How does the sitagliptin component of metformin + sitagliptin lower blood glucose?

In metformin + sitagliptin (Janumet), sitagliptin inhibits DPP-4, increasing active GLP-1 and GIP to enhance insulin synthesis and release when glucose is normal or elevated, while GLP-1 reduces glucagon and liver glucose production; when glucose is low, insulin release is not enhanced and glucagon secretion is not suppressed.