Comparison of phosphatidylinositol-3-kinase signalling within a panel of human colorectal cancer cell lines with mutant or wild-type PIK3CA

Crosstalk between PI3K/Akt and Wnt/β-catenin pathways promote colorectal cancer progression regardless of mutational status

The PI3K/Akt and Wnt/β-catenin pathways play an important role in the acquisition of the malignant phenotype in cancer. However, there are few data regarding the role of the interplay between both pathways in colorectal cancer (CRC) progression. The mutational status and the clinicopathological characteristics of PI3K/Akt and Wnt/β-catenin pathways were accessed by bioinformatic analysis whereas that the impact of the interplay between the activity of both pathways to explain tumorigenic potential was performed in vitro using IGF-1 and Wnt3a treatments in CRC cell models. The mutational status of these pathways did not influence the survival of CRC patients, but an association between clinicopathological characteristics in patients with mutations in one, but not in both pathways was observed. A potentiating effect on the activation of both pathways and enhanced cellular migration and proliferation was observed when both pathways were activated simultaneously with IGF-1 and Wnt3a. In addition, these effects were hindered after pretreatment with LY294002, a specific PI3K inhibitor, suggesting some dependence between these two signaling cascades. Our findings show that, regardless of mutational status, there is an interplay between the activity of PI3K/Akt and Wnt/β-catenin pathways that contributes to events related to CRC progression and that the reversal of such events using a PI3K inhibitor highlights the value of targeting these pathways for potential directed therapies in CRC patients.

BRG1 regulates lipid metabolism in hepatocellular carcinoma through the PIK3AP1/PI3K/AKT pathway by mediating GLMP expression

BACKGROUND

Brahma-related gene 1 (BRG1) is essential for embryogenesis and cellular metabolism. A deficiency of BRG1 in vivo decreases lipid droplets, but the molecular mechanism underlying its role in lipid metabolism associated with hepatocellular carcinoma (HCC) remains unknown.

AIMS

We aimed to determine the role of BRG1 in lipid metabolism in HCC.

METHODS

We assessed the differential expression of BRG1 in HCC and adjacent non-tumorous tissues using tissue microarrays. We stained lipid droplets in HCC cells with Bodipy fluorescence and Oil Red O, and verified BRG1 binding to the promoter region of glycosylated lysosomal membrane protein (GLMP) using chromatin immunoprecipitation.

RESULTS

The expression of GLMP, a potential lipid metabolism regulator, was suppressed by BRG1 via transcriptional activity. Knockdown of BRG1 decreased lipid droplets, increased GLMP expression and altered the phosphoinositide-3-kinase adaptor protein 1 (PIK3AP1)/phosphatidylinositol-3 kinase (PI3K)/protein kinase B (AKT) pathway in HCC, which further GLMP knockdown partially restored. Thus, GLMP knockdown increased lipid droplets and differentially altered the PI3K/AKT pathway.

CONCLUSIONS

Downregulating BRG1 decreased lipid droplet deposition in HCC cells by upregulating GLMP and altering the PI3K/AKT pathway. Both BRG1 and GLMP might serve as therapeutic targets for disorders associated with dysregulated lipid metabolism, such as NAFLD and NAFLD-associated HCC.

JNK Pathway Mediates Low Oxygen Level Induced Epithelial-Mesenchymal Transition and Stemness Maintenance in Colorectal Cancer Cells

