Adipocyte differentiation is modulated by secreted delta-like (dlk) variants and requires the expression of membrane-associated dlk

Previous studies demonstrate that the deltalike (dlk) sad preadipocyte fact or 1 (Pref-1) genes encode similar proteins. Pref-1 is downregulated during adipocyte differentiation, and expression of ectopic Pref-1 inhibits adipo genesis. We explored whether dlk functions similarly to Pref-1 and studied the role of alternately spliced dlk variants encoding membrane-associated o r secreted forms. We also studied whether enforced downregulation of dlk/Pr ef-1 may enhance the differentiation response of non-committed cells. Ectop ic expression of a potentially secreted dlk variant, conditioned media from dlk expressing cells or several individual epidermal-growth-factor-dlk pep tides inhibited 3T3-L1 differentiation. This demonstrates that dlk and Pref -1 are functionally equivalent, dlk gene mRNA encoding for secreted variant s decreased much faster than total dlk gene mRNA during differentiation of 3T3-L1 cells. In fact, total dlk or membrane-associated dlk protein express ion increased during the first hours of differentiation. Cells sorted for l owest levels of dlk protein diminished or lost their ability to differentia te. These data suggest that membrane and secreted dlk protein variants play opposite roles in the control of adipogenesis. In addition, enforced downr egulation of dlk protein expression in the weakly adipogenic Balb/c 3T3 cel l line dramatically enhanced adipogenesis in response to insulin. These res ults indicate that dlk protein not only participates in processes leading t o inhibition of adipogenesis but that the control of its expression and dif ferent spliced variants is essential for the adipogenic response to extrace llular signals.