PIK3CA and obesity due to melanocortin 4 receptor deficiency: Mice with PTPN1 inhibition exhibited augmented phosphorylation of INSR and enhanced PI3K/AKT signalling, protecting against diet‐induced obesity, improving whole‐body glucose homoeostasis, decreasing TG and TC and diminishing expression of the lipogenic gene SREBP1.12, 41, 42, 43