The enzymatic activity of SR protein kinases 1 and 1a is negatively affected by interaction with scaffold attachment factors B1 and 2.

SR protein kinases (SRPKs) phosphorylate Ser/Arg dipeptide-containing proteins that play crucial roles in a broad spectrum of basic cellular processes. Phosphorylation by SRPKs constitutes a major way of regulating such cellular mechanisms. In the past, we have shown that SRPK1a interacts with the nuclear matrix protein scaffold attachment factor B1 (SAFB1) via its unique N-terminal domain, which differentiates it from SRPK1. In this study, we show that SAFB1 inhibits the activity of both SRPK1a and SRPK1 in vitro and that its RE-rich region is redundant for the observed inhibition. We demonstrate that kinase activity inhibition is caused by direct binding of SAFB1 to SRPK1a and SRPK1, and we also present evidence for the in vitro binding of SAFB2 to the two kinases, albeit with different affinity. Moreover, we show that both SR protein kinases can form complexes with both scaffold attachment factors B in living cells and that this interaction is capable of inhibiting their activity, depending on the tenacity of the complex formed. Finally, we present data demonstrating that SRPK/SAFB complexes are present in the nucleus of HeLa cells and that the enzymatic activity of the nuclear matrixlocalized SRPK1 is repressed. These results suggest a new role for SAFB proteins as regulators of SRPK activity and underline the importance of the assembly of transient intranuclear complexes in cellular regulation.

MgcRacGAP interacts with HIF-1alpha and regulates its transcriptional activity.

HIF-1alpha is the inducible subunit of the dimeric transcription factor HIF-1 (Hypoxia Inducible Factor 1). It is induced by hypoxia and hypoxia-mimetics in most cell types, as well as non-hypoxic signals such as growth factors, cytokines and oncogenes, often in a cell specific manner. HIF-1 is present in virtually all cells of higher eukaryotes and its function is of great biomedical relevance since it is highly involved in development, tumor progression and tissue ischemia. Intracellular signaling to HIF-1alpha, as well as its further action, involves its participation in numerous protein complexes. Using the yeast two-hybrid system we have identified MgcRacGAP (male germ cell Rac GTPase Activating Protein) as a HIF-1alpha interacting protein. The MgcRacGAP protein is a regulator of Rho proteins, which are principally involved in cytoskeletal organization. We have verified specific binding of HIF-1alpha and MgcRacGAP in vitro and in vivo in mammalian cells. We have additionally shown that MgcRacGAP overexpression inhibits HIF-1alpha transcriptional activity, without lowering HIF-1alpha protein levels, or altering its subcellular localization. Moreover, this inhibition is dependent on the MgcRacGAP domain that interacts with HIF-1alpha. In conclusion, our findings demonstrate that HIF-1alpha function is negatively affected by its interaction with MgcRacGAP.

Exposure of differentiated airway smooth muscle cells to serum stimulates both induction of hypoxia-inducible factor-1{alpha} and airway responsiveness to ACh.

Airway smooth muscle (ASM) cells are characterized by phenotypic plasticity and can switch between differentiated and proliferative phenotypes. In rabbit tracheal ASM cells that had been differentiated in vitro by serum starvation, readdition of FBS caused initiation of proliferation and induction of nuclear and transcriptionally active hypoxia-inducible factor (HIF)-1alpha. In addition, FBS stimulated the induction of HIF-1alpha by the hypoxia mimetic cobalt. Treatment with actinomycin D, cycloheximide, the phosphatidylinositol 3-kinase inhibitors LY-294002 and wortmannin or the reactive oxygen species scavenger diphenyleneiodonium inhibited the FBS-dependent induction of HIF-1alpha. These data indicate that, in differentiated ASM cells, FBS upregulates HIF-1alpha by a transcription-, translation-, phosphatidylinositol 3-kinase-, and reactive oxygen species-dependent mechanism. Interestingly, addition of FBS and cobalt also induced HIF-1alpha in organ cultures of rabbit trachea strips and synergistically increased their contractile response to ACh, suggesting that HIF-1alpha might be implicated in airway hypercontractility.

Identification of MAPK phosphorylation sites and their role in the localization and activity of hypoxia-inducible factor-1alpha.

Hypoxia-inducible factor 1 (HIF-1) controls the expression of most genes induced by hypoxic conditions. Regulation of expression and activity of its inducible subunit, HIF-1alpha, involves several post-translational modifications. To study HIF-1alpha phosphorylation, we have used human full-length recombinant HIF-1alpha as a substrate in kinase assays. We show that at least two different nuclear protein kinases, one of them identified as p42/p44 MAPK, can modify HIF-1alpha. Analysis of in vitro phosphorylated HIF-1alpha by mass spectroscopy revealed residues Ser-641 and Ser-643 as possible MAPK phosphorylation sites. Site-directed mutagenesis of these residues reduced significantly the phosphorylation of HIF-1alpha. When these mutant forms of HIF-1alpha were expressed in HeLa cells, they exhibited much lower transcriptional activity than the wild-type form. However, expression of the same mutants in yeast revealed that their capacity to stimulate transcription was not significantly compromised. Localization of the green fluorescent protein-tagged HIF-1alpha mutants in HeLa cells showed their exclusion from the nucleus in contrast to wild-type HIF-1alpha. Treatment of the cells with leptomycin B, an inhibitor of the major exportin CRM1, reversed this exclusion and led to nuclear accumulation and partial recovery of the activity of the HIF-1alpha mutants. Moreover, inhibition of the MAPK pathway by PD98059 impaired the phosphorylation, nuclear accumulation, and activity of wild-type GFP-HIF-1alpha. Overall, these data suggest that phosphorylation of Ser-641/643 by MAPK promotes the nuclear accumulation and transcriptional activity of HIF-1alpha by blocking its CRM1-dependent nuclear export.

Bacterially produced human HIF-1alpha is competent for heterodimerization and specific DNA-binding.

Hypoxia-inducible factor 1alpha (HIF-1alpha) is the regulatory subunit of HIF-1, the transcriptional activator and key mediator of the cellular response to hypoxia. Regulation of HIF-1alpha occurs at multiple levels and involves several different post-translational modifications. In order to examine the importance of these modifications for the basic function of HIF-1alpha, we have produced in bacteria recombinant full-length human HIF-1alpha using different expression systems. We show that this unmodified form of HIF-1alpha is able to form a stable heterodimer with the second subunit of HIF-1 (HIF-1beta or ARNT) when both proteins are co-expressed in Escherichia coli. Furthermore, this bacterially reconstituted heterodimer exhibits specific DNA-binding activity. These data indicate that post-translational modification of HIF-1alpha is not essential for its interaction with ARNT and DNA, and provide an in vitro system for the characterization of the molecular properties of HIF-1alpha.