It enhances cellular glucose uptake by regulating expression of genes that code for glucose transporters.
It is secreted by the beta cells of islets of Langerhans and acts mainly on hepatocytes and adipocytes.
It stimulates the conversion of glucose to glycogen in the hepatocytes.
In mammals, it is synthesized as a prohormone that contains an extra stretch called C peptide.
Insulin is a water soluble hormone which binds to the surface receptors. It does not regulate the gene expression but, acts through the secondary messengers inside the cell.
Steroid hormones, on the other hand regulate gene expression by binding to intracellular receptors.