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Sp1 phosphorylation regulates apoptosis via extracellular FasL-Fas engagement
M M Kavurma1, F S Santiago, E Bonfoco
1Centre for Thrombosis and Vascular Research, The University of New South Wales, Sydney NSW 2052, Australia.
Abstract:
Apoptosis of smooth muscle cells (SMC) in atherosclerotic vessels can destabilize the atheromatus plaque and result in rupture, thrombosis, and sudden death. In efforts to understand the molecular processes regulating apoptosis in this cell type, we have defined a novel mechanism involving the ubiquitously expressed transcription factor Sp1. Subtypes of SMC expressing abundant levels of Sp1 produce the death agonist, Fas ligand (FasL) and undergo greater spontaneous apoptosis. Sp1 activates the FasL promoter via a distinct nucleotide recognition element whose integrity is crucial for inducible expression. Inducible FasL promoter activation is also inhibited by a dominant-negative form of Sp1. Increased SMC apoptosis is preceded by Sp1 phosphorylation, increased FasL transcription, and the autocrine/paracrine engagement of FasL with its cell-surface receptor, Fas. Inducible FasL transcription and apoptosis are blocked by dominant-negative protein kinase C-zeta, whose wild-type counterpart phosphorylates Sp1. Thus, Sp1 phosphorylation is a proapoptotic transcriptional event in vascular SMC and, given the wide distribution of this housekeeping transcription factor, may be a common regulatory theme in apoptotic signal transduction.
Insights
Smooth muscle cell (SMC) apoptosis, a factor in atherosclerotic plaque rupture, is regulated by the transcription factor Sp1. Sp1 phosphorylation promotes Fas ligand (FasL) transcription, increasing SMC apoptosis.
Area of Science:
- Molecular Biology
- Cardiovascular Research
- Cell Biology
Background:
- Smooth muscle cell (SMC) apoptosis contributes to atherosclerotic plaque instability, rupture, and thrombosis.
- Understanding the molecular mechanisms regulating SMC apoptosis is crucial for cardiovascular disease research.
Purpose of the Study:
- To elucidate a novel mechanism regulating smooth muscle cell apoptosis involving the transcription factor Sp1.
- To investigate the role of Sp1 phosphorylation and its downstream effects on Fas ligand (FasL) expression and apoptosis.
Main Methods:
- Analysis of Sp1's role in activating the FasL promoter in vascular SMC.
- Investigation of Sp1 phosphorylation and its correlation with FasL transcription and apoptosis.
- Utilizing dominant-negative constructs of Sp1 and protein kinase C-zeta to assess their impact on apoptosis.
Main Results:
- Vascular SMCs with high Sp1 levels exhibit increased FasL production and spontaneous apoptosis.
- Sp1 directly activates the FasL promoter through a specific nucleotide recognition element.
- Sp1 phosphorylation precedes increased FasL transcription and subsequent SMC apoptosis, which can be blocked by inhibiting protein kinase C-zeta.
Conclusions:
- Sp1 phosphorylation represents a proapoptotic transcriptional event in vascular SMCs.
- This Sp1-mediated pathway, involving FasL and Fas signaling, is a key regulator of vascular SMC apoptosis.
- Given Sp1's ubiquitous expression, this mechanism may represent a common regulatory theme in apoptotic signal transduction across various cell types.