Bulky PP1 analogs exert cellular effects independently from analog-sensitive kinase inhibition

To circumvent the general lack of selectivity of protein kinase inhibitors, a chemical genetics approach has been developed to allow the selective targeting of engineered kinases by bulky ATP analogs, most of which derived from the pyrazolo[3,4-d]pyrimidine 1 (PP1) inhibitor. Although designed to selectively inhibit so-called analog-sensitive (AS) kinases presenting enlarged active sites, bulky PP1 analogs were shown to inhibit a number of wild-type protein kinases in vitro. Here, we examine the effects of 5 bulky PP1 analogs on the migration, invasive potential, proliferation, cell cycle, viability and differentiation of non-tumoral and tumoral cell lines that do not express an AS kinase. We show that the three inhibitors that have been employed the most so far (1NA-, 1NM-, 3MB-PP1) produce conspicuous effects on these cellular processes, sometimes at lower concentrations than those often used in AS kinase studies. Our work calls for caution when interpreting some cellular effects obtained with PP1 analogs used at concentrations ≥5 µM or even 2 µM for the least specific of them. Whenever possible, it advocates the use of lower concentrations of 3IB- or 3MSB-PP1 that produce less non-specific effects, as predicted from previously published in vitro data.