Daniel E. Frigo

Development of a small molecule serum and glucocorticoid-regulated kinase 1 antagonist and its evaluation as a prostate cancer therapeutic

Androgens, through their actions on the androgen receptor (AR), are required for the development of the prostate and contribute to the pathologic growth dysregulation observed in prostate cancers. Consequently, androgen ablation has become an essential component of the pharmacotherapy of prostate cancer. In this study, we explored the utility of targeting processes downstream of AR as an alternate approach for therapy. Specifically, we show that the serum and glucocorticoid-regulated kinase 1 (SGK1) gene is an androgen-regulated target gene in cellular models of prostate cancer. Furthermore, functional serum- and glucocorticoid-regulated kinase 1 (SGK1) protein, as determined by the phosphorylation of its target Nedd4-2, was also increased with androgen treatment. Importantly, we determined that RNA interference-mediated knockdown of SGK1 expression attenuates the androgen-mediated growth of the prostate cancer cell line LNCaP. Given these findings, we explored the utility of SGK1 as a therapeutic target in prostate cancer by developing and evaluating a small-molecule inhibitor of this enzyme. From these studies emerged GSK650394, a competitive inhibitor that quantitatively blocks the effect of androgens on LNCaP cell growth. Thus, in addition to androgen ablation, inhibition of pathways downstream of AR is likely to have therapeutic utility in prostate cancer.

Androgens regulate prostate cancer cell growth via an AMPK-PGC-1α-mediated metabolic switch

Prostate cancer is the most commonly diagnosed malignancy among men in industrialized countries, accounting for the second leading cause of cancer-related deaths. Although we now know that the androgen receptor (AR) is important for progression to the deadly advanced stages of the disease, it is poorly understood what AR-regulated processes drive this pathology. Here we demonstrate that AR regulates prostate cancer cell growth via the metabolic sensor 5′-AMP-activated protein kinase (AMPK), a kinase that classically regulates cellular energy homeostasis. In patients, activation of AMPK correlated with prostate cancer progression. Using a combination of radiolabeled assays and emerging metabolomic approaches, we also show that prostate cancer cells respond to androgen treatment by increasing not only rates of glycolysis, as is commonly seen in many cancers, but also glucose and fatty acid oxidation. Importantly, this effect was dependent on androgen-mediated AMPK activity. Our results further indicate that the AMPK-mediated metabolic changes increased intracellular ATP levels and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α)-mediated mitochondrial biogenesis, affording distinct growth advantages to the prostate cancer cells. Correspondingly, we used outlier analysis to determine that PGC-1α is overexpressed in a subpopulation of clinical cancer samples. This was in contrast to what was observed in immortalized benign human prostate cells and a testosterone-induced rat model of benign prostatic hyperplasia. Taken together, our findings converge to demonstrate that androgens can co-opt the AMPK-PGC-1α signaling cascade, a known homeostatic mechanism, to increase prostate cancer cell growth. The current study points to the potential utility of developing metabolic-targeted therapies directed toward the AMPK-PGC-1α signaling axis for the treatment of prostate cancer.

CaM Kinase Kinase β-Mediated Activation of the Growth Regulatory Kinase AMPK Is Required for Androgen-Dependent Migration of Prostate Cancer Cells

While patients with advanced prostate cancer initially respond favorably to androgen ablation therapy, most experience a relapse of the disease within 1-2 years. Although hormone-refractory disease is unresponsive to androgen-deprivation, androgen receptor (AR)-regulated signaling pathways remain active and are necessary for cancer progression. Thus, both AR itself and the processes downstream of the receptor remain viable targets for therapeutic intervention. Microarray analysis of multiple clinical cohorts showed that the serine/threonine kinase Ca2+/calmodulin-dependent protein kinase kinase β (CaMKKβ) is both highly expressed in the prostate and further elevated in prostate cancers. Using cellular models of prostate cancer, we have determined that androgens (a) directly increase the expression of a CaMKKβ splice variant and (b) increase functional CaMKKβ protein levels as determined by the phosphorylation of both CaMKI and AMP-activated protein kinase (AMPK), two of CaMKKβs primary substrates. Importantly, inhibition of the CaMKKβ-AMPK, but not CaMKI, signaling axis in prostate cancer cells by pharmacological inhibitors or siRNA-mediated knockdown blocks androgen-mediated migration and invasion. Conversely, overexpression of CaMKKβ alone leads to both increased AMPK phosphorylation and cell migration. Given the key roles of CaMKKβ and AMPK in the biology of prostate cancer cells, we propose that these enzymes are potential therapeutic targets in prostate cancer.

Regulation of the pentose phosphate pathway by an androgen receptor-mTOR-mediated mechanism and its role in prostate cancer cell growth.

