The BCR/ABL oncogenic tyrosine kinase activates phosphatidylinositol 3-kinase (PI-3k) by a mechanism that requires binding of BCR/ABL to p85, the regulatory subunit of PI-3k, and an intact BCR/ABL SH2 domain. SH2 domain BCR/ABL mutants deficient in PI-3k activation failed to stimulate Akt kinase, a recently identified PI-3k downstream effector with oncogenic potential, but did activate p21 RAS and p70 S6 kinase. The PI-3k/Akt pathway is essential for BCR/ABL leukemogenesis as indicated by experiments demonstrating that wortmannin, a PI-3k specific inhibitor at low concentrations, suppressed BCR/ABL-dependent colony formation of murine marrow cells, and that a kinase-deficient Akt mutant with dominant-negative activity inhibited BCR/ABL-dependent transformation of murine bone marrow cells in vitro and suppressed leukemia development in SCID mice. In complementation assays using mouse marrow progenitor cells, the ability of transformation-defective SH2 domain BCR/ABL mutants to induce growth factor-independent colony formation and leukemia in SCID mice was markedly enhanced by expression of constitutively active Akt. In retrovirally infected mouse marrow cells, the BCR/ABL mutant lacking the SH2 domain was unable to upregulate the expression of c-Myc and Bcl-2; in contrast, expression of a constitutively active Akt mutant induced Bcl-2 and c-Myc expression, and stimulated the transcription activation function of c-Myc. Together, these data demonstrate the requirement for the BCR/ABL SH2 domain in PI-3k activation and document the essential role of the PI-3k/Akt pathway in BCR/ABL leukemogenesis.

Melanoma is a cancer that exhibits one of the most aggressive and heterogeneous features. The incidence rate escalates. A high number of clones harboring various mutations contribute to an exceptional level of intratumor heterogeneity of melanoma. It also refers to metastases which may originate from different subclones of primary lesion. Such component of the neoplasm biology is termed intertumor and intratumor heterogeneity. These levels of tumor heterogeneity hinder accurate diagnosis and effective treatment. The increasing number of research on the topic reflects the need for understanding limitation or failure of contemporary therapies. Majority of analyses concentrate on mutations in cancer-related genes. Novel high-throughput techniques reveal even higher degree of variations within a lesion. Consolidation of theories and researches indicates new routes for treatment options such as targets for immunotherapy. The demand for personalized approach in melanoma treatment requires extensive knowledge on intratumor and intertumor heterogeneity on the level of genome, transcriptome/proteome, and epigenome. Thus, achievements in exploration of melanoma variety are described in details. Particularly, the issue of tumor heterogeneity or homogeneity given BRAF mutations is discussed.

Phosphoinositides have a pivotal role as precursors to important second messengers and as bona fide signaling and scaffold targeting molecules. Phosphatidylinositol 4-kinases (PtdIns 4-kinases or PI4Ks) are at the apex of the phosphoinsitide cascade. Sequence analysis revealed that mammalian cells contain two type II PtdIns 4-kinase isoforms, now termed PI4KII␣ and PI4KII␤. PI4KII␣ was cloned first. It is tightly membrane-associated and behaves as an integral membrane protein. In this study, we cloned PI4KII␤ and compared the two isoforms by monitoring the distribution of endogenous and overexpressed proteins, their modes of association with membranes, their response to growth factor stimulation or Rac-GTP activation, and their kinetic properties. We find that the two kinases have different properties. PI4KII␤ is primarily cytosolic, and it associates peripherally with plasma membranes, endoplasmic reticulum, and the Golgi. In contrast, PI4KII␣ is primarily Golgi-associated. Platelet-derived growth factor promotes PI4KII␤ recruitment to membrane ruffles. This effect is potentially mediated through Rac; overexpression of the constitutively active RacV12 induces membrane ruffling, increases PI4KII␤ translocation to the plasma membrane, and stimulates its activity. The dominant-negative RacN17 blocks plasma membrane association and inhibits activity. RacV12 does not boost the catalytic activity of PI4KII␣ further, probably because it is constitutively membrane-bound and already activated. Membrane recruitment is an important mechanism for PI4KII␤ activation, because microsome-bound PI4KII␤ is 16 times more active than cytosolic PI4KII␤. Membrane-associated PI4KII␤ is as active as membraneassociated PI4KII␣ and has essentially identical kinetic properties. We conclude that PI4KII␣ and PI4KII␤ may have partially overlapping, but not identical, functions. PI4KII␤ is activated strongly by membrane association to stimulate phosphatidylinositol 4,5-bisphosphate synthesis at the plasma membrane. These findings provide new insight into how phosphoinositide cascades are propagated in cells.

No abstract

No abstract

No abstract

Here we identify the first two cysteines in the CCPCC motif as the principal sites of palmitoylation under basal conditions, and we demonstrate the importance of the central proline for enzymatic activity, although not for membrane binding. We further show that palmitoylation is critical for targeting PI4KII␣ to the trans-Golgi network and for enhancement of its association with low buoyant density membrane fractions, commonly termed lipid rafts. Replacement of the four cysteines in CCPCC with a hydrophobic residue, phenylalanine, substantially restores catalytic activity of PI4KII␣ in vitro and in cells without restoring integral membrane binding. Although this FFPFF mutant displays a perinuclear distribution, it does not strongly co-localize with wild-type PI4KII␣ and associates more weakly with lipid rafts.

The phenotype of hematopoietic cells transformed by the BCR͞ABL oncoprotein of the Philadelphia chromosome is characterized by growth factor-independent proliferation, reduced susceptibility to apoptosis, and altered adhesion and motility. The mechanisms underlying this phenotype are not fully understood, but there is evidence that some of the properties of BCR͞ABL-expressing cells are dependent on the activation of downstream effector molecules such as RAS, PI-3k, and bcl-2. We show here that the small GTP-binding protein Rac is activated by BCR͞ABL in a tyrosine kinase-dependent manner. Upon transfection with a vector carrying the dominant-negative N17Rac, BCR͞ABL-expressing myeloid precursor 32Dcl3 cells retained the resistance to growth factor deprivation-induced apoptosis but showed a decrease in proliferative potential in the absence of interleukin-3 (IL-3) and markedly reduced invasive properties. Moreover, compared with BCR͞ABL-expressing cells, fewer BCR͞ABL plus N17Rac double transfectants were capable of homing to bone marrow and spleen. Consistent with these findings, survival of SCID mice injected with the BCR͞ABL plus N17Rac double transfectants was markedly prolonged as compared with that of mice injected with BCR͞ ABL-expressing cells. Together, these data support the important role of a Rac-dependent pathway(s) controlling motility in BCR͞ABL-mediated leukemogenesis.