Function of pref-1 as an inhibitor of adipocyte differentiation

During conversion of preadipocytes to adipocytes, growth arrest and subsequ ent activation of adipocyte genes by the transcription factors, C/EBP alpha and PPAR gamma, lead to adipogenesis, During differentiation, these cells not only start expressing those genes necessary for adipocyte function, but also undergo changes in morphology to become rounded lipid filled adipocyt es. Various factors in cell-cell communication or cell-matrix interaction m ay govern whether preadipocytes are kept in an undifferentiated state or un dergo differentiation. In an attempt to identify molecules that play critic al roles in the conversion of preadipocytes to adipocytes, we cloned by dif ferential screening several regulatory molecules, including pref-1. Pref-1 is an inhibitor of adipocyte differentiation and is synthesized as a plasma membrane protein containing 6 EGF-repeats in the extracellular domain. Pre f-1 is highly expressed in 3T3-L1 preadipocytes, but is not detectable in m ature fat cells. Dexamethasone, a component of standard differentiation age nts, inhibits pref-1 transcription and thereby promotes adipogenesis. Downr egulation of pref-1 is required for adipose conversion and constitutive exp ression of pref-1 inhibits adipogenesis. Conversely, decreasing pref-1 leve ls by antisense pref-1 transfection greatly enhances adipogenesis. The ecto domain of pref-1 is cleaved to generate a biologically active 50 kDa solubl e form. There are four major forms of membrane pref-1 resulting from altern ate splicing. Two of these forms which have a deletion that includes the pu tative processing site proximal to the membrane do not produce a biological ly active soluble form. This indicates that alternate splicing may determin e the range of action, juxtacrine or paracrine, of pref-1.