AKT phosphorylation as a predictive biomarker for PI3K/mTOR dual inhibition-induced proteolytic cleavage of mTOR companion proteins in small cell lung cancer

Abstract

Background Constitutive activation of PI3K signaling has been well recognized in a subset of small cell lung cancer (SCLC), the cancer type which has the most aggressive clinical course amongst pulmonary tumors. Whereas cancers that acquire a mutation/copy gain in PIK3CA or loss of PTEN have been implicated in enhanced sensitivity to inhibitors targeting the PI3K/AKT/mTOR pathway, the complexities of the pathway and corresponding feedback loops hamper clear predictions as to the response of tumors presenting these genomic features. Methods The correlation between the expression profile of proteins involved in the PI3K/AKT/mTOR signaling and cell viability in response to treatment with small molecule inhibitors targeting isoform-specific PI3Ks, AKT, and mTOR was assessed in 13 SCLC cancer cell lines. Athymic nude mice were used to determine the effect of PI3K/mTOR dual inhibition on the growth of xenograft SCLC tumors in vivo. The activation of caspase signaling and proteolytic cleavages of mTOR companion proteins were assessed using recombinant caspases assays and Western blot analyses. Results Our results indicate that the sensitivity of these SCLC cell lines to GSK2126458, a dual PI3K/mTOR inhibitor, is positively correlated with the expression levels of phosphorylated AKT (p-AKT) at Thr308 and Ser473. Inhibition of pan-class I PI3Ks or PI3K/mTOR dual inhibition was shown to induce proteolytic cleavage of RICTOR and RPTOR, which were respectively dependent on Caspase-6 and Caspase-3. A combination of a clinically approved PI3Kα-selective inhibitor and an mTORC1 inhibitor was shown to have synergistic effects in inducing the death of SCLC cells with high p-AKT. We observed no clear correlation between PTEN levels and the survival of SCLCs in response to PI3K/mTOR dual inhibition; however, PTEN depletion was shown to increase the susceptibility of low p-AKT SCLC cells to dual PI3K/mTOR inhibitor-induced cell death as well as the proteolytic cleavage of RICTOR. Conclusions These results suggest the level of p-AKT can be a companion diagnostic biomarker for the treatment of SCLC involving the combinational use of clinically approved isoform-specific PI3K and mTOR inhibitors.

Tumor biomarkers for diagnosis, prognosis and targeted therapy

“… 302 The level of pAKT is a diagnostic biomarker for the treatment of SCLC involving the combination of clinically approved inhibitors against isoform-specific PI3K and mTOR. 345 In addition, the class I isoform of PI3K, the most well-known PI3K protein, contains four distinct isoforms of catalytic structural domain: p110α (PIK3CA), p110β (PIK3CB), p110γ (PIK3CG), and p110δ (PIK3CD). 343 pIK3CA and PTEN aberrations lead to the activation of the PI3K-AKT-mTOR pathway.…”

“…342 In cancer cells, the PI3K-AKT-mTOR signaling pathway is abnormally activated via the stimulation of tyrosine kinase growth factor receptors, 343 the loss of PTEN functions, and the mutations of PIK3CA, thereby promoting tumorigenesis in a wide variety of human cancers. 302,342 The PI3K-AKT-mTOR pathway exerts significant impacts on multiple cancers including lung cancer, 344,345 ovarian cancer, 302 breast cancer, 346 and NPC. 347,348 The PI3K-AKT-mTOR has been proven to be crucial in ovarian tumorigenesis and drug resistance.…”

Actionable Driver Events in Small Cell Lung Cancer

“…Blockade of the PI3K/AKT/mTOR pathway has shown encouraging pre-clinical results, indicating a potential strategy for SCLC treatment through a combination of clinically approved PI3K and mTOR inhibitors [ 128 ]. The activation of this pathway has been described as a potential mechanism underlying therapeutic resistance [ 129 ]; thus, this approach holds promise for reversing resistance to standard therapy in SCLC.…”

Untitled

“…PI3K regulates cellular metabolism and cellular proliferation by phosphorylating downstream effectors and adapters via the second messenger, PIP3 12 . However, consistent activation and/or overexpression of PI3K reasons disruption of mobile characteristics main to most cancers [13][14][15][16][17] . PI3Ks are categorized into 3 groups 3 , I, II and III, primarily based totally on their collection and substrate specificity [18][19] .…”