Phosphorylation of three regulatory serines of Tob by Erk1 and Erk2 is required for Ras-mediated cell proliferation and transformation

Abstract

tob is a member of an emerging family of genes with antiproliferative function. Tob is rapidly phosphorylated at Ser 152, Ser 154, and Ser 164 by Erk1 and Erk2 upon growth-factor stimulation. Oncogenic Ras-induced transformation and growth-factor-induced cell proliferation are efficiently suppressed by mutant Tob that carries alanines but not glutamates, mimicking phosphoserines, at these sites. Wild-type Tob inhibits cell growth when the three serine residues are not phosphorylated but is less inhibitory when the serines are phosphorylated. Because growth of Rb-deficient cells was not affected by Tob, Tob appears to function upstream of Rb. Intriguingly, cyclin D1 expression is elevated in serum-starved tob −/− cells. Reintroduction of wild-type Tob and mutant Tob with serine-to-alanine but not to glutamate mutations on the Erk phosphorylation sites in these cells restores the suppression of cyclin D1 expression. Finally, the S-phase population was significantly increased in serum-starved tob −/− cells as compared with that in wild-type cells. Thus, Tob inhibits cell growth by suppressing cyclin D1 expression, which is canceled by Erk1-and Erk2-mediated Tob phosphorylation. We propose that Tob is critically involved in the control of early G 1 progression.

IL‐4‐induced gene 1 maintains high Tob1 expression that contributes to TCR unresponsiveness in human T helper 17 cells

“…Finally, it has already been shown that Tob1 can inhibit the classical pathway of CD28 costimulatory signaling [19]. Of note, the results of this study also demonstrate that human Th17 cells exhibit reduced levels of Skp2, which interacts with Tob1 and promotes its ubiquitin-dependent degradation [18]. Although the mechanisms responsible for the reduced Skp2 expression in Th17 cells remain presently unclear, this reduced expression may also contribute to the maintenance of high Tob1 levels in human Th17 cells.…”

“…In contrast, elimination of steady-state expression of Tob1 reduces the threshold of CD4 + T-cell activation required for optimal cellular proliferation and clonal expansion [19]. It seems that downregulation of Tob1 expression during cell-cycle entry is mediated at both mRNA and protein levels [18][19][20].…”

“…In contrast, elimination of steady-state expression of Tob1 reduces the threshold of CD4 + T-cell activation required for optimal cellular proliferation and clonal expansion [19]. It seems that downregulation of Tob1 expression during cell-cycle entry is mediated at both mRNA and protein levels [18][19][20].In this study, we demonstrate that IL4I1 maintains high Tob1 expression in human Th17 cells when compared with that in classic Th1 cells, suggesting an additional mechanism responsible for an impaired Th17-cell proliferation response. …”

“…Consistent with such a function, the levels of these proteins change dynamically during the cell cycle. In fibroblasts, Tob1 protein is detectable during the G0 phase, declines dramatically during the G1 and S phase and returns to base line during the G2 phase [18]. In CD4 + T lymphocytes, Tob1 protein must be degraded rapidly during early G1 phase to ensure progression through late G1 phase.…”

“…These data clearly demonstrate that the high Tob1 mRNA levels found in Th17 cells are associated with their impairment in cell-cycle entry and progression. Recently, Tob1 protein has been reported to have a quick turnover through the ubiquitin-proteasome pathway and it has been shown that Skp2, an F-box protein that promotes degradation of the cyclin-dependent kinase inhibitor p27, interacts with Tob1 and promotes its ubiquitin-dependent degradation [18]. In order to establish whether the high Tob1 mRNA levels found in human Th17 cells are associated with a reduced expression of Skp2 in the same cells, Skp2 mRNA levels were measured by quantitative RT-PCR in Th17-and classic Th1-cell clones.…”