1996; Shieh et al. phosphorylation of Ser 62, is usually associated with degradation of Myc. Further analysis demonstrates that this Ras-dependent PI-3K pathway is also critical for controlling Myc protein accumulation, likely through the control of GSK-3 activity. These observations thus define a synergistic role for multiple Ras-mediated phosphorylation pathways in the control of Myc protein accumulation during the initial stage of cell proliferation. proto-oncogene encodes a RU-302 small GTP-binding protein that plays a critical role in cell growth control as a central component of mitogenic signaling events (White et al. 1995). Ras activation initiates a complex array of signal transduction pathways including the Raf/MAPK (ERK) pathway, primarily involved in plasma membrane-to-nucleus signaling crucial for mitogen-induced cell proliferation (Seger and Krebs 1995; Lavoie et al. 1996), the PI3 kinase/AKT pathway, which is usually involved in cell survival signaling (Kauffmann-Zeh et al. 1997), the Rac/Rho pathway, involved in cytoskeletal remodeling (Lamarche et al. 1996), and the Rac/JNK and Rac/p38 pathways, both of which appear to be involved in cell stress responses, growth inhibition, and apoptotic signals (Coso et al. 1995; Minden et al. 1995; Xia et al. 1995). Activation of Ras-signaling pathways has been shown to be essential for Keratin 10 antibody cells both to leave a quiescent state and to pass through G1 phase of the cell cycle (Peeper et al. 1997). c-Myc is the most ubiquitous and best studied member of a family of proteins that includes N-Myc, L-Myc, S-Myc, and B-Myc. The N terminus of Myc proteins contains the transcriptional activation domain name, within which are two 20Camino acid segments termed Myc boxes 1 and 2 that are conserved in most Myc family proteins and appear in most RU-302 cases to be crucial for all biological activities (Sakamuro and Prendergast 1999). The C terminus of Myc includes the basic/helixCloopChelix/leucine zipper (b/HLH/Z) motif that mediates oligomerization with the small b/HLH/Z partner protein Max and sequence-specific DNA recognition of E-box motifs (Luscher and Larsson 1999). The Max protein also acts as a heterodimeric partner for the Mad family of b/HLH/Z proteins that form transcriptional repressors on the same E-box sequence elements and that can antagonize Myc function (Foley and Eisenman 1999). While Max is usually ubiquitously and constitutively expressed, both Myc and Mad expression is usually tightly regulated in relation to cell growth; Myc levels are high in cycling cells but decrease as cells cease to proliferate and differentiate, and Mad expression follows the opposite pattern. Thus, precise regulation of the levels of Myc and Mad expression is critical to determine the formation of either Myc/Max or Mad/Max heterodimers and consequently cell growth or inhibition, respectively. A variety of studies demonstrate that tight regulation of Myc protein levels is essential for normal cell function. Whereas homozygous deletion of genes results in embryonic lethality (Charron et al. 1992; Davis et al. 1993), constitutive overexpression of Myc proteins in cultured cells as well as in transgenic animals blocks differentiation, induces neoplastic transformation, and can initiate apoptosis (Coppola and Cole 1986; Evan et al. 1992). Moreover, a wide variety of naturally occurring tumors exhibit both chromosomal translocations and amplification of the c-locus that result in constitutive overexpression of Myc proteins (Cole 1986; Spencer and Groudine 1991). Perhaps some of the best evidence demonstrating the important RU-302 role that fluctuations in Myc protein levels play in Myc function comes from studies in mice carrying inducible transgenes. It was observed that enforced expression of c-Myc in either skin or hematopoietic lineages in transgenic mice leads to neoplastic premalignant and malignant phenotypes, respectively, but when Myc expression is usually turned off in these systems, spontaneous regression of the neoplastic and malignant changes occurs (Felsher and Bishop 1999; Pelengaris et al. 1999). Numerous studies have documented the growth-regulated accumulation of RNA (Kelly et al. 1983; Luscher and Eisenman 1990) resulting from increases in gene transcription and an increase in RNA stability (Jones and Cole 1987; Luscher and Eisenman 1990). However, posttranslational control of Myc protein levels has also recently been shown to contribute to the regulated accumulation of Myc activity. Myc protein exhibits an extremely short half-life, 30 min in growing cells (Hann and Eisenman 1984; Ramsay et al. 1986), because of its regulated destruction via the ubiquitin/26S proteasome pathway (Ciechanover et al. 1991; Flinn et al. 1998; Gross-Mesilaty et al. 1998; Salghetti et al. 1999). Moreover, Myc turnover by the ubiquitin/proteasome system appears to be regulated. It was shown that Myc half-life is usually decreased during erythroleukemia.