The Ras/Raf/MEK/ERK signaling cascade that integrates an extreme variety of extracellular stimuli into key biological responses controlling cell proliferation, differentiation or death is one of the most studied intracellular pathways. Here we present some evidences that have been accumulated over the last 15 years proving the requirement of ERK in the control of cell proliferation. In this review we focus (i) on the spatio-temporal control of ERK signaling, (ii) on the key cellular components linking extracellular signals to the induction and activation of cell cycle events controlling G1 to S-phase transition and (iii) on the role of ERK in the growth factor-independent G2/M phase of the cell cycle. As ERK pathway is often co-activated with the PI3 kinase signaling, we highlight some of the key points of convergence leading to a full activation of mTOR via ERK and AKT synergies. Finally, ERK and AKT targets being constitutively activated in so many human cancers, we briefly touched the cure issue of using more specific drugs in rationally selected cancer patients.

The CD98/LAT1 complex is overexpressed in aggressive human cancers and is thereby described as a potential therapeutic target. This complex promotes tumorigenesis with CD98 (4F2hc) engaging b-integrin signaling while LAT1 (SLC7A5) imports essential amino acids (EAA) and promotes mTORC1 activity. However, it is unclear as to which member of the heterodimer carries the most prevalent protumoral action. To answer this question, we explored the tumoral potential of each member by gene disruption of CD98, LAT1, or both and by inhibition of LAT1 with the selective inhibitor (JPH203) in six human cancer cell lines from colon, lung, and kidney. Each knockout respectively ablated 90% (CD98 KO ) and 100% (LAT1 KO ) of Na þ -independent leucine transport activity. LAT1 KO or JPH203-treated cells presented an amino acid stress response with ATF4, GCN2 activation, mTORC1 inhibition, and severe in vitro and in vivo tumor growth arrest. We show that this severe growth phenotype is independent of the level of expression of CD98 in the six tumor cell lines. Surprisingly, CD98KO cells with only 10% EAA transport activity displayed a normal growth phenotype, with mTORC1 activity and tumor growth rate undistinguishable from wild-type cells. However, CD98KO cells became extremely sensitive to inhibition or genetic disruption of LAT1 (CD98 KO /LAT1 KO ). This finding demonstrates that the tumoral potential of CD98 KO cells is due to residual LAT1 transport activity. Therefore, these findings clearly establish that LAT1 transport activity is the key growthlimiting step of the heterodimer and advocate the pharmacology development of LAT1 transporter inhibitors as a very promising anticancer target. Cancer Res; 76(15);

The proteins ERK1 and ERK2 are highly similar, are ubiquitously expressed, and share activators and substrates; however, erk2 gene invalidation is lethal in mice, while erk1 inactivation is not. We ablated ERK1 and/or ERK2 by RNA interference and explored their relative roles in cell proliferation and immediate-early gene (IEG) expression. Reducing expression of either ERK1 or ERK2 lowered IEG induction by serum; however, silencing of only ERK2 slowed down cell proliferation. When both isoforms were silenced simultaneously, compensating activation of the residual pool of ERK1/2 masked a more deleterious effect on cell proliferation. It was only when ERK2 activation was clamped at a limiting level that we demonstrated the positive contribution of ERK1 to cell proliferation. We then established that ERK isoforms are activated indiscriminately and that their expression ratio correlated exactly with their activation ratio. Furthermore, we determined for the first time that ERK1 and ERK2 kinase activities are indistinguishable in vitro and that erk gene dosage is essential for survival of mice. We propose that the expression levels of ERK1 and ERK2 drive their apparent biological differences. Indeed, ERK1 is dispensable in some vertebrates, since it is absent from chicken and frog genomes despite being present in all mammals and fishes sequenced so far.Numerous cell surface agonists activate the signaling cascade Ras/Raf/MEK/ERK. In contrast to the upstream activation steps of the cascade, which have few known substrates, ERK phosphorylates hundreds of substrates on serine and threonine residues (52). ERK substrates are localized in all cell compartments: at the membrane, (e.g., epidermal growth factor receptor), in the cytosol (e.g., DUSP6-MKP-3), on the cytoskeleton (e.g., cortactin), and in the nucleus (e.g., Elk-1). The serine/threonine protein kinase ERKs play key roles in cell proliferation, cell differentiation, and cell death. The spatio-temporal regulation of ERK activation dictates the biological outcome (34). Therefore, for all these reasons, ERK activation is a key signaling step for the regulation of many biological responses.Two isoforms convey ERK activity in many vertebrates; these are ERK1 and ERK2, which are 84% identical at the amino acid level (8). Among the mitogen-activated protein kinases, ERK5 (55) is the closest kinase to ERK1/2, with all three kinases being activated on the threonine and tyrosine residues of the TEY sequence. However, ERK5 cannot be considered an ERK1/2 isoform, since ERK5 and ERK1/2 are activated by different kinase modules. ERK5 is activated by the kinase MEK5 (55), unlike ERK1 and -2, which are activated by MEK1/2 (23).Many observations indicate that ERK1 and ERK2 are very similar. ERK1 and ERK2 are ubiquitously expressed; however, their relative distributions across tissues differ (8). ERK1 and -2 display the same subcellular localization; both isoforms translocate from the cytosol to the nucleus upon stimulation of resting cells (25). ERK1 and -2 are serine/threonine pr...