A transcription-independent function of FOXO1 in inhibition of androgen-independent activation of the androgen receptor in prostate cancer cells

Activation of FOXO1 is critical for the anticancer effect of methylseleninic acid in prostate cancer cells

BACKGROUND

Previous studies have demonstrated that physiological concentrations of methylseleninic acid (MSA) inhibits the growth of prostate cancer cells. The growth inhibitory effect could be attributed to cell cycle block and apoptosis induction. The current study was designed to investigate the involvement of forkhead box O1 (FOXO1) in the anticancer effect of MSA.

METHODS

LNCaP and LAPC-4 cells were treated with 10 microM MSA for various time points, and the expression of FOXO1 was analyzed by qRT-PCR and Western blotting. FOXO1 activity was determined by a luciferase construct containing FOXO binding sites. The trans-activation activity of the androgen receptor (AR) was determined by the ARE-luciferase assay. FOXO1 gene silencing was achieved by using a small interfering RNA (siRNA).

RESULTS

MSA treatment led to a rapid and robust increase of FOXO1 expression, as well as an increase of the FOXO1 transcriptional activity. Blocking FOXO1 activation by gene silencing abolished apoptosis induction by MSA, suggesting FOXO1 plays a critical role in mediating the apoptotic effect of MSA. Recent studies have shown that FOXO1 and AR antagonize the actions of each other. We examined the consequence of FOXO1 induction on AR activity. Consistent with previous reports, we found that ectopic expression of FOXO1 suppressed the transcriptional activity of AR. Furthermore, FOXO1 silencing attenuated MSA suppression of AR activity, suggesting that FOXO1 induction contributes to suppression of AR signaling by MSA.

CONCLUSIONS

In prostate cancer cells, MSA activates the FOXO1 signaling pathway. FOXO1 activation is critical for the anticancer effects of MSA.

Histone deacetylase 3 antagonizes aspirin-stimulated endothelial nitric oxide production by reversing aspirin-induced lysine acetylation of endothelial nitric oxide synthase

RATIONALE

Low-dose acetylsalicylic acid (aspirin) is widely used in the treatment and prevention of vascular atherothrombosis. Cardiovascular doses of aspirin also reduce systemic blood pressure and improve endothelium-dependent vasorelaxation in patients with atherosclerosis or risk factors for atherosclerosis. Aspirin can acetylate proteins, other than its pharmacological target cyclooxygenase, at lysine residues. The role of lysine acetylation in mediating the effects of low-dose aspirin on the endothelium is not known.

OBJECTIVE

To determine the role of lysine acetylation of endothelial nitric oxide synthase (eNOS) in the regulation of endothelial NO production by low-dose aspirin and to examine whether the lysine deacetylase histone deacetylase (HDAC)3 antagonizes the effect of low-dose aspirin on endothelial NO production by reversing acetylation of functionally critical eNOS lysine residues.

METHODS AND RESULTS

Low concentrations of aspirin induce lysine acetylation of eNOS, stimulating eNOS enzymatic activity and endothelial NO production in a cyclooxygenase-1-independent fashion. Low-dose aspirin in vivo also increases bioavailable vascular NO in an eNOS-dependent and cyclooxygenase-1-independent manner. Low-dose aspirin promotes the binding of eNOS to calmodulin. Lysine 609 in the calmodulin autoinhibitory domain of bovine eNOS mediates aspirin-stimulated binding of eNOS to calmodulin and eNOS-derived NO production. HDAC3 inhibits aspirin-stimulated (1) lysine acetylation of eNOS, (2) eNOS enzymatic activity, (3) eNOS-derived NO, and (4) binding of eNOS to calmodulin. Conversely, downregulation of HDAC3 promotes lysine acetylation of eNOS and endothelial NO generation.

CONCLUSIONS

Lysine acetylation of eNOS is a posttranslational protein modification supporting low-dose aspirin-induced vasoprotection. HDAC3, by deacetylating aspirin-acetylated eNOS, antagonizes aspirin-stimulated endothelial production of NO.

Cytosolic FoxO1 is essential for the induction of autophagy and tumour suppressor activity

Autophagy is characterized by the sequestration of bulk cytoplasm, including damaged proteins and organelles, and delivery of the cargo to lysosomes for degradation. Although the autophagic pathway is also linked to tumour suppression activity, the mechanism is not yet clear. Here we report that cytosolic FoxO1, a forkhead O family protein, is a mediator of autophagy. Endogenous FoxO1 was required for autophagy in human cancer cell lines in response to oxidative stress or serum starvation, but this process was independent of the transcriptional activity of FoxO1. In response to stress, FoxO1 was acetylated by dissociation from sirtuin-2 (SIRT2), a NAD(+)-dependent histone deacetylase, and the acetylated FoxO1 bound to Atg7, an E1-like protein, to influence the autophagic process leading to cell death. This FoxO1-modulated cell death is associated with tumour suppressor activity in human colon tumours and a xenograft mouse model. Our finding links the anti-neoplastic activity of FoxO1 and the process of autophagy.

