In the present study, we found that AZ men with low sperm motility had significantly higher SP IGF-I, IGF-II, IGFBP-2 and PSA levels than NZ and OZ men implying that increased SP levels of these peptides was associated with decreased sperm motility, but not decreased sperm count. Conversely, SP IGFBP-3 levels (consisting mainly of low molecular weight IGFBP-3 fragments) were similar across the three groups suggesting that this peptide does not contribute to any meaningful effect in the pathogenesis of male infertility. The presence of IGF-I, IGF-II, and IGF-I and IGF-II receptors in mature motile spermatozoa suggests a direct role of these peptides in modulating in vivo sperm motility, and SP IGF-II, IGFBP-2 and PSA, but not SP IGF-I, levels increase with aging suggesting that SP IGF-II may be an implicating factor in the development of BPH and prostate cancer as men age.
Previous studies have investigated the role of IGF-I in sperm function, semen quality, and infertility in terms of a mitogenic, metabolic, and differentiating polypeptide with endocrine, paracrine, and autocrine effects [8, 30]. Animal studies have demonstrated that the proportion of normal spermatozoa was related to serum IGF-I levels in bulls [31]. Additionally, bovine SP IGF-I levels were related to semen quality [32] and motility in buffalos [33] and rams [34], and exogenous IGF-I administration improved sperm parameters in buffalo bulls [35]. Seminal plasma, the non-cellular component of semen, is a heterogeneous composite fluid built by secretions of the testis, the epididymis and the accessory sexual glands that is relevant for fertility [36]. Early human studies have shown the presence of IGF-I, IGF-II, IGFBP-2, IGFBP-3 fragments, IGFBP-4, IGFBP-5, IGFBP- protease activities and PSA in SP [14, 18, 19, 27, 37-39], while recent studies have reported that SP IGF-I levels do not differ between men with normal and abnormal semen parameters [15, 20]. Evidence that vasectomized patients had lower SP IGF-I levels than intact patients suggests that SP IGF-I is mainly of testicular origin [39], whereas in patients with varicocele (a common cause of male infertility), higher SP IGF-I levels than healthy fertile men have been observed implying a role of localized IGF-I in the pathogenesis of male infertility [17].
We and others [30, 40] have shown that spermatozoa contains IGF-I receptors, so it is possible that the increased SP IGF-I levels can directly modulate sperm motility through these receptors. Although our data are in line with some studies where higher SP IGF-I levels are associated with abnormal sperm parameters [14, 17], others have reported either improved sperm parameters with increased SP IGF-I levels [9, 30, 33, 41] or unchanged SP IGF-I levels with abnormal sperm parameters [15, 20]. These discordant results suggest that there might be other local factors (e.g., IGF-II, IGFBPs and IGFBP proteolytic activity) that can influence the bioavailability and bioactivity of SP IGF-I.
To our knowledge, the present study is the first to investigate whether SP IGF-II might have any relationship with sperm function. The levels of IGF-II observed in the SP of fertile NZ men (1130 ± 79 ng/mL) are comparable with those reported by Ramasharma et al. [18] (1575 ± 66 ng/mL), and are notably 15-20 fold higher than the SP IGF-I levels. Our results concerning SP from AZ men with low sperm motility and increased IGF-II levels suggest that IGF-II, together with IGF-I, may act synergistically to inhibit sperm motility. Our findings are consistent with those reported by Henricks et al. [40] in which exogenous IGF-I and IGF-II increased the motility of washed bovine spermatozoa in an in vitro system devoid of IGFBPs. However, the in vitro system used in this study does not fully represent the in vivo environment because substantial amounts of IGFBPs are present in bovine SP that may modify the half-life and receptor interactions of IGFs.
Our study demonstrated that SP IGFBP-2 levels were significantly higher in AZ compared to NZ and OZ men. In normal men, there were no differences in SP IGFBP-2 levels before and after vasectomy, implying decreased IGFBP-2 secretion occurs in the testis rather than in the accessory sex glands [39]. This is consistent with the findings by Cohen et al. [42] who reported that IGFBP-2 was found in prostate epithelial cell-conditioned medium. Studies from both in vivo and in vitro models suggest that IGFBP-2 exerts inhibitory effects on IGF-mediated functions [43]. Hence, it is likely that SP IGFBP-2 may not induce any meaningful inhibitory effects on IGF-I and IGF-II actions in affecting sperm motility. Our data also indicated that SP IGFBP-3 levels were similar across the three groups. We have previously shown that SP IGFBP-3 is proteolyzed into low molecular weight fragments by PSA and other local proteases, resulting in a loss of intact IGFBP-3 [27]. These low molecular weight IGFBP-3 fragments do not to bind to IGF-I or IGF-II, and were comparable between normal and vasectomized males, and patients with idiopathic azoospermia, suggesting that the testicular contribution of SP IGFBP-3 is probably minimal [27]. Seminal plasma of proteolyzed IGFBP-3 would lead to increased IGF bioavailability [38] because intact IGFBP-3 would bind strongly to IGFs. Hence, the proteolyzed IGFBP-3 fragments in SP might not have a direct effect compared to the intact IGFBP-3 on sperm motility because other IGF independent actions of IGFBP-3 requires the intact molecule. The fact that no changes of IGFBP-3 levels in the SP of AZ, OZ and NZ men were observed in the present study suggests that proteolyzed IGFBP-3 did not exert any direct effects on sperm motility in vivo. Conversely, elevated PSA levels were observed in the SP of AZ men, but whether there is a direct relationship between elevated SP PSA levels and reduced sperm motility remains to be clarified.
