Basal phosphorylation of the PEST domain in the I(kappa)B(beta) regulates its functional interaction with the c-rel proto-oncogene product.

The product of the c-rel proto-oncogene (c-Rel) belongs to the NF-kappaB/Rel family of polypeptides and has been implicated in the transcriptional control of cell proliferation and immune function. In human T lymphocytes, c-Rel is sequestered in the cytoplasmic compartment by constitutively phosphorylated inhibitors, including I(kappa)B(alpha) and I(kappa)B(beta). Studies with bacterially expressed forms of these inhibitory proteins revealed that unphosphorylated I(kappa)B(alpha) but not I(kappa)B(beta) assembles with c-Rel and inhibits its DNA binding activity. Furthermore, latent I(kappa)B(beta)-c-Rel complexes derived from mammalian cells were sensitive to phosphatase treatment, whereas I(kappa)B(alpha)-c-Rel complexes were resistant. We have identified a constitutive protein kinase in unstimulated T cells that associates with and phosphorylates I(kappa)B(beta) in vitro. The substrate specificity, electrophoretic mobility, and antigenic properties of this I(kappa)B(beta)-associated kinase (BAK) suggest identity with casein kinase II (CKII), an enzyme known to mediate basal phosphorylation of I(kappa)B(alpha). Phosphorylation of recombinant I(kappa)B(beta) by either BAK or CKII restored the capacity of this inhibitor to antagonize the DNA binding activity of c-Rel. Peptide mapping and mutational analyses localized the bulk of the basal phosphorylation sites in I(kappa)B(beta) to the C-terminal PEST domain, which contains two potential acceptors for CKII-mediated phosphoryl group transfer (Ser-313 and Ser-315). Point mutations introduced into the full-length inhibitor at Ser-313 and Ser-315 led to a significant reduction in the phosphorylation of I(kappa)B(beta) and severely impaired its c-Rel inhibitory function in vivo. Taken together, these findings strongly suggest that basal phosphorylation of the PEST domain of I(kappa)B(beta) at consensus CKII sites is required for the efficient formation of latent I(kappa)B(beta)-c-Rel complexes.

[Effects of berberine on synthesis of platelet TXA2 and plasma PGI2 in rabbits].

Using radioimmunoassay, we found that berberine (0.50 mol.L-1) inhibited dose-dependently collagen-, ADP-, and arachidonic acid (AA)-induced TXA2 release from platelets. Berberine (25 mg.kg-1) iv lowered rabbit plasma level of PGI2 from 0.92 +/- 0.20 to 0.61 +/- 0.08 ng.ml-1 after 60 min. The results suggest that berberine inhibits AA release from cell membrane phospholipids and its metabolism.

Phosphorylation of the PEST domain of IkappaBbeta regulates the function of NF-kappaB/IkappaBbeta complexes.

Activation of transcription factor NF-kappaB involves the signal-dependent degradation of basally phosphorylated inhibitors such as IkappaBalpha and IkappaBbeta. The gene encoding IkappaBalpha is under NF-kappaB control, which provides a negative feedback loop to terminate the induced NF-kappaB response. However, recent studies have identified a hypophosphorylated pool of IkappaBbeta that shields nuclear NF-kappaB from inhibition by newly synthesized IkappaBalpha. In the present work, we provide three lines of evidence indicating that this protection mechanism is regulated by the C-terminal PEST domain of IkappaBbeta. First, disruption of two basal phosphoacceptors present in the IkappaBbeta PEST domain (Ser-313 and Ser-315) yields a mutant that forms ternary complexes with NF-kappaB and its target DNA-binding site. Second, based on in vitro mixing experiments, these ternary complexes are resistant to the inhibitory action of IkappaBalpha. Third, mutants of IkappaBbeta that are defective for phosphorylation at Ser-313 and Ser-315 fail to efficiently block NF-kappaB-directed transcription in vivo, whereas replacement of these two IkappaBbeta residues with a phosphoserine mimetic generates a fully functional repressor. Taken together, our findings suggest that the functional fate of NF-kappaB when bound to IkappaBbeta is critically dependent on the phosphorylation status of the IkappaBbeta PEST domain.

Erythroid-specific processing of human beta spectrin I pre-mRNA.

Erythroid cells express a unique form of beta spectrin I as a result of tissue-specific alternative pre-mRNA processing. Nonerythroid cells that express the beta spectrin I gene include four additional exons at the 3' end of the mature transcript, leading to elongation of the carboxyl terminus of the protein. The nonerythroid beta spectrin I isoform is not present in the red blood cell membrane skeleton; the erythroid isoform is not detected in other cell types. Therefore, developing erythroid cells acquire this tissue-specific pre-mRNA processing activity during differentiation. In the present study, we investigated the developmental timing of erythroid-specific pre-mRNA processing in human erythroid precursors. Partially purified human peripheral blood burst forming uniterythroid (BFU-E) cells were grown in culture for 5 to 12 days. beta Spectrin I mRNA transcripts were analyzed at different time points by S1 nuclease mapping. The processing of beta spectrin I transcripts was found to be exclusively erythroid from day 5 onward, indicating that erythroid-specific processing is not linked temporally to assembly of the mature erythroid membrane skeleton. Human erythroleukemia (HEL) cells had both erythroid and nonerythroid transcripts, indicating that both processing patterns can coexist. Induction of erythroid differentiation in HEL cells using hemin resulted in a partial switch toward the erythroid processing pattern of beta spectrin I transcripts. Using a genomic S1 probe that spans the erythroid polyadenylation signal, we found that a substantial portion of the transcripts detected by the erythroid cDNA S1 probe (in both cultured BFU-E and HEL cells) is incompletely processed pre-mRNA precursors. Poly(A) RNA selection before S1 analysis showed that the unprocessed transcripts are not polyadenylated. We conclude that (1) erythroid-specific pre-mRNA processing activity is present early in erythroid differentiation; (2) beta spectrin I transcripts that are unprocessed at the 3' end accumulate, awaiting either erythroid or nonerythroid processing pathways, from which observation we infer that the regulated alternative pathways are both inefficient; and (3) HEL cells offer a human cell culture model in which to study the balance between the two pre-mRNA processing pathways. We speculate that erythroid cells evolved this tissue-specific pre-mRNA processing machinery for other erythroid genes in addition to beta spectrin I.

