Title: RPGRIP1L and FTO – genes implicated in the effects of FTO intronic sequence variants on food intake – also affect adipogenesis and adipocyte biology
Description: Single nucleotides in the first intron of FTO convey effects on adiposity by mechanisms that remain unclear, but appear to include modulation of expression of FTO itself, as well as other genes (e.g. RPGRIP1L, IRX3) in the vicinity of FTO. This locus affects food intake, the browning of white adipose tissue and risk of type 2 diabetes (independent of its effects on body weight). FTO and RPGRIP1L expression are decreased in fibroblasts and iPSC-derived human neurons of individuals segregating for obesity risk alleles of FTO at rs8050136 and rs1421085. These alleles exhibit decreased binding of isoform p110 of the CUX1 transcription factor. This isoform activates transcription of both FTO and RPGRIP1L. The FTO locus conveys effects on adiposity via hyperphagia, in part, by regulating FTO and RPGRIP1L expression in the hypothalamus. We examined whether FTO and RPGRIP1L also modify adipogenesis and adipose tissue lipid storage. Such effects would influence systemic consequences of the hyperphagia driven by the actions of the genes in the hypothalamus.

Given the role in energy homeostasis of genes encoding elements of the primary cilium, we hypothesized that mice hypomorphic for Rpgrip1l would display increased adiposity. In confirmation, we find that Rpgrip1l+/− mice are hyperphagic and obese, and display diminished suppression of food intake in response to leptin administration. These findings suggest that RPGRIP1L may be partly or exclusively responsible for the obesity susceptibility signal at the FTO intronic locus. We describe effects of Rpgrip1l in adipocytes which may contribute to the adiposity phenotype observed in these animals, and possibly humans. Loss of Rpgrip1l in 3T3-L1 preadipocytes increased the number of cells capable of differentiating into mature adipocytes. Knockout of Rpgrip1l in mature adipocytes (using Adipoq-Cre) did not increase adiposity in mice fed chow or high fat diet. Neither did we observe any effects of Rpgrip1l knockdown in mature 3T3-L1 adipocytes in vitro. Thus, to the extent that Rpgrip1l affects cell-autonomous adipose tissue function, it appears to do so by effects conveyed in preadipocytes, a cell type in which the primary cilium – as a mediator of developmental signals – may have functional importance.

We propose that decreased RPGRIP1L expression in preadipocytes in humans segregating for FTO-associated obesity risk alleles increases the potential storage capacity of adipose tissue. Such capacity would influence the metabolic consequences of positive energy balance due to the action of these alleles within the brain. Fto expression is upregulated during adipogenesis in murine and human cells in vitro, and is more highly expressed in isolated mouse adipocytes than in preadipocytes. Here we demonstrate that FTO is required for the maintenance of adipocyte lipid filling and endocrine function in murine 3T3-L1 cells and human adipose tissue-derived stromal cells. RNAseq analysis indicates that this effect on adipocyte programming is conveyed in part by modulation of C/ebpβ- and C/ebpδ-regulated transcription, consistent with reports that Fto acts a transcriptional coactivator. Fto-/- mice have normal fat mass in early life, but spontaneously lose adipose tissue as they age. We propose that Fto is required to maintain adipocyte viability, a function critical to the prevention of ectopic lipid accumulation in obese states.

Such accumulation – both total and in specific anatomic regions – has adverse metabolic consequences. In addition to the developmental effects on adiposity mediated by RPGRIP1L, and the effects conveyed on adipocyte function related to FTO, the FTO locus could also impact systemic energy homeostasis by modifying production of humoral signals that are integrated centrally to regulate energy balance. We explored molecular modifiers of adipocyte production of leptin identified by GWAS that may modify obesity risk. The FTO locus was associated with circulating leptin concentration, but this association was abrogated when corrected for BMI, indicating that this locus does not contribute to adiposity by dysregulating leptin production. Our in vitro findings are consistent in this regard, as knockdown of Rpgrip1l and Fto in 3T3-L1 cells did not affect leptin production per adipocyte. These results, however, are not inconsistent with a role for FTO in maintenance of adipocyte viability.