Characterization of structural, biochemical, pharmacokinetic, and pharmacodynamic properties of the LSD1 inhibitor bomedemstat in preclinical models.

INTRODUCTION: Lysine-specific demethylase 1 (LSD1) is emerging as a critical mediator of tumor progression in metastatic castration-resistant prostate cancer (mCRPC). Neuroendocrine prostate cancer (NEPC) is increasingly recognized as an adaptive mechanism of resistance in mCRPC patients failing androgen receptor axis-targeted therapies. Safe and effective LSD1 inhibitors are necessary to determine antitumor response in prostate cancer models. For this reason, we characterize the LSD1 inhibitor bomedemstat to assess its clinical potential in NEPC as well as other mCRPC pathological subtypes. METHODS: Bomedemstat was characterized via crystallization, flavine adenine dinucleotide spectrophotometry, and enzyme kinetics. On-target effects were assessed in relevant prostate cancer cell models by measuring proliferation and H3K4 methylation using western blot analysis. In vivo, pharmacokinetic (PK) and pharmacodynamic (PD) profiles of bomedemstat are also described. RESULTS: Structural, biochemical, and PK/PD properties of bomedemstat, an irreversible, orally-bioavailable inhibitor of LSD1 are reported. Our data demonstrate bomedemstat has >2500-fold greater specificity for LSD1 over monoamine oxidase (MAO)-A and -B. Bomedemstat also demonstrates activity against several models of advanced CRPC, including NEPC patient-derived xenografts. Significant intra-tumoral accumulation of orally-administered bomedemstat is measured with micromolar levels achieved in vivo (1.2 ± 0.45 µM at the 7.5 mg/kg dose and 3.76 ± 0.43 µM at the 15 mg/kg dose). Daily oral dosing of bomedemstat at 40 mg/kg/day is well-tolerated, with on-target thrombocytopenia observed that is rapidly reversible following treatment cessation. CONCLUSIONS: Bomedemstat provides enhanced specificity against LSD1, as revealed by structural and biochemical data. PK/PD data display an overall safety profile with manageable side effects resulting from LSD1 inhibition using bomedemstat in preclinical models. Altogether, our results support clinical testing of bomedemstat in the setting of mCRPC.

Overexpression of Fibroblast Activation Protein (FAP) in stroma of proliferative inflammatory atrophy (PIA) and primary adenocarcinoma of the prostate.

Fibroblast activation protein (FAP) is a serine protease upregulated at sites of tissue remodeling and cancer that represents a promising therapeutic and molecular imaging target. In prostate cancer, studies of FAP expression using tissue microarrays are conflicting, such that its clinical potential is unclear. Furthermore, little is known regarding FAP expression in benign prostatic tissues. Here we demonstrated, using a novel iterative multiplex IHC assay in standard tissue sections, that FAP was nearly absent in normal regions, but was increased consistently in regions of proliferative inflammatory atrophy (PIA). In carcinoma, FAP was expressed in all cases, but was highly heterogeneous. High FAP levels were associated with increased pathological stage and cribriform morphology. We verified that FAP levels in cancer correlated with CD163+ M2 macrophage density. In this first report to quantify FAP protein in benign prostate and primary tumors, using standard large tissue sections, we clarify that FAP is present in all primary prostatic carcinomas, supporting its potential clinical relevance. The finding of high levels of FAP within PIA supports the injury/regeneration model for its pathogenesis and suggests that it harbors a protumorigenic stroma. Yet, high levels of FAP in benign regions could lead to false positive FAP-based molecular imaging results in clinically localized prostate cancer.

Third generation quinoline-3-carboxamide transcriptional disrupter of HDAC4, HIF-1α, and MEF-2 signaling for metastatic castration-resistant prostate cancer.

