Phenotypically Dormant and Immature Leukaemia Cells Display Increased Ribosomal Protein S6 Phosphorylation.

Monica Pallis,Nigel Russell,Tamsin Harvey,Ken Mills

doi:10.1371/journal.pone.0151480

Abstract

Mechanistic/mammalian target of rapamycin (mTOR) activity drives a number of key metabolic processes including growth and protein synthesis. Inhibition of the mTOR pathway promotes cellular dormancy. Since cells from patients with acute myeloid leukaemia (AML) can be phenotypically dormant (quiescent), we examined biomarkers of their mTOR pathway activity concurrently with Ki-67 and CD71 (indicators of cycling cells) by quantitative flow cytometry. Using antibodies to phosphorylated epitopes of mTOR (S2448) and its downstream targets ribosomal protein S6 (rpS6, S235/236) and 4E-BP1 (T36/45), we documented that these phosphorylations were negligible in lymphocytes, but evident in dormant as well as proliferating subsets of both mobilised normal stem cell harvest CD34+ cells and AML blasts. Although mTOR phosphorylation in AML blasts was lower than that of the normal CD34+ cells, p-4E-BP1 was 2.6-fold higher and p-rpS6 was 22-fold higher. Moreover, in contrast to 4E-BP1, rpS6 phosphorylation was higher in dormant than proliferating AML blasts, and was also higher in the immature CD34+CD38- blast subset. Data from the Cancer Genome Atlas show that rpS6 expression is associated with that of respiratory chain enzymes in AML. We conclude that phenotypic quiescence markers do not necessarily predict metabolic dormancy and that elevated rpS6 ser235/236 phosphorylation is characteristic of AML.

Highlights

Tumour cell growth is driven by active biosynthetic and glycolytic pathways [1] fuelling interest in finding anti-cancer uses for drugs which interfere with these processes [2,3,4,5]
We measured ribosomal protein S6 phosphorylated at serine 235/236 [22, 30], a downstream target of mTORC1
ERK activation can stimulate mammalian target of rapamycin (mTOR) activity through TSC2 [32], ribosomal protein S6 (rpS6) can be phosphorylated at S235/236 in an mTORC1 independent/ERK dependent manner by p90S6 kinase [33]

Summary

Introduction

Tumour cell growth is driven by active biosynthetic and glycolytic pathways [1] fuelling interest in finding anti-cancer uses for drugs which interfere with these processes [2,3,4,5]. Constitutive activation of mTOR is commonly found in tumour cells, but in quiescent normal cells mTOR activity and biosynthetic pathways are suppressed [1, 5]. This may happen in an energy-rich and nutrient-replete environment, such as in the case of circulating lymphocytes [9, 10], or may be a homeostatic response to nutrient or energy depletion in which AMPK is activated.

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Results

Conclusion