Cellular responses to silencing of PDIA3 (protein disulphide-isomerase A3): Effects on proliferation, migration, and genes in control of active vitamin D.

The active form of vitamin D, 1,25-dihydroxyvitamin D3, is known to act via VDR (vitamin D receptor), affecting several physiological processes. In addition, PDIA3 (protein disulphide-isomerase A3) has been associated with some of the functions of 1,25-dihydroxyvitamin D3. In the present study we used siRNA-mediated silencing of PDIA3 in osteosarcoma and prostate carcinoma cell lines to examine the role(s) of PDIA3 for 1,25-dihydroxyvitamin D3-dependent responses. PDIA3 silencing affected VDR target genes and significantly altered the 1,25-dihydroxyvitamin D3-dependent induction of CYP24A1, essential for elimination of excess 1,25-dihydroxyvitamin D3. Also, PDIA3 silencing significantly altered migration and proliferation in prostate PC3 cells, independently of 1,25-dihydroxyvitamin D3. 1,25-Dihydroxyvitamin D3 increased thermostability of PDIA3 in cellular thermal shift assay, supporting functional interaction between PDIA3 and 1,25-dihydroxyvitamin D3-dependent pathways. In summary, our data link PDIA3 to 1,25-dihydroxyvitamin D3-mediated signalling, underline and extend its role in proliferation and reveal a novel function in maintenance of 1,25-dihydroxyvitamin D3 levels.

Activated EGFR and PDGFR internalize in separate vesicles and downstream AKT and ERK1/2 signaling are differentially impacted by cholesterol depletion.

The interplay between membrane subregions and receptor tyrosine kinases (RTK) will influence signaling in both normal and pathological RTK conditions. In this study, epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor ß (PDGFR-ß) internalizations were investigated by immunofluorescent microscopy following simultaneous treatment with EGF and PDGF-BB. We found that the two receptors utilize separate routes of internalization, which merges in a common perinuclear endosomal compartment after 45 min of stimulation. This is further strengthened when contrasting the recruitment of either EGFR or PDGFR-ß to either clathrin or caveolin-1: PDGFR-ß dissociates from caveolin-1 upon stimulation, and engages clathrin, whilst an increased recruitment of EGFR, to both clathrin and caveolin-1, was observed upon EGF stimulation. The association between EGFR and caveolin-1 is supported by the observation that EGFR was localized in lipid raft associated fractions, whereas PDGFR-ß was not. We also found that disruption of lipid rafts using MßCD led to an increased EGFR dimerization and phosphorylation in response to ligand, as well as a dramatic decrease in AKT- and a smaller but robust decrease in ERK1/2 phosphorylation. This suggest that lipid rafts may be important to effectively connect the EGFR with downstream proteins to facilitate signaling. Our data implies that cholesterol depletion of the plasma membrane affect the signaling of EGFR and PDGFRß differently.

The E3 Ubiquitin Ligase TRIM21 Regulates Basal Levels of PDGFRß.

Activation of platelet-derived growth factor (PDGF) receptors α and ß (PDGFRα and PDGFRß) at the cell surface by binding of PDGF isoforms leads to internalization of receptors, which affects the amplitude and kinetics of signaling. Ubiquitination of PDGF receptors in response to ligand stimulation is mediated by the Casitas b-lineage lymphoma (Cbl) family of ubiquitin ligases, promoting internalization and serving as a sorting signal for vesicular trafficking of receptors. We report here that another E3 ligase, i.e., tripartite motif-containing protein 21 (TRIM21), contributes to the ubiquitination of PDGFRß in human primary fibroblasts AG1523 and the osteosarcoma cell line U2OS and regulates basal levels of PDGFRß. We found that siRNA-mediated depletion of TRIM21 led to decreased ubiquitination of PDGFRß in response to PDGF-BB stimulation, while internalization from the cell surface and the rate of ligand-induced degradation of the receptor were not affected. Moreover, induction of TRIM21 decreased the levels of PDGFRß in serum-starved cells, and even more in growing cells, in the absence of PDGF stimulation. Consistently, siRNA knockdown of TRIM21 caused accumulation of the total amount of PDGFRß, both in the cytoplasm and on the cell surface, without affecting mRNA levels of the receptor. We conclude that TRIM21 acts post-translationally and maintains basal levels of PDGFRß, thus suggesting that ubiquitination of PDGFRß by TRIM21 may direct a portion of receptor for degradation in growing cells in a ligand-independent manner.

