The physiological effect of PRP therapy on tissue damage is exerted through the integrated action of various GFs in platelet α-granules and GFs and glycoproteins (adhesion factors) contained in the plasma, while maintaining an equilibrium in vivo (Alsousou et al., 2009; Foster et al., 2009). GFs play a vital role in the recovery from injury of cartilage, ligaments, and tendons (Fortier et al., 2011; Molloy et al., 2003; Oliva et al., 2012). In this study, seven GFs in LP-PRP used in clinical practice were measured to analyze their correlations with age, gender, and platelet counts in PRP. This study provided the following information. First, a negative correlation was observed between age and the levels of PDGF-BB and IGF-1, two of the measured GFs. No correlation was detected between age and the other five GFs. Secondly, no correlation was detected between gender and levels of GFs in PRP. Finally, a positive correlation was observed between four of the seven GFs (PDGF-BB, TGF-β1, EGF, and HGF) and platelet counts in PRP. Additionally, GF levels were higher in subjects with greater platelet counts.
Some studies analyzed the relationship between age and gender and GFs contained in PRP. Evanson et al. (2014) reported that levels of PDGF-BB, TGF-β1, IGF-1, and EGF in the LP-PRP were statistically higher in subjects younger than 25 years old compared to those in subjects aged 26 years old or older. Additionally, some other studies reported a weak negative correlation between age and levels of PDGF-BB and IGF-1, but these studies did not describe the details of purified PRP (Cho et al., 2011; Dragoo et al., 2012; Weibrich et al., 2002). However, Anitua et al. reported that IGF-I levels were significantly reduced and HGF levels were increased in elderly individuals compared with the young group (Anitua et al., 2013). The results of this study indicate a correlation between age and levels of PDGF-BB and IGF-1 in the LP-PRP, consistent with previous studies. Moreover, the correlations between age and PDGF-BB (r = − 0.32) or IGF-1 (r = − 0.39) were weakly negative correlations, and no correlation was observed with the other five types of GFs (i.e., TGF-β1, VEGF, FGF, EGF, and HGF). PDGF is a potent mitogen and chemotactic factor for cells of mesenchymal origin, on which it demonstrates a major proliferative effect as well as inhibition of differentiation (Husmann et al., 1996). Also, PDGF-BB is one of the three isoforms of PDGF; with regard to muscle precursor cells in vitro, PDGF-BB showed a significant promotion of proliferation and inhibition of differentiation (Jin et al., 1990). IGF-1 also plays an important role in muscle regeneration, and its action has been clarified. It promotes both proliferation and differentiation of myoblasts, induces myofiber hypertrophy, and protects from atrophy. IGF-1 also increases myoblast proliferation (Barton-Davis et al., 1999) and myofiber protein synthesis (Bark et al., 1998). In other words, since both PDGF-BB and IGF-1 have a function to proliferate muscle cells in muscle regeneration, the results of this study suggest that LP-PRP prepared from young patients may be more effective for various musculoskeletal conditions in the clinical setting. In addition, PDGF-BB and IGF-1 inhibit nuclear factor-κB (NF-κB) activity and apoptosis via suppression of IL-1β (Montaseri et al., 2011). Since NF-κB plays a crucial role in OA, leading to cartilage destruction and articular damage (Roman-Blas & Jimenez, 2006), when using PRP for the treatment of OA, age must also be taken into consideration.
No significant difference was observed between gender and GFs in PRP in this study. A few studies reported a correlation between gender and GFs (Evanson et al., 2014; Weibrich et al., 2002; Xiong et al., 2017), but these results were inconsistent. Hormonal differences drive many sex-related differences between men and women. In particular, menstruation in women has a significant impact on hormones; increases in inflammatory markers were observed under the influence of menopause in some studies (Pfeilschifter et al., 2002; Singh & Newman, 2011). Subjects in this study were healthy adults between 21 and 48 years of age, and although the menstrual cycle was not taken into account, the female subjects were not affected by menopause. There was a possibility that the difference in the target age group influenced the results of this study.
