Inhibitors of the PI3K/mTOR pathway prevent STAT5 phosphorylation inJAK2V617Fmutated cells through PP2A/CIP2A axis

Abstract

Inhibition of the constitutively activated JAK/STAT pathway in JAK2V617F mutated cells by the JAK1/JAK2 inhibitor ruxolitinib resulted in clinical benefits in patients with myeloproliferative neoplasms. However, evidence of disease-modifying effects remains scanty; furthermore, some patients do not respond adequately to ruxolitinib, or have transient responses, thus novel treatment strategies are needed. Here we demonstrate that ruxolitinib causes incomplete inhibition of STAT5 in JAK2V617F mutated cells due to persistence of phosphorylated serine residues of STAT5b, that conversely are targeted by PI3K and mTORC1 inhibitors. We found that PI3K/mTOR-dependent phosphorylation of STAT5b serine residues involves Protein Phosphatase 2A and its repressor CIP2A. The levels of CIP2A were found increased in cells harboring the JAK2V617F mutation, and we provide evidence of a correlation between clinical responses and the extent of CIP2A downregulation in myelofibrosis patients receiving the mTOR inhibitor RAD001 in a phase II clinical trial. To achieve maximal inhibition of STAT5 phosphorylation, we combined ruxolitinib with BKM120, a PI3K inhibitor, and RAD001, an mTOR inhibitor, obtaining improved efficacy in JAK2V617F mutated cell lines, primary patients’ cells, and JAK2V617F knock-in mice. These findings contribute to understanding the effectiveness of PI3K/mTOR inhibitors in MPN and argue for the rationale to develop combination clinical trials.

Targeting Janus Kinases and Signal Transducer and Activator of Transcription 3 to Treat Inflammation, Fibrosis, and Cancer: Rationale, Progress, and Caution

“…The BRD4/BET/NF-kB signaling pathway also has been suggested to contribute to MPN, as the addition of BET inhibitors to Jakinibs has been shown to markedly reduce serum levels of inflammatory cytokines, accompanied by reduction in disease burden, and reversal of bone marrow fibrosis in preclinical models (Kleppe et al, 2018). In addition, simultaneous inhibition of STAT5 phosphorylation on tyrosine and serine residues had synergistic effects against JAK2V617F mutant-positive cell lines, suggesting that approaches that target both post-translational modifications of STAT5 may be of benefit to patients with PMF (Bartalucci et al, 2017).…”

The pan-PIM inhibitor INCB053914 displays potent synergy in combination with ruxolitinib in models of MPN

“…Targeting mTOR has also demonstrated positive anti-MPN effects as a single agent and in combination with JAK2 inhibition. 45,[65][66][67][68][69] The potency of the combination of INCB053914 and ruxolitinib was also manifested in in vivo MPN models driven by aberrant JAK2 signaling. The combination of INCB053914 and ruxolitinib antagonized the growth of SET2 cell xenograft tumors at doses of each drug that had little to no effect as monotherapies ( Figure 6A).…”

Effect of CIP2A and its mechanism of action in the malignant biological behavior of colorectal cancer

“…The mechanisms of CIP2A activation and overexpression in cancer cells have been investigated by several studies, in which some pathways were found to be associated with development of cancer, such as PI3K/mTOR and MAPK/ERK signaling pathway [27,35]. In the current study, we profiled the phosphokinase signaling pathways associated with CIP2A in different cell lines of CRC and found p-p53 (S392) and p-STAT5a (Y694) to be associated with the depletion of CIP2A expression.…”