Potentiation of glucocorticoid receptor-mediated gene expression by heat and chemical shock

FKBP51 reciprocally regulates GRα and PPARγ activation via the Akt-p38 pathway

FK506-binding protein 51 (FKBP51) is a negative regulator of glucocorticoid receptor-α (GRα), although the mechanism is unknown. We show here that FKBP51 is also a chaperone to peroxisome proliferator-activated receptor-γ (PPARγ), which is essential for activity, and uncover the mechanism underlying this differential regulation. In COS-7 cells, FKBP51 overexpression reduced GRα activity at a glucocorticoid response element-luciferase reporter, while increasing PPARγ activity at a peroxisome proliferator response element reporter. Conversely, FKBP51-deficient (knockout) (51KO) mouse embryonic fibroblasts (MEFs) showed elevated GRα but reduced PPARγ activities compared with those in wild-type MEFs. Phosphorylation is known to exert a similar pattern of reciprocal modulation of GRα and PPARγ. Knockdown of FKBP51 in 3T3-L1 preadipocytes increased phosphorylation of PPARγ at serine 112, a phospho-residue that inhibits activity. In 51KO cells, elevated phosphorylation of GRα at serines 220 and 234, phospho-residues that promote activity, was observed. Because FKBP51 is an essential chaperone to the Akt-specific phosphatase PH domain leucine-rich repeat protein phosphatase, Akt signaling was investigated. Elevated Akt activation and increased activation of p38 kinase, a downstream target of Akt that phosphorylates GRα and PPARγ, were seen in 51KO MEFs, causing activation and inhibition, respectively. Inactivation of p38 with PD169316 reversed the effects of FKBP51 deficiency on GRα and PPARγ activities and reduced PPARγ phosphorylation. Last, loss of FKBP51 caused a shift of PPARγ from cytoplasm to nucleus, as previously shown for GRα. A model is proposed in which FKBP51 loss reciprocally regulates GRα and PPARγ via 2 complementary mechanisms: activation of Akt-p38-mediated phosphorylation and redistribution of the receptors to the nucleus for direct targeting by p38.

RSUME enhances glucocorticoid receptor SUMOylation and transcriptional activity

Glucocorticoid receptor (GR) activity is modulated by posttranslational modifications, including phosphorylation, ubiquitination, and SUMOylation. The GR has three SUMOylation sites: lysine 297 (K297) and K313 in the N-terminal domain (NTD) and K721 within the ligand-binding domain. SUMOylation of the NTD sites mediates the negative effect of the synergy control motifs of GR on promoters with closely spaced GR binding sites. There is scarce evidence on the role of SUMO conjugation to K721 and its impact on GR transcriptional activity. We have previously shown that RSUME (RWD-containing SUMOylation enhancer) increases protein SUMOylation. We now demonstrate that RSUME interacts with the GR and increases its SUMOylation. RSUME regulates GR transcriptional activity and the expression of its endogenous target genes, FKBP51 and S100P. RSUME uncovers a positive role for the third SUMOylation site, K721, on GR-mediated transcription, demonstrating that GR SUMOylation acts positively in the presence of a SUMOylation enhancer. Both mutation of K721 and small interfering RNA-mediated RSUME knockdown diminish GRIP1 coactivator activity. RSUME, whose expression is induced under stress conditions, is a key factor in heat shock-induced GR SUMOylation. These results show that inhibitory and stimulatory SUMO sites are present in the GR and at higher SUMOylation levels the stimulatory one becomes dominant.

