ESPE Abstracts

Background: Pediatric patients with germline pathogenic variants in the tumor suppressor gene PTEN frequently develop cancer and adipose tissue overgrowth in the form of lipomas. While the canonical function of the phosphatase PTEN is to antagonize the growth promoting PI3K pathway, non-canonical PTEN functions e.g. in the nucleus are less well described. To uncover the mechanisms leading to lipoma formation related to PTEN mutations, we previously performed RNA-sequencing of adipose progenitor cells (APCs) with PTEN knockdown (KD). The so far uncharacterized small integral membrane protein 10 like 1 (SMIM10L1) was the most significantly downregulated gene. We aimed to uncover the role of SMIM10L1 downregulation in APCs in relation to adipose tissue overgrowth and cancer formation.

Methods: SMIM10L1 was downregulated via siRNA or overexpressed via expression vectors in APCs and effects on gene expression, adipogenesis, and cell death were assessed.

Results: We observed a downregulation of SMIM10L1 mRNA to 35.5 ± 2 % (P=0.001) after PTEN KD. This was independent of PI3K pathway activation, as stimulation with insulin-like growth factor 1 (IGF-1) or inhibition with wortmannin did not result in a SMIM10L1 regulation. Moreover SMIM10L1 KD itself led to a downregulation of PTEN on protein level (by 18.9 ± 7 %, P=0.023) and mRNA level (by 12.7 ± 3 %, P=0.0018) and a subsequent activation of the downstream signaling molecule AKT (1.9 ± 0.2 fold, P=0.11). SMIM10L1 KD led to an increase in the lipogenic transcription factor SREBP1 protein expression (1.4 ± 0.2 fold, P=0.096) and enhanced adipogenesis in APCs (1.6 ± 0.2 fold, P=0.032) similar to PTEN KD APCs. Overexpression of GFP tagged SMIM10L1 led to detachment of cells and PARP cleavage, indicating apoptosis, while transfection of a mutant SMIM10L1 version with a stop instead of a start codon or GFP alone did not.

Conclusion: SMIM10L1 and PTEN mutually affect their expression, while activation or inhibition of the PI3K pathway did not alter SMIM10L1 expression, suggesting regulation via non-canonical functions of PTEN. The increased adipogenesis in conditions of SMIM10L1 downregulation observed in adipose progenitors potentially contributes to the adipose tissue overgrowth in patients with PTEN germline mutations. The induction of apoptosis through SMIM10L1 overexpression points to a regulatory mechanism that might be overcome by downregulation in conditions of PTEN insufficiency.