(
) encodes a tumor-suppressive phosphatase with both lipid and protein phosphatase activity. The tumor-suppressive functions of PTEN are lost through a variety of mechanisms across a wide spectrum ...of human malignancies, including several rare cancers that affect pediatric and adult populations. Originally discovered and characterized as a negative regulator of the cytoplasmic, pro-oncogenic phosphoinositide-3-kinase (PI3K) pathway, PTEN is also localized to the nucleus where it can exert tumor-suppressive functions in a PI3K pathway-independent manner. Cancers can usurp the tumor-suppressive functions of PTEN to promote oncogenesis by disrupting homeostatic subcellular PTEN localization. The objective of this review is to describe the changes seen in PTEN subcellular localization during tumorigenesis, how PTEN enters the nucleus, and the spectrum of impacts and consequences arising from disrupted PTEN nuclear localization on tumor promotion. This review will highlight the immediate need in understanding not only the cytoplasmic but also the nuclear functions of PTEN to gain more complete insights into how important PTEN is in preventing human cancers.
Abstract
Fusion-positive rhabdomyosarcoma (FP-RMS) driven by the expression of the PAX3-FOXO1 (P3F) fusion oncoprotein is an aggressive subtype of pediatric rhabdomyosarcoma. FP-RMS histologically ...resembles developing muscle yet occurs throughout the body in areas devoid of skeletal muscle highlighting that FP-RMS is not derived from an exclusively myogenic cell of origin. Here we demonstrate that P3F reprograms mouse and human endothelial progenitors to FP-RMS. We show that P3F expression in
aP2-Cre
expressing cells reprograms endothelial progenitors to functional myogenic stem cells capable of regenerating injured muscle fibers. Further, we describe a FP-RMS mouse model driven by P3F expression and
Cdkn2a
loss in endothelial cells. Additionally, we show that P3F expression in
TP53
-null human iPSCs blocks endothelial-directed differentiation and guides cells to become myogenic cells that form FP-RMS tumors in immunocompromised mice. Together these findings demonstrate that FP-RMS can originate from aberrant development of non-myogenic cells driven by P3F.
PTEN promoter hypermethylation is nearly universal and PTEN copy number loss occurs in ~25% of fusion-negative rhabdomyosarcoma (FN-RMS). Here we show Pten deletion in a mouse model of FN-RMS results ...in less differentiated tumors more closely resembling human embryonal RMS. PTEN loss activated the PI3K pathway but did not increase mTOR activity. In wild-type tumors, PTEN was expressed in the nucleus suggesting loss of nuclear PTEN functions could account for these phenotypes. Pten deleted tumors had increased expression of transcription factors important in neural and skeletal muscle development including Dbx1 and Pax7. Pax7 deletion completely rescued the effects of Pten loss. Strikingly, these Pten;Pax7 deleted tumors were no longer FN-RMS but displayed smooth muscle differentiation similar to leiomyosarcoma. These data highlight how Pten loss in FN-RMS is connected to a PAX7 lineage-specific transcriptional output that creates a dependency or synthetic essentiality on the transcription factor PAX7 to maintain tumor identity.
Resistance and partial responses to targeted monotherapy are major obstacles in cancer treatment. Systematic approaches to identify efficacious drug combinations for cancer are not well established, ...especially in the context of genotype. To address this, we have tested pairwise combinations of an array of small-molecule inhibitors on early-passage melanoma cultures using combinatorial drug screening. Results reveal several inhibitor combinations effective for melanomas with activating RAS or BRAF mutations, including mutant BRAF melanomas with intrinsic or acquired resistance to vemurafenib. Inhibition of both EGF receptor and AKT sensitized treatment-resistant BRAF mutant melanoma cultures to vemurafenib. Melanomas with RAS mutations were more resistant to combination therapies relative to BRAF mutants, but were sensitive to combinations of statins and cyclin-dependent kinase inhibitors in vitro and in vivo. These results show the use of combinatorial drug screening for discovering unique treatment regimens that overcome resistance phenotypes of mutant BRAF- and RAS-driven melanomas.
