Role of PPARγ in Regulating a Cascade Expression of Cyclin-dependent Kinase Inhibitors, p18(INK4c) and p21(Waf1/Cip1), during Adipogenesis

Abstract

Molecular mechanisms coupling growth arrest and cell differentiation were examined during adipogenesis. Data are presented that document a cascade expression of members of two independent families of cyclin-dependent kinase inhibitors that define distinct states of growth arrest during 3T3-L1 preadipocyte differentiation. Exit from the cell cycle into a pre-differentiation state of post-mitotic growth arrest was characterized by significant increases in p21 and p27. During onset of irreversible growth arrest associated with terminal differentiation, the level of p21 declined with a concomitant, dramatic increase in p18 and a sustained level of p27. The expression of p18 and p21, regulated at the level of protein and mRNA accumulation, was directly coupled to differentiation. Stable cell lines were engineered to express adipogenic transcription factors to examine the active role of trans-acting elements in regulating these cell cycle inhibitors. Ectopic expression of peroxisome proliferator-activated receptor (PPAR) ␥ in non-precursor fibroblastic cell lines resulted in conversion to adipocytes and a coordinated increase in p18 and p21 mRNA and protein expression in a PPAR␥ ligandassociated manner. These data demonstrate a role for PPAR␥ in mediating the differentiation-dependent cascade expression of cyclin-dependent kinase inhibitors, thereby providing a molecular mechanism coupling growth arrest and adipocyte differentiation.

Prostaglandin F2α inhibits adipocyte differentiation via a Gαq‐Calcium‐Calcineurin‐Dependent signaling pathway

“…As shown in Figure 4B, MDI treatment of 3T3-L1 preadipocytes resulted in the expected time-dependent increase in expression of both p21 and p27, whose expression has previously been shown to correlate with the commitment of differentiating preadipocytes to terminal differentiation [Morrison and Farmer, 1999]. Notably, the presence of PGF2a did not appear to affect the expression of either of these cell cycle regulatory proteins.…”

“…Once preadipocytes have undergone MCE, they next exit the cell cycle and undergo terminal adipocyte differentiation, a step believed to be controlled by changes in the expression of a variety of cell cycle regulatory proteins including cyclin-dependent kinase inhibitors and members of the retinoblastomarelated family of proteins [Richon et al, 1997;Cowherd et al, 1999;Morrison and Farmer, 1999]. As shown in Figure 4B, MDI treatment of 3T3-L1 preadipocytes resulted in the expected time-dependent increase in expression of both p21 and p27, whose expression has previously been shown to correlate with the commitment of differentiating preadipocytes to terminal differentiation [Morrison and Farmer, 1999].…”

Peroxisome proliferator-activated receptor γ ligand-induced growth inhibition of human hepatocellular carcinoma

“…In a recent study, troglitazone induced G1 arrest in pancreatic carcinoma cell line PK-1 was found to be associated with increased p27 Kipl but unchanged p21 Cipl/Wafl and p18 Ink4c levels . Cell cycle withdrawal during PPARγ induced adipogenesis was also found associated with an initial increase in both p27 Kipl and p21 Cipl/Wafl , followed by a decline in p21 Cipl/Wafl , increase in p18 Ink4c with sustained p27 Kipl level (Morrison and Farmer, 1999). Difference in the extent of G1 or G2 cell cycle arrest in Hep G2 cells with troglitazone, pioglitazone or 15d-PGJ 2 may be due to differences in cascade expression of the CDK inhibitors.…”

“…However, CDK inhibitors other than p27 Kipl , p21 Cipl/Wafl and p18 Ink4c may also be involved in the PPARγ ligands induced cell cycle arrest. Possibility of an initial increased p21 Cipl/Wafl followed by declined level in troglitazone treated Hep G2 cells as reported in coupling growth arrest and adipocyte differentiation (Morrison and Farmer, 1999) can not be excluded until further time course studies are done. Further studies are also required to reveal whether inhibition of the p27 Kipl or p18 Ink4c increase could inhibit the growth arrest in Hep G2 as found in PK-1 cells .…”

Inhibition of human chondrosarcoma cell growth via apoptosis by peroxisome proliferator-activated receptor-γ

“…It has been shown that PPARg has regulatory functions in the cell cycle by altering cell growth (Motomura et al, 2000;Okura et al, 2000;Morrison and Farmer, 1999). Recently, Motomura et al (2000) reported that activation of PPARg by troglitazone inhibited cell growth and G1 arrest through the increase of cycline dependent kinase inhibitor p27 Kip1 in human pancreatic carcinoma cells.…”