Chemopreventive mechanisms of α-keto acid metabolites of naturally occurring organoselenium compounds

Previous studies on the chemopreventive mechanisms of dietary selenium have focused on its incorporation into antioxidative selenoproteins, such as glutathione peroxidase and thioredoxin reductase. Several studies, however, have revealed that dietary selenium in the form of L-selenomethionine and the 21st amino acid, selenocysteine, also have intrinsic anti-cancer properties. Biochemical mechanisms previously investigated to contribute to their anticancer effects involve β- and γ-lyase reactions. Some pyridoxal 5'-phosphate (PLP)-containing enzymes can catalyze a β-lyase reaction with Se-methyl-L-selenocysteine (MSC) generating pyruvate and ammonia. Other PLP-enzymes can catalyze a γ-lyase reaction with L-selenomethionine (SM) generating α-ketobutyrate and ammonia. In both cases, a purported third product is methylselenol (CH(3)SeH). Although not directly quantifiable, as a result of its extreme hydrophobicity and high vapor pressure, CH(3)SeH has been indirectly observed to act through the alteration of protein-sulfhydryl moieties on redox-responsive signal and transcription factors, thereby maintaining a non-proliferative intracellular environment. We have considered the possibility that α-keto acid analogues of MSC (i.e., methylselenopyruvate; MSP) and SM (i.e., α-keto-γ-methylselenobutyrate; KMSB), generated via a transamination and/or L-amino acid oxidase reaction may also be chemoprotective. Indeed, these compounds were shown to increase the level of histone-H3 acetylation in human prostate and colon cancer cells. MSP and KMSB structurally resemble butyrate, an inhibitor of several histone deacetylases. Thus, the seleno α-keto acid metabolites of MSC and SM, along with CH(3)SeH derived from β- and γ-lyase reactions, may be potential direct-acting metabolites of organoselenium that lead to de-repression of silenced tumor suppressor proteins and/or regulation of genes and signaling molecules.

Inhibition of the androgen receptor as a novel mechanism of taxol chemotherapy in prostate cancer

Taxol chemotherapy is one of the few therapeutic options for men with castration-resistant prostate cancer (CRPC). However, the working mechanisms for Taxol are not fully understood. Here, we showed that treatment of 22Rv1, a PTEN-positive CRPC cell line, with paclitaxel and its semisynthetic analogue docetaxel decreases expression of the androgen receptor (AR)-activated genes prostate-specific antigen (PSA) and Nkx3.1 but increases expression of the AR repression gene maspin, suggesting that Taxol treatment inhibits AR activity. This was further supported by the observation that the activity of AR luciferase reporter genes was inhibited by paclitaxel. In contrast, paclitaxel treatment failed to inhibit AR activity in the PTEN-null CRPC cell line C4-2. However, pretreatment of C4-2 cells with the phosphoinositide 3-kinase inhibitor LY294002 restored paclitaxel inhibition of the AR. Treatment of 22Rv1 xenografts in mice with docetaxel induced mitotic arrest and a decrease in PSA expression in tumor cells adjacent to vascular vessels. We further showed that paclitaxel induces nuclear accumulation of FOXO1, a known AR suppressive nuclear factor, and increases the association of FOXO1 with AR proteins in the nucleus. FOXO1 knockdown with small interfering RNA attenuated the inhibitory effect of paclitaxel on AR transcriptional activity, expression of PSA and Nkx3.1, and cell survival. These data reveal a previously uncharacterized, FOXO1-mediated, AR-inhibitory effect of Taxol in CRPC cells that may play an important role in Taxol-mediated inhibition of CRPC growth.

Cyclin D1 promotes anchorage-independent cell survival by inhibiting FOXO-mediated anoikis

O-class forkhead box (FOXO) transcription factors are critical regulators of diverse cellular processes, including apoptosis, cell-cycle arrest, DNA damage repair and oxidative stress resistance. Here, we show that FOXO1 and FOXO3a have an essential function in promoting cell detachment-induced anoikis, resistance to which is implicated in cancer development and metastasis. In contrast, the oncoprotein cyclin D1 inhibits anoikis. We further show that cyclin D1 interacts with FOXO proteins and impedes their transcriptional regulatory and anoikis-promoting functions. This effect of cyclin D1 requires its transcription repression domain but is independent of cyclin-dependent kinases CDK4 and CDK6. Moreover, we show that cancer-derived mutants of cyclin D1 are much more stable than wild-type cyclin D1 under anchorage-independent conditions and possess a greater antagonistic effect on FOXO-regulated anoikis and anchorage-independent growth of cancer cells. These data suggest that cyclin D1 may have a critical function in tumorigenesis and cancer metastasis by inhibiting the anoikis-promoting function of FOXO proteins.