Deregulated MAPK Activity Prevents Adipocyte Differentiation of Fibroblasts Lacking the Retinoblastoma Protein

Abstract

A functional retinoblastoma protein (pRB) is required for adipose conversion of preadipocyte cell lines and primary mouse embryo fibroblasts (MEFs) in response to treatment with standard adipogenic inducers. Interestingly, lack of functional pRB in MEFs was recently linked to elevated Ras activity. Ras-dependent signaling plays a significant, although incompletely understood, role in adipocyte differentiation, because activated Ras has been reported to either promote or inhibit adipogenesis depending on the cellular context. In various cell types activation of Ras leads to activation of the mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase 1/2 (ERK1/2), and protein kinase B (PKB)/Akt, which exert opposing effects on adipogenesis, with ERK1/2 inhibiting and PKB/Akt promoting terminal differentiation. Here we report that the levels of activated ERK1/2 and PKB/Akt are significantly increased in pRB-deficient MEFs both before and after the addition of adipogenic inducers. Consistently, we detected higher levels of activated Ras in MEFs lacking pRB. Suppression of ERK1/2 activation by the MEK inhibitor UO126 restored the ability of pRB-deficient MEFs to undergo adipocyte differentiation, as manifested by expression of adipocyte marker genes and lipid accumulation. Furthermore and reflecting the elevated levels of activated PKB/Akt in the pRB-deficient MEFs, differentiation proceeded in an insulin-independent manner. In conclusion, we suggest that pRB plays a pivotal role in adipogenesis by suppressing MAPK activity.The conversion of fibroblast-like precursor cells to fully differentiated adipocytes is a process controlled by a complex network of signaling pathways and is tightly regulated by numerous transcription factors, including members of the peroxisome proliferator-activated receptor (PPAR) 1 and CCAAT/ enhancer-binding protein (C/EBP) families, and the adipocyte determination and differentiation-dependent factor-1/sterol regulatory element-binding protein-1 (for review, see Refs.

Arachidonic acid-dependent inhibition of adipocyte differentiation requires PKA activity and is associated with sustained expression of cyclooxygenases

“…Thorough research by Tang, Otto, and Lane (58) has provided evidence that clonal expansion is an integral part of the differentiation process and that adipocyte differentiation is prevented in the absence of clonal expansion. However, it has also been reported that clonal expansion, at least under certain conditions, is dispensable (53,54,57). Our finding is compatible with this view, although we cannot rule out the possibility that some ERK1/2 activity remains in the U0126-treated cells, allowing some degree of clonal expansion.…”

“…It is generally agreed that a transient upregulation of ERK1/2 MAPK activity is a prerequisite for clonal expansion and promotes adipogenesis (51-53), whereas sustained activation of the ERK1/2 has been demonstrated to inhibit adipogenesis (54)(55)(56)(57). We show that addition of arachidonic acid enhances ERK1/2 activation following induction of adipocyte differentiation by MDI and prolongs the period during which ERK2, in particular, remains in the active state.…”

Alterations of the RB1 gene in dedifferentiated liposarcoma

“…pRB has been shown to promote adipocyte differentiation by enhancing the DNA-binding and transactivation activity of C/EBPb, and/or by inhibiting Ras signaling to suppress the activation of the ERK1/2 MAPKs. 33,34 These findings support our hypothesis.…”

Protein Kinase B/AKT 1 Plays a Pivotal Role in Insulin-like Growth Factor-1 Receptor Signaling Induced 3T3-L1 Adipocyte Differentiation

“…The function of the MAP kinase system in IGF-1 receptor signal-induced 3T3-L1 cell adipogenesis is not clear. It is further complicated by the different effects exhibited by p38 MAP kinase and ERK1/2 on 3T3-L1 cell adipogenesis (5)(6)(7)(8)(9)(10)(11)(12). More molecular studies are required in order to understand the relationship between MAP kinase system and 3T3-L1 cell adipogenesis.…”