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Roles of autocrine TGF-beta receptor and Smad signaling in adipocyte differentiation
L Choy1, J Skillington, R Derynck
1Department of Growth and Development, Programs in Cell Biology and Developmental Biology, University of California at San Francisco, San Francisco, California 94143-0640, USA.
Abstract:
TGF-beta inhibits adipocyte differentiation, yet is expressed by adipocytes. The function of TGF-beta in adipogenesis, and its mechanism of action, is unknown. To address the role of TGF-beta signaling in adipocyte differentiation, we characterized the expression of the TGF-beta receptors, and the Smads which transmit or inhibit TGF-beta signals, during adipogenesis in 3T3-F442A cells. We found that the cell-surface availability of TGF-beta receptors strongly decreased as adipogenesis proceeds. Whereas mRNA levels for Smads 2, 3, and 4 were unchanged during differentiation, mRNA levels for Smads 6 and 7, which are known to inhibit TGF-beta responses, decreased severely. Dominant negative interference with TGF-beta receptor signaling, by stably expressing a truncated type II TGF-beta receptor, enhanced differentiation and decreased growth. Stable overexpression of Smad2 or Smad3 inhibited differentiation and dominant negative inhibition of Smad3 function, but not Smad2 function, enhanced adipogenesis. Increased Smad6 and Smad7 levels blocked differentiation and enhanced TGF-beta-induced responses. The inhibitory effect of Smad7 on adipocyte differentiation and its cooperation with TGF-beta was associated with the C-domain of Smad7. Our results indicate that endogenous TGF-beta signaling regulates the rate of adipogenesis, and that Smad2 and Smad3 have distinct functions in this endogenous control of differentiation. Smad6 and Smad7 act as negative regulators of adipogenesis and, even though known to inhibit TGF-beta responses, enhance the effects of TGF-beta on these cells.
Insights
Transforming Growth Factor-beta (TGF-β) signaling regulates adipocyte differentiation. Smad2 and Smad3 have distinct roles, while Smad6 and Smad7 inhibit adipogenesis, paradoxically enhancing TGF-β effects.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Transforming Growth Factor-beta (TGF-β) is known to inhibit adipocyte differentiation but is expressed by adipocytes, creating a functional paradox.
- The precise role and mechanism of TGF-β signaling in adipogenesis remain largely uncharacterized.
Purpose of the Study:
- To investigate the function and mechanism of TGF-β signaling in adipocyte differentiation.
- To characterize the expression and roles of TGF-β receptors and Smad proteins during adipogenesis.
Main Methods:
- Utilized 3T3-F442A cells to study adipogenesis.
- Analyzed mRNA levels of Smads 2, 3, 4, 6, and 7 during differentiation.
- Employed dominant-negative interference and stable overexpression of TGF-β receptors and Smads.
Main Results:
- TGF-β receptor availability decreased during adipogenesis.
- Smad6 and Smad7 mRNA levels significantly decreased, while Smad2, 3, and 4 remained unchanged.
- Inhibition of TGF-β receptor signaling enhanced differentiation; Smad3 inhibition promoted adipogenesis, while Smad2 overexpression inhibited it.
- Overexpression of Smad6 and Smad7 blocked differentiation and enhanced TGF-β responses.
Conclusions:
- Endogenous TGF-β signaling modulates the rate of adipogenesis.
- Smad2 and Smad3 exhibit distinct functions in regulating adipocyte differentiation.
- Smad6 and Smad7 function as negative regulators of adipogenesis, enhancing TGF-β effects despite their inhibitory nature.