Dysregulation of the expression levels and the activity of kinases/phosphatases is an intrinsic hallmark of tumor transformation and progression, as either as a primary cause or consequence. The myosin phosphatase (MP)/protein arginine methyltransferase 5 (PRMT5)/histone (H4) pathway is an oncogenic signaling pathway downregulating the gene expression of tumor suppressors. However, the upstream regulators of the pathway are unknown. We show that the Mg²⁺-dependent protein phosphatase 1 B (PP2Cb or PPM1B) interacts and regulates MP through the MYPT1 regulatory subunit, and this interplay results in the inactivation of the tumorigenic pathway driven by PRMT5. The phospho-Thr696 inhibitory residues of the MYPT1 regulatory subunit of MP was dephosphorylated by PPM1B. The inhibition of PPM1B by sanguinarine resulted in the deactivation of MP and the increased activity of PRMT5 leading to increased symmetric dimethylation of histone H4 in HeLa cells. The overexpression of the PPM1B had the opposite action. The overexpression of PPM1B decreased the colonization activity of HeLa cells through modulation of MP. Finally, human cervical carcinoma biopsies showed almost complete elimination of PPM1B compared to their healthy control counterparts. The phosphorylation of the inhibitory MYPT1^pT696 and the regulatory PRMT5^pT80 residues and the symmetric dimethylation of H4 were elevated in the cancer biopsies and it resulted in a decrease in retinoblastoma protein expression. The results indicate a tumor suppressor role of the PPM1B/MP axis via inhibition of PRMT5, thereby regulating gene expression through H4 arginine dimethylation. Collectively, PPM1B is a tumor suppressor and a possible tumor marker for cervical carcinoma.

Pulmonary carcinoma remains a highly aggressive malignancy driven by complex signaling and epigenetic dysregulation. This study investigates a novel oncogenic pathway involving the Mg^2+/Mn²⁺-dependent protein phosphatase 1B PPM1B/myosin phosphatase (MP)/protein arginine methyltransferase 5 (PRMT5) axis, which promotes carcinogenesis by symmetrically dimethylating histone H2A and suppressing tumor suppressor genes. We hypothesized that loss of PPM1B would activate this pathway and drive tumorigenesis. Western blotting, PCR, and immunohistochemistry revealed a significant reduction in PPM1B expression in both squamous cell carcinoma (SCC) and human lung adenocarcinoma (ADC) compared to normal lung tissues, which correlated with worse patient survival. Despite an increase in total MYPT1, the regulatory subunit of MP, its inhibitory phosphorylation at Thr853 was significantly elevated in both tumor types. The inactivation of MP corresponded with a significant increase in the activating phosphorylation of PRMT5 at Thr80, especially in SCC, which was linked to a particularly poor prognosis. Downstream, this resulted in a dramatic elevation in the symmetric dimethylation of histone H2A, leading to decreased expression of retinoblastoma protein. Our findings demonstrate that decreased PPM1B expression drives the oncogenic activation of the MP/PRMT5 axis. This mechanism contributes to the aggressive nature of SCC, establishing PPM1B as a promising prognostic marker in lung cancer.