Enhancement of cytokine expression in transiently transfected cells by magnetoliposome mediated hyperthermia

We investigated the enhancement of cytokine expression by heat treatment in transiently transfected glioma cells. The cells were transfected with plasmid bearing the interferon (IFN)-β gene under control of the MMTV promoter, which is inducible by glucocorticoid (dexamethasone). Then magnetite particles (10 nm diameter) as intracellular heating material were incorporated to the cells as the form of magnetoliposome. After 5 hours of incorporation, alternative magnetic field (384Oe, 118kHz) was applied for intracellular heating. IFN-β secreted in the medium was assayed and its concentration was compared to the extracellular heating induced expression, both in the presence and absence of dexamethasone. Higher IFN-β concentration was detected in intracellular heating even at lower temperature, 39 °C, than 43 °C in extracellular heating. The IFN-β expression level reached in the presence of dexamethasone was about three times higher than in the absence of inducer. In intracellular heating of 60 min, the surviving cell number reduced until 20%.

ARRB1-mediated regulation of E2F target genes in nicotine-induced growth of lung tumors

BACKGROUND

Nicotine induces the proliferation of non-small cell lung cancer (NSCLC) cells via nicotinic acetylcholine receptors and the arrestin, β1 (ARRB1) protein. However, whether ARRB1 translocates to the nucleus upon nicotinic acetylcholine receptor activation and how it regulates growth of human NSCLCs are not known.

METHODS

We investigated nuclear localization of ARRB1 in human NSCLC cell lines (A549 and H1650), normal lung cell lines (NHBE and SAEC), and lung cancer tissue microarray. A549 cells were transfected with ARRB1-specific short hairpin RNA (A549-sh) to knockdown ARRB1 expression, or with empty vector (A549-EV), to examine the role of ARRB1 in the mitogenic and antiapoptotic effects of nicotine, binding of ARRB1 to E2F transcription factors, and the role of ARRB1 in nicotine-induced expression of E2F-regulated survival and proliferative genes cell division cycle 6 homolog (CDC6), thymidylate synthetase (TYMS), and baculoviral IAP repeat-containing 5 (BIRC5). Real-time polymerase chain reaction was performed for quantitative analysis of mRNA expression. Chromatin immunoprecipitation assays were performed on A549 cells and fresh-frozen human NSCLC tumors (n = 8) to examine the binding of ARRB1, E1A binding protein (EP300), and acetylated histone 3 (Ac-H3) on the E2F-regulated genes. All statistical tests were two-sided.

RESULTS

Nicotine induced the nuclear translocation of ARRB1 in NSCLC and normal lung cells, and lung tumor tissues from smokers showed an increased nuclear localization. The mitogenic and antiapoptotic effects of nicotine were reduced in A549-sh cells. Nuclear ARRB1 bound to E2F transcription factors in normal lung cells, NSCLC cells, and tumors. Nicotine treatment induced a statistically significant increased expression of E2F-regulated genes in A549-EV but not in A549-sh cells; the maximum difference being observed in BIRC5 (A549-EV vs A549-sh, mean fold-increase in mRNA level upon nicotine treatment = 20.7-fold, 95% confidence interval = 19.2- to 22.2-fold, vs mean = 0.8-fold, 95% confidence interval= 0.78- to 0.82-fold, P < .001). Furthermore, nicotine induced the binding of ARRB1, EP300, and Ac-H3 on E2F-regulated genes.

CONCLUSION

Nicotine induced the nuclear translocation of ARRB1 and showed increased expression of proliferative and survival genes, thereby contributing to the growth and progression of NSCLCs.

Co-chaperones are limiting in a depleted chaperone network

To probe the limiting nodes in the chaperoning network which maintains cellular proteostasis, we expressed a dominant negative mutant of heat shock factor 1 (dnHSF1), the regulator of the cytoplasmic proteotoxic stress response. Microarray analysis of non-stressed dnHSF1 cells showed a two- or more fold decrease in the transcript level of 10 genes, amongst which are the (co-)chaperone genes HSP90AA1, HSPA6, DNAJB1 and HSPB1. Glucocorticoid signaling, which requires the Hsp70 and the Hsp90 folding machines, was severely impaired by dnHSF1, but fully rescued by expression of DNAJA1 or DNAJB1, and partially by ST13. Expression of DNAJB6, DNAJB8, HSPA1A, HSPB1, HSPB8, or STIP1 had no effect while HSP90AA1 even inhibited. PTGES3 (p23) inhibited only in control cells. Our results suggest that the DNAJ co-chaperones in particular become limiting in a depleted chaperoning network. Our results also suggest a difference between the transcriptomes of cells lacking HSF1 and cells expressing dnHSF1.

