Transforming Growth Factor β Integrates Smad 3 to Mechanistic Target of Rapamycin Complexes to Arrest Deptor Abundance for Glomerular Mesangial Cell Hypertrophy

In many renal diseases, transforming growth factor beta (TGF beta)-stimulated canonical Smad 3 and noncanonical mechanistic target of rapamycin (mTOR) promote increased protein synthesis and mesangial cell hypertrophy. The cellular underpinnings involving these signaling molecules to regulate mesangial cell hypertrophy are not fully understood. Deptor has recently been identified as an mTOR interacting protein and functions as an endogenous inhibitor of the kinase activity for both TORC1 and TORC2. Prolonged incubation of mesangial cells with TGF beta reduced the levels of deptor concomitant with an increase in TORC1 and TORC2 activity. Sustained TGF beta activation was required to inhibit association of deptor with mTOR, whereas rapid activation had no effect. Using the mTOR inhibitor PP242, we found that TGF beta-induced both early and sustained activation of TORC1 and TORC2 was necessary for deptor suppression. PP242-induced reversal of deptor suppression by TGF beta was associated with a significant inhibition of TGF beta-stimulated protein synthesis and hypertrophy. Interestingly, expression of siRNA against Smad 3 or Smad 7, which blocks TGF beta receptor-specific Smad 3 signaling, prevented TGF beta-induced suppression of deptor abundance and TORC1/2 activities. Furthermore, overexpression of Smad 3 decreased deptor expression similar to TGF beta stimulation concomitant with increased TORC1 and TORC2 activities. Finally, knockdown of deptor reversed Smad 7-mediated inhibition of protein synthesis and mesangial cell hypertrophy induced by TGF beta. These data reveal the requirement of both early and late activation of mTOR for TGF beta-induced protein synthesis. Our results support that TGF beta-stimulated Smad 3 acts as a key node to instill a feedback loop between deptor down-regulation and TORC1/2 activation in driving mesangial cell hypertrophy.