(1) Background: Epithelial-mesenchymal transition (EMT) and cancer cell stemness maintenance (SM) are important factors for cancer metastasis. Although hypoxia has been considered as a possible factor for EMT induction and promotion of SM, studies in this area, apart from hypoxia-inducible factor (HIF) pathways and severe hypoxia, are scant. This study aimed to evaluate the effects of different oxygen levels on EMT induction and SM and elucidate the signaling pathways involved in colorectal cancer cells. (2) Methods: Cell morphological analysis, migration assay, immunofluorescence staining of cytoskeleton and Western blotting were performed on human colorectal cancer cells HT-29, DLD-1, and SW-480 cultured at 1%, 10%, and normal (21%) O₂ levels. The role played by c-Jun N-terminal kinase (JNK) was evaluated through the use of the specific JNK inhibitor SP600125. (3) Results: This study evaluated 1% and 10% O₂ are possible conditions for EMT induction and SM. This study also demonstrated the partial relieve of EMT induction and SM by SP600125, showing the importance of the JNK pathway in these processes. Furthermore, this study proposed a novel pathway on the regulation of Akt by JNK-c-Jun. (4) Conclusions: This study suggests 10% O₂ as another possible condition for EMT induction, and SM and JNK pathways play important roles in these processes through multiple factors. Inhibition of JNK could be explored as treatment for inhibiting metastasis in colorectal cancer cells.

Effect of API-1 and FR180204 on cell proliferation and apoptosis in human DLD-1 and LoVo colorectal cancer cells

The activation of the phosphatidylinositol-3 kinase/v-akt murine thymoma viral oncogene homolog (Akt) and mitogen activated protein kinase kinase/extracellular signal-regulated kinase (ERK) pathways are implicated in the majority of cancers. Selective inhibition of Akt and ERK represents a potential approach for cancer therapy. Therefore, the present study aimed to investigate the apoptotic and anti-proliferative effects of the novel and selective Akt inhibitor 4-amino-5,8-dihydro-5-oxo-8-β-D-ribofuranosyl-pyrido[2,3-d]pyrimidine-6-carboxamide (API-1) and selective ERK1/2 inhibitor FR180204 (FR) alone and in combination on colorectal cancer (CRC) cells (DLD-1 and LoVo). In addition, the effects of API-1 and FR on Akt and ERK signaling pathways were also investigated. The effects of the agents on DLD-1 and LoVo cells were evaluated in terms of cell viability, cytotoxicity, DNA synthesis rate, DNA fragmentation and caspase-3 activity levels. In addition, quantitative reverse transcription-polymerase chain reaction and western blot analysis were performed to examine relevant mRNA and protein levels. The present study observed that the combination of FR with API-1 resulted in significant apoptosis and cytotoxicity compared with any single agent alone in a time-dependent manner in these cells. Also, treatment with FR and API-1 in combination decreased the expression levels of B-cell lymphoma-2 (BCL2), Bcl-2-like1, cyclin D1 and cMYC, and increased the expression levels of BCL2-associated X protein and BCL2 antagonist/killer via phosphorylated Akt and phosphorylated ERK1/2 downregulation. The combination of Akt and ERK1/2 inhibitors resulted in enhanced apoptotic and anti-proliferative effects against CRC cells. The present study hypothesizes that the combination of FR and API-1 in CRC cells may contribute toward potential anti-carcinogenic effects. Additional analyses using other cancer cell lines and animal models are required to confirm these findings in vitro and in vivo.

PI3K and AKT: Unfaithful Partners in Cancer

The phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway regulates multiple cellular processes. An overactivation of the pathway is frequently present in human malignancies and plays a key role in cancer progression. Hence, its inhibition has become a promising approach in cancer therapy. However, the development of resistances, such as the abrogation of negative feedback mechanisms or the activation of other proliferative signaling pathways, has considerably limited the anticancer efficacy of PI3K/AKT inhibitors. In addition, emerging evidence points out that although AKT is acknowledged as the major downstream effector of PI3K, both PI3K and AKT can operate independently of each other in cancer, revealing another level of complexity in this pathway. Here, we highlight the complex relationship between PI3K and AKT in cancer and further discuss the consequences of this relationship for cancer therapy.

