TGF-β Concentration Specifies Differential Signaling Profiles of Growth Arrest/Differentiation and Apoptosis in Podocytes

Podocyte depletion occurs in most progressive glomerular diseases and is thought to result from podocyte loss while the remaining podocytes are unable to proliferate. The underlying mechanisms for podocyte growth arrest/differentiation and depletion remain poorly understood but may involve TGF-β, which is typically upregulated in injured glomeruli. The TGF-β are multifunctional cytokines that regulate growth, differentiation, and apoptosis in most cells. Determinants of functional specificity of TGF-β signaling in cell-cycle control and apoptosis remain poorly understood. Using a unique system of conditionally immortalized podocytes, it is demonstrated that autocrine TGF-β2 induces G0/G1 arrest and differentiation under nonpermissive culture through Smad3-dependent induction of the cyclin-dependent kinase inhibitor p15 Ink4b (Cdkn2b). When exposed to recombinant TGF-β1 (or TGF-β2), nonpermissive culture podocytes switch to G2/M arrest and apoptosis, selectively at advanced TGF-β concentrations and specifically in association with suppression of Cdkn2b and activation of proapoptotic p38 mitogen-activated protein kinase. Thus, distinct signaling profiles activated in a concentration-dependent manner by TGF-β were identified. Autocrine TGF-β2/Smad3/Cdkn2b signaling in podocytes specifies G0/G1 arrest associated with podocyte differentiation, whereas increasing TGF-β concentrations beyond a critical threshold induces G2/M block and apoptosis associated with selective p38 mitogen-activated protein kinase activation and with suppression of Cdkn2b. In summary, the results suggest a new functional requirement of TGF-β2 in growth arrest and differentiation of murine podocytes in vitro and demonstrate that a critical TGF-β concentration threshold may specify a molecular switch to proapoptotic signaling profiles and apoptosis.