RNA binding protein HuD regulates translation and turnover of target mRNAs, thereby affecting gene expression at the posttranscriptional level in mainly neuronal as well as pancreatic beta-cells. Here, we identified insulinoma-associated 1 (INSM1), an essential factor governing differentiation and proliferation of neuroendocrine cells, as a novel target of HuD and demonstrated the regulatory mechanism of INSM1 expression by HuD. HuD bound to 3'untranslated region (3'UTR) of Insm1 mRNA and negatively regulated its expression; knockdown of HuD increased INSM1 expression, while HuD overexpression repressed it by destabilizing its mRNA. In addition, we further demonstrated that HuD enhanced reduction of INSM1 by miR-203a, a novel miRNA targeting Insm1 mRNA 3'UTR. These results suggest that HuD and miR-203a cooperatively regulate INSM1 expression and it provides a novel regulatory mechanism of INSM1 expression by HuD and miR-203a. (C) 2019 Elsevier Inc. All rights reserved.