AOD9604 and hGH reduce obesity via beta(3)-AR upregulation, not direct receptor activation

Obesity remains difficult to treat pharmacologically without metabolic side effects. Human growth hormone (hGH) stimulates lipolysis but carries risks including insulin resistance, prompting development of targeted fragments. AOD9604 is a synthetic C-terminal fragment of hGH engineered to retain lipolytic activity without IGF-1-mediated growth effects. A key unanswered question was whether its fat-mobilising action required the beta(3)-adrenergic receptor (beta3-AR), the dominant lipolytic receptor in adipose tissue — whose expression is known to be suppressed in diet-induced obesity. Clarifying this mechanism matters for designing safer obesity pharmacotherapies.

Obese mice received hGH or AOD9604 via chronic intraperitoneal (ip) administration for 14 days; body weight, body fat, and beta(3)-AR RNA expression (qPCR) were measured against lean and obese wild-type controls. To probe receptor dependency, the same chronic protocol was applied to beta(3)-AR knock-out mice, with body weight and lipolysis as primary endpoints. An additional acute experiment in knock-out mice assessed energy expenditure and fat oxidation following AOD9604 dosing. Control arms included untreated obese wild-type and untreated knock-out cohorts.

Both hGH and AOD9604 significantly reduced body weight and body fat in obese mice after 14 days of chronic ip treatment. beta(3)-AR RNA expression — which is repressed in obese mice — was increased by both compounds to levels comparable with those observed in lean mice, suggesting a transcriptional normalisation effect on the key adipose lipolytic receptor.

Critically, in beta(3)-AR knock-out mice, chronic treatment with hGH and AOD9604 failed to produce the body-weight reduction and increased lipolysis seen in wild-type controls, confirming that intact beta3-AR signalling is required for the full chronic lipolytic response.

However, in the acute experiment, AOD9604 alone was still capable of increasing energy expenditure and fat oxidation in beta(3)-AR knock-out mice, indicating a short-term lipolytic mechanism that operates independently of beta3-AR. The combined evidence therefore shows that neither compound acts directly through beta3-AR, but that both upregulate its expression, which likely amplifies lipolytic sensitivity over time.