ABSTRACT
The protein Lgl1 has key roles in the regulation of cell polarity. We have shown that Lgl1 is inactivated by hyperphosphorylation in glioblastoma as a consequence of PTEN loss and aberrant activation of the PI 3-kinase pathway; this contributes to glioblastoma pathogenesis both by promoting invasion and repressing glioblastoma cell differentiation. Lgl1 is phosphorylated by atypical protein kinase C in a complex with Par6 and activated Rac. Here we have investigated the role of specific Rac guanine nucleotide exchange factors in Lgl1 hyperphosphorylation in glioblastoma. We used CRISPR/Cas9 to knockout PREX1, a PI 3-kinase pathway-responsive Rac guanine nucleotide exchange factor, in patient-derived glioblastoma cells. Knockout cells had reduced Lgl1 phosphorylation which could be reversed by re-expressing PREX1. PREX1 knockout cells showed reduced motility and an altered phenotype suggestive of partial neuronal differentiation; consistent with this, RNA-seq analyses identified sets of PREX1-regulated genes associated with changes in cell motility and neuronal differentiation. PREX1 knockout in glioblastoma cells from a second patient did not affect Lgl1 phosphorylation. These cells overexpressed a short isoform of the Rac guanine nucleotide exchange factor TIAM1; knockdown of TIAM1 in PREX1-knockout cells from this patient reduced Lgl1 phosphorylation. These data show that PREX1 links aberrant PI 3-kinase signaling to Lgl1 phosphorylation in glioblastoma, but that TIAM1 can also promote Lgl phosphorylation in a subset of patients. While this shows redundant mechanisms for Lgl1 phosphorylation, PREX1 appears to have a non-redundant role in glioblastoma cell motility, as this was impaired in PREX1 knockout cells from both patients.
Competing Interest Statement
The authors have declared no competing interest.