Suppression of 11β-hydroxysteroid dehydrogenase type 1 with RNA interference substantially attenuates 3T3-L1 adipogenesis

11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1), which regulates the local level of glucocorticoids, has been suggested to be involved in the development of obesity. A definitive functional role for 11β-HSD1 in adipogenesis, however, remains to be established. We developed 3T3-L1 cell lines stably transfected with a small hairpin RNA (shRNA) targeting 11β-HSD1. A shRNA containing two nucleotide substitutions was used as a control. Silencing of 11β-HSD1 substantially attenuated the accumulation of lipid droplets and the expression of adipogenesis marker genes, which was induced by a mixture containing either corticosterone or dexamethasone. Silencing of 11β-HSD1 increased the concentration of 11-dehydrocorticosterone in the culture supernatant but did not significantly affect the levels of corticosterone or dexamethasone. Translocation of glucocorticoid receptors to the nucleus in response to glucocorticoids was significantly attenuated by silencing 11β-HSD1. The number of cells entering the S phase of the cell cycle following the induction of adipogenesis was significantly reduced by silencing 11β-HSD1. 11β-HSD1 shRNA delivered by lentiviral vectors after the induction of differentiation, however, did not affect the progression of adipogenesis. These results indicate that 11β-HSD1 plays a significant functional role in the initiation of 3T3-L1 adipogenesis and provide new mechanistic insights into the role of 11β-HSD1 in the development of obesity and related diseases.

Antidepressant fluoxetine enhances glucocorticoid receptor function in vitro by modulating membrane steroid transporters

1. Incubation of LMCAT fibroblast cells with antidepressants potentiates glucocorticoid receptor (GR)-mediated gene transcription in the presence of dexamethasone and cortisol, but not of corticosterone. We have shown that antidepressants do so by inhibiting the LMCAT cell membrane steroid transporter (which is virtually identical to the multidrug resistance P-glycoprotein) and thus by increasing dexamethasone or cortisol intracellular concentrations. However, previous experiments with the antidepressant fluoxetine in the presence of dexamethasone have produced negative results (Pariante et al. (2001). Br. J. Pharmacol., 134, 1335-1343). 2. We have since re-examined the effects of fluoxetine on GR-mediated gene transcription in the presence of dexamethasone. Moreover, we have examined the effects of fluoxetine on GR-mediated gene transcription in the presence of cortisol and corticosterone, and on the intracellular accumulation of radioactive cortisol and corticosterone. Finally, we have examined the effects of fluoxetine on inhibition of P-glycoprotein activity in Caco-2 cells. 3. We now find that fluoxetine (1-10 micro M) enhances GR-mediated gene transcription in the presence of dexamethasone and cortisol (+140-170%), but not of corticosterone, and increases the intracellular accumulation of (3)H-cortisol (+5-15%), but not of (3)H-corticosterone. Moreover, fluoxetine (10 micro M) induces approximately 30% inhibition of PGP activity in Caco-2 cells. 4. Our results show that fluoxetine, like other antidepressants, inhibits membrane steroid transporters.

