PTHrP inhibits adipocyte differentiation by down-regulating PPARγ activity via a MAPK-dependent pathway

Abstract

We examined the capacity of PTHrP to modulate the terminal differentiation of the preadipocytic cell line, 3T3-L1. These cells express endogenous PTHrP and its receptor, but expression levels were undetectable after differentiation into mature adipocytes. Cells stably overexpressing PTHrP failed to differentiate when induced to undergo adipogenesis and proliferated at a faster rate. MAPK activity was elevated in PTHrP-transfected 3T3-L1 cells, and treatment with the PKA inhibitor H-8 decreased this activity. Inhibition of MAPK kinase with PD098059 permitted terminal differentiation of PTHrP-transfected 3T3-L1 cells to proceed. Although PPARγ gene expression levels remained relatively constant in the PTHrP-transfected cells, PPARγ phosphorylation was enhanced. Furthermore, the capacity of PPARγ to stimulate transcription in the presence of troglitazone was diminished by PTHrP. Expression of the PPARγ-regulated adipocyte-specific gene aP2 transiently rose and then fell in PTHrP-transfected cells. These results indicate that PTHrP can increase MAPK activity in 3T3-L1 cells via the PKA pathway, thereby enhancing PPARγ phosphorylation. This modification can inactivate the transcriptional enhancing activity of PPARγ and diminish the expression of adipocyte-specific genes. These studies therefore demonstrate that PTHrP may inhibit the terminal differentiation of preadipocytes and describe a molecular pathway by which this action can be achieved.