML-IAP protection against ICE- and TNF-α-induced apoptosis, indicating that JNK1 activation is necessary for the antiapoptotic effect of these proteins. The MAP3 kinase, TAK1, appears to be an essential component of this antiapoptotic pathway since IAP-mediated activation of JNK1, as well as protection against TNF-α- and ICE-induced apoptosis, is inhibited when catalytically inactive TAK1 is expressed. In addition, XIAP, NAIP, and JNK1 bind to TAK1. Importantly, expression of catalytically inactive TAK1 did not affect XIAP inhibition of caspase activity. These data suggest that XIAP's antiapoptotic activity is achieved by two separate mechanisms: one requiring TAK1-dependent JNK1 activation and the second involving caspase inhibition.

IAP suppression of apoptosis involves distinct mechanisms: the TAK1/JNK1 signaling cascade and caspase inhibition

The antiapoptotic properties of the inhibitor of apoptosis (IAP) family of proteins have been linked to caspase inhibition. We have previously described an alternative mechanism of XIAP inhibition of apoptosis that depends on the selective activation of JNK1. Here we report that two other members of the IAP family, NAIP and ML-IAP, both activate JNK1. Expression of catalytically inactive JNK1 blocks NAIP and ML-IAP protection against ICE- and TNF-alpha-induced apoptosis, indicating that JNK1 activation is necessary for the antiapoptotic effect of these proteins. The MAP3 kinase, TAK1, appears to be an essential component of this antiapoptotic pathway since IAP-mediated activation of JNK1, as well as protection against TNF-alpha- and ICE-induced apoptosis, is inhibited when catalytically inactive TAK1 is expressed. In addition, XIAP, NAIP, and JNK1 bind to TAK1. Importantly, expression of catalytically inactive TAK1 did not affect XIAP inhibition of caspase activity. These data suggest that XIAP's antiapoptotic activity is achieved by two separate mechanisms: one requiring TAK1-dependent JNK1 activation and the second involving caspase inhibition.