Functions of the C/EBPβ isoforms in breast cancer

Breast cancer is the most commonly diagnosed type of cancer in women and the survival outcome is strongly dependent on the stage of breast cancer at diagnosis. Therefore, it is essential to identify oncogenic factors that contribute to breast cancer development and metastasis. The transcription factor C/EBPβ is known to regulate cell proliferation and differentiation in a variety of tissues. The transactivation capacity of C/EBPβ is largely determined by the ratio between the isoforms C/EBPβ-LAP and C/EBPβ-LIP. Previous studies have shown that C/EBPβ-LIP-depleted mice display improved metabolic health, a prolonged lifespan, and reduced cancer incidence. Other studies have found high expression of LIP in grade III, Estrogen receptor (ER) and Progesterone (PR) receptor negative human breast cancer and have linked high LIP expression with increased mammary epithelial proliferation. We find that cell lines derived from triple-negative breast cancer (TNBC, lacking the Estrogen and Progesterone receptors and HER2 expression) have a very high LIP/LAP ratio. Reducing the LIP/LAP ratio by exogenous expression of LAP in TNBC cell lines reduces migration and invasion of the breast cancer cells. Moreover, the overexpression of LIP promotes cell migration of untransformed mammary epithelial cells. Transcriptomics data obtained from TNBC cells with CEBPB knockout show a regulation of migration and extracellular matrix-related genes. Our data propose a role for the LIP/LAP ratio in the regulation of breast cancer cell migration and ECM remodelling, two key characteristics that are associated with the aggressive phenotype of TNBC cells

C/EBP beta-LIP induces cancer-type metabolic reprogramming by regulating the let-7/LIN28B circuit in mice

The transcription factors LAP1, LAP2 and LIP are derived from the Cebpb-mRNA through the use of alternative start codons. High LIP expression has been associated with human cancer and increased cancer incidence in mice. However, how LIP contributes to cellular transformation is poorly understood. Here we present that LIP induces aerobic glycolysis and mitochondrial respiration reminiscent of cancer metabolism. We show that LIP-induced metabolic programming is dependent on the RNA-binding protein LIN28B, a translational regulator of glycolytic and mitochondrial enzymes with known oncogenic function. LIP activates LIN28B through repression of the let-7 microRNA family that targets the Lin28b-mRNA. Transgenic mice overexpressing LIP have reduced levels of let-7 and increased LIN28B expression, which is associated with metabolic reprogramming as shown in primary bone marrow cells, and with hyperplasia in the skin. This study establishes LIP as an inducer of cancer-type metabolic reprogramming and as a regulator of the let-7/LIN28B regulatory circuit

C/EBPβ isoform-specific regulation of migration and invasion in triple-negative breast cancer cells

The transcription factor C/EBPβ is a master regulator of mammary gland development and tissue remodelling during lactation. The CEBPB-mRNA is translated into three distinct protein isoforms named C/EBPβ-LAP1, -LAP2 and -LIP that are functionally different. The smaller isoform LIP lacks the N-terminal transactivation domains and is considered to act as an inhibitor of the transactivating LAP1/2 isoforms by competitive binding for the same DNA recognition sequences. Aberrantly high expression of LIP is associated with mammary epithelial proliferation and is found in grade III, estrogen receptor (ER) and progesterone (PR) receptor-negative human breast cancer. Here, we show that reverting the high LIP/LAP ratios in triple-negative breast cancer (TNBC) cell lines into low LIP/LAP ratios by overexpression of LAP reduces migration and matrix invasion of these TNBC cells. In addition, in untransformed MCF10A human mammary epithelial cells overexpression of LIP stimulates migration. Knockout of CEBPB in TNBC cells where LIP expression prevails, resulted in strongly reduced migration that was accompanied by a downregulation of genes involved in cell migration, extracellular matrix production and cytoskeletal remodelling, many of which are epithelial to mesenchymal transition (EMT) marker genes. Together, this study suggests that the LIP/LAP ratio is involved in regulating breast cancer cell migration and invasion. This study together with studies from others shows that understanding the functions the C/EBPβ-isoforms in breast cancer development may reveal new avenues of treatment

C/EBPβ-LIP induces cancer-type metabolic reprogramming by regulating the let-7/LIN28B circuit in mice

The transcription factors LAP1, LAP2 and LIP are derived from the Cebpb-mRNA through the use of alternative start codons. High LIP expression has been associated with human cancer and increased cancer incidence in mice. However, how LIP contributes to cellular transformation is poorly understood. Here we present that LIP induces aerobic glycolysis and mitochondrial respiration reminiscent of cancer metabolism. We show that LIP-induced metabolic programming is dependent on the RNA-binding protein LIN28B, a translational regulator of glycolytic and mitochondrial enzymes with known oncogenic function. LIP activates LIN28B through repression of the let-7 microRNA family that targets the Lin28b-mRNA. Transgenic mice overexpressing LIP have reduced levels of let-7 and increased LIN28B expression, which is associated with metabolic reprogramming as shown in primary bone marrow cells, and with hyperplasia in the skin. This study establishes LIP as an inducer of cancer-type metabolic reprogramming and as a regulator of the let-7/LIN28B regulatory circuit