We have used drug combinatorial screening to identify effective combinations for mutant BRAF melanomas, including those resistant to vemurafenib, and mutant RAS melanomas that are resistant to many therapies. Mechanisms governing the interactions of the drug combinations are proposed, and in vivo xenografts show the enhanced benefit and tolerability of a mutant RAS -selective combination, which is currently lacking in the clinic.
Melanoma development involves members of the AGC kinase family, including AKT, PKC, and, most recently, PDK1, as elucidated recently in studies of Braf::Pten mutant melanomas. Here, we report that ...PDK1 contributes functionally to skin pigmentation and to the development of melanomas harboring a wild-type PTEN genotype, which occurs in about 70% of human melanomas. The PDK1 substrate SGK3 was determined to be an important mediator of PDK1 activities in melanoma cells. Genetic or pharmacologic inhibition of PDK1 and SGK3 attenuated melanoma growth by inducing G1 phase cell-cycle arrest. In a synthetic lethal screen, pan-PI3K inhibition synergized with PDK1 inhibition to suppress melanoma growth, suggesting that focused blockade of PDK1/PI3K signaling might offer a new therapeutic modality for wild-type PTEN tumors. We also noted that responsiveness to PDK1 inhibition associated with decreased expression of pigmentation genes and increased expression of cytokines and inflammatory genes, suggesting a method to stratify patients with melanoma for PDK1-based therapies. Overall, our work highlights the potential significance of PDK1 as a therapeutic target to improve melanoma treatment.
Despite improvements in survival in metastatic melanoma with combined BRAF and mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor treatment, the overwhelming majority of ...patients eventually acquire resistance to both agents. Consequently, new targets for therapy in resistant tumors are currently being evaluated. Previous studies have identified p90 subfamily of ribosomal S6 kinase (p90RSK) family kinases as key factors for growth and proliferation, as well as protein synthesis via assembly of the 7-methyl-guanosine triphosphate cap-dependent translation complex. We sought to evaluate inhibitors of p90RSK family members: BI-D1870 and BRD7389, for their ability to inhibit both proliferation and protein synthesis in patient-derived melanoma cell lines with acquired resistance to combined treatment with the BRAF inhibitor vemurafenib and the mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor selumetinib. We found that the RSK inhibitors blocked cell proliferation and protein synthesis in multiple dual-resistant melanoma lines. In addition, single agent RSK inhibitor treatment was effective in drug-naïve lines, two of which are innately vemurafenib resistant. We also used Reverse Phase Protein Array screening to identify differential protein expression that correlates with BI-D1870 sensitivity, and identified prognostic biomarkers for survival in human melanoma patients. These findings establish p90RSK inhibition as a therapeutic strategy in treatment-resistant melanoma and provide insight into the mechanism of action.
Fusion-negative rhabdomyosarcoma (FN-RMS) is molecularly heterogeneous with few universal alterations except for Phosphatase and tensin homolog (PTEN) promoter hypermethylation. We demonstrate that ...losing Pten in FN-RMS engages an aberrant transcriptional program key in tumor maintenance and cell identity. These results highlight the importance between transcriptional state, cell of origin, and genetic perturbation in tumorigenesis.
This study evaluates the use of HMG‐CoA reductase inhibitors, or statins, as an adjunctive to BRAF and MEK inhibition as a treatment in melanomas and other tumors with driver mutations in the MAPK ...pathway. Experiments used simvastatin in conjunction with vemurafenib and selumetinib in vitro and simvastatin with vemurafenib in vivo to demonstrate additional growth abrogation beyond MAPK blockade alone. Additional studies demonstrated that statin anti‐tumor effects appeared to depend on inhibition of isoprenoid synthesis given rescue with add‐back of downstream metabolites. Ultimately, we concluded that statins represent a possible useful adjunctive therapy in MAPK‐driven tumors when given with current approved targeted therapy.
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