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2014 ; 12
(9
): 1205-15
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Targeting mTORC1-mediated metabolic addiction overcomes fludarabine resistance in
malignant B cells
#MMPMID25061101
Sharma A
; Janocha AJ
; Hill BT
; Smith MR
; Erzurum SC
; Almasan A
Mol Cancer Res
2014[Sep]; 12
(9
): 1205-15
PMID25061101
show ga
MTOR complex-1(mTORC1) activation occurs frequently in cancers, yet clinical
efficacy of rapalogs is limited because of the associated activation of upstream
survival pathways. An alternative approach is to inhibit downstream of mTORC1;
therefore, acquired resistance to fludarabine (Flu), a purine analogue and
antimetabolite chemotherapy, active agent for chronic lymphocytic leukemia (CLL)
was investigated. Elevated phospho-p70S6K, also known as RPS6KB1 (ribosomal
protein S6 kinase, 70kDa, polypeptide 1) (T389), an mTORC1 activation marker,
predicted Flu resistance in a panel of B-cell lines, isogenic Flu-resistant
(FluR) derivatives, and primary human CLL cells. Consistent with the anabolic
role of mTORC1, FluR cells had higher rates of glycolysis and oxidative
phosphorylation than Flu-sensitive (FluS) cells. Rapalogs (everolimus and
rapamycin) induced moderate cell death in FluR and primary CLL cells, and
everolimus significantly inhibited glycolysis and oxidative phosphorylation in
FluR cells. Strikingly, the higher oxidative phosphorylation in FluR cells was
not coupled to higher ATP synthesis. Instead, it contributed primarily to an
essential, dihydroorotate dehydrogenase catalyzed, step in de novo pyrimidine
biosynthesis. mTORC1 promotes pyrimidine biosynthesis by p70S6 kinase-mediated
phosphorylation of CAD (carbamoyl-phosphate synthetase 2, aspartate
transcarbamylase, and dihydroorotase; Ser1859) and favors S-phase cell-cycle
progression. We found increased phospho-CAD (S1859) and higher S-phase population
in FluR cells. Pharmacological inhibition of de novo pyrimidine biosynthesis
using N-phosphonacetyl-l-aspartate and leflunomide, RNAi-mediated knockdown of
p70S6K, and inhibition of mitochondrial respiration were selectively cytotoxic to
FluR, but not FluS, cells. These results reveal a novel link between
mTORC1-mediated metabolic reprogramming and Flu resistance identifying
mitochondrial respiration and de novo pyrimidine biosynthesis as potential
therapeutic targets. IMPLICATIONS: This study provides the first evidence for
mTORC1/p70S6K-dependent regulation of pyrimidine biosynthesis in a relevant
disease setting.