BMX acts downstream of PI3K to promote colorectal cancer cell survival and pathway inhibition sensitizes to the BH3 mimetic ABT-737

Evasion of apoptosis is a hallmark of cancer, and reversing this process by inhibition of survival signaling pathways is a potential therapeutic strategy. Phosphoinositide 3-kinase (PI3K) signaling can promote cell survival and is upregulated in solid tumor types, including colorectal cancer (CRC), although these effects are context dependent. The role of PI3K in tumorigenesis combined with their amenability to specific inhibition makes them attractive drug targets. However, we observed that inhibition of PI3K in HCT116, DLD-1, and SW620 CRC cells did not induce apoptotic cell death. Moreover, these cells were relatively resistant to the Bcl-2 homology domain 3 (BH3) mimetic ABT-737, which directly targets the Bcl-2 family of apoptosis regulators. To test the hypothesis that PI3K inhibition lowers the apoptotic threshold without causing apoptosis per se, PI3K inhibitors were combined with ABT-737. PI3K inhibition enhanced ABT-737-induced apoptosis by 2.3- to 4.5-fold and reduced expression levels of MCL-1, the resistance biomarker for ABT-737. PI3K inhibition enhanced ABT-737-induced apoptosis a further 1.4- to 2.4-fold in CRC cells with small interfering RNA-depleted MCL-1, indicative of additional sensitizing mechanisms. The observation that ABT-737-induced apoptosis was unaffected by inhibition of PI3K downstream effectors AKT and mTOR, implicated a novel PI3K-dependant pathway. To elucidate this, an RNA interference (RNAi) screen of potential downstream effectors of PI3K signaling was conducted, which demonstrated that knockdown of the TEC kinase BMX sensitized to ABT-737. This suggests that BMX is an antiapoptotic downstream effector of PI3K, independent of AKT.

P110α-mediated constitutive PI3K signaling limits the efficacy of p110δ-selective inhibition in mantle cell lymphoma, particularly with multiple relapse

Phosphoinositide-3 kinase (PI3K) pathway activation contributes to mantle cell lymphoma (MCL) pathogenesis, but early-phase studies of the PI3K p110δ inhibitor GS-1101 have reported inferior responses in MCL compared with other non-Hodgkin lymphomas. Because the relative importance of the class IA PI3K isoforms p110α, p110β, and p110δ in MCL is not clear, we studied expression of these isoforms and assessed their contribution to PI3K signaling in this disease. We found that although p110δ was highly expressed in MCL, p110α showed wide variation and expression increased significantly with relapse. Loss of phosphatase and tensin homolog expression was found in 16% (22/138) of cases, whereas PIK3CA and PIK3R1 mutations were absent. Although p110δ inhibition was sufficient to block B-cell receptor-mediated PI3K activation, combined p110α and p110δ inhibition was necessary to abolish constitutive PI3K activation. In addition, GDC-0941, a predominantly p110α/δ inhibitor, was significantly more active compared with GS-1101 against MCL cell lines and primary samples. We found that a high PIK3CA/PIK3CD ratio identified a subset of primary MCLs resistant to GS-1101 and this ratio increased significantly with relapse. These findings support the use of dual p110α/p110δ inhibitors in MCL and suggest a role for p110α in disease progression.

PI3K keeps the balance between metabolism and cancer

Epidemiological studies have established a positive correlation between cancer and metabolic disorders, suggesting that aberrant cell metabolism is a common feature of nearly all tumors. To meet their demand of building block molecules, cancer cells switch to a heavily glucose-dependent metabolism. As insulin triggers glucose uptake, most tumors are or become insulin-dependent. However, the effects of insulin and of other similar growth factors are not only limited to metabolic control but also favor tumor growth by stimulating proliferation and survival. A key signaling event mediating these metabolic and proliferative responses is the activation of the phosphatidylinositol-3 kinases (PI3K) pathway. In this review, we will thus discuss the current concepts of tumor metabolism and the opportunity of PI3K-targeted therapies to exploit the "sweet tooth" of cancer cells.

Comparison of growth factor signalling pathway utilisation in cultured normal melanocytes and melanoma cell lines

BACKGROUND

The phosphatidylinositol-3-kinase (PI3K-PKB), mitogen activated protein kinase (MEK-ERK) and the mammalian target of rapamycin (mTOR- p70S6K), are thought to regulate many aspects of tumour cell proliferation and survival. We have examined the utilisation of these three signalling pathways in a number of cell lines derived from patients with metastatic malignant melanoma of known PIK3CA, PTEN, NRAS and BRAF mutational status.