The tax oncoprotein of human T-cell leukemia virus type 1 associates with and persistently activates IkappaB kinases containing IKKalpha and IKKbeta.

The Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV1) chronically activates transcription factor NF-kappaB by a mechanism involving degradation of IkappaBalpha, an NF-kappaB-associated cytoplasmic inhibitor. Tax-induced breakdown of IkappaBalpha requires phosphorylation of the inhibitor at Ser-32 and Ser-36, which is also a prerequisite for the transient activation of NF-kappaB in cytokine-treated T lymphocytes. However, it remained unclear how Tax interfaces with the cellular NF-kappaB/IkappaB signaling machinery to generate a chronic rather than a transient NF-kappaB response. We now demonstrate that Tax associates with cytokine-inducible IkappaB kinase (IKK) complexes containing catalytic subunits IKKalpha and IKKbeta, which mediate phosphorylation of IkappaBalpha at Ser-32 and Ser-36. Unlike their transiently activated counterparts in cytokine-treated cells, Tax-associated forms of IKK are constitutively active in either Tax transfectants or HTLV1-infected T lymphocytes. Moreover, point mutations in Tax that ablate its IKK-binding function also prevent Tax-mediated activation of IKK and NF-kappaB. Together, these findings suggest that the persistent activation of NF-kappaB in HTLV1-infected T-cells is mediated by a direct Tax/IKK coupling mechanism.

A novel enhancer of the Apaf1 apoptosome involved in cytochrome c-dependent caspase activation and apoptosis.

Apaf1/CED4 family members play central roles in apoptosis regulation as activators of caspase family cell death proteases. These proteins contain a nucleotide-binding (NB) self-oligomerization domain and a caspase recruitment domain (CARD). A novel human protein was identified, NAC, that contains an NB domain and CARD. The CARD of NAC interacts selectively with the CARD domain of Apaf1, a caspase-activating protein that couples mitochondria-released cytochrome c (cyt-c) to activation of cytosolic caspases. Cyt-c-mediated activation of caspases in cytosolic extracts and in cells is enhanced by overexpressing NAC and inhibited by reducing NAC using antisense/DNAzymes. Furthermore, association of NAC with Apaf1 is cyt c-inducible, resulting in a mega-complex (>1 MDa) containing both NAC and Apaf1 and correlating with enhanced recruitment and proteolytic processing of pro-caspase-9. NAC also collaborates with Apaf1 in inducing caspase activation and apoptosis in intact cells, whereas fragments of NAC representing only the CARD or NB domain suppress Apaf1-dependent apoptosis induction. NAC expression in vivo is associated with terminal differentiation of short lived cells in epithelia and some other tissues. The ability of NAC to enhance Apaf1-apoptosome function reveals a novel paradigm for apoptosis regulation.

Suppression of tumor necrosis factor-induced cell death by inhibitor of apoptosis c-IAP2 is under NF-kappaB control.

Members of the NF-kappaB/Rel and inhibitor of apoptosis (IAP) protein families have been implicated in signal transduction programs that prevent cell death elicited by the cytokine tumor necrosis factor alpha (TNF). Although NF-kappaB appears to stimulate the expression of specific protective genes, neither the identities of these genes nor the precise role of IAP proteins in this anti-apoptotic process are known. We demonstrate here that NF-kappaB is required for TNF-mediated induction of the gene encoding human c-IAP2. When overexpressed in mammalian cells, c-IAP2 activates NF-kappaB and suppresses TNF cytotoxicity. Both of these c-IAP2 activities are blocked in vivo by coexpressing a dominant form of IkappaB that is resistant to TNF-induced degradation. In contrast to wild-type c-IAP2, a mutant lacking the C-terminal RING domain inhibits NF-kappaB induction by TNF and enhances TNF killing. These findings suggest that c-IAP2 is critically involved in TNF signaling and exerts positive feedback control on NF-kappaB via an IkappaB targeting mechanism. Functional coupling of NF-kappaB and c-IAP2 during the TNF response may provide a signal amplification loop that promotes cell survival rather than death.

IKKgamma mediates the interaction of cellular IkappaB kinases with the tax transforming protein of human T cell leukemia virus type 1.

The Tax oncoprotein of human T cell leukemia virus type 1 constitutively activates transcription factor NF-kappaB by a mechanism involving Tax-induced phosphorylation of IkappaBalpha, a labile cytoplasmic inhibitor of NF-kappaB. To trigger this signaling cascade, Tax associates stably with and persistently activates a cellular IkappaB kinase (IKK) containing both catalytic (IKKalpha and IKKbeta) and noncatalytic (IKKgamma) subunits. We now demonstrate that IKKgamma enables Tax to dock with the IKKbeta catalytic subunit, resulting in chronic IkappaB kinase activation. Mutations in either IKKgamma or Tax that prevent formation of these higher order Tax.IKK complexes also interfere with the ability of Tax to induce IKKbeta catalytic function in vivo. Deletion mapping studies indicate that amino acids 1-100 of IKKgamma are required for this Tax targeting function. Together, these findings identify IKKgamma as an adaptor protein that directs the stable formation of pathologic Tax.IKK complexes in virally infected T cells.