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.