SUMOylation of PDGF receptor α affects signaling via PLCγ and STAT3, and cell proliferation.

BACKGROUND: The platelet-derived growth factor (PDGF) family of ligands exerts their cellular effects by binding to α- and ß-tyrosine kinase receptors (PDGFRα and PDGFRß, respectively). SUMOylation is an important posttranslational modification (PTM) which regulates protein stability, localization, activation and protein interactions. A mass spectrometry screen has demonstrated SUMOylation of PDGFRα. However, the functional role of SUMOylation of PDGFRα has remained unknown. RESULTS: In the present study, we validated that PDGFRα is SUMOylated on lysine residue 917 as was previously reported using a mass spectrometry approach. Mutation of lysine residue 917 to arginine (K917R) in PDGFRα substantially decreased SUMOylation, indicating that this amino acid residue is a major SUMOylation site. Whereas no difference in the stability of wild-type and mutant receptor was observed, the K917R mutant PDGFRα was less ubiquitinated than wild-type PDGFRα. The internalization and trafficking of the receptor to early and late endosomes were not affected by the mutation, neither was the localization of the PDGFRα to Golgi. However, the K917R mutant PDGFRα showed delayed activation of PLC-γ and enhanced activation of STAT3. Functional assays showed that the mutation of K917 of PDGFRα decreased cell proliferation in response to PDGF-BB stimulation. CONCLUSIONS: SUMOylation of PDGFRα decreases ubiquitination of the receptor and affects ligand-induced signaling and cell proliferation.

Effects of 1α,25-dihydroxyvitamin D3 and tacalcitol on cell signaling and anchorage-independent growth in T98G and U251 glioblastoma cells.

The active hormonal form of vitamin D, 1α,25-dihydroxyvitamin D3, is reported to have 1000s of biological targets. The growth-suppressive properties of 1α,25-dihydroxyvitamin D3 and its synthetic analogs have attracted interest for the development of treatment and/or prevention of cancer. We examined effects of 1α,25-dihydroxyvitamin D3 and the vitamin D analog tacalcitol on signaling pathways and anchorage-independent growth in T98G and U251 glioblastoma cells. Assay of signaling proteins important for cellular growth indicated suppression of p70-S6 kinase levels by 1α,25-dihydroxyvitamin D3 and tacalcitol in T98G cells, whereas the levels of PLCγ, a target for phospholipid signaling, was slightly increased. Activation of STAT3, an important regulator of malignancy, was suppressed by 1α,25-dihydroxyvitamin D3 and tacalcitol in T98G and U251 cells. However, despite the close structural similarity of these compounds, suppression was stronger by tacalcitol (1α,24-dihydroxyvitamin D3), indicating that even minor modifications of a vitamin D analog can impact its effects on signaling. Experiments using soft agar colony formation assay in T98G and U251 cells revealed significant suppression by 1α,25-dihydroxyvitamin D3 and tacalcitol on anchorage-independent growth, a property for cancer invasion and metastasis known to correlate with tumorigenicity. These findings indicate that vitamin D and its analogs may be able to counteract the oncogenic transformation, invasion and metastatic potential of glioblastoma and prompt further study of these compounds in the development of improved therapy for brain cancer.

Differential impact of lipid raft depletion on platelet-derived growth factor (PDGF)-induced ERK1/2 MAP-kinase, SRC and AKT signaling.

It has become clear that lipid rafts functions as signaling hotspots connecting cell surface receptors to intracellular signaling pathways. However, the exact involvement of lipid rafts in receptor tyrosine kinase signaling is still poorly understood. In this study, we have analyzed platelet-derived growth factor (PDGF) receptor ß (PDGFR-ß) signaling in two different cell lines depleted of cholesterol, and as a consequence, disruption of lipid rafts. Cholesterol depletion of BJ-hTERT fibroblasts using methyl-ß-cyclodextrin (MßCD) did not affect PDGFR-ß activation as measured by its tyrosine phosphorylation. However, we did observe a small reduction in AKT phosphorylation and a more robust decrease of ERK1/2 activation. In contrast, in the osteosarcoma cell line U2OS, we noticed a deficient receptor activation. Interestingly, in U2OS cells, the ERK1/2 pathway was unaffected, but instead AKT and SRC signaling was reduced. These results suggest that cell type specific wiring of signaling pathways can lead to differential sensitivity to cholesterol depletion. Furthermore, MßCD treatment had a much more pronounced morphological effect on U2OS compared to BJ-hTERT cells. This is consistent with a previous report claiming that cancer cells are more sensitive to cholesterol depletion than normal cells. Our data supports the possibility that cholesterol lowering drugs may impede tumor growth.

