Introduction

Colorectal cancer (CRC) is the second most common cause of cancer death in the world, which responsible for ∼655,000 deaths globally each year [1]. As the major metastasis organ, synchronous liver metastases exist in up to 20% of newly diagnosed CRC patients [2], and only 13% of 5-year survival rate of CRC patients has been reported [3]. Luckily, due to numerous studies revealing alterations in oncogenes and tumor suppressor genes contributed to the tumorigenesis and development of CRC, the clinical treatment and drug targeted therapies of CRC have been greatly improved over the last decades [4]. However, the detailed mechanisms underlying CRC progression and metastasis are still far from being completely elucidated.

Cyclin-dependent kinases (CDKs), the critical regulatory enzymes controlling all cell cycle transitions, have been suggested as the potent therapeutic targets for cancers [5]. Among the identified CDKs, CDK7 has a general role in the phosphorylation of the RNA polymerase II carboxyterminal domain and therefore regulates the initiation and elongation of transcription [6, 7]. Although the detailed mechanisms remain poorly understood, inhibition of the CDK7 can regulate Pol II promoter occupancy [8, 9], promoter-proximal pausing [10,11,12] and mRNA stability [13]. Targeted inhibition of CDK7 via a newly developed covalent inhibitor THZ1 can negatively impact gene expression, suppress the proliferation of cancer cells, and inhibit progression of animal xenograft models [10, 12, 43]. A recent study showed that MYC mediates phosphorylation of S5 on the carboxy-terminal domain of RNA Pol II through recruitment of CDK7, which leads to the activation of targeted transcripts [44]. Whether MYC or other transcription factors are involved in CDK7 regulated transcription of PKD1 via similar approach needs further investigation.

In vivo study and clinical analysis confirmed the suppression effects of CDK7 on the metastasis of CRC. We revealed that expression of CDK7 was elevated in CRC tissue as compared with the adjacent normal tissues. This was consistent with recent reports that CDK7 was increased in cancer tissues [45, 46]. We observed a strong negative correlation of CDK7 and mesenchymal markers including VIM, FN1, N-Cad, MMP2 and MMP9 expression in CRC patients. The expression of CDK7 was negatively correlated with the in vivo metastasis of CRC patients. Although there was no significant difference for the overall survival rate of patients with CDK7 expression level, CRC patients with less CDK7/SNAI1 or PKD1/SNAI1 showed poorer survival rate as compared with those with greater levels. In estrogen receptor positive (ER+) breast cancer, high-level expression of CDK7 was associated with longer survival in univariate and multivariate analyses [45]. However, patients with high expression of CDK7 exhibited worse overall survival as compared to those with low CDK7 in hepatocellular carcinoma (HCC) [47], gastric [48] and triple negative breast cancer [26]. For an unstratified cohort of 2656 breast cancer patients, no significant association was observed between CDK7 expression and recurrence-free survival [26]. It suggested that the roles of CDK7 on cancer progression might be cancer type specific.

Collectively, our data suggested that CDK7 contributes to the growth of CRC during tumorigenesis. However, CDK7 can suppress the metastasis during cancer development via a PKD1/Snail-dependent manner. The promotive effects on cancer metastasis of THZ1 in the present study imposed great challenge that targeted inhibition of CDK7 is a potential approach for cancer treatment, at least for CRC patients.

Availability of supplementary data

Detailed methods and materials, Figure S1 to S5, supplementary table S1 to S2 are attached.