The PI3K/mTOR inhibitor GSK2126458 is effective for treating TSC solid renal tumours

Tuberous sclerosis (TSC) is an inherited tumour syndrome that shares clinical similarities with Birt-Hogg-Dube Syndrome. It is caused by mutations in TSC1 or TSC2 that lead to aberrant activation of mTOR, affecting multiple organs, including the kidney and lung. In the kidney, lesions such as multiple renal cysts and renal cell carcinoma (RCC) can occur. Tumour reduction in TSC patients after treatment with rapamycin, an inhibitor of mTOR, is partial and reversible probably due to feedback activation of Akt. In their new study, Narov et al. (2017) test the efficacy of GSK2126458, an inhibitor of PI3K/mTOR, in comparison to rapamycin, for treatment of renal tumours in genetically engineered Tsc2+/- mice, that spontaneously develop various lesions in the kidneys. Both GSK2126458 and rapamycin caused significant reduction in number and size of solid renal tumours. GSK2126458 inhibited both PI3K and mTOR while rapamycin exerted stronger inhibitory effect on mTORC1 in renal tumours. Both GSK2126458 and rapamycin suppressed proliferation of tumour cells. However, GSK2126458 increased apoptosis of solid tumours but rapamycin did not. Further investigations are needed to test whether rapamycin in combination with GSK2126458 can improve anti-tumour therapy.

The kidney lesions of adult Tsc2+/- mice had aberrant activation of the mTOR complex 1 (mTORC1) and mTORC2. The MAPK pathway was also activated in these lesions. To test the anti-tumour efficacy of GSK2126458, the authors first determined the maximum tolerated dose (MTD) of GSK2126458 in Tsc2+/- mice. After this, adult mice were treated for two months with vehicle, the MTD dose of GSK2126458 or rapamycin. Both GSK2126458 and rapamycin significantly reduced total number, size and cellular area of solid renal tumours and other lesion types. Rapamycin caused greater reduction than GSK2126458 in number, size and cellular area of all types of lesions but the difference in reduction of solid tumour burden was not significant. IHC of kidney sections was used to investigate PI3K/Akt/mTOR signalling. GSK2126458 reduced phosphorylation of Akt at T308 but rapamycin did not. It is not known whether this reduction in Akt phosphorylation contributes to antitumour efficacy. The phosphorylation of a part of the Erk1/2 signalling pathway, was reduced in GSK2126458 treated solid renal tumours. The Erk1/2 signalling was inhibited by rapamycin in solid renal tumours. In contrast, reduced phosphorylation of mTOR at S2481, an indicator of mTORC2 activation, was detected in rapamycin treated solid tumours. Ki67 staining was used to assess proliferation of renal tumour cells on treated mice. Both GSK2126458 and rapamycin markedly reduced the median percentage of Ki67-positive cells. Rapamycin inhibited proliferation of tumour cells to a greater extent than GSK2126458. Active caspase 3 was used to test whether treatment induced apoptosis in tumour cells. Interestingly, GSK2126458 significantly increased the median total area of active caspase 3-positive tumour cells but rapamycin did not. Similar results were observed when other apoptosis marker was analysed. To investigate the mechanism behind the increased apoptosis associated with GSK2126458 treatment, expression of p53 and phosphorylation of MDM2 at S166 were analysed by IHC. Renal lesions had a lower level of p53 and decreased phosphorylation of MDM2 in rapamycin treated mice, but not GSK2126458 treated mice, compared to vehicle treated mice. Similar results were observed by Western analysis.

In conclusion, the authors demonstrated that GSK2126458 was effective for treating solid renal tumours. A Phase I clinical trial has recently reported that GSK2126458 is well tolerated in patients treated for multiple solid malignancy types and tumour responses and disease stabilization were observed (Munster et al., 2016). The authors found that GSK2126458 inhibited both mTORC1 and mTORC2 in all types of renal lesions in Tsc2+/- mice but the inhibitory effect of GSK2126458 on mTORC1 was weaker than that of rapamycin. Both GSK2126458 and rapamycin reduced proliferation of tumour cells. However, GSK2126458 increased apoptosis of solid tumours but rapamycin did not.

The clinical similarities between BHD and TSC suggest that FLCN and TSC proteins may function within a common pathway. It may be worthwhile investigating whether combination of GSK2126458 with rapamycin could improve anti-tumour therapy through increased tumour cell death in different disease models.

  • Narov, K., Yang, J., Samsel, P., Jones, A., Sampson, J., & Hong Shen, M. (2017). The dual PI3K/mTOR inhibitor GSK2126458 is effective for treating solid renal tumours in Tsc2+/- mice through suppression of cell proliferation and induction of apoptosis Oncotarget DOI: 10.18632/oncotarget.17215

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