Deubiquitinating enzymes USP4 and USP17 finetune the trafficking of PDGFRß and affect PDGF-BB-induced STAT3 signalling.

Interaction of platelet-derived growth factor (PDGF) isoforms with their receptors results in activation and internalization of receptors, with a concomitant activation of downstream signalling pathways. Ubiquitination of PDGFRs serves as a mark to direct the internalization and sorting of the receptors. By overexpressing a panel of deubiquitinating enzymes (DUBs), we found that USP17 and USP4 efficiently deubiquitinate PDGF receptor ß (PDGFRß) and are able to remove both Lys63 and Lys48-linked polyubiquitin chains from the receptor. Deubiquitination of PDGFRß did not affect its stability, but regulated the timing of its trafficking, whereby USP17 prolonged the presence of the receptor at the cell surface, while USP4 affected the speed of trafficking towards early endosomes. Induction of each of the DUBs in BJhTERT fibroblasts and U2OS osteosarcoma cells led to prolonged and/or shifted activation of STAT3 in response to PDGF-BB stimulation, which in turn led to increased transcriptional activity of STAT3. Induction of USP17 promoted acute upregulation of the mRNA expression of STAT3-inducible genes STAT3, CSF1, junB and c-myc, while causing long-term changes in the expression of myc and CDKN1A. Deletion of USP17 was lethal to fibroblasts, while deletion of USP4 led to a decreased proliferative response to stimulation by PDGF-BB. Thus, USP17- and USP4-mediated changes in ubiquitination of PDFGRß lead to dysregulated signalling and transcription downstream of STAT3, resulting in defects in the control of cell proliferation.

Erratum: Specific targeting of PDGFRß in the stroma inhibits growth and angiogenesis in tumors with high PDGF-BB expression: Erratum.

[This corrects the article DOI: 10.7150/thno.37851.].

Specific targeting of PDGFRß in the stroma inhibits growth and angiogenesis in tumors with high PDGF-BB expression.

PDGF-BB/PDGFRß signaling plays an important role during vascularization by mediating pericyte recruitment to the vasculature, promoting the integrity and function of vessels. Until now it has not been possible to assess the specific role of PDGFRß signaling in tumor progression and angiogenesis due to lack of appropriate animal models and molecular tools. Methods: In the present study, we used a transgenic knock-in mouse strain carrying a silent mutation in the PDGFRß ATP binding site that allows specific targeting of PDGFRß using the compound 1-NaPP1. To evaluate the impact of selective PDGFRß inhibition of stromal cells on tumor growth we investigated four tumor cell lines with no or low PDGFRß expression, i.e. Lewis lung carcinoma (LLC), EO771 breast carcinoma, B16 melanoma and a version of B16 that had been engineered to overexpress PDGF-BB (B16/PDGF-BB). Results: We found that specific impairment of PDGFRß kinase activity by 1-NaPP1 treatment efficiently suppressed growth in tumors with high expression of PDGF-BB, i.e. LLC and B16/PDGF-BB, while the clinically used PDGFRß kinase inhibitor imatinib did not suppress tumor growth. Notably, tumors with low levels of PDGF-BB, i.e. EO771 and B16, neither responded to 1-NaPP1 nor to imatinib treatment. Inhibition of PDGFRß by either drug impaired tumor vascularization and also affected pericyte coverage; however, specific targeting of PDGFRß by 1-NaPP1 resulted in a more pronounced decrease in vessel function with increased vessel apoptosis in high PDGF-BB expressing tumors, compared to treatment with imatinib. In vitro analysis of PDGFRß ASKA mouse embryo fibroblasts and the mesenchymal progenitor cell line 10T1/2 revealed that PDGF-BB induced NG2 expression, consistent with the in vivo data. Conclusion: Specific targeting of PDGFRß signaling significantly inhibits tumor progression and angiogenesis depending on PDGF-BB expression. Our data suggest that targeting PDGFRß in the tumor stroma could have therapeutic value in patients with high tumor PDGF-BB expression.

Structure-based discovery of novel small molecule inhibitors of platelet-derived growth factor-B.

