retinolRBP4

Retinol-binding protein 4 (RBP4) is a secreted transport protein of the lipocalin family that binds retinol, the alcohol form of vitamin A, and carries it through the bloodstream to peripheral tissues. It is primarily produced by the liver, with additional contributions from adipose tissue and other organs. The holo form of RBP4 carries retinol to cells that require vitamin A for signaling and metabolism.

In plasma, RBP4 often forms a complex with transthyretin (TTR). This ternary holo-RBP4–retinol–TTR assembly increases the complex’s stability and prevents filtration by the kidneys, prolonging circulatory residence. Cellular uptake of retinol from RBP4 can occur via receptors such as STRA6 on various cell types, enabling the delivery of retinol for conversion to retinal and subsequently to retinoic acid, which regulates gene expression and development.

Functionally, RBP4 serves as the main systemic carrier of retinol, supporting vision, growth, immune function, and overall retinoid signaling. Its circulating levels reflect vitamin A status and can be influenced by nutritional state and metabolic conditions. In some studies, elevated RBP4 has been linked to insulin resistance and metabolic syndrome, though findings can vary by context and population.

clinically, measurement of RBP4 can aid in assessing vitamin A status and retinoid biology. Abnormal RBP4 levels may arise from genetic factors, liver function, kidney handling, or adipose tissue dynamics, highlighting the protein’s role at the intersection of nutrition, metabolism, and endocrine signaling.