BACKGROUND: The quinoline-3-carboxamide, Tasquinimod (TasQ), is orally active as a maintenance therapy with an on-target mechanism-of-action via allosteric binding to HDAC4. This prevents formation of the HDAC4/NCoR1/HDAC3 complex, disrupting HIF-1α transcriptional activation and repressing MEF-2 target genes needed for adaptive survival signaling in the compromised tumor micro environment. In phase 3 clinical testing against metastatic castration-resistant prostate cancer(mCRPC), TasQ (1 mg/day) increased time-to-progression, but not overall survival. METHODS: TasQ analogs were chemically synthesized and tested for activity compared to the parental compound. These included HDAC4 enzymatic assays, qRT-PCR and western blot analyses of gene and protein expression following treatment, in vitro and in vivo efficacy against multiple prostate cancer models including PDXs, pharmacokinetic analyses,AHR binding and agonist assays, SPR analyses of binding to HDAC4 and NCoR1, RNAseq analysis of in vivo tumors, 3D endothelial sprouting assays, and a targeted kinase screen. Genetic knockout or knockdown controls were used when appropriate. RESULTS: Here, we document that, on this regimen (1 mg/day), TasQ blood levels are 10-fold lower than the optimal concentration (≥2 µM) needed for anticancer activity, suggesting higher daily doses are needed. Unfortunately, we also demonstrate that TasQ is an arylhydrocarbon receptor (AHR) agonist, which binds with an EC50 of 1 µM to produce unwanted off-target side effects. Therefore, we screened a library of TasQ analogsto maximize on-target versus off-target activity. Using this approach, we identified ESATA-20, which has ~10-fold lower AHR agonism and 5-fold greater potency against prostate cancer patient-derived xenografts. CONCLUSION: This increased therapeuticindex nominates ESATA-20 as a lead candidate forclinical development as an orally active third generation quinoline-3-carboxamide analog thatretains its on-target ability to disrupt HDAC4/HIF-1α/MEF-2-dependent adaptive survival signaling in the compromisedtumor microenvironment found in mCRPC.

Androgen receptor activity in prostate cancer dictates efficacy of bipolar androgen therapy through MYC.

Testosterone is the canonical growth factor of prostate cancer but can paradoxically suppress its growth when present at supraphysiological levels. We have previously demonstrated that the cyclical administration of supraphysiological androgen (SPA), termed bipolar androgen therapy (BAT), can result in tumor regression and clinical benefit for patients with castration-resistant prostate cancer. However, predictors and mechanisms of response and resistance have been ill defined. Here, we show that growth inhibition of prostate cancer models by SPA required high androgen receptor (AR) activity and were driven in part by downregulation of MYC. Using matched sequential patient biopsies, we show that high pretreatment AR activity predicted downregulation of MYC, improved clinical response, and prolonged progression-free and overall survival for patients on BAT. BAT induced strong downregulation of AR in all patients, which is shown to be a primary mechanism of acquired resistance to SPA. Acquired resistance was overcome by alternating SPA with the AR inhibitor enzalutamide, which induced adaptive upregulation of AR and resensitized prostate cancer to SPA. This work identifies high AR activity as a predictive biomarker of response to BAT and supports a treatment paradigm for prostate cancer involving alternating between AR inhibition and activation.

Resistance to androgen receptor signaling inhibition does not necessitate development of neuroendocrine prostate cancer.

Resistance to AR signaling inhibitors (ARSis) in a subset of metastatic castration-resistant prostate cancers (mCRPCs) occurs with the emergence of AR- neuroendocrine prostate cancer (NEPC) coupled with mutations/deletions in PTEN, TP53, and RB1 and the overexpression of DNMTs, EZH2, and/or SOX2. To resolve whether the lack of AR is the driving factor for the emergence of the NE phenotype, molecular, cell, and tumor biology analyses were performed on 23 xenografts derived from patients with PC, recapitulating the full spectrum of genetic alterations proposed to drive NE differentiation. Additionally, phenotypic response to CRISPR/Cas9-mediated AR KO in AR+ CRPC cells was evaluated. These analyses document that (a) ARSi-resistant NEPC developed without androgen deprivation treatment; (b) ARS in ARSi-resistant AR+/NE+ double-positive "amphicrine" mCRPCs did not suppress NE differentiation; (c) the lack of AR expression did not necessitate acquiring a NE phenotype, despite concomitant mutations/deletions in PTEN and TP53, and the loss of RB1 but occurred via emergence of an AR-/NE- double-negative PC (DNPC); (d) despite DNPC cells having homogeneous genetic driver mutations, they were phenotypically heterogeneous, expressing basal lineage markers alone or in combination with luminal lineage markers; and (e) AR loss was associated with AR promoter hypermethylation in NEPCs but not in DNPCs.

Overcoming stromal barriers to immuno-oncological responses via fibroblast activation protein-targeted therapy.