Platelet-derived growth factor (PDGF) is a family of growth factors with mitogenic and chemotactic activity. However, uncontrolled and overactivated PDGF signaling has been implicated in a variety of diseases, such as cancers and atherosclerosis. In this context, inhibition of PDGF-PDGFR signaling is of paramount importance in progression of such diseases. The purpose of the current study was to identify novel PDGF-B inhibitors using virtual screening methods. To this end, a combination of molecular modeling techniques such as molecular docking and dynamics simulation, as well as drug likeness filtering criteria, was applied to select anti-PDGF peptidomimetic candidates based on crystallography solved structure of an anti-PDGF-B monoclonal antibody named, MOR8457. In vitro biological assays of the selected compounds revealed two of them being active at micromolar IC50 concentrations. The presented work can provide a framework for systematic peptidomimetic identification for anti-PDGF-B agents from large chemical libraries.

Dual specificity phosphatase (DUSP)-4 is induced by platelet-derived growth factor -BB in an Erk1/2-, STAT3- and p53-dependent manner.

Dual specificity phosphatase (DUSP) 4 has been described as a negative regulator of MAP kinase signaling, in particular for the ERK1/2 and JNK pathways. We found that DUSP4 expression was upregulated in response to prolonged platelet-derived growth factor (PDGF)-BB stimulation. The PDGF-BB-induced DUSP4 expression was dependent on ERK1/2, STAT3 and p53. We found that inhibition of ERK1/2 effectively reduced DUSP4 mRNA levels, whereas STAT3 was necessary for maintaining p53 expression. p53 has binding sites in the DUSP4 promoter and was found to promote DUSP4 expression.

Dynamin inhibitors impair platelet-derived growth factor ß-receptor dimerization and signaling.

The role of plasma membrane composition and dynamics in the activation process of receptor tyrosine kinases (RTKs) is still poorly understood. In this study we have investigated how signaling via the RTK, platelet-derived growth factor ß-receptor (PDGFR-ß) is affected by Dynasore or Dyngo-4a, which are commonly used dynamin inhibitors. PDGFR-ß preferentially internalizes via clathrin-coated pits and in this pathway, Dynamin II has a major role in the formation and release of vesicles from the plasma membrane by performing the membrane scission. We have found that dynamin inhibitors impedes the activation of PDGFR-ß by impairing ligand-induced dimerization of the receptor monomers, which leads to a subsequent lack of phosphorylation and activation both of receptors and downstream effectors, such as ERK1/2 and AKT. In contrast, dynamin inhibitors did not affect epidermal growth factor receptor (EGFR) dimerization and phosphorylation. Our findings suggest that there is a link between plasma membrane dynamics and PDGFR-ß activation, and that this link is not shared with the epidermal growth factor receptor.

PDGFRß translocates to the nucleus and regulates chromatin remodeling via TATA element-modifying factor 1.

Translocation of full-length or fragments of receptors to the nucleus has been reported for several tyrosine kinase receptors. In this paper, we show that a fraction of full-length cell surface platelet-derived growth factor (PDGF) receptor ß (PDGFRß) accumulates in the nucleus at the chromatin and the nuclear matrix after ligand stimulation. Nuclear translocation of PDGFRß was dependent on PDGF-BB-induced receptor dimerization, clathrin-mediated endocytosis, ß-importin, and intact Golgi, occurring in both normal and cancer cells. In the nucleus, PDGFRß formed ligand-inducible complexes with the tyrosine kinase Fer and its substrate, TATA element-modifying factor 1 (TMF-1). PDGF-BB stimulation decreased TMF-1 binding to the transcriptional regulator Brahma-related gene 1 (Brg-1) and released Brg-1 from the SWI-SNF chromatin remodeling complex. Moreover, knockdown of TMF-1 by small interfering RNA decreased nuclear translocation of PDGFRß and caused significant up-regulation of the Brg-1/p53-regulated cell cycle inhibitor CDKN1A (encoding p21) without affecting PDGFRß-inducible immediate-early genes. In conclusion, nuclear interactions of PDGFRß control proliferation by chromatin remodeling and regulation of p21 levels.

The PDGF/PDGFR pathway as a drug target.

Platelet-derived growth factors (PDGF) promotes cell proliferation, survival and migration, primarily of cells of mesenchymal origin. Dysfunction of PDGF signaling has been observed in a wide array of pathological conditions, such as cancer, fibrosis, neurological conditions and atherosclerosis. Reported abnormalities of the PDGF pathway include overexpression or amplification of PDGF receptors (PDGFRs), gain of function point mutations or activating chromosomal translocations. Current development of therapeutic drugs often aims at producing compounds that specifically target interaction between PDGFs and their receptors by specific DNA aptamers and ligand traps, or downregulate PDGFRs with blocking antibodies, or inhibit tyrosine kinase activity of PDGFRs with small molecules. In this review, we discuss some of the approaches taken to interfere with PDGF signaling, review a panel of existing therapeutic drugs, and consider clinically successful cases and remaining challenges.

