Claudin 1, a major tight junction protein, is highly expressed in various types of tumors such as thyroid, breast, and colorectal cancers. Moreover, claudin 1 is frequently found in the cytoplasm in various types of tumor cells. However, the cytoplasmic function of claudin 1 in tumors still remains largely unknown. Here, we investigated the novel function of cytoplasmic claudin 1 in autophagy. The mRNA expression level of claudin 1 was higher in several types of tumors than in normal tissues. Furthermore, colon tumor tissues showed increased autophagy compared with the adjacent normal tissues. Both endogenous and exogenous claudin 1 showed a cytoplasmic punctate staining pattern and were co-stained with the lysosome-associated membrane protein 1 (LAMP1). Importantly, autophagy-induced conditions, including starvation, increased the protein stability of claudin 1. Moreover, the increased level of claudin 1 stimulated autophagy by decreasing the level of the autophagy substrate, sequestosome1/p62 (SQSTM1), under autophagy-inducing conditions; activation of AMP-activated protein kinase (AMPK) and inhibition of mammalian target of rapamycin (mTOR). Taken together, we demonstrate that the novel function of cytoplasmic claudin 1 is related to autophagy. This study is the first to show a cytoplasmic function of claudin 1 as an autophagy regulator and provides the evidence that claudin 1 mediated autophagy regulation is an integral part of the mechanism by which claudin 1 regulates cancer progression. (C) 2018 Elsevier Inc. All rights reserved.