METHODS

Western blotting was used to compare the phosphorylation status of components of the PI3K-PKB, MEK-ERK and mTOR-p70S6K signalling pathways, as indices of pathway utilisation.

RESULTS

Normal melanocytes could not be distinguished from melanoma cells on the basis of pathway utilisation when grown in the presence of serum, but could be distinguished upon serum starvation, where signalling protein phosphorylation was generally abrogated. Surprisingly, the differential utilisation of individual pathways was not consistently associated with the presence of an oncogenic or tumour suppressor mutation of genes in these pathways.

CONCLUSION

Utilisation of the PI3K-PKB, MEK-ERK and mTOR-p70S6K signalling pathways in melanoma, as determined by phosphorylation of signalling components, varies widely across a series of cell lines, and does not directly reflect mutation of genes coding these components. The main difference between cultured normal melanocytes and melanoma cells is not the pathway utilisation itself, but rather in the serum dependence of pathway utilisation.

Both p110α and p110β isoforms of PI3K can modulate the impact of loss-of-function of the PTEN tumour suppressor

The PI3K (phosphoinositide 3-kinase) pathway is commonly activated in cancer as a consequence of inactivation of the tumour suppressor PTEN (phosphatase and tensin homologue deleted on chromosome 10), a major negative regulator of PI3K signalling. In line with this important role of PTEN, mice that are heterozygous for a PTEN-null allele (PTEN+/− mice) spontaneously develop a variety of tumours in multiple organs. PTEN is a phosphatase with selectivity for PtdIns(3,4,5)P3, which is produced by the class I isoforms of PI3K (p110α, p110β, p110γ and p110δ). Previous studies indicated that PTEN-deficient cancer cell lines mainly depend on p110β, and that p110β, but not p110α, controls mouse prostate cancer development driven by PTEN loss. In the present study, we investigated whether the ubiquitously expressed p110α can also functionally interact with PTEN in cancer. Using genetic mouse models that mimic systemic administration of p110α- or p110β-selective inhibitors, we confirm that inactivation of p110β, but not p110α, inhibits prostate cancer development in PTEN+/− mice, but also find that p110α inactivation protects from glomerulonephritis, pheochromocytoma and thyroid cancer induced by PTEN loss. This indicates that p110α can modulate the impact of PTEN loss in disease and tumourigenesis. In primary and immortalized mouse fibroblast cell lines, both p110α and p110β controlled steady-state PtdIns(3,4,5)P3 levels and Akt signalling induced by heterozygous PTEN loss. In contrast, no correlation was found in primary mouse tissues between PtdIns(3,4,5)P3 levels, PI3K/PTEN genotype and cancer development. Taken together, our results from the present study show that inactivation of either p110α or p110β can counteract the impact of PTEN inactivation. The potential implications of these findings for PI3K-targeted therapy of cancer are discussed.

Phosphoinositide 3-kinases in health and disease

In the last decade, the availability of genetically modified animals has revealed interesting roles for phosphoinositide 3-kinases (PI3Ks) as signaling platforms orchestrating multiple cellular responses, both in health and pathology. By acting downstream distinct receptor types, PI3Ks nucleate complex signaling assemblies controlling several biological process, ranging from cell proliferation and survival to immunity, cancer, metabolism and cardiovascular control. While the involvement of these kinases in modulating immune reactions and neoplastic transformation has long been accepted, recent progress from our group and others has highlighted new and unforeseen roles of PI3Ks in controlling cardiovascular function. Hence, the view is emerging that pharmacological targeting of distinct PI3K isoforms could be successful in treating disorders such as myocardial infarction and heart failure, besides inflammatory diseases and cancer. Currently, PI3Ks represent attractive drug targets for companies interested in the development of novel and safe treatments for such diseases. Numerous hit and lead compounds are now becoming available and, for some of them, clinical trials can be envisaged in the near future. In the following sections, we will outline the impact of specific PI3K isoforms in regulating different cellular contexts, including immunity, metabolism, cancer and cardiovascular system, both in physiological and disease conditions.