The tumor microenvironment contributes to disease progression through multiple mechanisms, including immune suppression mediated in part by fibroblast activation protein (FAP)-expressing cells. Herein, a review of FAP biology is presented, supplemented with primary data. This includes FAP expression in prostate cancer and activation of latent reservoirs of TGF-ß and VEGF to produce a positive feedback loop. This collectively suggests a normal wound repair process subverted during cancer pathophysiology. There has been immense interest in targeting FAP for diagnostic, monitoring and therapeutic purposes. Until recently, this development has outpaced an understanding of the biology; impeding optimal translation into the clinic. A summary of these applications is provided with an emphasis on eliminating tumor-infiltrating FAP-positive cells to overcome stromal barriers to immuno-oncological responses.

Microparticle Encapsulation of a Prostate-targeted Biologic for the Treatment of Liver Metastases in a Preclinical Model of Castration-resistant Prostate Cancer.

PRX302 is a highly potent, mutant bacterial pore-forming biologic protoxin engineered for selective activation by PSA, a serine protease expressed by benign and malignant prostate epithelial cells. Although being developed as a local therapy for benign prostatic hyperplasia and localized prostate cancer, PRX302 cannot be administered systemically as a treatment for metastatic disease due to binding to ubiquitously expressed glycosylphosphatidylinositol (GPI)-anchored proteins, which leads to poor accumulation within the tumor microenvironment. To overcome this limitation, poly-lactic-co-glycolic acid (PLGA) microparticles encapsulating the protoxin were developed, which are known to accumulate in the liver, a major site of metastasis for prostate cancer and other solid tumors. A highly sensitive and reproducible sandwich ELISA to quantify PRX302 released from microparticles was developed. Utilizing this assay, PRX302 release from different microparticle formulations was assessed over multiple days. Hemolysis assays documented PSA-dependent pore formation and lytic potential (i.e., function) of the released protoxin. MTT assays demonstrated that conditioned supernatant from PRX302-loaded, but not blank (i.e., unloaded), PLGA microparticles was highly cytotoxic to PC3 and DU145 human prostate cancer cells in the presence of exogenous PSA. Microparticle encapsulation prevented PRX302 from immediately interacting with GPI-anchored proteins as demonstrated in a competition assay, which resulted in an increased therapeutic index and significant antitumor efficacy following a single dose of PRX302-loaded microparticles in a preclinical model of prostate cancer liver metastasis with no obvious toxicity. These results document that PRX302 released from PLGA microparticles demonstrate in vivo antitumor efficacy in a clinically relevant preclinical model of metastatic prostate cancer.

Mesenchymal stem cell infiltration during neoplastic transformation of the human prostate.

Mesenchymal Stem Cells (MSCs) have been identified in prostate cancer, raising the critical question of their physical and temporal source. Therefore, MSCs were quantified and characterized in benign and malignant prostate tissue representing different disease states and a wide range of age groups from fetal development through adult death using analytical and functional methodologies. In contrast to lineage-restricted Mesenchymal Progenitor Cells (MPCs) found in normal prostate tissue, MSCs with tri-lineage differentiation potential (adipogenesis, osteogenesis, and chondrogenesis) are identified in prostate tissue from a subset of men with prostate cancer, consistent with an influx of more stem-like progenitors (i.e. MSCs) from the bone marrow. Additionally, prostate tissue from a subset of these patients is highly enriched in MSCs, suggesting their enumeration may have prognostic value for identifying men with aggressive disease. This influx is an ongoing process continuing throughout disease progression as documented by the presence of MSCs in metastatic lesions from multiple organ sites harvested at the time of death in metastatic castration-resistant prostate cancer (mCRPC) patients. This infiltration of MSCs from systemic circulation provides the rationale for their use as a cell-based vector to deliver therapeutic agents.

Low p16INK4a Expression in Early Passage Human Prostate Basal Epithelial Cells Enables Immortalization by Telomerase Expression Alone.

