ESPE Abstracts

Background and Aim: Germline mutations in the tumor suppressor gene PTEN cause PTEN Hamartoma Tumor Syndrome (PHTS). Pediatric patients frequently develop lipomas. PTEN antagonizes the growth promoting PI3K/AKT/mTOR pathway. There is no current treatment option except surgery. Treatment attempts with the mTORC1 inhibitor Rapamycin could not reverse lipoma growth. Recently, lipomas associated with a related syndrome caused by mosaic activating PI3K mutations (PIK3CA-related overgrowth syndrome, PROS) were successfully treated with the novel PI3K inhibitor alpelisib. Here we tested whether or not alpelisib has growth restrictive effects and induces apoptosis in lipoma cells from a pediatric patient with PHTS

Methods: We used PTEN haploinsufficient lipoma cells (LipPD1) and treated them with alpelisib alone or in combination with rapamycin. We tested viability, proliferation, apoptosis and signaling in those cells at different alpelisib concentrations (0.1-100 µM).

Results: Alpelisib reduced the viability of LipPD1 cells in a concentration- (P<0.0001) and time- (P < 0.0001) dependent manner as shown by WST-1 assays after 24, 48, 72, 96, 120 and 144 h. After 48 h cell count was decreased (0.75 fold at 10 µM, P= 0.0001), as well as the fraction of the proliferation marker Ki-67 positive cells (0.45 fold at 10 µM, P=0.0346). A combination with 10 nM rapamycin decreased cell viability further (0.76 fold after 72 h, P=0.0283). After 24 h Annexin V/Propidium iodide apoptosis assay was negative. Western blots revealed a reduced phosphorylation of AKT (0.06 fold at 100 µM), mTOR (0.59 fold at 100 µM) and ribosomal protein S6 (0.5 fold at 100 µM) in the alpelisib treated cells. While rapamycin treatment alone led to decreased levels of mTOR phosphorylation (0.43 fold at 10 nM), the AKT phosphorylation was increased (9.33 fold at 10 nM). This effect could be reversed by combining rapamycin with 100 µM alpelisib (0.22 fold compared to solvent control).

Discussion: The reduced activation of AKT through inhibition of PI3K with alpelisib reduced cell viability and proliferation of PTEN deficient lipoma cells. After 24 h treatment we did not observe apoptosis, but later time points remain to be tested. Rapamycin activated AKT through a negative feedback loop which was prevented by simultaneous treatment with alpelisib.

Conclusion: Since alpelisib was well tolerated in first clinical trials, this drug could be a potential new treatment for PHTS-related adipose tissue hyperplasia.