The emerging mechanisms of isoform-specific PI3K signalling

Phosphoinositide 3-kinases (PI3Ks) function early in intracellular signal transduction pathways and affect many biological functions. A further level of complexity derives from the existence of eight PI3K isoforms, which are divided into class I, class II and class III PI3Ks. PI3K signalling has been implicated in metabolic control, immunity, angiogenesis and cardiovascular homeostasis, and is one of the most frequently deregulated pathways in cancer. PI3K inhibitors have recently entered clinical trials in oncology. A better understanding of how the different PI3K isoforms are regulated and control signalling could uncover their roles in pathology and reveal in which disease contexts their blockade could be most beneficial.

Natural sphingadienes inhibit Akt-dependent signaling and prevent intestinal tumorigenesis

Sphingolipid metabolites regulate cell proliferation, migration, and stress responses. Alterations in sphingolipid metabolism have been proposed to contribute to carcinogenesis, cancer progression, and drug resistance. We identified a family of natural sphingolipids called sphingadienes and investigated their effects in colon cancer. We find that sphingadienes induce colon cancer cell death in vitro and prevent intestinal tumorigenesis in vivo. Sphingadienes exert their influence by blocking Akt translocation from the cytosol to the membrane, thereby inhibiting protein translation and promoting apoptosis and autophagy. Sphingadienes are orally available, are slowly metabolized through the sphingolipid degradative pathway, and show limited short-term toxicity. Thus, sphingadienes represent a new class of therapeutic and/or chemopreventive agents that blocks Akt signaling in neoplastic and preneoplastic cells.

PI3K: from the bench to the clinic and back

From humble beginnings over 25 years ago as a lipid kinase activity associated with certain oncoproteins, PI3K (phosphoinositide 3-kinase) has been catapulted to the forefront of drug development in cancer, immunity and thrombosis, with the first clinical trials of PI3K pathway inhibitors now in progress. Here, we give a brief overview of some key discoveries in the PI3K area and their impact, and include thoughts on the current state of the field, and where it could go from here.PI3K has become a very intense area of research, with over 2,000 publications on PI3K in PubMed for 2009 alone. The expectations for a therapeutic impact of intervention with PI3K activity are high, and progress in the clinical arena is being monitored by many. However, targeted therapies almost invariably encounter roadblocks, often exposing unresolved questions in the basic understanding of the target. PI3K will most likely be no exception. Below, we describe some of these early "surprises" and how these inform and shape basic science investigations.

Intrinsic resistance to the MEK1/2 inhibitor AZD6244 (ARRY-142886) is associated with weak ERK1/2 signalling and/or strong PI3K signalling in colorectal cancer cell lines

Mutations in KRAS or BRAF frequently manifest in constitutive activation of the MEK1/2-ERK1/2 signalling pathway. The MEK1/2-selective inhibitor, AZD6244 (ARRY-142886), blocks ERK1/2 activation and is currently undergoing clinical evaluation. Tumour cells can vary markedly in their response to MAPK or ERK kinase (MEK) inhibitors, and the presence of a BRAF mutation is thought to predict sensitivity, with the RAS mutations being associated with intrinsic resistance. We analysed cell proliferation in a panel of 19 colorectal cancer cell lines and found no simple correlation between BRAF or KRAS mutation and sensitivity to AZD6244, though cells that harbour neither mutation tended to be resistant. Cells that were sensitive arrested in G(1) and/or underwent apoptosis and the presence of BRAF or KRAS mutation was not sufficient to predict either fate. Cell lines that were resistant to AZD6244 exhibited low or no ERK1/2 activation or exhibited coincident activation of ERK1/2 and protein kinase B (PKB), the latter indicative of activation of the PI3K pathway. In cell lines with coincident ERK1/2 and PKB activation, sensitivity to AZD6244 could be re-imposed by any of the 3 distinct PI3K/mTOR inhibitors. We conclude that AZD6244 is effective in colorectal cancer cell lines with BRAF or KRAS mutations. Sensitivity to MEK1/2 inhibition correlates with a biochemical signature; those cells with high ERK1/2 activity (whether mutant for BRAF or KRAS) evolve a dependency upon that pathway and tend to be sensitive to AZD6244 but this can be offset by high PI3K-dependent signalling. This may have implications for the use of MEK inhibitors in combination with PI3K inhibitors.