BACKGROUND: There are two principal senescence barriers that must be overcome to successfully immortalize primary human epithelial cells in culture, stress-induced senescence, and replicative senescence. The p16INK4a /retinoblastoma protein (p16/Rb) pathway mediates stress-induced senescence, and is generally upregulated by primary epithelial cells in response to the artificial conditions from tissue culture. Replicative senescence is associated with telomere loss. Following each round of cell division, telomeres progressively shorten. Once telomeres shorten to a critical length, the DNA damage response pathway is activated, and the tumor suppressor p53 pathway triggers replicative senescence. Exogenous expression of telomerase in normal human epithelial cells extends the replicative capacity of cells, and in some cases, immortalizes cells. However reliable immortalization of epithelial cells usually requires telomerase activity coupled with inactivation of the p16/Rb pathway. METHODS: A lentiviral vector, pLOX-TERT-iresTK (Addgene #12245), containing a CMV promoter upstream of a bicistronic coding cassette that includes loxP sites flanking the catalytic subunit of human telomerase gene (TERT) and herpes simplex virus type-1 thymidine kinase gene (HSV1-tk) was used to transduce normal prostate basal epithelial cells (PrECs) initiated in cell culture from prostate cancer patients undergoing radical prostatectomies. RESULTS: Transduction of early (i.e., <7) passage PrECs with TERT led to successful immortalization. However, attempts to immortalize late (i.e., >7) passage PrECs were unsuccessful. Late passage PrECs, which acquired elevated p16, were unable to overcome the senescence barrier. Immortalized PrECs (TERT-PrECs) retained a normal male karyotype and low p16 expression. Additionally, TERT-PrECs were non-tumorigenic when inoculated into intact male immunodeficient NSG mice. CONCLUSIONS: The present studies document that early passage human PrECs have sufficiently low p16 to permit immortalization by TERT expression alone. TERT-PrECs developed using this transduction approach provides an appropriate and experimentally facile model for clarifying the molecular mechanism(s) involved in both immortalization of human PrECs, as well as identifying genetic/epigenetic "drivers" for conversion of these immortalized non-tumorigenic cells into fully lethal prostate cancers. Notably, loxP sites flank the exogenous TERT gene in the TERT-PrECs. Cre recombinase can be used to excise TERT, and resolve whether TERT expression is required for these cells to be fully transformed into lethal cancer. Prostate 77: 374-384, 2017. © 2016 Wiley Periodicals, Inc.

Androgen receptor (AR) suppresses normal human prostate epithelial cell proliferation via AR/ß-catenin/TCF-4 complex inhibition of c-MYC transcription.

INTRODUCTION: Physiologic testosterone continuously stimulates prostate stromal cell secretion of paracrine growth factors (PGFs), which if unopposed would induce hyperplastic overgrowth of normal prostate epithelial cells (PrECs). METHODS: Lentiviral shRNA stable knock down of c-MYC, ß-catenin, or TCF-4 completely inhibits normal (i.e., non-transformed) human PrECs growth. c-MYC enhancer driven reporter expression and growth is inhibited by two chemically distinct molecules, which prevent ß-catenin signaling either by blocking TCF-4 binding (i.e., toxoflavin) or by stimulating degradation (i.e., AVX939). Recombinant DKK1 protein at a dose, which inhibits activation of canonical Wnt signaling does not inhibit PrEC growth. Nuclear ß-catenin translocation and PrEC growth is prevented by both lack of PGFs or Akt inhibitor-I. Growth inhibition induced by lack of PGFs, toxoflavin, or Akt inhibitor-I is overcome by constitutive c-MYC transcription. RESULTS: In the presence of continuous PGF signaling, PrEC hyperplasia is prevented by androgen binding to AR suppressing c-MYC transcription, resulting in G0 arrest/terminal differentiation independent of Rb, p21, p27, FoxP3, or down regulation of growth factors receptors and instead involves androgen-induced formation of AR/ß-catenin/TCF-4 complexes, which suppress c-MYC transcription. Such suppression does not occur when AR is mutated in its zinc-finger binding domain. DISCUSSION: Proliferation of non-transformed human PrECs is dependent upon c-MYC transcription via formation/binding of ß-catenin/TCF-4 complexes at both 5' and 3' c-MYC enhancers stimulated by Wnt-independent, PGF induced Akt signaling. In the presence of continuous PGF signaling, PrEC hyperplasia is prevented by androgen-induced formation of AR/ß-catenin/TCF-4 complexes, which retains binding to 3' c-MYC enhancer, but now suppresses c-MYC transcription.

Tasquinimod Is an Allosteric Modulator of HDAC4 survival signaling within the compromised cancer microenvironment.