Cancer cells display an increased demand for glucose. Therefore, identifying the specific aspects of glucose metabolism that are involved in the pathogenesis of cancer may uncover novel therapeutic nodes. Recently, there has been a renewed interest in the role of the pentose phosphate pathway in cancer. This metabolic pathway is advantageous for rapidly growing cells because it provides nucleotide precursors and helps regenerate the reducing agent NADPH, which can contribute to reactive oxygen species (ROS) scavenging. Correspondingly, clinical data suggest glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme of the pentose phosphate pathway, is upregulated in prostate cancer. We hypothesized that androgen receptor (AR) signaling, which plays an essential role in the disease, mediated prostate cancer cell growth in part by increasing flux through the pentose phosphate pathway. Here, we determined that G6PD, NADPH and ribose synthesis were all increased by AR signaling. Further, this process was necessary to modulate ROS levels. Pharmacological or molecular inhibition of G6PD abolished these effects and blocked androgen-mediated cell growth. Mechanistically, regulation of G6PD via AR in both hormone-sensitive and castration-resistant models of prostate cancer was abolished following rapamycin treatment, indicating that AR increased flux through the pentose phosphate pathway by the mammalian target of rapamycin (mTOR)-mediated upregulation of G6PD. Accordingly, in two separate mouse models of Pten deletion/elevated mTOR signaling, Pb-Cre;Ptenf/f and K8-CreERT2;Ptenf/f, G6PD levels correlated with prostate cancer progression in vivo. Importantly, G6PD levels remained high during progression to castration-resistant prostate cancer. Taken together, our data suggest that AR signaling can promote prostate cancer through the upregulation of G6PD and therefore, the flux of sugars through the pentose phosphate pathway. Hence, these findings support a vital role for other metabolic pathways (that is, not glycolysis) in prostate cancer cell growth and maintenance.

Induction of Krüppel-Like Factor 5 Expression by Androgens Results in Increased CXCR4-Dependent Migration of Prostate Cancer Cells in Vitro

Advanced prostate cancers preferentially metastasize to bone, suggesting that this tissue produces factors that provide a suitable microenvironment for prostate cancer cells. Recently, it has become clear that even in antiandrogen-resistant cancers, the androgen receptor (AR)-signaling axis is required for prostate cancer progression. Therefore, we hypothesized that AR may be involved in the regulation of pathways that are responsible for the homing of prostate cancer cells to select microenvironments. In support of this hypothesis, we have determined that chemokine (C-X-C motif) receptor 4 (CXCR4), the receptor for the chemokine CXCL12, is up-regulated in prostate cancer cells in response to androgens. Given that the levels of CXCL12 are elevated at sites of known prostate cancer metastases such as bone, these results suggest that androgens may influence prostate cancer metastasis. Specifically, we demonstrate that androgens increase the levels of both CXCR4 mRNA and functional protein in LNCaP prostate cancer cells. Importantly, androgens enhanced the migration of LNCaP cells toward a CXCL12 gradient, an effect that could be blocked by the specific CXCR4 antagonist AMD3100. Interestingly, CXCR4 is not directly regulated by androgens but rather is positively up-regulated by Krüppel-like factor 5 (KLF5), a transcription factor that we have shown to be an early, direct target of AR. Further, KLF5 is both required and sufficient for androgen-mediated CXCR4 expression and migration toward CXCL12. Taken together, these findings demonstrate that AR can utilize the CXCL12/CXCR4 axis through induction of KLF5 expression to promote prostate cancer progression and highlight the potential utility of CXCR4 antagonists as prostate cancer therapeutics.

Fatty Acid Oxidation-Driven Src Links Mitochondrial Energy Reprogramming and Oncogenic Properties in Triple-Negative Breast Cancer

Transmitochondrial cybrids and multiple OMICs approaches were used to understand mitochondrial reprogramming and mitochondria-regulated cancer pathways in triple-negative breast cancer (TNBC). Analysis of cybrids and established breast cancer (BC) cell lines showed that metastatic TNBC maintains high levels of ATP through fatty acid β oxidation (FAO) and activates Src oncoprotein through autophosphorylation at Y419. Manipulation of FAO including the knocking down of carnitine palmitoyltransferase-1A (CPT1) and 2 (CPT2), the rate-limiting proteins of FAO, and analysis of patient-derived xenograft models confirmed the role of mitochondrial FAO in Src activation and metastasis. Analysis of TCGA and other independent BC clinical data further reaffirmed the role of mitochondrial FAO and CPT genes in Src regulation and their significance in BC metastasis.

