The mitogen-activated protein kinase (MAPK) cascade controls phosphatase and tensin homolog (PTEN) expression through multiple mechanisms

: The mitogen-activated protein kinase (MAPK) and PI3K pathways are regulated by extensive crosstalk, occurring at different levels. In tumors, transactivation of the alternate pathway is a frequent "escape" mechanism, suggesting that combined inhibition of both pathways may achieve synergistic antitumor activity. Here we show that, in the M14 melanoma model, simultaneous inhibition of both MEK and mammalian target of rapamycin (mTOR) achieves synergistic effects at suboptimal concentrations, but becomes frankly antagonistic in the presence of relatively high concentrations of MEK inhibitors. This observation led to the identification of a novel crosstalk mechanism, by which either pharmacologic or genetic inhibition of constitutive MEK signaling restores phosphatase and tensin homolog (PTEN) expression, both in vitro and in vivo, and inhibits downstream signaling through AKT and mTOR, thus bypassing the need for double pathway blockade. This appears to be a general regulatory mechanism and is mediated by multiple mechanisms, such as MAPK-dependent c-Jun and miR-25 regulation. Finally, PTEN upregulation appears to be a major effector of MEK inhibitors' antitumor activity, as cancer cells in which PTEN is inactivated are consistently more resistant to the growth inhibitory and anti-angiogenic effects of MEK blockade

Chronic and acute alcohol exposure prevents monocyte-derived dendritic cells from differentiating and maturing

Increasing evidence suggests that alcohol abuse may be linked to adverse immunomodulatory effects on immune responses. Our study was undertaken to clarify the immunological consequences of chronic and acute alcohol exposure on differentiation and maturation of human dendritic cells (DCs). Using immunochemical and cytofluorimetric analysis we determined the phenotype and functions of monocyte-derived DCs from alcoholics and healthy subjects and analyzed their ability to respond to lipopolysaccharide (LPS) in the presence or absence of ethanol (EtOH) exposure. Our results showed that alcoholics' monocytes differentiated to immature DCs with altered phenotype and functions (alciDCs). Alc-iDCs showed fewer CD1a+ cells, weaker CD86 expression and higher HLA-DR expression associated with lower endocytosis and allostimulatory functions than iDCs from healthy subjects (control-iDCs). Despite these impairments, alc-iDCs produced TNF-α and IL-6 in large amounts. LPS stimulation failed to induce full phenotypical and functional alc-iDC maturation. In vitro acute EtOH exposure also prevented alc-iDCs and control-iDCs from maturing in response to LPS. T-cell priming experiments showed that EtOH treatment prevented LPS-stimulated control-iDCs from priming and polarizing naïve allogeneic T cells into Th1 cells, thus favouring a predominant Th2 environment. Collectively, our results provide evidence that chronic and acute alcohol exposure prevents DCs from differentiating and maturing in response to a microbial stimulus. Copyright © by BIOLIFE, s.a.s

A vertical combination strategy hitting multiple steps along the MAPK cascade: Molecular mechanisms of action and putative genetic determinants of synergism

Preclinical evidence shows that hitting a single point along the RAF/MEK/ERK cascade disrupts intra-pathway negative feedback loop, may cause paradoxical pathway activation and may lead to functional resistance. Thus, the \u201cvertical\u201d combination of agents simultaneously inhibiting RAF and MEK has been proposed as a strategy to synergistically inhibit tumor growth and delay resistance. Experimental procedures: Molecular and functional effects of single and combined MEK (using trametinib, T), BRAF (using dabrafenib, D), and RAF (using the pan-RAF inhibitor, RAF265, R) inhibition were dissected by WB analysis and conservative isobologram analysis to assess functional synergism, using a fixed dose-ratio experimental design. Summary data: We examined the in vitro molecular and functional consequences of D and T, alone or in combination, in a panel of different human BRAFV600E melanoma cell lines; both drugs inhibited cell growth and inactivated the MAPK pathway, but little or no synergistic growth inhibition was observed with their combination (CI: 0.7 - 1.3x106). Conversely, combined D+T suppressed malignant growth with highly synergistic effects in KRAS-mutant lung (2/5 cell lines tested) and pancreatic (4/6 cell lines tested) cancer models (CI: 0.1 - 0.7), in which selective BRAF inhibition induced hyperphosphorylation of MEK, ERK, and p90RSK (paradox effect). At concentrations inhibiting both BRAF and CRAF, R did not induce paradox MAPK activation and did not result in growth inhibitory synergism when combined with T. To define the role of RAS gene status in determining sensitivity/resistance to single and combined RAF and MEK blockade, we analyzed two isogenic tumor cell line models: H1299 expressing individual codon 12 KRAS mutants and isogenic HCT116 clones differing for the presence of a homo or heterozygous G13D KRAS mutation. Conversely, in lung cancer models driven by either EGFR mutations (HCC827, H1650) or HER-2 overexpression (Calu-3), selective BRAF inhibition also induced a paradox MAPK activation, which could be blocked using a reversible EGFR/HER-2 inhibitor (Lapatinib); in this context, combination (D+T) resulted in a non-synergic growth inhibitory effects. Conclusions: Overall, our data indicate that, in appropriate cellular contexts, vertical RAF/MEK inhibition-based combination strategies exert highly synergistic antitumor effects across different cancer models

THERAPEUTIC TARGETING OF RAF-INDUCED PARADOXICAL ERK ACTIVATION WITH A VERTICAL COMBINATION HITTING MULTIPLE STEPS ALONG THE MAPK CASCADE

Aim: Mounting evidence suggests that RAF-mediated MEK activation plays a crucial role in paradox MAPK (re)activation, leading to resistance and therapeutic failure with agents hitting a single step along the MAPK cascade. Methods: We examined the molecular and functional effects of single and combined MEK (trametinib, T), BRAF(dabrafenib, D), and pan-RAF (RAF265, R) inhibition, using WB and conservative isobologram analysis to assess functional synergism and explored genetic determinants of synergistic interactions. Results: In BRAF-mut models, both D and T effectively inactivated MAPK and inhibited cell growth, with no synergistic growth inhibition with their combination (CI: 0.7-1.3x106). Conversely, in KRAS-mut lung (2/5 cell lines) and pancreatic (4/6 cell lines) cancer models, in which selective BRAF inhibition induced paradox hyperphosphorylation of MEK, ERK, and p90RSK, combined D+T synergistically suppressed malignant growth (CI: 0.1-0.7). At concentrations inhibiting all RAF isoforms, R did not induce paradox MAPK activation and did not result in growth inhibitory synergism when combined withT. Highly synergistic in vitro growth inhibition was also observed with D+T in 2/5 patient-derived lung cancer stem cells. KRAS mutations appeared to be necessary, but not sufficient, to determine paradox MAPK activation and functional synergism with D+T, as assessed in two KRAS isogenic cell lines (H1299 expressing individual codon 12 mutants and HCT116 clones differing for homo or heterozygous G13D). Conversely, in lung cancer models driven by EGFR family activation (EGFR-mut HCC827 and H1650 cells and HER-2 overexpressing Calu-3 cells), paradox MAPK activation upon selective BRAF inhibition was dependent on upstream receptor activity and could be efficiently blocked by the EGFR/HER-2 inhibitor Lapatinib. Conclusions: Overall, our data indicate that RAF-mediated, paradox MAPK activation may be sustained by both upstream (RTK activation) and downstream (KRAS mutations) mechanisms and that in appropriate cellular contexts vertical RAF/MEK inhibition-based combination strategies may exert highly synergistic antitumor effects