Functional interplay between c‐Myc and Max in B lymphocyte differentiation

Abstract

The Myc family of oncogenic transcription factors regulates myriad cellular functions. Myc proteins contain a basic region/helix-loop-helix/leucine zipper domain that mediates DNA binding and heterodimerization with its partner Max. Among the Myc proteins, c-Myc is the most widely expressed and relevant in primary B lymphocytes. There is evidence suggesting that c-Myc can perform some of its functions in the absence of Max in different cellular contexts. However, the functional interplay between c-Myc and Max during B lymphocyte differentiation is not well understood. Using and models, we show that while c-Myc requires Max in primary B lymphocytes, several key biological processes, such as cell differentiation and DNA replication, can initially progress without the formation of c-Myc/Max heterodimers. We also describe that B lymphocytes lacking Myc, Max, or both show upregulation of signaling pathways associated with the B-cell receptor. These data suggest that c-Myc/Max heterodimers are not essential for the initiation of a subset of important biological processes in B lymphocytes, but are required for fine-tuning the initial response after activation.

An early Myc‐dependent transcriptional program orchestrates cell growth during B‐cell activation

“…Most importantly in this context, the differential occurrence of consensus binding motifs suggests that other transcription factors, such as E2F or NFY, may have predominant roles-or be redundant with Myc [49]-in regulating Myc-independent genes ( Fig EV4). It is noteworthy here that several of the Myc-dependent ontological categories (e.g., RNA metabolism, ribosome biogenesis, nucleotide biosynthesis) were also enriched among Myc-regulated genes in different cell types [8,29,31], in germinal center B-cells [10], or in primary B-cells at longer time-points of LPS stimulation (72 h) [16]. Yet, the overlaps between these and our gene lists were only modest ( Fig EV5A, Dataset EV5).…”

“… Venn diagrams representing the overlap between Myc‐dependent LPS‐induced genes defined in this study (either all genes, or only those in CL1) with other lists of Myc‐regulated genes. From left to right, and top to bottom: genes less expressed in Myc/Max KO B‐cells activated with LPS for 72 h, compared to wt cells ; genes more expressed in Myc‐positive versus negative germinal center B‐cells Myc‐dependent serum‐responsive (MDSR) genes in fibroblasts ; genes down‐regulated in Myc‐depleted U2OS , K562 and HCT116 cells . Venn diagrams representing the overlap between Myc‐dependent LPS‐induced genes defined in this study with genes induced in Eμ‐ myc B‐cells at the pre‐tumoral (left) or tumoral stage (right). Data information: The P ‐value of the overlap between Myc‐dependent LPS‐induced genes and the dataset of interest is shown below each diagram.…”

“…Column D: MYC/MAX KO B-cells [16] Column E: GFP-MYC+ GC B-cells [10] Column F: Myc-dependent serum responsive [8] Column G: Myc-dependent in U2OS cells [29] (q-val < 0.01; log 2 FC < À1)…”

“…A key regulator in this overall process is the Myc transcription factor, encoded by the c-myc proto-oncogene: indeed, Myc is directly induced by mitogenic signals and, in turn, is thought to orchestrate the plethora of transcriptional changes that foster cell growth and proliferation, as exemplified in cultured mouse fibroblasts [7,8]. In either B or T lymphocytes, c-myc serves as a direct sensor of activating signals [3,[9][10][11][12][13] and is required for multiple facets of cellular activation, including metabolic reprogramming, ATP production, ATP-dependent chromatin decompaction, RNA and biomass accumulation, and cell growth [3][4][5]11,[13][14][15][16][17][18]. However, how Myc activity impacts on those diverse cellular features remains largely unclear.…”

An early Myc-dependent transcriptional program underlies enhanced macromolecular biosynthesis and cell growth during B-cell activation

“…A key regulator in this overall process is the Myc transcription factor, encoded by the cmyc proto-oncogene: indeed, Myc is directly induced by mitogenic signals and, in turn, is thought to orchestrate the plethora of transcriptional changes that foster cell growth and proliferation, as exemplified in cultured mouse fibroblasts (Perna, Faga et al, 2012, Winkles, 1998. In either B or T lymphocytes, c-myc serves as a direct sensor of activating signals (Caro-Maldonado, Wang et al, 2014, Dominguez-Sola, Victora et al, 2012, Kelly, Cochran et al, 1983, Luo, Weisel et al, 2018, Nie et al, 2012, Wang, Dillon et al, 2011 and is required for multiple facets of cellular activation, including metabolic reprogramming, ATP production, ATP-dependent chromatin decompaction, RNA and biomass accumulation, cell growth, etc… (Caro-Maldonado et al, 2014, de Alboran, O'Hagan et al, 2001, De Silva & Klein, 2015, Kieffer-Kwon et al, 2017, Link & Hurlin, 2015, Murn, Mlinaric-Rascan et al, 2009, Nie et al, 2012, Perez-Olivares, Trento et al, 2018, Sabò et al, 2014, Wang et al, 2011. However, how Myc activity impacts on those diverse cellular features remains largely unclear.…”