Gla-Rich Protein Is a Potential New Vitamin K Target in Cancer: Evidences for a Direct GRP-Mineral Interaction

Abstract

Gla-rich protein (GRP) was described in sturgeon as a new vitamin-K-dependent protein (VKDP) with a high density of Gla residues and associated with ectopic calcifications in humans. Although VKDPs function has been related with γ-carboxylation, the Gla status of GRP in humans is still unknown. Here, we investigated the expression of recently identified GRP spliced transcripts, the γ-carboxylation status, and its association with ectopic calcifications, in skin basal cell and breast carcinomas. GRP-F1 was identified as the predominant splice variant expressed in healthy and cancer tissues. Patterns of γ-carboxylated GRP (cGRP)/undercarboxylated GRP (ucGRP) accumulation in healthy and cancer tissues were determined by immunohistochemistry, using newly developed conformation-specific antibodies. Both GRP protein forms were found colocalized in healthy tissues, while ucGRP was the predominant form associated with tumor cells. Both cGRP and ucGRP found at sites of microcalcifications were shown to have in vitro calcium mineral-binding capacity. The decreased levels of cGRP and predominance of ucGRP in tumor cells suggest that GRP may represent a new target for the anticancer potential of vitamin K. Also, the direct interaction of cGRP and ucGRP with BCP crystals provides a possible mechanism explaining GRP association with pathological mineralization.

New Perspectives for the Nutritional Value of Vitamin K in Human Health

“…However, the present dietary reference values for vitamin K (90 µg/day for women and 120 µg/day for men) [10] are based on proper functioning of the blood coagulation factors to maintain normal haemostasis, and not on the γ-carboxylation status of other VKDPs such as matrix Gla protein (MGP), Gla-rich protein (GRP), and osteocalcin (OC), known to be of vital importance in bone and/or vascular health [2][3][4][5][6][7]18,24]. Interestingly, extra-hepatic Gla proteins have been shown to be present as incompletely γ-carboxylated forms in the majority of healthy adults [4,25], and thus the biological activity of these proteins could be considered sub-optimal. Since VKOR is a dithiol dependent enzyme known to be inhibited by 4-hydroxycoumarin anticoagulant drugs, such as warfarin, acenocoumarol, and phenprocoumon, the widely use of these oral anticoagulants acting as vitamin K antagonists (VKAs), has also been linked to unwanted side effects in several extra-hepatic tissues with adverse clinical outcomes [4,[7][8][9]11,12].…”

“…The metal binding properties of Gla residues within the VKDP family have been associated with binding of calcium ions or calcium crystals, either through Ca 2+ coordination in the Ca 2+-dependent binding of coagulation factors to anionic phospholipids membrane surfaces [68], or through binding to HA crystals, the major mineral component present in mineralized extracellular matrix, and associated either with physiological (eg., in bone) or pathological processes in soft tissues [69]. Fully γ-carboxylated GRP in human can include 15 Gla residues, which confers to this protein outstanding calcium and mineral binding capacity [25,63,64]. Undercarboxylation of GRP has been recently associated with several pathological calcification related diseases, as calcific aortic valve disease [66], osteoarthritis [67] and certain cancers [25], while only γ-carboxylated GRP has shown to display antimineralization capacity [66,67].…”

“…Fully γ-carboxylated GRP in human can include 15 Gla residues, which confers to this protein outstanding calcium and mineral binding capacity [25,63,64]. Undercarboxylation of GRP has been recently associated with several pathological calcification related diseases, as calcific aortic valve disease [66], osteoarthritis [67] and certain cancers [25], while only γ-carboxylated GRP has shown to display antimineralization capacity [66,67]. Interestingly, we have recently shown that GRP is involved in the crosstalk between inflammation and calcification of articular tissues in osteoarthritis, acting as an antiinflammatory agent [67].…”

Cardiovascular Calcification and Bone: A Comparison of the Effects of Dietary and Serum Vitamin K and its Dependent Proteins

“…This calcium binding property may determine whether calcium ends up in bone (full carboxylation) or soft tissue (undercarboxylation) although the study of MGP carboxylation has now become more complicated by the parallel assessment of its serine phosphorylation, thought to promote the cellular release of MGP, so that both γ-carboxylation and serine phosphorylation can impact CV calcification 12 . The recent discovery of Gla-rich protein (GRP) may additionally be of relevance since it also has calcium-binding properties and regulated extracellular calcium metabolism 13 ; both cGRP and ucGRP have been found at sites of microcalcification 14 . Similarly the vitamin K-dependent protein Gas-6 affects apoptosis of VSMCs and bone metabolism, while periostin regulates angiogenesis 14 , although very limited studies have so far been carried out.…”

“…The recent discovery of Gla-rich protein (GRP) may additionally be of relevance since it also has calcium-binding properties and regulated extracellular calcium metabolism 13 ; both cGRP and ucGRP have been found at sites of microcalcification 14 . Similarly the vitamin K-dependent protein Gas-6 affects apoptosis of VSMCs and bone metabolism, while periostin regulates angiogenesis 14 , although very limited studies have so far been carried out.…”