miR‐9 Restricts Insulin Secretion by Targeting Rab34, Which Mediates Lysosomal Degradation of Proinsulin

ABSTRACT Insulin secretion is a complex, vesicular transport process. Rab34 is a key regulator of intracellular vesicle transport; however, its role in insulin secretion has not yet been reported. miRNA‐9 is vital for the development and progression of the diagnosis and treatment of type 2 diabetes. This study aimed to investigate whether miR‐9 targets Rab34 to regulate insulin secretion in beta (β) cells and its molecular mechanism. We used miR‐9 mimics, miR‐9 inhibitors, and dual‐luciferase reporter gene detection to prove that miR‐9 is regulated by targeting Rab34 in Min6 cells. Transmission electron microscopy revealed a decrease in the number of insulin particles when Rab34 was overexpressed in β cells. Regarding the mechanism, Rab34 overexpression induced a decrease in the number of proinsulin‐containing particles in β cells and significantly promoted proinsulin degradation. Immunofluorescence microscopy revealed that Rab34 overexpression resulted in insulin granule clustering. Conversely, Rab34 depletion maintained proinsulin and increased insulin secretion. miR‐9 overexpression inhibited insulin secretion, and miR‐9 downregulation promoted insulin secretion. Immunofluorescence results revealed that Rab34 induced insulin secretory vesicles toward the autophagic degradation pathway. Therefore, we concluded that miR‐9 inhibits insulin secretion by promoting Rab34 expression, which regulates insulin secretion by mediating the degradation of proinsulin and autophagic degradation pathways in β cells.