Tasquinimod is an orally active antiangiogenic drug that is currently in phase III clinical trials for the treatment of castration-resistant prostate cancer. However, the target of this drug has remained unclear. In this study, we applied diverse strategies to identify the histone deacetylase HDAC4 as a target for the antiangiogenic activity of tasquinimod. Our comprehensive analysis revealed allosteric binding (Kd 10-30 nmol/L) to the regulatory Zn(2+) binding domain of HDAC4 that locks the protein in a conformation preventing HDAC4/N-CoR/HDAC3 complex formation. This binding inhibited colocalization of N-CoR/HDAC3, thereby inhibiting deacetylation of histones and HDAC4 client transcription factors, such as HIF-1α, which are bound at promoter/enhancers where epigenetic reprogramming is required for cancer cell survival and angiogenic response. Through this mechanism, tasquinimod is effective as a monotherapeutic agent against human prostate, breast, bladder, and colon tumor xenografts, where its efficacy could be further enhanced in combination with a targeted thapsigargin prodrug (G202) that selectively kills tumor endothelial cells. Together, our findings define a mechanism of action of tasquinimod and offer a perspective on how its clinical activity might be leveraged in combination with other drugs that target the tumor microenvironment. Cancer Res; 73(4); 1386-99. ©2012 AACR.

Adaptive auto-regulation of androgen receptor provides a paradigm shifting rationale for bipolar androgen therapy (BAT) for castrate resistant human prostate cancer.

Cell culture/xenograft and gene arrays of clinical material document that development of castration resistant prostate cancer (CRPC) cells involves acquisition of adaptive auto-regulation resulting in >25-fold increase in Androgen Receptor (AR) protein expression in a low androgen environment. Such adaptive AR increase paradoxically is a liability in castrated hosts, however, when supraphysiologic androgen is acutely replaced. Cell synchronization/anti-androgen response is due to AR binding to replication complexes (RC) at origin of replication sites (ORS) in early G1 associated with licensing/restricting DNA for single round of duplication during S-phase. When CRPC cells are acutely exposed to supraphysiologic androgen, adaptively increased nuclear AR is over-stabilized, preventing sufficient degradation in mitosis, inhibiting DNA re-licensing, and thus death in the subsequent cell cycle. These mechanistic results and the fact that AR/RC binding occurs in metastatic CRPCs directly from patients provides a paradigm shifting rationale for bipolar androgen therapy (BAT) in patient progressing on chronic androgen ablation. BAT involves giving sequential cycles alternating between periods of acute supraphysiologic androgen followed by acute ablation to take advantage of vulnerability produced by adaptive auto-regulation and binding of AR to RC in CRPC cells. BAT therapy is effective in xenografts and based upon positive results has entered clinical testing.

Loss of androgen receptor-dependent growth suppression by prostate cancer cells can occur independently from acquiring oncogenic addiction to androgen receptor signaling.

The conversion of androgen receptor (AR) signaling as a mechanism of growth suppression of normal prostate epithelial cells to that of growth stimulation in prostate cancer cells is often associated with AR mutation, amplification and over-expression. Thus, down-regulation of AR signaling is commonly therapeutic for prostate cancer. The E006AA cell line was established from a hormone naïve, localized prostate cancer. E006AA cells are genetically aneuploid and grow equally well when xenografted into either intact or castrated male NOG but not nude mice. These cells exhibit: 1) X chromosome duplication and AR gene amplification, although paradoxically not coupled with increased AR expression, and 2) somatic, dominant-negative Serine-599-Glycine loss-of-function mutation within the dimerization surface of the DNA binding domain of the AR gene. No effect on the growth of E006AA cells is observed using targeted knockdown of endogenous mutant AR, ectopic expression of wild-type AR, or treatment with androgens or anti-androgens. E006AA cells represent a prototype for a newly identified subtype of prostate cancer cells that exhibit a dominant-negative AR loss-of-function in a hormonally naïve patient. Such loss-of-function eliminates AR-mediated growth suppression normally induced by normal physiological levels of androgens, thus producing a selective growth advantage for these malignant cells in hormonally naïve patients. These data highlight that loss of AR-mediated growth suppression is an independent process, and that, without additional changes, is insufficient for acquiring oncogene addiction to AR signaling. Thus, patients with prostate cancer cells harboring such AR loss-of-function mutations will not benefit from aggressive hormone or anti-AR therapies even though they express AR protein.

