Linda S. Steelman

Involvement of PI3K/Akt pathway in cell cycle progression, apoptosis, and neoplastic transformation: a target for cancer chemotherapy

The PI3K/Akt signal transduction cascade has been investigated extensively for its roles in oncogenic transformation. Initial studies implicated both PI3K and Akt in prevention of apoptosis. However, more recent evidence has also associated this pathway with regulation of cell cycle progression. Uncovering the signaling network spanning from extracellular environment to the nucleus should illuminate biochemical events contributing to malignant transformation. Here, we discuss PI3K/Akt-mediated signal transduction including its mechanisms of activation, signal transducing molecules, and effects on gene expression that contribute to tumorigenesis. Effects of PI3K/Akt signaling on important proteins controlling cellular proliferation are emphasized. These targets include cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors. Furthermore, strategies used to inhibit the PI3K/Akt pathway are presented. The potential for cancer treatment with agents inhibiting this pathway is also addressed.

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention.

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

Targeting survival cascades induced by activation of Ras/Raf/MEK/ERK, PI3K/PTEN/Akt/mTOR and Jak/STAT pathways for effective leukemia therapy

The Raf/MEK/ERK, PI3K/PTEN/Akt/mTOR and Jak/STAT pathways are frequently activated in leukemia and other hematopoietic disorders by upstream mutations in cytokine receptors, aberrant chromosomal translocations as well as other genetic mechanisms. The Jak2 kinase is frequently mutated in many myeloproliferative disorders. Effective targeting of these pathways may result in suppression of cell growth and death of leukemic cells. Furthermore it may be possible to combine various chemotherapeutic and antibody-based therapies with low molecular weight, cell membrane-permeable inhibitors which target the Raf/MEK/ERK, PI3K/PTEN/Akt/mTOR and Jak/STAT pathways to ultimately suppress the survival pathways, induce apoptosis and inhibit leukemic growth. In this review, we summarize how suppression of these pathways may inhibit key survival networks important in leukemogenesis and leukemia therapy as well as the treatment of other hematopoietic disorders. Targeting of these and additional cascades may also improve the therapy of chronic myelogenous leukemia, which are resistant to BCR-ABL inhibitors. Furthermore, we discuss how targeting of the leukemia microenvironment and the leukemia stem cell are emerging fields and challenges in targeted therapies.

Targeting the leukemic stem cell: the Holy Grail of leukemia therapy

Since the discovery of leukemic stem cells (LSCs) over a decade ago, many of their critical biological properties have been elucidated, including their distinct replicative properties, cell surface phenotypes, their increased resistance to chemotherapeutic drugs and the involvement of growth-promoting chromosomal translocations. Of particular importance is their ability to transfer malignancy to non-obese diabetic-severe combined immunodeficient (NOD-SCID) mice. Furthermore, numerous studies demonstrate that acute myeloid leukemia arises from mutations at the level of stem cell, and chronic myeloid leukemia is also a stem cell disease. In this review, we will evaluate the main characteristics of LSCs elucidated in several well-documented leukemias. In addition, we will discuss points of therapeutic intervention. Promising therapeutic approaches include the targeting of key signal transduction pathways (for example, PI3K, Rac and Wnt) with small-molecule inhibitors and specific cell surface molecules (for example, CD33, CD44 and CD123), with effective cytotoxic antibodies. Also, statins, which are already widely therapeutically used for a variety of diseases, show potential in targeting LSCs. In addition, drugs that inhibit ATP-binding cassette transporter proteins are being extensively studied, as they are important in drug resistance—a frequent characteristic of LSCs. Although the specific targeting of LSCs is a relatively new field, it is a highly promising battleground that may reveal the Holy Grail of cancer therapy.

Roles of the Ras/Raf/MEK/ERK pathway in leukemia therapy

The Ras/Raf/mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway is often implicated in sensitivity and resistance to leukemia therapy. Dysregulated signaling through the Ras/Raf/MEK/ERK pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Unrestricted leukemia proliferation and decreased sensitivity to apoptotic-inducing agents and chemoresistance are typically associated with activation of pro-survival pathways. Mutations in this pathway and upstream signaling molecules can alter sensitivity to small molecule inhibitors targeting components of this cascade as well as to inhibitors targeting other key pathways (for example, phosphatidylinositol 3 kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome 10 (PTEN)/Akt/mammalian target of rapamycin (mTOR)) activated in leukemia. Similarly, PI3K mutations can result in resistance to inhibitors targeting the Ras/Raf/MEK/ERK pathway, indicating important interaction points between the pathways (cross-talk). Furthermore, the Ras/Raf/MEK/ERK pathway can be activated by chemotherapeutic drugs commonly used in leukemia therapy. This review discusses the mechanisms by which abnormal expression of the Ras/Raf/MEK/ERK pathway can contribute to drug resistance as well as resistance to targeted leukemia therapy. Controlling the expression of this pathway could improve leukemia therapy and ameliorate human health.