Synergistic effects of combining proteasome inhibitors with chemotherapeutic drugs in lung cancer cells.

BACKGROUND: The prognosis for patients with disseminated lung cancer is poor and current treatments have limited survival benefit as resistance often occurs, and is often associated with significant toxicity. A possible strategy to improve treatment and evade chemoresistance may be to find new combinations of drugs. The aim of this study was to analyze the potential of combining proteasome inhibitors (PIs) with chemotherapeutic drugs used in the routine treatment for lung cancer patients. RESULTS: The median-effect method was applied to the Fluorometric Microculture Cytotoxicity Assay (FMCA) to evaluate effects of combining two different PIs (bortezomib and b-AP15) with clinically used chemotherapeutic drugs representing different mechanisms of action (cisplatin, gefitinib, gemcitabine and vinorelbine) in two lung cancer cell lines (one sensitive and one resistant). Proteasome inhibition in combination with cisplatin, gemcitabine or vinorelbine had synergistic effects in at least one of the tested cell lines. Furthermore, the effect of gefitinib appeared strongly potentiated by the PI in the least resistant lung cancer cell line, although the level of synergy could not be determined with the median-effect method. CONCLUSIONS: Combining PIs with cisplatin, gefitinib, gemcitabine or vinorelbine show potential as new combination chemotherapy for the treatment of lung cancer.

PDGF-BB enhances collagen gel contraction through a PI3K-PLCγ-PKC-cofilin pathway.

Cell-mediated contraction of collagenous matrices is modulated by various growth factors and cytokines, such as platelet-derived growth factor-BB (PDGF-BB). Here we used a genetic cell model to delineate defined signaling pathways that enhance collagen gel contraction downstream of ligand-stimulated platelet-derived growth factor receptor-ß (PDGF-Rß). Our data show that PDGF BB-enhanced activations of phosphatidylinositol 3'-kinase (PI3K) and phospholipase Cγ (PLCγ) were necessary for PDGF-enhanced collagen gel contraction. Importantly, other defined signaling pathways down-stream of PDGF-Rß were, however, dispensable. The decisive roles for PI3K and PLCγ were corroborated by experiments using selective inhibitors. Furthermore, we show that de-phosphorylation and thereby activation of cofilin that is important for the turnover of actin filaments, is depended on PI3K and PLCγ down-stream of PDGF-Rß. Moreover, inhibition of protein kinase C (PKC) by GÖ6976 and bisindolylmaleimide-II abolished cofilin de-phosphorylation, as well as PDGF-enhanced contraction. In contrast, activation of the PKC protein family by 4ß-phorbol 12-myristate 13-acetate (PMA) did not accelerate collagen gel contraction although it induced long-term cofilin de-phosphorylation, showing the need of a dynamic control of cofilin de-phosphorylation for PDGF-enhanced collagen gel contraction. Taken together, our data point to the involvement of a PI3K/PLCγ-PKC-cofilin pathway in both PDGF-enhanced cofilin de-phosphorylation and PDGF-enhanced collagen gel contraction.

Platelet-derived growth factor (PDGF)-induced activation of Erk5 MAP-kinase is dependent on Mekk2, Mek1/2, PKC and PI3-kinase, and affects BMP signaling.

Platelet-derived growth factor-BB (PDGF-BB) binds to its tyrosine kinase receptors (PDGFRs) and stimulates mitogenicity and survival of cells of mesenchymal origin. Activation of PDGFRs initiates a number of downstream signaling pathways, including phosphatidyl 3'-inositol kinase (PI3-kinase), phospholipase Cγ and MAP kinase pathways. In this report, we show that Erk5 MAP kinase is activated in response to PDGF-BB in the smooth muscle cell line MOVAS in a manner dependent on Mekk2, Mek1/2, Mek5, PI3-kinase and protein kinase C (PKC). The co-operation of Mek1/2 and Mekk2 in the activation of Erk5, suggests a close co-regulation between the Erk1/2 and Erk5 MAP kinase pathways. Furthermore, we found that classical PKCs are important for Erk5 activation. In addition, we found that PKCζ interacts with Erk5 and may exert a negative feed-back effect. We observed no nuclear accumulation of Erk5 in response to PDGF-BB stimulation, however, we identified a mechanism by which cytoplasmic Erk5 influences gene expression; Erk5 was essential for PDGF-BB-mediated Smad1/5/8 signaling by stimulating release and/or activation of bone morphogenetic protein(s) (BMPs). Thus, PDGF-BB-induced Erk5 activation involves parallel stimulatory and inhibitory pathways and promotes Smad1/5/8 signaling.