Conformational diversity in prion protein variants influences intermolecular beta-sheet formation.

A conformational transition of normal cellular prion protein (PrP(C)) to its pathogenic form (PrP(Sc)) is believed to be a central event in the transmission of the devastating neurological diseases known as spongiform encephalopathies. The common methionine/valine polymorphism at residue 129 in the PrP influences disease susceptibility and phenotype. We report here seven crystal structures of human PrP variants: three of wild-type (WT) PrP containing V129, and four of the familial variants D178N and F198S, containing either M129 or V129. Comparison of these structures with each other and with previously published WT PrP structures containing M129 revealed that only WT PrPs were found to crystallize as domain-swapped dimers or closed monomers; the four mutant PrPs crystallized as non-swapped dimers. Three of the four mutant PrPs aligned to form intermolecular beta-sheets. Several regions of structural variability were identified, and analysis of their conformations provides an explanation for the structural features, which can influence the formation and conformation of intermolecular beta-sheets involving the M/V129 polymorphic residue.

Cell-autonomous intracellular androgen receptor signaling drives the growth of human prostate cancer initiating cells.

BACKGROUND: The lethality of prostate cancer is due to the continuous growth of cancer initiating cells (CICs) which are often stimulated by androgen receptor (AR) signaling. However, the underlying molecular mechanism(s) for such AR-mediated growth stimulation are not fully understood. Such mechanisms may involve cancer cell-dependent induction of tumor stromal cells to produce paracrine growth factors or could involve cancer cell autonomous autocrine and/or intracellular AR signaling pathways. METHODS: We utilized clinical samples, animal models and a series of AR-positive human prostate cancer cell lines to evaluate AR-mediated growth stimulation of prostate CICs. RESULTS: The present studies document that stromal AR expression is not required for prostate cancer growth, since tumor stroma surrounding AR-positive human prostate cancer metastases (N = 127) are characteristically AR-negative. This lack of a requirement for AR expression in tumor stromal cells is also documented by the fact that human AR-positive prostate cancer cells grow equally well when xenografted in wild-type versus AR-null nude mice. AR-dependent growth stimulation was documented to involve secretion, extracellular binding, and signaling by autocrine growth factors. Orthotopic xenograft animal studies documented that the cellautonomous autocrine growth factors which stimulate prostate CIC growth are not the andromedins secreted by normal prostate stromal cells. Such cell autonomous and extracellular autocrine signaling is necessary but not sufficient for the optimal growth of prostate CICs based upon the response to anti-androgen plus/or minus preconditioned media. CONCLUSIONS: AR-induced growth stimulation of human prostate CICs requires AR-dependent intracellular pathways. The identification of such AR-dependent intracellular pathways offers new leads for the development of effective therapies for prostate cancer.

Amino acid containing thapsigargin analogues deplete androgen receptor protein via synthesis inhibition and induce the death of prostate cancer cells.

There are quantitative and/or qualitative mechanisms allowing androgen receptor (AR) growth signaling in androgen ablation refractory prostate cancer cells. Regardless of the mechanism, agents that deplete AR protein expression prevent such AR growth signaling. Thapsigargin (TG) is a highly cell-penetrant sequiterpene-lactone that once inside cells inhibits (IC(50), ∼ 10 nmol/L) critically important housekeeping SERCA 2b calcium pumps in the endoplasmic reticulum. Using a series of five genetically diverse androgen ablation refractory human prostate cancer lines (LNCaP, LAPC-4, VCaP, MDA-PCa-2b, and CWR22Rv1), TG inhibition of SERCA pumps consistently results in depletion of the endoplasmic reticulum Ca(+2) coupled with µmol/L elevation in the intracellular free Ca(+2) initiating a molecular cascade that: (a) inhibits Cap-dependent AR protein synthesis resulting in 90% depletion of AR protein by 24 hours of TG exposure, (b) arrests the cells in G(0), and (c) induces their apoptotic death. Unfortunately, due to its highly lipophilic nature, TG is not deliverable as a systemic agent without host toxicity. Therefore, TG analogues containing amino acids were developed, which retain ability to deplete AR protein and induce cell death and which can be covalently linked to peptide carriers producing water soluble prodrugs for systemic delivery. Specific amino acid sequences are used to restrict the liberation of cytotoxic amino acid containing TG analogues from the peptide prodrug by prostate-specific proteases, such as prostate-specific antigen and prostate-specific membrane antigen, or cancer-specific proteases, such as fibroblast activation protein, so that toxicity of these prodrugs is selectively targeted to metastatic sites of prostate cancer. Based on these results, these prodrugs are undergoing clinical development.

