Protein phosphatase 6 activates NF-κB to confer sensitivity to MAPK pathway inhibitors in - and -mutant cancer cells.

The Ras-mitogen-activated protein kinase (MAPK) pathway is a major target for cancer treatment. To better understand the genetic pathways that modulate cancer cell sensitivity to MAPK pathway inhibitors, we performed a CRISPR knockout screen with MAPK pathway inhibitors on a colorectal cancer (CRC) cell line carrying mutant KRAS. Genetic deletion of the catalytic subunit of protein phosphatase 6 (PP6), encoded by , rendered - and -mutant CRC and -mutant melanoma cells more resistant to these inhibitors. In the absence of MAPK pathway inhibition, deletion in CRC cells decreased cell proliferation in two-dimensional (2D) adherent cultures but accelerated the growth of tumor spheroids in 3D culture and tumor xenografts in vivo. deletion enhanced the activation of nuclear factor κB (NF-κB) signaling in CRC and melanoma cells and circumvented the cell cycle arrest and decreased cyclin D1 abundance induced by MAPK pathway blockade in CRC cells. Inhibiting NF-κB activity by genetic and pharmacological means restored the sensitivity of -deficient cells to MAPK pathway inhibition in CRC and melanoma cells in vitro and in CRC cells in vivo. Furthermore, a R264 point mutation in PPP6C conferred loss of function in CRC cells, phenocopying the enhanced NF-κB activation and resistance to MAPK pathway inhibition observed for deletion. These findings demonstrate that PP6 constrains the growth of - and -mutant cancer cells, implicates the PP6-NF-κB axis as a modulator of MAPK pathway output, and presents a rationale for cotargeting the NF-κB pathway in -mutant cancer cells.