Enigmatic role of RIPK1 in leukemia

Introduction Cell survival, inflammation and death are essential physiological events for maintaining cellular homeostasis and for preventing disease, especially cancer. These processes may be regulated by receptor-interacting protein kinase 1 (RIPK1), whose activity depends on post-translational modifications [1]. While several advances in describing the molecular mechanisms involving RIPK1 are evident, the complex crosstalk in the regulation of its different functions in cancer makes it difficult to determine the precise events [1]. Although the activity of RIPK1 in a wide range of diseases and in tumorigenesis has been demonstrated, its role in leukemia is still highly debated and therefore unclear [2]. The heterogeneity of RIPK1 expression in leukemia patients and its enigmatic functions suggest to rapidly define the molecular mechanisms in hematologic malignancies [3]. Materials and methods To study the degree of RIPK1 expression in several human leukemia cell lines we performed Western blot and qRT-PCR analyses. To gain further mechanistic insights, RIPK1 expression was evaluated in nutrient deficiency (% serum in medium). The proteasome inhibitor MG132 was used to define the molecular mechanism. More sensitive detection methods (Immunoprecipitation and biosensors) have been used to assess the relative amount of RIPK1. Results RIPK1 protein in different leukemia cell models shows variable expression levels independent of its constant mRNA levels. Notably, despite the slight increase in PI positivity or alteration in cell cycle phases, U937 cells displayed divergent expression of RIPK1 protein after cell dilution and stress conditions. Indeed, nutrient deficiency downregulates RIPK1 which is restored after MG132 treatment. Furthermore, the more sensitive detection methods confirmed a reduced concentration of the target protein. Conclusions Our experiments confirm the variability of RIPK1 protein expression in leukemia and underline the hypothesis of a stress sensor whose mechanism depends on proteasome activity. Thus, these preliminary experiments are ideal for increasing our understanding of the transcriptomic and proteomic events underlying important biological processes that are not yet well characterized in leukemia.