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

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.