Differential effects of prostate cancer therapeutics on neuroendocrine transdifferentiation

Androgen ablation therapy is widely used for the treatment of advanced prostate cancer. However, the effectiveness of this intervention strategy is generally short-lived as the disease ultimately progresses to a hormone-refractory state. In recent years, it has become clear that even in antiandrogen-resistant cancers the androgen receptor (AR) signaling axis is intact and is required for prostate cancer growth. Thus, there is a heightened interest in developing small molecules that function in part by down-regulating AR expression in tumors. Paradoxically, AR expression has been shown to be important in preventing the transdifferentiation of epithelial prostate cancer cells toward a neuroendocrine phenotype associated with tumor progression. Consequently, we have evaluated the relative effect of prostate cancer therapeutics that function in part by depleting AR levels on neuroendocrine differentiation in established cellular models of prostate cancer. These studies reveal that although histone deacetylase inhibitors can down-regulate AR expression they increase the expression of neuroendocrine markers and alter cellular morphology. Inhibition of AR signaling using classic AR antagonists or small interfering RNA–mediated AR ablation induces incomplete neuroendocrine differentiation. Importantly, the Hsp90 inhibitor geldanamycin effectively down-regulates AR expression while having no effect on neuroendocrine differentiation. Taken together, these data show that the phenotypic responses to pharmacologic agents used in the clinic to prevent the progression of prostate cancer are not equivalent, a finding of significant therapeutic importance. [Mol Cancer Ther 2008;7(3):659–69]

Androgens Promote Prostate Cancer Cell Growth through Induction of Autophagy

Androgens regulate both the physiological development of the prostate and the pathology of prostatic diseases. However, the mechanisms by which androgens exert their regulatory activities on these processes are poorly understood. In this study, we have determined that androgens regulate overall cell metabolism and cell growth, in part, by increasing autophagy in prostate cancer cells. Importantly, inhibition of autophagy using either pharmacological or molecular inhibitors significantly abrogated androgen-induced prostate cancer cell growth. Mechanistically, androgen-mediated autophagy appears to promote cell growth by augmenting intracellular lipid accumulation, an effect previously demonstrated to be necessary for prostate cancer cell growth. Further, autophagy and subsequent cell growth is potentiated, in part, by androgen-mediated increases in reactive oxygen species. These findings demonstrate a role for increased fat metabolism and autophagy in prostatic neoplasias and highlight the potential of targeting underexplored metabolic pathways for the development of novel therapeutics.

Chitosan nanoparticle-mediated delivery of miRNA-34a decreases prostate tumor growth in the bone and its expression induces non-canonical autophagy

While several new therapies are FDA-approved for bone-metastatic prostate cancer (PCa), patient survival has only improved marginally. Here, we report that chitosan nanoparticle-mediated delivery of miR-34a, a tumor suppressive microRNA that downregulates multiple gene products involved in PCa progression and metastasis, inhibited prostate tumor growth and preserved bone integrity in a xenograft model representative of established PCa bone metastasis. Expression of miR-34a induced apoptosis in PCa cells, and, in accord with downregulation of targets associated with PCa growth, including MET and Axl and c-Myc, also induced a form of non-canonical autophagy that is independent of Beclin-1, ATG4, ATG5 and ATG7. MiR-34a-induced autophagy is anti-proliferative in prostate cancer cells, as blocking apoptosis still resulted in growth inhibition of tumor cells. Thus, combined effects of autophagy and apoptosis are responsible for miR-34a-mediated prostate tumor growth inhibition, and have translational impact, as this non-canonical form of autophagy is tumor inhibitory. Together, these results provide a new understanding of the biological effects of miR-34a and highlight the clinical potential for miR-34a delivery as a treatment for bone metastatic prostate cancer.

Organochlorine-mediated potentiation of the general coactivator p300 through p38 mitogen-activated protein kinase

The activity of nuclear transcription factors is often regulated by specific kinase-signaling pathways. We have previously shown that the organochlorine pesticide dichlorodiphenyltrichloroethane (DDT) stimulates activator protein-1 activity through the p38 mitogen-activated protein kinase (MAPK). Here, we show that DDT and its metabolites also stimulate the transcriptional activity of cyclic adenosine monophosphate response element-binding protein and Elk1 and potentiate gene expression through cyclic adenosine monophosphate and hypoxia response elements. Because DDT stimulates gene expression through various transcription factors and hence multiple response elements, we hypothesized that p38 signaling targets a common shared transcriptional activator. Here, we demonstrate using both pharmacological and molecular techniques, the general coactivator p300 is phosphorylated and potentiated by the p38 MAPK signaling cascade. We further show that p38 directly phosphorylates p300 in its N-terminus. These results, together with our previous work, suggest that p38 stimulates downstream transcription factors in part by targeting the general coactivator p300.