Androgen receptor as a licensing factor for DNA replication in androgen-sensitive prostate cancer cells.

Androgen receptor (AR) protein expression and function are critical for survival and proliferation of androgen-sensitive (AS) prostate cancer cells. Besides its ability to function as a transcription factor, experimental observations suggest that AR becomes a licensing factor for DNA replication in AS prostate cancer cells and thus must be degraded during each cell cycle in these cells to allow reinitiation of DNA replication in the next cell cycle. This possibility was tested by using the AS human prostate cancer cell lines, LNCaP, CWR22Rv1, and LAPC-4. These studies demonstrated that AR levels fluctuate both within and between various phases of the cell cycle in each of these AS lines. Consistent with its licensing ability, AR is degraded during mitosis via a proteasome-dependent pathway in these AS prostate cancer cells. In contrast, proteasome-dependent degradation of AR during mitosis is not observed in AR-expressing but androgen-insensitive human prostate stromal cells, in which AR does not function as a licensing factor for DNA replication. To evaluate mitotic degradation of AR in vivo, the same series of human AS prostate cancers growing as xenografts in nude mice and malignant tissues obtained directly from prostate cancer patients were evaluated by dual Ki-67 and AR immunohistochemistry for AR expression in mitosis. These results document that AR is also down-regulated during mitosis in vivo. Thus, AS prostate cancer cells do not express AR protein during mitosis, either in vitro or in vivo, consistent with AR functioning as a licensing factor for DNA replication in AS prostate cancer cells.

Low-calcium serum-free defined medium selects for growth of normal prostatic epithelial stem cells.

Stage-specific differentiation markers were used to evaluate the cellular composition and the origin of nonimmortalized (PrEC) and immortalized (PZ-HPV7, CA-HPV10, RWPE-1, and 957E/hTERT) human prostate cell lines. These studies documented that immortalized and nonimmortalized prostate epithelial cells established and maintained in low (i.e., <300 micromol/L) Ca(2+) serum-free defined (SFD) medium were all derived from normal nonmalignant prostate tissues and contain CD133(+)/ABCG2(+)/alpha(2)beta(1)(Hi)/p63(-)/PSCA(-)/AR(-)/PSA(-) prostate stem cells. In these cultures, prostate stem cells are able to self-renew and generate two distinct cell lineages: the minor proliferatively quiescent neuroendocrine lineage and the major transit-amplifying cell lineage. Subsequently, CD133(-)/ABCG2(-)/alpha(2)beta(1)(Hi)/p63(+)/PSCA(-)/AR(-)/PSA(-) transit-amplifying cells proliferate frequently and eventually mature into proliferatively quiescent CD133(-)/ABCG2(-)/alpha(2)beta(1)(Lo)/p63(-)/PSCA(+)/AR(-)/PSA(-) intermediate cells. Such proliferatively quiescent intermediate cells, however, do not complete their full maturation into CD133(-)/ABCG2(-)/alpha(2)beta(1)(Lo)/p63(-)/PSCA(-)/AR(+)/PSA(+) luminal-secretory cells in low Ca(2+) SFD medium. Addition of universal type I IFN and synthetic androgen (R1881) to culture medium resulted in up-regulation of androgen receptor protein expression. However, it failed to induce full differentiation of intermediate cells into AR(+)/PSA(+) luminal-secretory cells. Our results indicate that such inability of prostate epithelial cells to complete their differentiation is due to continuous expression of Notch-1 receptor and its downstream effector, Hey-1 protein, which actively suppresses differentiation via its ability to transcriptionally repress a series of genes, including the GATA family of transcription factors.

PC3, but not DU145, human prostate cancer cells retain the coregulators required for tumor suppressor ability of androgen receptor.