Blocking phosphoinositide 3-kinase activity in colorectal cancer cells reduces proliferation but does not increase apoptosis alone or in combination with cytotoxic drugs

In response to growth factors, class IA phosphoinositide 3-kinases (PI3K) phosphorylate phosphatidylinositol-4,5-bisphosphate, converting it to phosphatidylinositol-3,4,5-trisphosphate to activate protein kinase B/Akt. This is widely reported to promote tumorigenesis via increased cell survival, proliferation, migration, and invasion, and many tumor types, including colorectal cancer, exhibit increased PI3K signaling. To investigate the effect of inhibiting PI3K and as an alternative to the use of small molecular inhibitors of PI3K with varying degrees of selectivity, HT29 and HCT116 colorectal cancer cells bearing mutant PIK3CA were generated that could be induced with doxycycline to express synchronously a dominant negative subunit of PI3K, Deltap85alpha. On induction, decreased levels of phosphorylated protein kinase B were detected, confirming PI3K signaling impairment. Induction of Deltap85alpha in vitro reduced cell number via accumulation in G(0)-G(1) phase of the cell cycle in the absence of increased apoptosis. These effects were recapitulated in vivo. HT29 cells expressing Deltap85alpha and grown as tumor xenografts had a significantly slower growth rate on administration of doxycycline with reduced Ki67 staining without increased levels of apoptotic tissue biomarkers. Furthermore, in vitro Deltap85alpha expression did not sensitize HT29 cells to oxaliplatin- or etoposide-induced apoptosis, irrespective of drug treatment schedule. Further analysis comparing isogenic HCT116 cells with and without mutation in PIK3CA showed no effect of the mutation in either proliferative or apoptotic response to PI3K inhibition. These data show in colorectal cancer cells that PI3K inhibition does not provoke apoptosis per se nor enhance oxaliplatin- or etoposide-induced cell death.

Knockdown of Ron kinase inhibits mutant phosphatidylinositol 3-kinase and reduces metastasis in human colon carcinoma

Abnormal accumulation and activation of receptor tyrosine kinase Ron (recepteur d'origine nantais) has been demonstrated in a variety of primary human cancers. We show that RNA interference-mediated knockdown of Ron kinase in a highly tumorigenic colon cancer cell line led to reduced proliferation as compared with the control cells. Decreased Ron expression sensitized HCT116 cells to growth factor deprivation stress-induced apoptosis as reflected by increased DNA fragmentation and caspase 3 activation. In addition, cell motility was decreased in Ron knockdown cells as measured by wound healing assays and transwell assays. HCT116 cells are heterozygous for gain of function mutant PIK3CA H1047R. Analysis of signaling proteins that are affected by Ron knockdown revealed that phosphatidylinositol 3-kinase (PI3K) activity of the mutant PI3K as well as AKT phosphorylation was substantially reduced in the Ron knockdown cells compared with the control cells. Moreover, we demonstrated in vivo that knockdown of Ron expression significantly reduced lung metastasis as compared with the control cells in the orthotopic models. In summary, our results demonstrate that Ron plays an essential role in maintaining malignant phenotypes of colon cancer cells through regulating mutant PI3K activity. Therefore, targeting Ron kinase could be a potential strategy for colon cancer treatment, especially in patients bearing gain of function mutant PI3K activity.

