Anticipating resistance to KRAS inhibition: a novel role for USP21 in macropinocytosis regulation

KRAS-Dependency in Pancreatic Ductal Adenocarcinoma: Mechanisms of Escaping in Resistance to KRAS Inhibitors and Perspectives of Therapy

Pancreatic ductal adenocarcinoma (PDAC) is still one of the deadliest cancers in oncology because of its increasing incidence and poor survival rate. More than 90% of PDAC patients are KRAS mutated (KRASmu), with KRASG12D and KRASG12V being the most common mutations. Despite this critical role, its characteristics have made direct targeting of the RAS protein extremely difficult. KRAS regulates development, cell growth, epigenetically dysregulated differentiation, and survival in PDAC through activation of key downstream pathways, such as MAPK-ERK and PI3K-AKT-mammalian target of rapamycin (mTOR) signaling, in a KRAS-dependent manner. KRASmu induces the occurrence of acinar-to-ductal metaplasia (ADM) and pancreatic intraepithelial neoplasia (PanIN) and leads to an immunosuppressive tumor microenvironment (TME). In this context, the oncogenic mutation of KRAS induces an epigenetic program that leads to the initiation of PDAC. Several studies have identified multiple direct and indirect inhibitors of KRAS signaling. Therefore, KRAS dependency is so essential in KRASmu PDAC that cancer cells have secured several compensatory escape mechanisms to counteract the efficacy of KRAS inhibitors, such as activation of MEK/ERK signaling or YAP1 upregulation. This review will provide insights into KRAS dependency in PDAC and analyze recent data on inhibitors of KRAS signaling, focusing on how cancer cells establish compensatory escape mechanisms.

MARK3 kinase: Regulation and physiologic roles

Microtubule affinity-regulating kinase 3 (MARK3), a member of the MARK family, regulates several essential pathways, including the cell cycle, ciliated cell differentiation, and osteoclast differentiation. It is important to understand the control of their activities as MARK3 contains an N-terminal serine/threonine kinase domain, ubiquitin-associated domain, and C-terminal kinase-associated domain, which perform multiple regulatory functions. These functions include post-translational modification (e.g., phosphorylation) and interaction with scaffolding and other proteins. Differences in the amino acid sequence and domain position result in different three-dimensional protein structures and affect the function of MARK3, which distinguish it from the other MARK family members. Recent data indicate a potential role of MARK3 in several pathological conditions, including congenital blepharophimosis syndrome, osteoporosis, and tumorigenesis. The present review focuses on the physiological and pathological role of MARK3, its regulation, and recent developments in the small molecule inhibitors of the MARK3 signalling cascade.

USP21 Promotes the Progression of Nasopharyngeal Carcinoma by Regulating FOXM1

The purpose of this work was to explore the molecular mechanisms by which USP21 regulates nasopharyngeal carcinoma tumor growth and cancer cell stemness. In this study, the USP21 transcript data was obtained from TCGA database. Then, qPCR and western blot tests revealed that, in contrast to normal tissue or normal nasopharyngeal epithelial cells, the expression of USP21 was greater in nasopharyngeal carcinoma tissues or cell lines, respectively. CCK-8 and EdU immunofluorescent staining assays revealed that USP21 promoted the proliferation of nasopharyngeal carcinoma cells. Meanwhile, scratch and transwell assays showed that USP21 facilitated migration and invasion of nasopharyngeal carcinoma cells. Sphere formation assay was performed on nasopharyngeal carcinoma cells after knockdown of USP21, which revealed that knockdown of USP21 inhibited the stemness profiles of nasopharyngeal carcinoma cells. Then, the western blot assays indicated that knockdown of USP21 in nasopharyngeal carcinoma cells would inhibit FOXM1 expression, and overexpression of FOXM1 could reverse the cell proliferation ability, cell migration and invasion ability, and cell stemness profiles. Finally, a nasopharyngeal xenograft model suggested that USP21 facilitated tumor growth in mice. These findings proved that USP21 promoted tumor growth and cancer cell stemness in nasopharyngeal carcinoma by regulating FOXM1.

Insights Into the Properties, Biological Functions, and Regulation of USP21

Deubiquitylating enzymes (DUBs) antagonize ubiquitination by removing ubiquitin from their substrates. The role of DUBs in controlling various physiological and pathological processes has been extensively studied, and some members of DUBs have been identified as potential therapeutic targets in diseases ranging from tumors to neurodegeneration. Ubiquitin-specific protease 21 (USP21) is a member of the ubiquitin-specific protease family, the largest subfamily of DUBs. Although USP21 was discovered late and early research progress was slow, numerous studies in the last decade have gradually revealed the importance of USP21 in a wide variety of biological processes. In particular, the pro-carcinogenic effect of USP21 has been well elucidated in the last 2 years. In the present review, we provide a comprehensive overview of the current knowledge on USP21, including its properties, biological functions, pathophysiological roles, and cellular regulation. Limited pharmacological interventions for USP21 have also been introduced, highlighting the importance of developing novel and specific inhibitors targeting USP21.