Antidepressants enhance glucocorticoid receptor function in vitro by modulating the membrane steroid transporters

1. Previous data demonstrate that the tricyclic antidepressant, desipramine, induces glucocorticoid receptor (GR) translocation from the cytoplasm to the nucleus in L929 cells and increases dexamethasone-induced GR-mediated gene transcription in L929 cells stably transfected with the mouse mammary tumour virus-chloramphenicol acetyltransferase (MMTV-CAT) reporter gene (LMCAT cells) (Pariante et al., 1997). 2. To extend these findings, the present study has investigated the effects of 24 h coincubation of LMCAT cells with dexamethasone and amitriptyline, clomipramine, paroxetine, citalopram or fluoxetine. 3. All antidepressants, except fluoxetine, enhanced GR-mediated gene transcription, with clomipramine having the greatest effect (10 fold increase). Twenty-four hours coincubation of cells with desipramine, clomipramine or paroxetine, also enhanced GR function in the presence of cortisol, but not of corticosterone. 4. It is proposed that these effects are due to the antidepressants inhibiting the L929 membrane steroid transporter, which actively extrudes dexamethasone and cortisol from the cell, but not corticosterone. This is further confirmed by the fact that clomipramine failed to enhance GR-mediated gene transcription in the presence of dexamethasone when the membrane steroid transporter was blocked by verapamil. 5. The membrane steroid transporters that regulate access of glucocorticoids to the brain in vivo, like the multiple drug resistance p-glycoprotein, could be a fundamental target for antidepressant action.

Mutational analysis of Hsp90 function: interactions with a steroid receptor and a protein kinase

Hsp90 is a protein chaperone whose functions are focused on a specific set of target proteins. The nature of Hsp90's interactions with these proteins is poorly understood. To provide tools for examining these interactions, we have isolated eight broadly distributed temperature-sensitive (ts) point mutations in the Hsp90 gene (HSP82) of Saccharomyces cerevisiae. The mutants fall into two distinct classes. One has a classic ts phenotype, with nearly wild-type activity at 25 degrees C and a precipitous loss of function at 34 degrees C. The remaining seven mutants, in contrast, cause a general reduction in Hsp90 function and are ts because they do not provide the high level of function required for growth at high temperatures. The effects of these mutants on two target proteins, a transcription factor (glucocorticoid receptor) and a tyrosine kinase (pp60v-src), provided several insights on Hsp90 function. First, Hsp90 is not only required to help the glucocorticoid receptor achieve a hormone-activable state, it is continuously required to maintain that state. Second, Hsp90's function in the maturation of pp60v-src involves separable roles in protein accumulation and kinase activation. Thus, Hsp90 is an integral component of both the steroid receptor and kinase signaling pathways. Finally, all eight point mutants affect the activation of both the glucocorticoid receptor and pp60v-src, indicating that Hsp90 promotes the activity of these very different target proteins through common mechanisms.

Hormone-independent repression of AP-1-inducible collagenase promoter activity by glucocorticoid receptors

The role of the ligand in glucocorticoid receptor-mediated transactivation and transrepression of gene expression was investigated. Half-maximal transactivation of a mouse mammary tumor virus-chloramphenicol acetyltransferase reporter gene in transfected cells expressing the human glucocorticoid receptor mutant GRL753F, from which the rate of ligand dissociation is four to five times higher than the rate of dissociation from normal receptors, required a 200- to 300-fold-higher concentration of dexamethasone than was required in cells expressing the normal receptor. Immunocytochemical analysis demonstrated that this difference was not the result of a failure of the mutant receptor to accumulate in the nucleus after steroid treatment. In contrast, in cells cotransfected with a reporter gene containing the AP-1-inducible collagenase gene promoter, the concentration of dexamethasone required for 50% transrepression was the same for mutant and normal receptors. Efficient receptor-mediated transrepression was also observed with the double mutant GRL753F/C421Y, in which the first cysteine residue of the proximal zinc finger has been replaced by tyrosine, indicating that neither retention of the ligand nor direct binding of the receptor to DNA is required. RU38486 behaved as a full agonist with respect to transrepression. In addition, receptor-dependent transrepression, but not transactivation, was observed in transfected cells after heat shock in the absence of the ligand. Taken together, these results suggest that unlike transactivation, transrepression of AP-1 activity by the nuclear glucocorticoid receptor is ligand independent.