There are some reports on platelet counts in PRP and GF levels (Anitua et al., 2009; Eppley et al., 2004; Magalon et al., 2014; McCarrel & Fortier, 2009; Sanchez et al., 2007). Despite earlier reports in the literature, the platelet number in PRP is positively correlated with GF concentration, but it is reported that GFs are not enriched as much as platelets and that there is a difference in concentration rate depending on the type of GFs. Several GFs are stored not only in platelets but also in leukocytes (Boswell et al., 2012) and are affected by the number of WBC in the PRP (Sundman et al., 2011). In this study, we investigated PDGF-BB, TGF-β1, VEGF, EGF, FGF, and HGF as GFs contained in the α-granules of platelets and IGF-1 as a GF contained in plasma but not platelets. The GFs that correlated with the platelet count in LP-PRP were PDGF-BB, TGF-β1, EGF, and HGF; the GF that showed the strongest positive correlation among them was TGF-β1. TGF-β is a multifunctional cytokine, which plays an important role in regulating repair and remodeling following tissue injury (Husmann et al., 1996). Following trauma, TGF-β is expressed by regenerating muscles, suggesting that it may play a role in muscle regeneration (Bachl et al., 2009). However, TGF-β1 has the ability to inhibit myogenic differentiation, myoblast fusion, and the expression of various muscle-specific proteins (Florini et al., 1991; Liu et al., 2001). Moreover, since TGF-β1 contributes to increased collagen deposition, causing muscle fibrosis, experimental studies have been conducted to antagonize TGF in PRP to suppress myofibrosis (Li et al., 2016; Terada et al., 2013). On the other hand, for articular cartilage, TGF-β1 is considered as an anabolic factor and capable of activating the chondrogenic properties of reparative mesenchymal stem cells in vitro (Johnstone et al., 1998; Mackay et al., 1998). Also, TGF-β1 required for the normal development of a joint, and the lack of its signaling can cause a normal joint to develop into an osteoarthritic joint. When PRP is used for various musculoskeletal conditions, there are scenes where TGF-β1 is required and not necessary, and the optimum PRP varies depending on the environment in which it is used. A correlation was observed between platelet counts in PRP and VEGF levels in the above studies; however, although the above studies used leukocyte-rich PRP, and LP-PRP was used in this study, the average was only 300/μL. The lack of correlation with VEGF can be explained by the fact that only a few leukocytes are contained in PRP, as a positive correlation has been reported between GFs and leukocyte counts in PRP (Castillo et al., 2011; Magalon et al., 2014). Interestingly, a positive correlation was observed between platelet counts in PRP and platelet-derived HGF in this study, which has not been reported previously. HGF is an inhibitor of NF-κB, one of the main transcription factors that controls the inflammation process (Bendinelli et al., 2010). PRP contains various platelet-derived GFs, functioning in accordance with the environment of use and affecting the cells and tissues targeted by each GF. Treatment outcome may also be influenced by the combined effects of GFs in some cases (Engebretsen et al., 2010; Foster et al., 2009). Previously, PRP GF concentration was generally considered to be proportional to platelet concentration. However, the results of this study indicate that even for platelet-derived GFs, not all GF concentrations are necessarily proportional to platelet concentration. Thus, when implementing PRP treatment for tissues and organs targeted by these GFs, the GF concentration needs to be assessed separately from platelet concentration. In effect, the “more is better” theory does not appear to be valid for platelet concentration in PRP (Giusti et al., 2014). Rather, an appropriate type of PRP needs to be selected in accordance with the target tissue and environment of use.
While this study investigated inherent differences among individual patients in the PRP composition, host factors may also affect responses to PRP treatment. For example, the younger the age in this study, the higher the IGF-1 concentration in PRP, but the response of tissues to GFs varies based on host characteristics such as age and gender. Furthermore, subject age may affect the potential for regenerative responses. As previously mentioned, older, arthritic chondrocytes show a blunted response to IGF-1 (Loeser et al., 2014; Verschure et al., 1995). For future research, based on the results of this study, the impact of host factors on PRP treatment should also be considered.
This study has some limitations. First, the subject age groups were limited. As this study was designed to include subjects in their 20s, 30s, and 40s with athletes in mind, data obtained from this study do not provide information for the middle-aged population of 50 years of age or older. As PRP is used in injection therapy for osteoarthritis of the knee, data from the population of 50 years of age or older are required in the future. Secondly, the results of this study are mostly exploratory in nature because the sample size was insufficient to confirm the relationship between GFs and age and gender, respectively. Thus, the findings should be interpreted cautiously.