Targeting the translational apparatus to improve leukemia therapy: roles of the PI3K/PTEN/Akt/mTOR pathway

It has become apparent that regulation of protein translation is an important determinant in controlling cell growth and leukemic transformation. The phosphoinositide 3-kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome ten (PTEN)/Akt/mammalian target of rapamycin (mTOR) pathway is often implicated in sensitivity and resistance to therapy. Dysregulated signaling through the PI3K/PTEN/Akt/mTOR pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Furthermore, this pathway is activated by autocrine transformation mechanisms. PTEN is a critical tumor suppressor gene and its dysregulation results in the activation of Akt. PTEN is often mutated, silenced and is often haploinsufficient. The mTOR complex1 (mTORC1) regulates the assembly of the eukaryotic initiation factor4F complex, which is critical for the translation of mRNAs that are important for cell growth, prevention of apoptosis and transformation. These mRNAs have long 5′-untranslated regions that are G+C rich, rendering them difficult to translate. Elevated mTORC1 activity promotes the translation of these mRNAs via the phosphorylation of 4E-BP1. mTORC1 is a target of rapamycin and novel active-site inhibitors that directly target the TOR kinase activity. Although rapamycin and novel rapalogs are usually cytostatic and not cytotoxic for leukemic cells, novel inhibitors that target the kinase activities of PI3K and mTOR may prove more effective for leukemia therapy.

Signal transduction, cell cycle regulatory, and anti-apoptotic pathways regulated by IL-3 in hematopoietic cells: possible sites for intervention with anti-neoplastic drugs.

Over the past decade, there has been an exponential increase in our knowledge of how cytokines regulate signal transduction, cell cycle progression, differentiation and apoptosis. Research has focused on different biochemical and genetic aspects of these processes. Initially, cytokines were identified by clonogenic assays and purified by biochemical techniques. This soon led to the molecular cloning of the genes encoding the cytokines and their cognate receptors. Determining the structure and regulation of these genes in normal and malignant hematopoietic cells has furthered our understanding of neoplastic transformation. Furthermore, this has allowed the design of modified cytokines which are able to stimulate multiple receptors and be more effective in stimulating the repopulation of hematopoietic cells after myelosuppressive chemotherapy. The mechanisms by which cytokines transduce their regulatory signals have been evaluated by identifying the involvement of specific protein kinase cascades and their downstream transcription factor targets. The effects of cytokines on cell cycle regulatory molecules, which either promote or arrest cell cycle progression, have been more recently examined. In addition, the mechanisms by which cytokines regulate apoptotic proteins, which mediate survival vs death, are being elucidated. Identification and characterization of these complex, interconnected pathways has expanded our knowledge of leukemogenesis substantially. This information has the potential to guide the development of therapeutic drugs designed to target key intermediates in these pathways and effectively treat patients with leukemias and lymphomas. This review focuses on the current understanding of how hematopoietic cytokines such as IL-3, as well as its cognate receptor, are expressed and the mechanisms by which they transmit their growth regulatory signals. The effects of aberrant regulation of these molecules on signal transduction, cell cycle regulatory and apoptotic pathways in transformed hematopoietic cells are discussed. Finally, anti-neoplastic drugs that target crucial constituents in these pathways are evaluated.

Phosphatidylinositol 3′-Kinase Activation Leads to Multidrug Resistance Protein-1 Expression and Subsequent Chemoresistance in Advanced Prostate Cancer Cells

The development of androgen-independent growth in advanced carcinoma of the prostate (CaP) is associated with poor prognosis and few therapeutic options. Chemotherapeutic drugs offer the afflicted patient palliative benefits, but these are short-lived because of the chemoresistant nature of hormone-refractory prostate cancer. Given the high percentage of CaP patients with mutations in the PTEN tumor suppressor gene, we sought to determine the involvement of the phosphatidylinositol 3′-kinase (PI3K) cascade in the development of CaP drug resistance. PTEN-negative PC3 cells were observed to have increased resistance to both doxorubicin and paclitaxel when compared with PTEN-positive DU145 cells. Furthermore, modulation of PI3K activity with the use of constitutively active and dominant-negative inhibitors was found to affect the ability to CaP cells to respond to chemotoxic treatments. Additionally, inhibition of PI3K with a small molecular weight inhibitor (LY294002) was able to potentiate the antineoplastic activity of both doxorubicin and paclitaxel in CaP cells. Interestingly, multidrug resistance protein-1 (MRP-1) expression, but not MDR-1 (p-glycoprotein), was observed to be induced as a consequence of PI3K activation in these cell types. Inhibition of MRP-1 expression via siRNA was observed to synergistically sensitize CaP cells to chemotoxic drugs while having no appreciable effect on cell growth in the absence of these compounds. Taken together, these data suggest that PI3K activation can lead to the development of chemoresistant cells in prostatic carcinomas through the up-regulation of MRP-1. Thus, inhibition of PI3K activity with concomitant administration of chemotoxic compounds may prove beneficial in preventing the development of drug resistance in patients with hormone-refractory prostate cancer.

Roles of the RassRafsMEKsERK pathway in leukemia therapy

The Ras/Raf/mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway is often implicated in sensitivity and resistance to leukemia therapy. Dysregulated signaling through the Ras/Raf/MEK/ERK pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Unrestricted leukemia proliferation and decreased sensitivity to apoptotic-inducing agents and chemoresistance are typically associated with activation of pro-survival pathways. Mutations in this pathway and upstream signaling molecules can alter sensitivity to small molecule inhibitors targeting components of this cascade as well as to inhibitors targeting other key pathways (for example, phosphatidylinositol 3 kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome 10 (PTEN)/Akt/mammalian target of rapamycin (mTOR)) activated in leukemia. Similarly, PI3K mutations can result in resistance to inhibitors targeting the Ras/Raf/MEK/ERK pathway, indicating important interaction points between the pathways (cross-talk). Furthermore, the Ras/Raf/MEK/ERK pathway can be activated by chemotherapeutic drugs commonly used in leukemia therapy. This review discusses the mechanisms by which abnormal expression of the Ras/Raf/MEK/ERK pathway can contribute to drug resistance as well as resistance to targeted leukemia therapy. Controlling the expression of this pathway could improve leukemia therapy and ameliorate human health.