Miao et al. [16] first investigated the direct effects of IGFs and IGFBPs on in vitro sperm motility by using the ‘swim up’ technique utilizing washed sperm technique combined with computer video tracking methods. These authors found that IGF-I and IGFBP-3 caused significant and differing changes in sperm motility parameters, whereas IGF-II, IGFBP-2 and IGF-I/IGFBP-3 had no significant effects [16]. The mechanisms by which the IGF-I and IGFBP-3 exert these in vivo and in vitro effects on mature sperm is difficult to explain because of the uniqueness of the mature spermatozoa in not having a normal functioning nucleus, nor many of the normal processes of RNA transcription and translation. Additionally, the presence of IGF-I and IGF-II mRNA in the male prostate gland and testis have been reported [42, 44], but little is known about their function in mature spermatozoa. In light of these findings, we sought to evaluate the expression of mRNAs for the IGFs and IGF receptors in mature motile human spermatozoa. We found that IGF-I mRNA was detectable in low amounts in mature motile spermatozoa from fertile volunteers, whereas IGF-II mRNA was highly expressed. Furthermore, we found that spermatozoa can express type I and type II IGF receptors. Animal studies in bovine species have shown an increase in motile sperm when exposed to exogenous recombinant IGF-I and IGF-II [45]. The effects of IGF-I on sperm motility suggest increased flagella motion and thrust that is important in facilitating penetration of the zona pellucida [46] and in the transport through oviductal mucus [47]. Early studies have shown that mRNA for IGF-I and IGF-I receptor are expressed in oviducts, uteri and embryos [48-50]. Bongso et al. [51, 52] reported a strong correlation of sperm motility with fertilization, and showed that in vitro fertilization rates were higher when female tubal cells were added to sperm and oocytes in a co-culture system. The demonstration of the presence of IGFs and their receptors imply that complex interactions between the oviductal IGFs, sperm cell IGF receptors and sperm micro-anatomical structures in the tail/flagellum (e.g., axoneme, microtubules, and tubulin) may be at play in the regulation of sperm motility.
Previous studies have reported an association between male infertility and future prostate cancer [53, 54]. Dong et al. [55] also reported that IGF-II mRNA concentrations are 10-fold higher in prostate stromal cells from patients with BPH compared to normal men. Our findings that SP IGF-II increases with age suggest that the autocrine/paracrine actions of IGF-II may directly modulate prostate growth and possibly play a role in malignant prostatic changes. Our finding also helps to explain the paradox that although serum IGF-I has been shown to be a risk factor for prostate cancer, the incidence of BPH and prostate cancer increases whereas serum IGF-I levels decline with aging.
The limitations of our study are the small study numbers due to the rarity of AZ and OZ men in the general population and the fact that these men did not undergo specific examinations of the prostate and seminal vesicles by ultrasound or biochemical markers for improper function. The strengths, on the other hand, are the prospective design and the strict inclusion criteria of relatively healthy men over a wide age range with abnormalities of either sperm count or sperm motility, and no other ongoing medical conditions. Our data, thus, apply to men with stable causes, and not to those with transient dynamic causes of male infertility.
In summary, our study has demonstrated that changes in SP IGFs, IGFBPs and PSA profiles are associated with decreased sperm motility but not sperm count, and that the presence of IGF-I, IGF-II, IGF-I receptor and IGF-II receptor in mature motile spermatozoa suggests a role via direct effects on sperm motility. The mechanisms of how differences in SP IGF-I, IGF-II, IGFBP-2, and PSA levels between AZ and OZ men come about remains to be elucidated. Furthermore, our results also suggest a possible autocrine/paracrine role of prostatic IGF-II and the development of prostate diseases with male aging. Further studies are needed to better understand the clinical significance and biological functions of these SP peptides on sperm characteristics in male infertility and prostate diseases as men age.