PTEN deficiency causes dyschondroplasia in mice by enhanced hypoxia-inducible factor 1α signaling and endoplasmic reticulum stress

Abstract

Chondrocytes within the growth plates acclimatize themselves to a variety of stresses that might otherwise disturb cell fate. The tumor suppressor PTEN (phosphatase and tensin homolog deleted from chromosome 10) has been implicated in the maintenance of cell homeostasis. However, the functions of PTEN in regulating chondrocytic adaptation to stresses remain largely unknown. In this study, we have created chondrocyte-specific Pten knockout mice (Pten co/co ;Col2a1-Cre) using the Cre-loxP system. Following AKT activation, Pten mutant mice exhibited dyschondroplasia resembling human enchondroma. Cartilaginous nodules originated from Pten mutant resting chondrocytes that suffered from impaired proliferation and differentiation, and this was coupled with enhanced endoplasmic reticulum (ER) stress. We further found that ER stress in Pten mutant chondrocytes only occurred under hypoxic stress, characterized by an upregulation of unfolded protein response-related genes as well as an engorged and fragmented ER in which collagens were trapped. An upregulation of hypoxia-inducible factor 1α (HIF1α) and downstream targets followed by ER stress induction was also observed in Pten mutant growth plates and in cultured chondrocytes, suggesting that PI3K/AKT signaling modulates chondrocytic adaptation to hypoxic stress via regulation of the HIF1α pathway. These data demonstrate that PTEN function in chondrocytes is essential for their adaptation to stresses and for the inhibition of dyschondroplasia.KEY WORDS: PTEN, Dyschondroplasia, ER stress, HIF1α, Knockout mouse Development 135, 3587-3597 (2008) has been shown to lead to impaired chondrocyte proliferation, differentiation and apoptosis (Oliver et al., 2005;Yang et al., 2005;Yang et al., 2007). A number of studies have shown that deregulation of ER homeostasis is correlated with malformed skeleton development (chondrodysplasia) caused by mutations in genes encoding extracellular matrix (ECM) proteins (Hashimoto et al., 2003;Ho et al., 2007;Pirog-Garcia et al., 2007;Vranka et al., 2001). Chondrocytes expressing a mutant type-X collagen tolerate ER stress, experience delayed terminal differentiation and exhibit chondrodysplasia . A recent study has shown that site-1 protease (S1P; MBTPS1 -Mouse Genome Informatics) is necessary for a specialized ER stress response by chondrocytes that is required for the genesis of normal cartilage (Patra et al., 2007). However, the precise function of the ER stress response in cartilage tumor formation remains largely unknown.The tumor suppressor PTEN (phosphatase and tensin homolog deleted from chromosome 10) is a lipid phosphatase, the major substrate of which is phosphatidylinositol 3,4,5-triphosphate (PIP3), a secondary messenger generated by phosphatidylinositol-3-kinase (PI3K). Loss of PTEN function leads to an accumulation of PIP3 and an activation of its downstream effectors, acute transforming retrovirus thymoma [AKT; also known as protein kinase B (PKB) and AKT1]. As a serine/threonine protein kinase, AKT phosphorylates key intermed...

Conditional ablation of Pten in osteoprogenitors stimulates FGF signaling

“…Several recent studies (Ford-Hutchinson et al, 2007;Hsieh et al, 2009) reported the use of collagen2a1 cre to delete Pten in the cartilage of developing mice and saw defects in growth plate organization along with an increase in chondrocyte differentiation and increased bone formation resulting in skeletal overgrowth. Similar experiments carried out by Yang et al (Yang et al, 2008) showed that the growth plate defects in collagen2a1 cre Pten cko mice resulted from increased endoplasmic reticulum stress in Pten-null resting chondrocytes. Other investigators (Liu et al, 2007) used an osteocalcin cre to delete Pten in mature osteoblasts.…”

“…Recent data demonstrate that PTEN is essential for normal growth and development of skeletal cells. Conditional deletion of Pten in chondrocytes using collagen2a1 cre deleter mice leads to activation of stress response pathways and disruption of chondrocyte differentiation with development of a chondrodysplasia along with increased lipoma formation (Ford-Hutchinson et al, 2007;Yang et al, 2008;Hsieh et al, 2009). Pten deletion in end-stage, mature osteoblasts using osteocalcin cre leads to an increase in bone mass with a parallel increase in cell proliferation (Liu et al, 2007).…”

Sox9 sustains chondrocyte survival and hypertrophy in part through Pik3ca-Akt pathways

“…Thus, the deletion of Pten results in the accumulation of PtdIns(3,4,5)P 3 , leading to forced activation of Akt. Col2a1-Cre; Pten flox/flox mice show increased phosphorylation of Akt (Ford-Hutchinson et al, 2007;Yang et al, 2008). At 14.5 dpc, Alcian Blue staining of the skeleton showed that the degree of deformity of humeral cartilage in ; Pten +/+ proliferative chondrocytes.…”

Zbtb20 regulates the terminal differentiation of hypertrophic chondrocytes via repression of Sox9

“…To explore the role of Zbtb20 in skeletal development in vivo, we deleted the gene specifically in the cartilaginous template of endochondral bones by mating mice with a floxed Zbtb20 allele (Xie et al, 2008) with Col2-Cre transgenic mice, a line that was reported to mediate loxP recombination specifically in cartilaginous structures at E13.5 (Yang et al, 2008;Zhang et al, 2005), when Zbtb20 protein expression is not detectable in chondrocytes. Immunohistochemical analysis showed that Zbtb20 protein is undetectable in chondrocytes of Zbtb20 chondrocyte-specific knockout embryos and postnatal mice (Zbtb20 flox/flox ;Col2-Cre, hereafter referred to as Col-ZB20KO) (Fig.…”