BACKGROUND: Androgen receptor (AR) functions in normal prostate epithelium as a tumor suppressor to inhibit continuous proliferation of these cells. Such tumor suppressor function of AR is lost in androgen depletion independent (ADI) prostate cancers. In type-I ADI cancers AR is not expressed, while in type-II ADI cancers AR is recaptured as an oncogene. The PC3 and DU145 human prostate cancer cell lines are representative of the earlier type-I ADI prostate cancers. While these cells do not express AR, it is unclear whether they retained the coactivators necessary for AR-dependent tumor suppression. To answer this question the response to AR protein expression by PC3 and DU145 cells was evaluated. METHODS: To do this, a lentiviral AR (Lenti-AR) expression system was engineered to encode an AR transcript which includes appropriate 5' and 3' untranslated regions (UTRs) containing all previously identified post-transcriptional regulatory sequences. AR expression and transcriptional activity were evaluated in Lenti-AR transduced cells by Western blot and luciferase assay, respectively. Cell growth in culture and in mouse xenografts was evaluated in correlation to expression changes in p21, p27, and p45(SKP2) proteins. RESULTS: Lenti-AR transduced PC3 and DU145 lines expressed transcriptionally functional AR protein at appropriate physiological levels. Expression and engagement of AR protein in PC3-Lenti-AR cells resulted in transactivation of p21 and subsequent growth inhibition of these cells in culture and in mouse xenografts. Such inhibition was due to induced G1 arrest of these cells as documented by expression changes in p27 and p45(SKP2) proteins. Such growth inhibition was not observed in DU145-Lenti-AR cells. CONCLUSIONS: These results document that PC3, but not DU145 cells retain the coregulators needed for AR tumor suppressor ability.

Role of notch-1 and E-cadherin in the differential response to calcium in culturing normal versus malignant prostate cells.

A panel of expression markers was validated and used to document that, when radical prostatectomy specimens are cultured in low (i.e., <260 micromol/L)-calcium (Ca2+)-serum-free, growth factor-defined (SFD) medium, what grows out are not prostatic cancer cells but basally derived normal transit-amplifying prostatic epithelial cells. The selective outgrowth of the normal transit-amplifying versus prostatic cancer cells is due to the differential effect of low-Ca2+ medium on the structure of Notch-1 and E-cadherin signaling molecules. In low-Ca2+ medium, Notch-1 receptor is conformationally in a constitutively active, cell autonomous form not requiring reciprocal cell-cell (i.e., ligand) interaction for signaling. Such signaling is required for survival of transit-amplifying cells as shown by the death of transit-amplifying cells induced by treatment with a series of chemically distinct gamma-secretase inhibitors to prevent Notch-1 signaling. Conversely, in low-Ca2+ medium, E-cadherin is conformationally inactive preventing cell-cell homotypic interaction, but low cell density nonaggregated transit-amplifying cells still survived because Notch-1 is able to signal cell autonomously. In contrast, when medium Ca2+ is raised to >400 micromol/L, Notch-1 conformationally is no longer constitutively active but requires cell-cell contact for reciprocal binding of Jagged-1 ligands and Notch-1 receptors between adjacent transit-amplifying cells to activate their survival signaling. Such cell-cell contact is enhanced by the elevated Ca2+ inducing an E-cadherin conformation allowing homotypic interaction between transit-amplifying cells. Such Ca(2+)-dependent, E-cadherin-mediated interaction, however, results in cell aggregation, stratification, and inhibition of proliferation of transit-amplifying cells via contact inhibition-induced up-regulation of p27/kip1 protein. In addition, transit-amplifying cells not contacting other cells undergo squamous differentiation into cornified (i.e., 1% SDS insoluble) envelopes and death in the elevated Ca2+ medium. Stratification and contact inhibition induced by elevated Ca2+ are dependent on E-cadherin-mediated homotypic interaction between transit-amplifying cells as shown by their prevention in the presence of a cell-impermanent, E-cadherin neutralizing antibody. In contrast to growth inhibition of normal transit-amplifying cells, supplementation of low-Ca(2+)-SFD medium with 10% FCS and raising the Ca2+ to >600 micromol/L stimulates the growth of all prostate cancer cell lines tested. Additional results document that, at physiologic levels of Ca2+ (i.e., >600 micromol/L), prostatic cancer cells are not contact inhibited by E-cadherin interactions and Notch-1 signaling is no longer required for survival but instead becomes one of multiple signaling pathways for proliferation of prostatic cancer cells. These characteristic changes are consistent with prostate cancer cells' ability to metastasize to bone, a site of high-Ca2+ levels.