Cell cycle progression is associated with distinct patterns of phosphorylation of Op18

Abstract

Op18 is a highly conserved major cytosolic phosphoprotein which has been implicated in signal transduction in a wide variety of cell types. Freshly isolated peripheral blood lymphocytes (PBL) constitutively express low levels of mostly unphosphorylated Op18. Following mitogenic stimulation of PBL, Op18 synthesis is induced at a time when cells are entering S-phase. In this study we have characterized Op18 phosphorylation during progression of freshly isolated PBL through the cell cycle. Transition from G0 to G1 following activation with OKT3 was associated with an increase in a phosphorylated form designated Op18c. Progression of cells through G1 into S resulted in an increase in phosphorylated Op18 forms, designated Op18a and Op18b, which paralleled new Op18 synthesis. Transition of cells into G2 + M resulted in the appearance of the more acidic phosphorylated forms Op18d and Op18e. Calphostin C, a specific inhibitor of protein kinase C, dramatically decreased all forms of phosphorylated Op18 in OKT3 treated Jurkat cells. Our results suggest that Op18 phosphorylation is mediated in part by PKC activation as well as by other kinases yielding different phosphorylated forms at specific stages of the cell cycle.

Overexpression of the stathmin gene in a subset of human breast cancer

“…Importantly, phosphorylation of stathmin altered the affinity of stathmin for tubulin (Marklund et al, 1996;Curmi et al, 1997;Di Paolo et al, 1997;Horwitz et al, 1997;Larsson et al, 1997). Together, these results give insight into the observed physiological variations of stathmin phosphorylation during the mitotic cycle (Strahler et al, 1992;Brattsand et al, 1994) and argue for the search for stathmin dysregulations in tumours as well as an understanding of its mechanisms. In the present study, using genomic DNA, mRNA and protein analysis, we assessed, for the first time, stathmin expression in human breast cancer.…”

“…Interest in the study of stathmin in tumours grew with the observation that stathmin, a 19-kDa cytosolic phosphoprotein, is subject to marked variations in its expression and phosphorylation pattern during cell regulation, triggered by signals as diverse as hormones, growth factors and differentiation factors (Strahler et al, 1992;Larsson et al, 1995). Recently, attention was raised when it was discovered that stathmin is a major microtubule-destabilizing factor in vitro (Belmont and Mitchison, 1996) and in vivo.…”

“…Stathmin expression and phosphorylation are probably linked to the control of cell differentiation (Doye et al, 1992;Di Paolo et al, 1996) and proliferation (Braverman et al, 1986;Cooper et al, 1990;Koppel et al, 1993;Balogh et al, 1996) (for a review see Sobel, 1991). The state of stathmin phosphorylation changes markedly during the cell cycle (Strahler et al, 1992;Brattsand et al, 1994), and cell division also appears to require multisite phosphorylation of this protein (Larsson et al, 1995;Lawler et al, 1997). More obviously, it has recently been shown that stathmin interferes with the dynamic instability of microtubules by destabilizing them in vitro (Belmont and Mitchison, 1996) and in vivo (Marklund et al, 1996).…”

Identification of novel targets for cancer therapy using expression proteomics

“…[43][44][45][46] Op18 is expressed in proliferating cells of most cell lineages 47 and its level increases significantly during the S phase of the cell cycle. 48,49 The functionality of Op18 is also tightly controlled by phosphorylation during the cell cycle, 50 with its activity being down-regulated by increased phosphorylation on four serine (S16, S25, S38 and S63) residues. 46,51 Higher order phosphorylation is a prerequisite for progression through mitosis.…”

Detection of polypeptides associated with the histopathological differentiation of primary lung carcinoma

“…Other investigators suggest that new potential markers may occur in the molecular weight region 10-20 kDa (e.g. Op18/stathmin and nm23) (Strahler et al, 1992;Rosengard et al, 1989). Therefore, we extended the molecular weight range using a 10-13%T linear gradient.…”