Staurosporine inhibits phosphorylation of translational regulators linked to mTOR

Abstract

Treatment of Swiss 3T3 cells with staurosporine resulted in dephosphorylation of two proteins which play key roles in regulating mRNA translation. This occurred before the execution of apoptosis, assessed by caspase-3 activity. These translation regulators are p70 S6 kinase, which phosphorylates ribosomal protein S6, and eukaryotic initiation factor (eIF) 4E binding protein 1 (4E-BP1), which both lie downstream of the mammalian target of rapamycin (mTOR). This resulted in decreased p70 S6 kinase activity, dephosphorylation of ribosomal protein S6, increased binding of 4E-BP1 to eIF4E and a concomitant decrease in eIF4F complexes. Our data show that staurosporine impairs mTOR signalling in vivo but that this not due to direct inhibition of mTOR or to inhibition of protein kinase C. It is becoming clear that agents which cause apoptosis inactivate mTOR signalling as a common early response prior to the execution of apoptosis, i.e., before caspase activation. Cell Death and Differentiation (2001) 8, 841 ± 849.

UV irradiation inhibits ABC transporters via generation of ADP-ribose by concerted action of poly(ADP-ribose) polymerase-1 and glycohydrolase

“…However, it did not restore the fluorochrome export in UVB-irradiated cells (Figure 2b). Conversely, staurosporine, [19][20][21] that induced efficient and early lymphocyte death via apoptosis (Figure 2a) did even increase the fluorochrome export (Figure 2c), indicating that apoptosis per se does not downregulate the activity of ABC transporters.…”

Initiation factor modifications in the preapoptotic phase

“…Several previous studies have shown that during the early phase of the cellular response to inducers of apoptosis, the extent of phosphorylation of 4E-BP1 decreases, 83,101,124,125 and that the hypophosphorylated 4E-BP1 thus generated binds to eIF4E in competition with eIF4GI or II. This decreases the availability of the eIF4F complex for initiation of translation.…”

“…Both dephosphorylation and caspase-mediated cleavage of 4E-BP1 have been shown to occur in cells exposed to staurosporine and the DNA-damaging agent etoposide. [124][125][126] Cleavage occurs at Asp-24, close to the N-terminus of the protein (Figure 3). The activation of p53 also results in the cleavage of 4E-BP1, but in this case, the cleavage is a z-VAD.FMK-insensitive event, suggesting that it is mediated by another type of protease (C Constantinou and MJ Clemens, unpublished data).…”

The eukaryotic initiation factor 4E-binding proteins and apoptosis

“…The resulting major fragment of 4E-BP1 is indicated as 'D4E-BP1' apoptosis, also led to inhibition of mTOR signaling. 60 Since mTOR signaling positively regulates cell growth and proliferation, and ribosome biogenesis, this shut-down of mTOR signaling may represent a response of cells to adverse conditions that may ultimately compromise cell survival. By affecting both cap-dependent and -independent translation, it may also alter the balance of protein expression and thereby modulate the events that trigger commitment to apoptosis, an issue that is dealt with in more detail below.…”