PIK3CA mutations and PTEN loss correlate with similar prognostic factors and are not mutually exclusive in breast cancer

PURPOSE

The phosphatidylinositol 3'-kinase/Akt pathway is frequently altered in breast cancer. PTEN, a phosphatase that opposes the effect of phosphatidylinositol 3'-kinase, can be mutated or lost, whereas the PIK3CA gene is mutated. These have been proposed as alternative mechanisms, and their clinicalpathology significance is under discussion. In this study, we aimed to explore whether PIK3CA mutations and PTEN loss are mutually exclusive mechanisms, correlate with other known clinicopathologic markers, or have clinical implication in breast cancer.

EXPERIMENTAL DESIGN

Exons 9 and 20 of the PIK3CA gene were analyzed in 270 breast tumors, and mutations were detected by single-stranded conformational analysis followed by sequencing. The expression of PTEN was evaluated by immunohistochemistry in 201 tumors.

RESULTS

PIK3CA mutations were found in 24% of the tumors and associated with estrogen receptor(+) status, small size, negative HER2 status, high Akt1, and high cyclin D1 protein expression. PTEN was negative in 37% of the cases and PTEN loss was associated with PIK3CA mutations (P = 0.0024). Tumors presenting PTEN loss or both alterations were often estrogen receptor(+), small in size, and HER2(-). PIK3CA mutations predicted for longer local recurrence-free survival. Moreover, PTEN loss by itself or combined with mutated PIK3CA tended to confer radiosensitivity. In addition, the patients with high S-phase fraction had longer recurrence-free survival if they carried mutations in the PIK3CA gene and/or had lost PTEN, whereas the same alterations were associated with shorter recurrence-free survival among patients with low S-phase fraction.

CONCLUSIONS

PIK3CA mutations and PTEN loss were not mutually exclusive events and associated with similar prognostic factors.

Colon carcinoma cells harboring PIK3CA mutations display resistance to growth factor deprivation induced apoptosis

PIK3CA, encoding the p110alpha catalytic subunit of phosphatidylinositol 3-kinase (PI3K), is mutated in a variety of human cancers. We screened the colon cancer cell lines previously established in our laboratory for PIK3CA mutations and found that four of them harbored gain of function mutations. We have now compared a panel of mutant and wild-type cell lines for cell proliferation and survival in response to stress. There was little difference in PI3K activity between mutant PIK3CA-bearing cells (mutant cells) and wild-type PIK3CA-bearing cells (wild-type cells) under optimal growth conditions. However, the mutant cells showed constitutive PI3K activity during growth factor deprivation stress (GFDS), whereas PI3K activity decayed rapidly in the wild-type cells. Importantly, constitutively active PI3K rendered the mutant cells resistant to GFDS-induced apoptosis relative to the wild-type cells, indicating a biological advantage under stress conditions that is imparted by the mutant enzymes. Compared with the wild-type cells, the mutant cells were hypersensitive to the apoptosis induced by the PI3K inhibitor LY294002. In addition, PIK3CA small interfering RNA significantly decreased DNA synthesis and/or induced apoptosis in the mutant cells but not in the wild-type cells. Furthermore, ecotopic expression of a mutant PIK3CA in a nontumorigenic PIK3CA wild-type cell line resulted in resistance to GFDS-induced apoptosis, whereas transfection of wild-type PIK3CA or empty vector had little effect. Taken together, our studies show that mutant PIK3CA increases the capacity for proliferation and survival under environmental stresses, such as GFDS while also imparting greater dependency on the PI3K pathway for proliferation and survival.

Cancer-specific mutations in phosphatidylinositol 3-kinase

Cancer-specific mutations in the catalytic subunit of phosphatidylinositol 3-kinase (PI3K) p110 alpha occur in diverse tumors in frequencies that can exceed 30%. The majority of these mutations map to one of three hot spots in the gene, and the rest are distributed over much of the PI3K coding sequence. Most of the cancer-specific mutations induce a gain of function that results in oncogenicity, elevated lipid kinase activity and constitutive signaling through the kinases Akt and TOR. The location of the mutations on a model structure of p110 alpha indicates several distinct mechanisms for the gain of function. The mutated p110 alpha proteins are promising cancer targets. Although identification of mutant-specific small-molecule inhibitors seems technically challenging, the therapeutic benefits from such inhibitors could be extremely important.