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The cyclin-dependent kinase inhibitor p16 Ink4a plays a central role in cellular senescence in vitro. Although previous studies suggested cellular senescence is integrated in the systemic mechanisms of organismal aging, the localization and the dynamics of p16 Ink4a in tissues remain poorly understood, which hinders uncovering the role of p16 Ink4a under the in vivo context. One of the reasons is due to the lack of reliable reagents; as we also demonstrate here that commonly used antibodies raised against human p16 I NK 4 A barely recognize its murine ortholog. Here we generated a mouse model, in which the endogenous p16 Ink4a is HA-tagged at its N-terminus, to explore the protein expression of p16 Ink4a at the organismal level. p16 Ink4a was induced at the protein level along the course of senescence in primary embryonic fibroblasts derived from the mice, consistently to its transcriptional level. Remarkably, however, p16 Ink4a was not detected in the tissues of the mice exposed to pro-senescence conditions including genotoxic stress and activation of oncogenic signaling pathways, indicating that there is only subtle p16 Ink4a proteins induced. These results in our mouse model highlight the need for caution in evaluating p16 Ink4a protein expression in vivo., (Copyright © 2022 Elsevier Inc. All rights reserved.)

KCP-4 is a cisplatin-resistant cell line established from human epidermoid carcinoma KB-3-1 cells. Although our previous study revealed that one of the mechanisms for cisplatin resistance in KCP-4 cells is the activation of NF-κB, its high resistance is considered to be induced by multiple mechanisms. In the present study, we explored other factors involved in the development of cisplatin resistance in KCP-4 cells. Since it has been reported that an unknown efflux pump exports cisplatin from KCP-4 cells in an ATP-dependent manner, we examined 48 types of ATP-binding cassette proteins as candidate cisplatin efflux transporters. The mRNA expression levels of ABCA1, ABCA3, ABCA7 and ABCB10 in KCP-4 cells were higher when compared to those in KB-3-1 cells. These expression levels in cisplatin-sensitive revertant KCP-4 cells (KCP-4R cells), were reduced in parallel with the sensitivity of these cells to cisplatin and their intracellular accumulation of cisplatin. Next, we investigated the occurrence of mutations in p53 in KCP-4 cells. We found a heterozygous missense mutation at codon 72 (p.Pro72Arg) in p53 of both KCP-4 and KB-3-1 cells, but the protein expression level of p53 in KCP-4 cells was higher when compared to that in KB-3-1. These results suggest that ABCA1, ABCA3, ABCA7 and ABCB10 are candidate genes for the cisplatin efflux transporter that is involved in the cisplatin resistance of KCP-4 cells, and that the mutation at codon 72 of p53 may contribute to the development of cisplatin resistance.