The Ubiquitin Ligases c-Cbl and Cbl-b Negatively Regulate Platelet-derived Growth Factor (PDGF) BB-induced Chemotaxis by Affecting PDGF Receptor ß (PDGFRß) Internalization and Signaling.

Protein ubiquitination controls protein stability and subcellular localization of tyrosine kinase receptors, hence affecting signaling both quantitatively and qualitatively. In this report, we demonstrate that, after ligand stimulation, the PDGF ß receptor (PDGFRß) becomes ubiquitinated in a manner requiring both the c-Cbl and Cbl-b ubiquitin ligases. Simultaneous depletion of c-Cbl and Cbl-b resulted in reduced ligand-induced PDGFRß clearance from the cell surface because of reduced endocytosis of the receptor. Cbl-b formed a complex with c-Cbl, as well as with the PDGFRß, in response to PDGF-BB stimulation. We were unable to find a direct interaction between the receptor and c-Cbl, raising the possibility that Cbl-b is necessary for c-Cbl to interact with PDGFRß. Phosphorylated Tyr-1021 in PDGFRß was the primary interaction site for Cbl-b, with some contribution from Tyr-1009. Depletion of c-Cbl and Cbl-b led to an increased ligand-induced tyrosine phosphorylation of the receptor. Several tyrosine residues with elevated phosphorylation (i.e. Tyr-579, Tyr-581, Tyr-1009, and Tyr-1021) have previously been shown to interact with Src kinases and PLCγ. Indeed, in cells depleted of c-Cbl and Cbl-b, both Src and PLCγ phosphorylation were enhanced, whereas activation of other pathways, such as Erk1/2 MAP kinase and Akt, were not affected. In addition, Stat3 phosphorylation, which has been connected to Src activity, was also elevated in cells lacking c-Cbl and Cbl-b. Functionally, we found that cells depleted of c-Cbl and Cbl-b were more prone to migrate toward PDGF-BB, whereas no reproducible effect on cell proliferation could be observed. In conclusion, internalization as well as signaling via PDGFRß are controlled by ubiquitination.

Functional Characterization of Germline Mutations in PDGFB and PDGFRB in Primary Familial Brain Calcification.

Primary Familial Brain Calcification (PFBC), a neurodegenerative disease characterized by progressive pericapillary calcifications, has recently been linked to heterozygous mutations in PDGFB and PDGFRB genes. Here, we functionally analyzed several of these mutations in vitro. All six analyzed PDGFB mutations led to complete loss of PDGF-B function either through abolished protein synthesis or through defective binding and/or stimulation of PDGF-Rß. The three analyzed PDGFRB mutations had more diverse consequences. Whereas PDGF-Rß autophosphorylation was almost totally abolished in the PDGFRB L658P mutation, the two sporadic PDGFRB mutations R987W and E1071V caused reductions in protein levels and specific changes in the intensity and kinetics of PLCγ activation, respectively. Since at least some of the PDGFB mutations were predicted to act through haploinsufficiency, we explored the consequences of reduced Pdgfb or Pdgfrb transcript and protein levels in mice. Heterozygous Pdgfb or Pdgfrb knockouts, as well as double Pdgfb+/-;Pdgfrb+/- mice did not develop brain calcification, nor did Pdgfrbredeye/redeye mice, which show a 90% reduction of PDGFRß protein levels. In contrast, Pdgfbret/ret mice, which have altered tissue distribution of PDGF-B protein due to loss of a proteoglycan binding motif, developed brain calcifications. We also determined pericyte coverage in calcification-prone and non-calcification-prone brain regions in Pdgfbret/ret mice. Surprisingly and contrary to our hypothesis, we found that the calcification-prone brain regions in Pdgfbret/ret mice model had a higher pericyte coverage and a more intact blood-brain barrier (BBB) compared to non-calcification-prone brain regions. While our findings provide clear evidence that loss-of-function mutations in PDGFB or PDGFRB cause PFBC, they also demonstrate species differences in the threshold levels of PDGF-B/PDGF-Rß signaling that protect against small-vessel calcification in the brain. They further implicate region-specific susceptibility factor(s) in PFBC pathogenesis that are distinct from pericyte and BBB deficiency.